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Laser Molecular Spectroscopy CHE466 Fall 2009 David L. Cedeño, Ph.D. Illinois State University Department of Chemistry Lasers LASERS Definition Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The first laser was designed and made by Theodore Maiman in 1960. Lasers work on the basis of stimulated emission, in which excited molecules are stimulated to emit by a photon with frequency equivalent to the energy difference between the excited state and a lower energy state (not necessarily the ground state). LASERS Stimulated Emission and Light Amplification For a laser to work efficiently, the number of excited molecules (or atoms) must exceed the number of molecules in the lower energy state. This is a process called population inversion and is not spontaneous. Population inversion condition is required for a laser to work, because otherwise the incident photon would be absorbed by the sample (recall that the rate of photon absorption is equal to the rate of stimulated emission, B0n = Bn0). Population inversion requires external work (i.e. energy input) via a process called pumping. Incident photons Pumping Emitted photons Population Inversion Stimulated Emission LASERS Population Inversion Schemes The efficiency of a laser is related to its ability to sustain a population inversion and regenerate a population in the low energy state to initiate a new pumping cycle. As you may see a 2 level laser would be very inefficient, because a population inversion may not be acquired as a result of population equilibration. An additional level is then necessary to quickly move the pumped molecules to another level in which population inversion is achieved. Stimulated emission then occurs from this level. A four-level system is also common, and much more efficient because it allows for fast replenishing of molecules in the lowest energy level for efficient pumping. 3 level laser system 4 level laser system LASERS Light Amplification and Optical Cavities The other process necessary for a laser is the effect of amplifying the stimulated emission process. For such a process the lasing medium must be enclosed in an accurately aligned optical cavity, in which the emitted photons are confined to bounce back to the sample to stimulate further emission (optical feedback). An optical cavity consists of a closed containment ended on polished surfaces or mirrors that allow photon feedback and output. Mirrors could be planar or spherical. Lasing medium Output mirror (R2 < R1) Mirror (R1) d Optical cavities are also called resonators because the constructive interference of the travelling photons will create standing photon waves that resonate at a frequency (n) related to the length of the cavity (d): n nc 2d Where n is the number of half-wavelengths that are built within the cavity and c is the speed of light. LASERS Light Amplification and Optical Cavities Example: Calculate the length of the optical cavity required to have one million half wavelengths of photons with a wavelength of 337.1 nm. nc n 106 337.1 nm d 16.855 cm 2n 2 2 Laser Cavity Modes The cavity itself has different modes of oscillation of two types transverse (normal to photon propagation) and longitudinal (along photon propagation). Cavity modes perturb the propagation of the photons creating an interference pattern that is reflected in the output beam. The transverse modes (also called TEM) affect the intensity distribution of the laser beam along the plane normal to its propagation direction. LASERS Laser Cavity Modes Laser cavities are designed to obtain a TEM00, in which there are no nodes and a Gaussian intensity distribution. Images from Photon Inc., www.photon-inc.com Axial modes are usually very important in the design of a laser. Lasers operate only if n takes a value in which the resonant frequency n of the cavity corresponds to the line width of the lasing transition. The axial cavity modes interfere with the photons and “split” (modulate) them into packets that are temporally separated with a frequency: n c 2d LASERS n Laser cavity modes Transition band width Modulated laser output Intensity Laser Output and Longitudinal Modes: A laser usually operates in a multimodal way. Note, however that it is possible to reduce the bandwidth of a laser transition by selecting a single mode operation. This requires a shortening of the cavity at the