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All-Optical Alignment and Orientation of Neutral Molecules & Attosecond Pulse Generation Holly Herbert Department of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland Summer of 2016 Abstract The central body of this report, labelled section I, focuses on the research conducted in the area of Molecular Alignment and Orientation, with section II addressing the research conducted in the area of Attosecond Pulse Generation. As detailed in section I, the alignment of a sample of gasous OCS molecules was achieved through the application of an intense single colour non-resonant laser field of frequency ω. The degree of alignment was probed via coulomb explosion of the sample molecules and subsequent measurement of the directions of the ejected fragment ions. The alignment parameter hcos2 θ i, where θ is the angle between the molecular axis and the polarisation axis of the applied field, was calculated to be 0.5791±0.001, which had increased from the background value of 0.54 beyond the margins of experimental error. Molecular orientation of the same sample was attempted via the addition of a second colour laser field of frequency 2ω, with the degree of orientation probed in the same manner. The orientation parameter hcosθ i was calculated to be 0.017±0.001, which did not increase from the background value of 0.016 beyond the margins of experimental error. Thus no conclusive evidence was found to verify the achievement of molecular orientation. The relationship between the degree of orientation achieved and the relative phase between the two applied laser fields was also investigated, with no conclusive evidence drawn to support that variations of the relative phase have any impact on the achieved degree of orientation. As detailed in section II, 40fs laser pulses outputted by a standard TiSap laser were temporally reduced to 5fs pulses through the employment of a hollow core fibre compressor and used to generate a pulse train containing 4 attosecond pulses via High Harmonic Generation from a sample of gaseous Argon. Double Optical Gating was then successfully employed in order to isolate a single attosecond pulse from this train. Section I - All Optical Molecular Alignment and Orientation Introduction symmetric top molecules. There are however a number of flaws associated with such traditional techniques. Firstly, brute force orientation requires extremely strong electrostatic fields, the presence of which often alters the chemical and physical properties of the molecules involved, thus presenting a large barrier to the accurate study of chemical and electronic steroedynamics using samples of oriented molecules. Hexapole focusing involves the spatial separation of the original sample based on rotational state, and thus provides samples of oriented molecules with densities too low for further experimentation. Finally, both techniques rely exclusively on the interaction between the electrostatic field and the permanent molecular dipole moment and thus are not applicable to non-polar molecules. In recent years, almost all molecular alignment and orientation techniques have turned to the employment of laser fields. The alignment of neutral iodine molecules [9] as well as many other molecular species [10] has been demonstrated by Sakai et al. via the application of a strong, non-resonant, linearly polarised laser field. This technique relies on the interaction between the laser field and the induced dipole moment of the molecules, and results in the alignment of the molecular axis along the The manipulation of molecules via the application of external fields has found itself to be, and indeed remains, one the most thriving areas of research in Physics. The potential to exert control over the translational and rotational degrees of freedom of molecules has wide ranging applications that span the breath and depth of Science, from allowing the investigation of chemical [1] and electronic [2] stereodynamics, to enabling the creation of molecular movies [3-5]. The resulting samples of aligned or oriented molecules offer fantastic prospects for novel experiments with complex molecules, for example High Harmonic Generation [6], and overall, for a large range of experiments in Chemistry and Physics, a high level of control over the external degrees of freedom of molecules is hugely beneficial. Many different molecular orientation techniques have been realised over the years. Brute force orientation [7] and hexapole focusing [8] were two of the first techniques developed. By Brute force orientation, a strong electrostatic field orients molecules with large dipole moments, and by hexapole focusing, inhomogeneous electrostatic fields, created by a hexapole focuser, orient state selected 1 All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation polarisation direction of the applied field. However, this method is incapable of discriminating between the parallel and anti-parallel configurations of the molecule, and so may not be used in order to achieve orientation. Many modern orientation techniques have been based on the proposal [11-12] that a combination of electrostatic field and intense non-resonant laser field may result in an enhanced degree of molecular orientation compared to that achieved by traditional methods. Techniques based on this combined field approach have been employed by the Sakai group, as well as many others, in order to demonstrate both 1[13,14] and 3[15] dimensional molecular orientation. While this approach does facilitate an enhanced degree of orientation when compared to the brute force method, it remains inapplicable to non-polar molecules due to the dependence of the approach on the interaction between the electrostatic field and the permanent molecular dipole moment, and also in depending on this interaction, the technique does not lend itself to development towards a method capable of achieving orientation in the absence of applied fields. With these issues in mind, the Sakai group proposed[16] and demonstrated [17] a novel technique for the achievement of all optical molecular orientation i.e. orientation in the absence of an electrostatic field. This technique, based on what is referred to as the two color laser field approach, does not rely on the permanent dipole moment of the sample molecules, nor on any resonant process and also offers an enhanced degree of orientation compared to traditional techniques. The mechanism underpinning the technique is based on the interaction between the laser field and the molecular anisotropic polarisability as well as the anisotropic hyperpolarisability. This technique offers numerous advantages, from enabling the control of the achieved molecular orientation through the variation of the relative phase between the two colours, to the applicability of the approach to almost non polar molecules. This method also lends it self to development towards a technique capable of achieving orientation in the completely field free regime, as has been suggested by Sakai et al. [18]. ment of a molecular axis to one hemisphere relative to the other, i.e. the external field preferentially selects either the parallel or anti-parallel configuration. The inversion symmetry of the distribution is broken, and the molecules are often described as behaving as single headed arrows. Molecular orientation is achieved through the application of a vertically asymmetric field, as illustrated by figure 1(c). Figure 1: (a) (b) (c) (a): Absence of external field- Random distribution (b): Application of vertically symmetric field- Alignment (c): Application of vertically asymmetric field- Orientation Basic Theory Traditional techniques designed for the achievement of molecular orientation involved exploitations of the Stark effect; the interaction between the permanent dipole moment of a molecule and an applied electric field. The permanent dipole moment ~µ of any molecule is given by ~µ = ∑ qi~ri , (1) i where the vectors ~r j give the positions of charges qi and the sum extends over both the electrons and nuclei in the molecule. The potential energy of a neutral molecule with permanent dipole moment ~µ, placed in an electric field ~E, is given by