expense of reducing the intensity of the output. Single mode operation is the preferred way for high resolution spectroscopy when laser band widths of 0.001 cm-1 are needed. n (Hz) LASERS Laser Threshold: This term refers to the required pumping ability to maintain lasing action. Since resonance inside a cavity decreases the potential output of a laser (call cavity loses), an additional amount of energy is required to overcome such losses. Pumping is performed using electrical power (electrical discharges), or photon energy (using flashlamps, arc lamps, or another laser). The threshold is quantified in terms of a gain threshold (gt): gt 1 ln( R1R2 ) 2d The expression above is in terms of cavity design parameters, namely the reflectances of the mirrors (R1 and R2) and the length of the lasing medium (d). It is also related to the characteristics of the transition and the population inversion excess (Nn – N0): 2 A0n gt N n N 0 4n bw is the wavelength of the transition, nbw is the bandwidth of the transition and A0n is the Einstein coefficient for spontaneous emission. Pumping of a laser could be pulsed or continuous, which then determines the mode of operation of the laser: pulsed or CW (continuous wave). Laser Power and Flux LASERS The spatial coherence and directionality of a laser makes it advantageous over other light sources, because it allows for large energy (or power) flux, i.e. the concentration of many photons in a very small area. The intensity of a laser is defined by the frequency and amount of photons leaving the optical cavity. The net power or energy can be measured using special detectors. The power (P) of a laser is defined as: P = E/t For a pulsed laser, E is the energy output measured using a Joule meter and t is the time duration of the pulse. In a CW laser, the power is measured directly with a power meter. The power flux is the power per unit area: F = P/A Example: Find the power and power flux of a pulsed Nd-YAG laser beam (1064 nm, 350 mJ/pulse, 10 ns pulse width) with a 5 mm beam diameter. E 350 10 3 J 7 P 3 . 5 10 W 35 MW t 10 10-9 s P 35 MW MW F 178 A (0.25) 2 cm 2 cm 2 LASERS Laser Power and Flux The spatial coherence and directionality of a laser makes it advantageous over other light sources, because it allows for large energy (or power) flux, i.e. the concentration of many photons in a very small area. The intensity of a laser is defined by the frequency and amount of photons leaving the optical cavity. The net power or energy can be measured using special detectors. The power (P) of a laser is defined as: P = E/t For a pulsed laser, E is the energy output measured using a Joule meter and t is the time duration of the pulse. In a CW laser, the power is measured directly with a power meter. The power flux is the power per unit area: F = P/A Example: Find the power and power flux of a pulsed Nd-YAG laser beam (1064 nm, 350 mJ/pulse, 10 ns pulse width) with a 5 mm beam diameter. E 350 10 3 J 7 P 3 . 5 10 W 35 MW t 10 10-9 s P 35 MW MW F 178 A (0.25) 2 cm 2 cm 2 LASERS Pulse Width Control One of the main advantages of a laser beam as an spectroscopic tool is the control over pulse width, in other words the possibility of tracking down ultrafast events which may be photoinitiated. The pulse width of some commercially available lasers could be below 100 x 10-15 s, which could be compressed even lower into the subfemtosecond regime. Q-Switching The quality factor of an optical cavity (Q) is defined as the ratio of the photon frequency to the width of the laser. Q = n/n Q is also related to the amount of energy stored in the cavity (Ec) and the amount of energy allowed to leak out (Et) in a given amount of time t: Q 2nEc t Et Q-Switching is an optical procedure that allows the compression of a pulse by decreasing/increasing the cavity quality for a very short time. Since the energy stored in the cavity is released in a shorter time, the power of the output pulse increases. LASERS Q-Switching The switching is usually made by using an fast electrooptical device. The most common is a crystalline material with a voltage dependent birefringence (i.e. doubly refracting). Without Q-switching Polarizer M1 Laser medium –V +V M2 Q-switched output Pockels Cell Q-switching produces pulse as short as 10 ns, only limited by the speed of the switching process. Mode Locking LASERS In order to obtain pico or femtosecond pulses, mode-locking is commonly used. This consists of modulating the photons to a particular longitudinal cavity mode of an optical cavity. The modulation guarantees phase and amplitude synchronization in which the amplitude of the edge modes is increased at expenses of the internal modes. The modulation is achieved by an acousto-optical device (a fancy light chopper) at a frequency equivalent to one complete round-trip of light inside the cavity: tr = 2d/c The pulse width is dependent on the number of modes excited (2N+1) and their frequency separation (n): 2 t (2 N 1)n LASERS Mode Locking The figures below show the concept of mode locking (Figures courtesy of D. T. Moore, www.unc.edu/~dtmoore) The output of a cavity contains a random distribution of longitudinal modes (red). Amplitude and phase modulation to one specific mode creates a mode locked output (blue) The dependence of the number of modes locked is shown here. The red trace shows wider pulses as a result of locking a smaller amount of modes. LASERS Laser Frequency Changes: Non-linear effects and harmonic generation Many spectroscopical applications require the acquisition of an spectrum in a wavelength range. Unfortunately, most lasers have near monochromatic output which limits the user’s choice of wavelength ranges. Highly polarizable materials allow us to obtain photons of different wavelengths via the interaction of the electric fields of the incident photon and the hyperpolarizability terms (non-linear terms of the induced dipole) of the material. The induced dipole is: m = aE + ½ bE·E + 1/6 gE·E·E + … With E = Asin(2nt) E2 = ½ A2(1-cos(2(2n)t) E3 = A3 (3/4 sin(2nt) – 1/4sin(2(3n)t) Thus second order scattered radiation form the material has a frequency that is doubled the incident frequency. This process is called harmonic generation and allows the doubling, tripling and quadrupling of laser radiation. Lasers Some typical lasers and their spectral output Lasers The Helium Neon Laser (Gas) 2p55s1 21S 3390 nm 2p54p1 23S 632.8 nm 2p54s1 1150 nm Electrical pumping He/Ne mixture (10:1 typical at low P) Bandwidth: 1.5 GHz (at 633 nm) CW operation 2p53p1 2p53s1 11S More info at Olympus He Ne 2p6 Lasers The Nd-YAG Laser (solid) 4F 5/2 Nd3+ is the lasing component. Ion is embedded in an Yttrium-Aluminum-Garnet matrix. Pulsed operation, Q-Switched in most applications (10 ns pulses), high output allow frequency doubling, tripling and quadrupling (532, 355, 266 nm) Optical pumping 4F 4I 9/2 3/2 1064 nm 4I 11/2 Lasers The Nitrogen Laser (gas) C3Pu First UV laser made, it operated with one mirror. Pulsed operation, with electrical pumping (spark gap or thrystor switching) does not require Q-switching, operates at ~ 1-10 ns widths. Used to pump dye lasers, and in MALDI-TOF Mass spectrometry. Electrical pumping 337.1 nm X1Sg+ B3Pg Lasers An excimer is a metastable excited state of a dimer molecule. In general the excited state is more stable than the ground state. Consider the Xe2 molecule. According to MO theory, it will not be stable. However, the excited state has better probability for living. Pulsed operation, with electrical pumping (thrystor switching) does not require Qswitching, operates at 10 ns widths and high frequencies. Common Excimer lasers: ArF: 193 nm KrF: 248 nm XeF: 351 nm KrCl: 222 nm XeCl: 308 nm XeBr: 282 nm Used in eye surgical procedures Electrical pumping The Excimer Laser (gas) Lasing X1S+ Lasers The Carbon Dioxide Laser (gas) 301 v=1 10.6 mm 9.6 mm Electrical pumping Near IR laser capable of delivering high power output. Can be operated pulsed or CW (1 kW). Pumping is done via collisional excitation with N2. 101 201 v=0 N2 CO2 202 Lasers The Dye Laser (solution phase) The best tunable laser. Requires optical pumping and works either CW or pulsed. Power is usually 10% (or less) of pump power at maximum of emission band. Highly fluorescent dyes are commonly used at small concentrations (mM). Pulses are usually 1-1000 ns depending on pump width. optical pumping S1 v’ = 0 XS0 v” = 0 For a list of commercially available dye lasers: http://www.exciton.com/ Lasers The Diode Laser (solid, semiconductor) The cheapest lasers available. Also the smallest ones. Based on n-p type juctions + p n – E’F Conduction band Impurity level closes the gap EF voltage E”F Valence band Semiconductor bands n-type: EF close to conduction p-type: EF close to valence Find more info at Molecular Expressions n j p