Ustark = −~µ · ~E = −µhcosθ i E = µe f f E, (2) where θ is the angle between the molecular dipole moment and the lab-fixed axis, as defined by the direction of the applied field ~E. When the applied field is spatially inhomogeneous, the molecule experiences a force given Alignment versus Orientation by In the absence of any external fields, a sample of linear ~F = − dU = µe f f dE , (3) dx dx molecules will be randomly distributed in space, as illustrated by figure 1(a). Molecular alignment refers to where µe f f depends explicitly on hCosθ i, which is a functhe confinement of a molecular axis to a laboratory-fixed tion of the rotational state of the molecule. This force axis, as defined by the direction of an applied electric allows the spatial separation of a molecular sample based field. The molecular fixed axis may align parallel or anti- on rotational state and is exploited by the Hexapole focusparallel with respect to the lab-fixed axis. This implies ing technique in order to generate state selected samples that the molecular ensemble will remain inversion sym- in which all molecules are oriented at a single angle θ metric, and the molecules are often described as behaving with respect to the lab-fixed axis. It is worth noting that as double headed arrows, in that neither the parallel nor molecules possess vibrational, rotational and electronic anti-parallel configuration is preferentially selected by the degrees of freedom, each with its own characteristic enexternal field. Molecular alignment is achieved through ergy scale. Interactions of an external electric field with the application of a vertically symmetric field as illus- a molecule are unlikely to perturb the electronic or vitrated by figure 1(b). brational structure of the molecule, only the rotational In contrast, molecular orientation refers to the confine- structure. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation In terms of Brute force orientation, spatially homogenius electrostatic fields are employed in order to align molecules via the interaction between the applied field and the permanent molecular dipole moment ~µ. In general for linear molecules, the permanent dipole moment will lie along the molecular axis. Under the influence of an applied electric field, the dipole moment and so molecular axis will precess about the lab-fixed axis in a manner analogous to a magnetic moment precessing about an applied magnetic field. The molecular axis in this way is confined to a small range as it swings to and fro about the field direction, and such a state of the molecule is referred to as a pendular state in which the molecule may be considered, on average, to be aligned with the direction of the applied field. Both the hexapole focusing and brute force alignment methods allow the achievement of modest orientation and rely on the interaction between the field and the permanent dipole moment of the molecule. In attempts to improve the degree of alignment and orientation achieved, attention has been turned to the employment of laser fields. When considering the interaction of a laser field with a molecule, a number of points should be noted. Firstly, for an electromagnetic wave, the relation between electric and magnetic field strength is given by ~E = c~B, (4) a linear molecule placed in an electric field of the form E~(t) = E0 (t)Cos(ωt), is given by Ĥ = B~J 2 − ~µ · ~E, (5) where B is the rotational constant of the molecule, ~J is the angular momentum operator and B~J 2 represents the unperturbed Hamiltonian of a linear molecule, approximated as a rigid motor. Upon the application of an electric field ~E, a molecule generally responds by redistributing its internal charge, leading to the formation of an induced dipole moment, µ~ind . Provided that the applied field is small, the induced dipole moment of a molecule takes the form µ~ind = α~E, (6) where α is the molecular polarisability. Thus in the presence of an electric field ~E, the total dipole moment of a polarisable molecule becomes µ~tot = ~µ + µ~ind = ~µ + α~E, (7) and the Hamiltonian of the system becomes Ĥ = B~J 2 − (~µ + α~E) · ~E (8) Due to the fact that the electric field changes direction every half optical cycle, the interaction with the permanent dipole moment averages to zero over one full optical thus justifying neglecting the interaction of the magnetic cycle and the −~µ · ~E term is dominated by the interaction field component with the molecule, as it is smaller than of the laser field with the induced dipole moment of the the interaction with the electric field by a factor of the molecule. Averaged over one optical cycle, the interaction speed of light. It should be noted that the laser field may term takes the form interact with the sample molecules in either the adiabatic 1 or non-adiabatic regime. This report focuses on tech−µ · E = − E2 ([α para − α perp ]cos2 θ + α perp ), (9) 2 niques involving adiabatic interactions, i.e. techniques 1 in which the duration of the applied laser pulse is far = − E2 (∆αcos2 θ + α perp ), (10) 2 larger than the rotational period of the sample molecules. Finally, laser-based alignment and orientation techniques where α para and α perp are the components of the polarisare greatly enhanced by the cooling of the initial rota- ability parallel and perpendicular to the molecular axis retional energy of the sample molecules, and the majority spectively and θ is the polar angle between the molecular of methods assume a high degree of rotational cooling axis and polarisation axis of the laser pulse. Provided that prior to the application of the alignment or orientation the polarisability is anisotropic, i.e. that α para − α perp 6= 0, technique. this interaction may be exploited in order to achieve Molecular alignment may be achieved through the ap- molecular alignment. The induced dipole moment is plication of a vertically symmetric, strong, non-resonant the same in both directions along a linear molecule and laser field of frequency ω. In order to understand the its interaction with the laser field creates a symmetric mechanism underlying this technique, it is necessary to double potential well. If α para < α perp , equation 10 shows consider the manner in which the laser field interacts minima around θ=0, π, thus creating a potential miniwith the molecule. A molecule may be treated as a spatial mum for the molecules along the polarisation direction distribution of charge, and thus may be characterised by of the laser field. The eigenstates of the molecules in the a multipole expansion; a series in which the first term is presence of the electric field are referred to as pendular referred to as the monopole, the second as the dipole and states in which the molecular axis liberates around the so forth. The various terms in the expansion are referred polarisation direction. Provided that the electric field is to as moments of the charge distribution. The monopole turned on adiabatically, the pendular states consist of corresponds to the total charge, which, for the purposes linear superpositions of the field free rotational states. of aligning neutral molecules, is zero, and as a result The anisotropy of the molecular polarisability does not the multipole expansion will be dominated by the dipole distinguish between the two ends of the molecule, with moment. The interaction energy associated with a dipole both the parallel and anti-parallel configurations equally moment ~µ in an electric field ~E is given by U = −~µ · ~E, probable, and so this technique may not be employed in as mentioned in equation 2. Thus, the Hamiltonian for order to achieve orientation. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation When orientation is sought, a vertically asymmetric interaction potential is required in order for one configuration of the molecule to be preferentially selected over the other. There exist two central methods by which this is achieved • The combined electrostatic and intense non-resonant laser field approach • The two colour intense non resonant laser field approach In the combined field approach, the application of a static electric field modifies the double potential well created by the laser field alone, effectively resulting in one well being lowered in energy with respect to the other. With one of the wells corresponding to θ=0 and the other to θ=π, one orientation will be preferentially selected over the other depending on the manner in which the symmetry of the potential is broken. In the two colour laser field approach, as pioneered by the Sakai group, a second laser field is employed such that the superposition of the two colours yields an asymmetric electric field and thus asymmetric potential. The total electric field takes the form E(t) = E0 (t)[Cos(ωt) + γCos(ωt + φ)], (11) where E0 (t) is the electric field amplitude, γ is the ratio of amplitudes and φ is the phase difference between the two colors. ω represents the frequency of the fundamental tone, and 2ω represents the frequency of the second colour. The Hamiltonian of the system makes clear the underlying physics of this technique. The key lies in the addition of the hyperpolarisability term to firstly, the expression for the induced dipole moment of the molecules in the presence of the laser field, and secondly to the Hamiltonian itself. As stated in equation 6, the induced dipole moment of a molecule is linear in the applied electric field strength ~E, provided that the field is small. Including the next higher order term in the expression for the induced dipole moment yields with respect to the polarisation axis, and the hyperpolarisability interaction is responsible for the asymmetry of the potential, which allows the achievement of orientation. The polarisability interaction takes the same form as mentioned in equation 10, with the hyperpolarisability interaction taking the form 1 − β E~2 · ~E = − [( β para − 3β perp )cos3 θ + 3β perp cosθ ] E3 , 6 (15) where β para and β perp are the hyperpolarisability components parallel and perpendicular to the molecular axis, respectively. In order to maximize the asymmetry of the potential to enable the selection of one configuration over the other, the hyperpolarisability interaction should be maximized. To achieve this, the parameter to be maximized is h E3 i = (16) where φ is the relative phase between the two colours. Maximal asymmetry is thus achieved by setting the relative phase difference equal to 0 or π. Figure 2 below illustrates the dependence of the asymmetry of the potential on the relative phase difference. With a phase of π2 , the symmetry of the single laser field induced potential is maintained, and no orientation should be achieved. With a completely random phase, the potential starts to change shape however not in a drastic manner. Finally with a phase of π, the potential becomes strongly vertically asymmetric, allowing the preferential selection of either the up or down orientation of the molecules as desired. In this manner, the extend to which the symmetry of the potential is broken is completely dependent on the phase between the two colors, and thus the degree of orientation may be controlled by varying the relative phase. Unbroken Vertical Symmetry : Phase = π/2 Potential (a.u.) 0 µ~ind = α~E + β~E2 , 3 γE0 3 cosφ 4 θ π Slight Vertical Asymmetry : Random Phase Maximum Vertical Asymmetry : Phase = 0 Potential (a.u.) 0 θ π Potential (a.u.) 0 (12) where β is the molecular hyperpolarisability. The interaction term −~µ · ~E to be added to the Hamiltonian in this case becomes Figure 2: Illustration of the dependence of the interaction potential −~µ · ~E = −(~µ + α~E + β E~2 ) · ~E, (13) yielding a Hamiltonian of the form ~2 ) · ~E Ĥ = B~J 2 − (~µ + α~E + β E (14) As before, the B~J 2 term represents the unperturbed rotational Hamiltonian of the molecule and the interaction with the permanent dipole moment averages to zero over one optical cycle. Thus, the polarisability interaction is responsible for the creation of a double potential well, preferentailly selecting neither configuration of the molecule on the relative phase between the two colours It is important to notice that the polarisability interaction, responsible for molecular alignment, depends explicitly on the term cos2 θ, with the hyperpolarisability interaction depending explicitly on cosθ. The angle θ between the molecular axis and the polarisation direction of the applied laser field is an experimentally measurable parameter, and the calculation of hcos2 θ i and hcosθ i for a given distribution of molecules allows the quantification of the experimentally achieved degree of alignment and orientation respectively. θ π All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation Experimental Set up The Lasers There was need for three laser beams in this experiment. Each beam was generated using either a Nd:YAG or TiSap laser, both of which employed chirped pulse amplification*1 . 1- Nd:YAG fundamental tone: needed to achieve molecular alignment • • • • • Vertically polarised pump pulse λ= 1064 nm (IR) Focus intensity= 1012 W/cm2 . Pulse Duration=12 ns Spatial and temporal gaussian profile*2 Central Features of the Optical Set Up • Two separate paths for the TiSap and Nd:YAG beams • Each path contained a series of optical elements (mirrors, lenses) in order to guide each beam to the central interaction chamber • Half wave plates were inserted into both paths in order to ensure the horizontal polarisation of the TiSap beam and the vertical polarisation of both harmonics comprising the Nd:YAG beam • A fused Silica plate was inserted into the path of the Nd:YAG beam only and was used to vary the relative phase between the two harmonics The Chambers Neutral OCS seeded in helium gas was used as a sample. The set up, as illustrated in figure 4, consisted of three 2- Nd:YAG second Harmonic: needed to break the symdifferentially pumped vacuum chambers, with pressures metry of the fundamental to achieve molecular orienta−4 −6 of 10 , 10 and 10−7 respectively, as viewed from left tion to right. • Generated through second order harmonic generation in a non-linear crystal (BBO) by the fundamental tone • Vertically polarised pump pulse • λ= 532 nm (green vis) • Focus intensity= 1012 W/cm2 • Pulse Duration= 8.5 ns • Spatial and temporal gaussian profile 3- TiSaphire: needed to probe the achieved degree of alignment or orientation via coulomb explosion of the Figure 4: Schematic representation of the involved vacuum chambers sample molecules and detection system • • • • • Horizontally polarised probe pulse λ= 800 nm Focus intensity= 2 × 1014 W/cm2 Pulse Duration= 35 fs Spatial and temporal gaussian profile The Beam Paths Figure 3: Schematic representation of the optical set up • Seeded OCS gas was pumped into the first chamber via an Evan Lavie valve • Supersonic expansion of the gas allowed cooling below 1K • A skimmer was appropriately positioned in order to pass only the coldest molecules into the second chamber • Within this chamber, the three laser beams were spatially overlapped and focused to a point which lay both directly in the path of the OCS beam, and between a set of electrodes • At this point, the Nd:YAG beam(s) aligned or oriented the OCS molecules, depending on whether the single or two colour approach was being employed • In the presence of the Nd:YAG pulse, the TiSap probe pulse dissociated the OCS molecules via coulomb explosion • The fragment ions were accelerated towards a TOF drift tube by the surrounding electrodes, allowing the temporal separation of the fragment ions based on their charge to mass ratio • The ejected ions reached an MCP, the anode of which was formed by a phosphor screen • The MCP was appropriately gated in order to image singly ionised sulfur, S+ All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation • Once struck by an ion, the phosphor screen fluo- TOF Mass Spectrometer resced, and was imaged by a CCD camera which The TOF mass spectrometer played the central role within outputted 2D distributions of S+ ions at the instant the detection system of enabling the imaging of singly of coulomb explosion ionised sulfur, S+ , only. This piece of apparatus allowed the temporal separation of the different fragment ions The Detection System present in the interaction chamber and facilitated the identification of the time at which each species impacted In order to investigate the achieved degree of alignment the phosphor screen, referred to as the time of flight and orientation, a detection system consisting of a TOF (TOF). This identification allowed the MCP to be gated mass spectrometer, MCP, Phosphor screen and CCD camsuch that the outputted 2D distributions were captured era was employed. The detection system functioned by for S+ only. imaging the fragment ions ejected from OCS molecules The manner in which the TOF mass spectrometer funcupon interaction with the TiSap probe pulse. The TiSap tioned may be understood in a number of simple steps. 14 probe pulse had an intensity of 10 W/cm2 , exceeding Firstly, upon coulomb explosion, the fragment ions were the ionisation threshold of OCS molecules. In this high accelerated towards the TOF drift tube by the electrostatic intensity regime, there were two central mechanisms via field generated by the surrounding electrodes, gaining which the fragmentation of the OCS molecules could energy W occur W = qEr, (19) • Photodissociation; The breaking of molecular bonds upon photon absorption • Coulomb explosion; A two step process involving ionisation, followed by bond breaking due to the repulsion between the molecular components of the same charge In the process of Coulomb Explosion, by the conservation of angular momentum, the fragment ions were ejected and recoiled along the molecular bond axis, thus maintaining the angle θ between the vertical polarisation axis of the Nd:YAG and the molecular axis. Imaging OCS molecules at the instant of Coulomb Explosion allowed the extraction of the angle θ from the 2D distributions, and so enabled the quantification of the achieved degree of alignment and orientation by allowing the calculation of the alignment and orientation parameters, hCos2 θ i and hCosθ i, respectively. It was decided that for this experiment, the Coulomb Explosion channel described by equations 17 and 18 would be focused on, with all generated 2D distributions representing singly ionised Sulfur only. where E is the electric field strength, q is the charge of the fragment ion and r is the distance from the point of ejection to the TOF drift tube. Assuming zero kinetic energy in the direction of motion prior to acceleration, the velocity of the fragment ion upon reaching the TOF drift tube was given by r v= 2qEr m (20) Within the TOF drift tube, no fields were present and the fragment ions were allowed to drift, in a presumed frictionless environment. Assuming that the TOF drift tube had a length L, the time taken for the respective fragment ions to reach the phosphor screen was governed exclusively by the charge to mass ratio and given by tto f = L =L v r m =k 2qEr r m q (21) Connecting the MCP to a digital Oscilloscope allowed the observation of the signal illustrated in figure 5 below. OCS+ → OCS2+ + e− (17) The displayed x axis represents time, with each peak corresponding to a different characteristic TOF, and so 2+ + + OCS → S + CO (18) to a different ionised species present in the interaction chamber. The polarisations of the Nd:YAG and TiSap probe pulses were pivotal in terms of maintaining the angle θ during the imaging process. Once ejected, the fragment ions were subject both to the electrostatic field accelerating them towards the TOF mass spectrometer, and also to the field of the TiSap pulse, accelerating them along the direction of polarisation. It was thus necessary that the TiSap pulse was polarised in the horizontal direction, normal to the detector plane and parallel with the axis of propagation of the ions. In this manner, any change in the trajectory of the ions due to the presence of the TiSap pulse would not effect the projection of the angle θ onto the plane of the detector. In addition, any overestimations of the achieved degree of alignment due to the presence Figure 5: Time of flight signal as outputted by the digital oscilloscope connected to the MCP of the TiSap pulse were avoided. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation In an ideal situation, this plot would show signals corre• t= 2µs: Voltage applied to MCP is increased from sponding only to species ejected from the fragmentation 1kV to 2kV for the purpose of gating for S+ of OCS, however due to imperfect pumping of the vac• t=49.2ms: Evan Lavie Valve is pulsed, injecting OCS uum chambers, many other species were also present. into chamber. It is desired that this happens 0.8ms The raw TOF data did not enable the direct linking of a before the emission of the TiSap pulse, however since peak to an ionised species as the displayed TOFs were the TiSap acts as the master signal, the operation of not absolute. In order to overcome this problem, a list the pump valve had to be timed in this manner. was generated with all of the possible molecular species • t=50ms: Next pulse is emitted from the TiSap laser present in the chamber and their corresponding charge to mass ratios. Of all of the species present, it was obvious Experimental Method that singly ionised hydrogen had the smallest charge to mass ratio and so shortest TOF, with singly ionised OCS There were three central aims in this experiment; having the longest TOF by the same argument. Know• To achieve and probe the alignment of OCS ing the TOF and charge to mass ratios associated with molecules, requiring the temporal and spatial overthe first and final peaks yielded a pair of simultaneous lap of the Nd:YAG fundamental and TiSap probe equations of the form pulses r m • To achieve and probe the orientation of OCS + b, (22) tto f = k q molecules, requiring the temporal and spatial overlap of the Nd:YAG fundamental, second harmonic where the constant b was added to take into account the and TiSap pulses fact that a time t=0 did not correspond to a TOF of zero. • To demonstrate the dependence of the degree of Solving the pair of simultaneous equations for k and b achieved orientation on the relative phase between gave the relation the two harmonics generated by the Nd:YAG, rer quiring the adjustment of the fused Silica plate m tto f = 0.4( ) + 0.16, (23) q Spatial Overlap of the Three Beams which was manipulated to give It was pivotal to the achievement and detection of molecular alignment and orientation that accurate spatial overlap m ( ) = 2.5(tto f ) − 0.4 (24) of the three beams was attained. A number of steps were q employed in order to achieve this Thus, the TOF associated with each peak provided infor• A glass plate was inserted into the path of both the mation on the charge to mass ratio of the ionised species Nd:YAG and TiSap beams, directly before entering responsible. Comparing the theoretically calculated value m the interaction chamber and directly after transmisof q with the list containing all of the possible species sion through the major lens responsible for focusing and their respective charge to mass ratios allowed the within the chamber mapping of a specific ionised species to each peak. Hav• The plate was positioned such that the beams were ing identified the peak associated with S+ , the MCP was incident at roughly 45◦ appropriately gated via a home built high voltage power • As governed by Fresnel’s laws, roughly 90% of the supply such that all outputted 2D distributions reprebeam intensity was transmitted to the chamber, with + sented S only. roughly 10% reflected through an angle of 90◦ . It was the reflected beam only that was further worked Relative Timings with • The focus length of the major lens was known and Molecular alignment and orientation are achieved while allowed the rough estimation of the focus point of the Nd:YAG pulse is present only, with the molecules the three beams. A sheet of paper was employed returning to their field free states in the time period beand moved into the path of the beams to verify this tween pulses. Thus it was necessary that the coulomb position explosion of OCS molecules occurred while the Nd:YAG • Some distance behind this focus point, a second fopulse was also present. To facilitate this, the Nd:YAG and cusing lens was used in order to re-focus the beams TiSap pulses were temporally overlapped prior to the beonto a photodiode ginning of the experiment via electronic synchronisation. • A sheet of foil containing a pinhole with a diameter The TiSap pulse was then treated as the master signal, on the order of micrometers was mounted on an and all other components of the set up were timed with automated stand and inserted into the beam path at respect to this. Each time a pulse was emitted from the the estimated focus position. The stand could also TiSap laser, the clock on the MCP was reset to t=0. be adjusted manually • t=0: TiSap pulse is emitted • A two-person, systematic approach was employed in • t=100ns: TiSap pulse reaches the interaction chamber order to pass each beam through the pinhole r All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation spatial overlap was seen to drift with time and had to be rechecked prior to beginning of each experiment 7 x 10 Ti:Sa and YAG Beams Overlap 2 1.5 Laser Intensity (a.u.) • Person 1 was positioned on eye level with the beam*, while Person 2 was positioned behind the pinhole, manually controlling its motion in the x any y directions • Person 1 observed the position of the beam on the foil which contained the pinhole • Person 2 adjusted the position of the pinhole in the x direction, until alerted by Person 1 that the beam had fallen outside of the boundary of the foil • Person 2 then adjusted the position of the pinhole in the y direction by one increment of the manual control knob • This process was repeated, and in this manner, the beam was systematically scanned across the entire area of the foil, until it was seen that light had passed through • This approach was applied firstly to the second harmonic of the Nd:YAG alone such that the beam being worked with could be seen • The TiSap beam was added afterwards, and in order for the photodiode to differentiate between the two beams, their temporal overlap was removed • The light passing through the pinhole was detected by the photodiode and the signals were displayed on a digital oscilloscope • By interpreting the oscilloscope output, further manual adjustments of the x, y and z positions of the pinhole were made in order to maximize the signals • Optimum spatial overlap of the three beams required that the position of the pinhole overlapped exactly with the focus position and so finer adjustments of the pinhole position in the z direction were needed • The photodiode was connected to the PC where, through the programme LABVIEW, the motion of the pinhole in the z direction was controlled and the beam profiles were imaged • Despite the employment of achromatic lenses, the three beams did not focus to exactly the same point, and so the focus of each beam was found separately • As the z-position of the pinhole approached the focus position, the beam signal was maximized, and the beam profile became increasingly gaussian • The z-position at which each beam displayed maximum intensity was located and the pinhole was placed at a position deemed to be in the centre of the individual focus points • In order to attain perfect spatial overlap, the manual adjustment of one or two pivotal optical elements within the set up was necessary • As these element were adjusted, the profiles of the beams were imaged, as illustrated in figure 6 • Via the interpretation of the outputted beam profiles, the directions in which the elements needed to be adjusted could be reasoned, and spatial overlap was achieved • Finally, the TiSap and Nd:YAG pulses were temporally overlapped, the glass plate was removed and the set up was fully prepared for use • Due to slight instabilities in the laser systems, the 1 0.5 0 50 0 Vertical Pinhole Position −50 −30 −20 30 20 10 0 −10 Horizontal Pinhole Position Figure 6: Overlapping beam profiles, as imaged by the photodiode *It was ensured that while on eye level with the Nd:YAG second harmonic beam that the power had been reduced, and that goggles designed to protect specifically against light in the 500nm range were worn Investigation of the Relative Phase between the Two Harmonics In order to investigate the relationship between the achieved degree of orientation and the relative phase between the fundamental and second harmonic, a fused Silica plate was employed. Due to the wavelength dependence of the refractive index, the fundamental and second harmonic produced by the Nd:YAG laser spent different periods of time within the fused Silica plate. The variation of the angle of inclination of the plate with respect to the beam resulted in the variation of the relative phase between the two harmonics, thus shaping the interaction potential seen by the molecules. It was known that orientation would be optimised for a relative phase of 0 or π, however the relative phase between the two harmonics at the instant of generation was not known, nor was the relation between adjustment of the inclination of the plate and the induced phase difference. In this experiment, the angle of inclination of the fused Silica plate was varied via the manual adjustment of a micrometer screw gauge present upon the apparatus. For each new screw gauge position, orientation of the OCS molecules was attempted via the application of both the fundamental and second harmonics, and the corresponding 2D distributions were outputted. From the distributions, the value of the orientation parameter hcosθ i was calculated in order to deduce whether the adjustment of the screw gauge had resulted in the reduction or enhancement of the achieved orientation. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation Results and Analysis The raw data collected in all experiments comprised 2D distributions of S+ ions at the instant of coulomb explosion. From such a 2D distribution, as illustrated in figure 7, the alignment parameter hcos2 θ i and the orientation parameter hcosθ i were extracted. Each point on the distribution corresponded to one impacting S+ ion. The centre of the distribution was outputted by LABVIEW, and using this center, the x and y positions of all points within the distribution were found. Using these positions and the formula for Cosine, Figure 8: 2D distribution of S+ ions in presence of the TiSap probe pulse only Cosθ = p x x2 + y2 , (25) the average value of cos2 θ and cosθ were calculated for the distribution, thus yielding the alignment and orientation parameters respectively. This analysis was carried out using matlab, and allowed the quantification of the degree of alignment or orientation achieved. Peaks may be noticed at values of θ=0 ,π and the values of hcos2 θ i and hcosθ i were calculated to be 0.541±0.001 and 0.004±0.003 respectively. The value of the alignment parameter was thus larger than the expected value of 0.5 by an amount that was non-negligible, and the blame for this discrepancy was placed on the imperfect horizontal polarisation of the TiSap laser. In an attempt to reduce the value of hcos2 θ i to 0.5, the polariser responsible for ensuring the horizontal polarisation of the TiSap beam was adjusted and the 2D distributions of the S+ ions were captured for each adjustment. From each 2D distribution, the value of hcos2 θ i was calculated and the position of the polariser corresponding to the value of hcos2 θ i nearest to 0.5 was selected for the duration of the experiment. Figure 8 corresponds to the distribution with the polariser in this optimum position. Since it was not possible to reduce the value of hcos2 θ i to 0.5, it was decided that 0.54 would be taken as a background value, and that in order to conclude the achievement of alignment upon the addition of the Nd:YAG pulse, an increase in the value of the alignment parameter beyond this would be needed. Alignment In order to achieve the alignment of OCS molecules, the Nd:YAG fundamental tone was applied. Figure 9 illustrates the analysed data corresponding to this situation. Figure 7: 2D distribution of S+ ions as imaged by the CCD camera Bar charts were generated with each bin corresponding to a given angle θ with respect to the vertical, and the height of each bin representing the relative number of S+ ions detected at this angle. Ideally, in the absence of the Nd:YAG pulse, such a bar chart should be completely flat, yielding no peak at any particular value of θ. The calculated values of the alignment and orientation parameters for the distribution should be 0.5 and 0 respectively, as the OCS molecules should be randomly distributed + in space. Figure 8 illustrates the chart generated in the Figure 9: 2D distribution of S ions in presence of the Nd:YAG fundamental tone and TiSap probe pulse presence of the TiSap pulse only. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation Both YAG Harmonics 450 400 Number of Data Points Peaks may be noticed at values of θ= 0, π. These peaks have clearly been enhanced with respect to the background image generated while the sample was in the presence of the TiSap pulse only. This is obvious upon the comparison of figures 8 and 9. The enhancement of the peaks corresponds to an enhanced number of S+ ions detected at the angles θ= 0, π, and was due to the successful alignment of the OCS molecules with the vertical direction. The value of hcos2 θ i was calculated to be 0.579 ±0.001 which clearly increased beyond the background value of 0.54, outside of the bounds of experimental error, allowing the conclusion that alignment had been achieved. The value of hcosθ i was calculated to be 0.016±0.001. An increase in this parameter was not expected upon the addition of the Nd:YAG fundamental pulse only. It was thus decided that in order to conclude the achievement of orientation upon the addition of the second harmonic, an increase in the value of the orientation parameter beyond this value of 0.016±0.001 would be needed. The relationship between pulse intensity and the achieved degree of alignment was also investigated. The second harmonic was constantly maintained at a lower power than the fundamental, generally in the region of 20mJ, with the fundamental in the region of 130mJ. Figure 10 illustrates the analysed data corresponding to the situation in which the second harmonic only was applied. 350 300 250 200 150 100 50 0 0 20 40 60 80 100 θ (Deg) 120 140 160 180 <cos2(θ)>=0.567±0.002 <cos(θ)>=0.017±0.004 Figure 11: 2D distribution of S+ ions in presence of both Nd:YAG harmonics and TiSap probe pulse The value of hcosθ i was calculated to be 0.017±0.001, which did not increase above the background value of 0.016, outside of the bounds of experimental error. Thus, in this experiment, it could not be concluded that orientation was achieved. Investigation of the Relative Phase between the Two Harmonics Figure 10: 2D distribution of S+ ions in presence of the Nd:YAG second harmonic and TiSap probe pulse In order to investigate the effect of the relative phase between the two harmonics on the achieved degree of orientation, the inclination of the fused Silica plate with respect to the beam was varied. 2D distributions in the presence of both Nd:YAG harmonics were generated for different positions of the micrometer screw gauge, and so different angles of inclination of the plate. From each 2D distribution, the values of hcos2 θ i and hcosθ i were extracted. The values of hcosθ i were used to investigate whether or not the achieved degree of orientation had been improved. Due to the fact that varying the relative phase between two harmonics should have no effect on the achieved degree of alignment, the values of hcos2 θ i were used as a reference, such that random fluctuations due to laser system instabilities could be differentiated from true increases in the orientation parameter. The value of hcos2 θ i was calculated to be 0.558±0.001 which increased from the background value of 0.54, outside of the bounds of experimental error. Thus it was concluded that alignment had been achieved via the application of the second harmonic. However, this value of hcos2 θ i was less than that achieved by the application of Figures 12 and 13 illustrate the values of hcos2 θ i and the fundamental tone, thus verifying the dependence of hcosθ i corresponding to different positions of the micromalignment on the intensity of the applied pulse. eter screw gauge. The screw gauge was uncalibrated, in that it was unknown by how much the relative phase Orientation between harmonics would change for a given adjustment When the orientation of the OCS molecules was sought, of the screw gauge. Figure 12 illustrates the values of both the fundamental and second harmonic of the the alignment and orientation parameters as the screw Nd:YAG were applied. Figure 11 illustrates the analysed gauge was adjusted in 0.5mm increments, with figure 13 data corresponding to this situation. illustrating the same situation for increments of 0.025mm. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation between relative phase between the two harmonics and the achieved degree of orientation. Section II Introduction The ability to probe and to image dynamical electronic and molecular processes on shorter and shorter timescales has lead to the development of the area of Attosecond Figure 12: hcos2 θ i and hcosθ i values for 0.5mm increments of the Physics. It is predicted that the control of a single, isolated attosecond pulse will allow the investigation of a vast screw gauge range of processes previously taking place on timescales too small to imagine, from the motion of electrons within their orbitals and indeed within semiconductors, to the processes at play in photosynthesis. The controlled production of single attosecond pulses has, in very recent years, been achieved [19] and since then has inspired the development of technology towards many noble applications, for example the use of single attosecond pulses for the generation of live bio-images as an alternative to Xray imaging. However, the generation of isolated attosecond pulses places stringent requirements on the driving laser, making the technology accessible to only a small number of laboratories across the world. In 2008, a method of isolation of single attosecond pulses known as Double Optical Gating was proposed [20] and is investigated in this experiment. 2 Figure 13: hcos θ i and hcosθ i values for 0.025mm increments of the screw gauge It was hoped that figures 12 and 13 would show at least one clear peak as the inclination of the fused Silica plate was varied, indicating that a relative phase between harmonics of 0 or π had been achieved. However, in figure 12 no increase in the value of hcosθ i which could not be written off as a random fluctuation by comparison with the corresponding hcos2 θ i value was seen. It is possible that this was due to the step size being too large, such that phase shifts introduced between the two harmonics were completely random and generally hit on values that were not optimum in terms of facilitating the achievement of orientation. In figure 13, there were two instances at which the value of hcosθ i was seen to increase, and which could not be written off as a random fluctuation by comparison with the corresponding hcos2 θ i value. This suggests that it is possible that the step size taken on the screw gauge was again too large to allow the observation of clear, gradual peaks in the values of hcosθ i, and that potentially, an optimum relative phase somewhere close to 0 or π happened to be hit on in the instances circled in figure 13. This would suggest that upon better calibration of the micrometer screw gauge, the variation of the inclination of the plate could be used to enhance the probability of achieving orientation. This data does not however provide any conclusive evidence supporting the relationship Theory High Harmonic Generation High Harmonic Generation (HHG) is a non-linear process which involves the illumination of a target material by an intense laser pulse and results in the emission of harmonics of the incident laser beam by the material. Although inherently quantum mechanical in nature, this process is often understood using the semiclassical three step model. 1 An electron within an atom is confined to the region of space surrounding the nucleus by the attractive Coulomb potential. An incident light pulse modifies this potential such that tunneling through the barrier is facilitated and the electron tunnels out into the continuum in a process known as tunnel ionisation. 2 The electron is accelerated along the direction of the laser field and follows a trajectory that leads first away from the atom, and then back. 3 Upon returning to the vicinity of the ion, the electron recombines and rids itself of the excess energy that it gained during its acceleration by emitting harmonic radiation. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation Figure 14: Three step model of High Harmonic Generation The success of this process strongly depends on the intensity and the polarisation of the incident light pulse. The relationship between HHG and incident intensity is obvious, with higher intensity beams increasing the probability of ionisation. With respect to the polarisation dependence, it has been shown [21] that linearly polarised light facilitates HHG, with elliptically polarised light reducing the success and finally with circularly polarised light permitting no HHG. This can be understood by considering that the trajectory followed by the ionised electron is determined by the direction of the incident light field. In this manner, circularly polarised light yields a trajectory that does not lead the electron back within the vicinity of the ion, thus preventing recombination and elliptically polarised light creates a trajectory that, by the same logic, dramatically reduces the probability of recombination. Attosecond Pulse Generation When atoms are driven by laser pulses containing multiple optical cycles, a train of attosecond pulses is generated [22] through repeated HHG. One attosecond pulse is generated every half cycle of the incident laser pulse, as illustrated in figure 15. Each half cycle enables HHG through the 3 step model. Beginning at position 1, the applied light field modifies the coulomb potential well confining the electron to the atom and the electron tunnels into the continuum. The electron is then accelerated away from the atom, until it reaches position 2. At this point, the direction of the light field reverses and the electron is accelerated back towards the ion. At position 3, the electron recombines with the ion and emits a pulse, with a duration on the order of attoseconds. The generated attosecond pulse train contains pulses temporally separated by a half period of the incident light field. Such pulse trains have limited experimental use, and isolated attosecond pulses are instead desired. Figure 15: Illustration of the manner in which attosecond pulses are generated Isolation of Single Attosecond Pulses via DOG DOG is an approach used to control the HHG process so that a single attosecond pulse may be generated using multiple cycle pulses. This technique employs a combination of Polarisation Gating and Two Colour Gating. • Polarisation Gating Two counter-rotating, circularly polarised pulses with an appropriate time delay are spatially overlapped in order to generate a single pulse that is linearly polarised for a small portion of its duration, and circularly polarised for the majority. Since HHG is only realised using linearly polarised incident light fields, only a small portion of the incident pulse is capable of generating an attosecond pulse. As a result, the number of attosecond pulses within the generated train is reduced. • Two Colour Gating A weak second harmonic field is added to the Polarisation Gating field in order to modulate the pulse shape. When appropriately modulated, this addition can reduce the number of attosecond pulses generated by reducing the intensity of the light field constituting the incident pulse at the beginning and ending of its duration. These parts of the pulse thus become incapable of generating attosecond pulses. The central part of the incident pulse wavepacket may simultaneously be enhanced in the same manner. This method utilises the dependence of HHG on incident pulse intensity. Experimental There were two major parts to the experimental set up. Firstly, pulses of appropriate temporal duration for use in attosecond pulse generation needed to be produced and secondly, these incident pulses needed to be guided to the target medium and the generated attosecond pulses needed to be detected. All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation Production of Incident Pulses of Appropriate Duration The TiSap laser was used to generate pulses with the following characteristics; • • • • • λ= 800nm Temporal duration=40fs Length of one optical cycle =τ = 800nm = 2.6fs c 40 ≈ 14 Number of optical cycles/pulse = 2.6 Number of attosecond pulses generated/pulse≈28 A pulse train containing 28 attosecond pulses is experimentally useless and difficult to reduce to a single isolated pulse using DOG. It was decided that a pulse with a temporal duration of 5fs was most appropriate. Within one 5fs pulse, there are two optical cycles and thus potentially four generated attosecond pulses, which can much more easily be reduced to a single pulse by DOG. In order to generate a 5fs pulse from a 40fs pulse, a hollow core fibre compressor in combination with chirped mirrors*3 was employed. A hollow core fibre compressor is used to spectrally broaden high energy input fs pulses by nonlinear interaction with a noble gas of adjustable pressure inside a hollow fibre. Chirped mirrors then compress the pulse after the fibre and the increased spectral width supports shorter pulse durations than the input pulses. The combination of the the hollow core fibre compressor and chirped pulse mirrors act to take a 40fs pulse, broaden its frequency spectrum, and then compress it temporally, to a duration far smaller than that which would have been supported before spectral broadening, 5fs in the case of this experiment. Attosecond Pulse Generation and Detection A schematic of the experimental setup from the point after which the 5fs pulses were produced is illustrated in figure 16. Argon gas was employed as the sample onto which the 5fs pulses were focused in order to generate the attosecond pulses via HHG. The set up included the following components; • Vacuum chamber containing Argon gas • Toroidal focusing mirror, used to focus the incident 5fs TiSap pulses to a point within the vacuum chamber • Aluminium membrane, used to screen 800nm light from continuing any further within the set up. The CCD camera was preferentially sensitive to visible light, making it necessary to screen all light which was transmitted through the sample of Argon gas without inducing the generation of attosecond pulses • Diffraction grating, used to disperse the different wavelength components of the generated attosecond pulses for analysis • CCD camera, used for imaging the dispersed attosecond pulses Figure 16: Schematic representation of the Experimental set up following the production of the 5fs pulses Results It is understood that a 5fs pulse incident on the sample of Argon gas should result in the generation of an attosecond pulse train containing four pulses, provided that the hollow core fibre compressor and chirped mirror combination were successful in reducing the duration of the incident pulse from 40fs to 5fs. Each attosecond pulse within the train will have a wavelength of 30nm and the pulses will be temporally separated by 1.3fs, half an optical cycle of the incident 800nm light. The CCD image illustrated in figure 17 displays the detected and outputted intensity versus wavelength spectrum corresponding to this situation. Figure 17: Wavelength versus intensity spectrum in the absence of Double Optical Gating Four peaks are clearly visible, as was expected. The reason for the set of four peaks at four different values of wavelength, despite our knowledge that each attosecond pulse is generated with a wavelength of 30nm, may be roughly understood by considering the four generated attosecond pulses to have undergone interference in the time domain. The pattern outputted appears as if a single 30nm beam was incident on four ’slits’, giving rise to four peaks in the wavelength domain. Three generated attosecond pulses by this logic could be equivalent to three slits and three peaks in the wavelength domain, and so forth. Naturally, this is purely an analogy, however it facilitates the understanding that four peaks in the wavelength domain corresponds to a generated attosecond pulse train consisting of four pulses. Figure 18 illustrates the spectrum detected and outputted when DOG was employed. In this case, a single broad peak, centered at 30nm is visible. This illustrates the situation for which only one attosecond pulse is present, and so only one ’slit’ in this interference model is present. It may thus be concluded via the comparison of figures 17 and 18 that the employment of DOG was All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation successful in isolating a single attosecond pulse. Figure 18: Wavelength versus intensity spectrum in the presence of Double Optical Gating Discussion and Conclusions In section I, the alignment of neutral OCS molecules was achieved via the application of the fundamental tone (λ=1064 nm) generated by an Nd:YAG laser with a peak power in the region of 130mJ. The value of the alignment parameter, hcos2 θ i, was calculated to be 0.579±0.001, which increased above the background value of 0.54, outside of the bounds of experimental error. The application of the second harmonic generated by the Nd:YAG laser (λ=532nm) with a peak power in the region of 20mJ also resulted in the achievement of alignment, with the value of hcos2 θ i calculated to be 0.558±0.001. This value increased by an amount substantially less that that resulting from the application of the Nd:YAG fundamental tone, and thus the dependence of the degree of achieved alignment on the intensity of the incident pulse was verified. Orientation of neutral OCS molecules was attempted via the application of both the Nd:YAG fundamental and second harmonics. The value of the orientation parameter, hcosθ i, was calculated to be 0.017±0.001, which did not increase above the background value of 0.016 outside of the bounds of experimental error. Thus it could not be concluded that orientation was achieved. It was thought that since the relative phase between the two harmonics was unknown, a relative phase which did not facilitate the achievement of orientation could have been responsible for this failure. Variations of the relative phase between the two harmonics with the aim of locating the inclination of the fused Silica plate which resulted in a relative phase of 0 or π were unsuccessful. Varying the relative phase between the two harmonics was not seen to improve the degree of orientation achieved, thus eliminating a relative phase between harmonics corresponding to a symmetric interaction potential as the cause for the failure of the achievement of orientation. Since it was unknown by how much a single adjustment of the micrometer screw gauge controlling the inclination of the fused Silica plate would change the relative phase between the two harmonics, adjusting the inclination of the plate in order to locate a relative phase of 0 or π was difficult. This lack of calibration of the plate potentially led to the adjustments made consistently being too large, meaning that from one adjustment to the next, completely random jumps in induced relative phase were made. The data generated did, on two instances, yield values of the orientation parameter which could not be written off as random fluctuations. Potentially in these two instances, a relative phase close to 0 or π was achieved, with all other measurements corresponding to relative phases which happened to be more in the region of π2 , yielding symmetric interaction potentials which did not facilitate orientation. This is however a very optimistic manner of viewing the results. In order to improve the probability of achieving orientation in the future, it would be beneficial to include a stark state selector within the set up such that only the coldest rotational states of the OCS molecules are permitted into the interaction chamber. The power of the beams should be further increased, as it has been verified that the degree of alignment achieved depends on the incident power. In addition, the 2D distributions contained a large amount of noise, which if removed, may allow the recovery of more promising results. In section II, 40fs pulses were successfully reduced to 5fs pulses via the employment of a hollow core fibre compressor and chirped mirrors. The produced 5fs pulses were used to generate an attosecond pulse train consisting of four pulses via HHG from a sample of gaseous Argon. Double optical gating was then successfully employed in order to isolate a single attosecond pulse from the train. In the future, it is hoped that the two areas of research may be linked and that samples of aligned molecules may be used as birefringent wave plates in order to control the polarisation state of the generated attosecond pulses. Due to the nature of the process, all generated attosecond pulses are linearly polarised, and it is hoped that upon aligning molecules at 45◦ to the vertical and passing the attosecond pulses through the aligned sample, that circularly polarised pulses may be produced. Acknowledgments The author would like to thank Professor Hirofumi Sakai, Assistant Professor Shinichirou Minemoto, Mr. Md. Maruf Hossain and Mr. Wataru Komatsubara for their acceptance into the group, their guidance, patience and for all of their help in the lab! The author would also like to thank Dr. Hiroki Mashiko for his insight into the area of Attosecond Pulse Generation, the ILO office for the fantastic organisation of the UTRIP programme, the Graduate School of Science Scholarship for the financial support as well as Mr. Niccolò Bigagli for the long hours of company in the lab as well as for all of the figures used in this report. Appendix *1 Chirped Pulse Amplification (CPA) With the advent of the mode locked laser came the ability to experimentally realise ultrashort laser pulses. Associated with such pulses are high elecric field strengths and All-Optical Alignment and Orientation of Neutral Molecule & Attosecond Pulse Generation peak powers; E τ Where E is the energy of a single pulse and τ is the pulse duration. As a result, attempting to amplify such pulses often yields optical damage of the gain medium through which the pulses propagate. In order to avoid such damage, an amplification technique aimed specifically at ultrashort pulse amplification was developed and is referred to as Chirped Pulse Amplification. The idea behind this technique is to manipulate the ultra short pulses in a reversible manner, such that the pulses are temporally stretched prior to being introduced to the gain medium and temporally compressed after exiting the gain medium. In this way, the ultra short pulses are never introduced to the gain medium and damage to the apparatus is avoided. The most common approach for the reversible stretching and compressing of ultra short pulses is the employment of a pair of diffraction gratings. Due to the broad spectral bandwidth of ultra short pulses, the stretcher operates by introducing dispersion of these components. The time delay of the different spectral components within the pulse gives rise to a long, chirped optical pulse, with a far lower peak power as a result. The compressor operates on the same principle, with dispersion that closely matches that produced by the stretcher, but opposite in sign. Thus, the ultrashort pulses generated by a mode locked laser will first undergo temporal stretching, amplification to high energies in the gain medium, and finally compression, ideally to the original pulse duration. Damage to the laser amplifier is avoided, and this method of ultrashort pulse amplification was employed by both the TiSap and Nd:YAG lasers in our experiments. Ppeak = *2 Gaussian Pulse In all conducted experiments, it is assumed that the employed pulses have a spatial and temporal Gaussian profile. Under this assumption, the pulse intensity takes the form I (r, t) = I0 exp[− 4ln2t2 ( FW HM) 2 ]exp[− 2r2 ], ω2 (26) where FWHM is the full width at half maximum height. Integrating the Intensity over space and time yields the energy of the pulse, Z +∞ exp[− 4ln(2t2 ) Z +∞ 2r2 ]dr, ω2 −∞ −∞ ( FW HM) (27) which clearly requires a standard Gaussian integral to solve, yielding √ FW HM π πω 2 √ E pulse = I0 ( ).( ) (28) 2 2 ln2 I0 E pulse ≈ ]dt × 2π 2 I0 πω 2 FW HM 2 γexp[− (29) *3 Chirped Pulse Mirror A chirped mirror is effectively an extended version of a dielectric mirror, which is made to reflect only a single frequency of light. It is composed of transparent materials, uniformly layered at a quarter of the wavelength of the light that the mirror is designed to reflect. 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