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Photomultipliers hf e PE effect e e Secondary electron emission e e e Electron multiplication Photomultiplier tube hf e Dynode -V • Combines PE effect with electron multiplication to provide very high detection sensitivity • Can detect single photons. Anode Microchannel plates • The principle of the photomultiplier tube can be extended to an array of photomultipliers • This way one can obtain spatial resolution • Biggest application is in night vision goggles for military and civilian use Microchannel plates •MCPs consist of arrays of tiny tubes •Each tube is coated with a photomultiplying film •The tubes are about 10 microns wide http://hea-www.harvard.edu/HRC/mcp/mcp.html http://hea-www.harvard.edu/HRC/mcp/mcp.html MCP array structure http://hea-www.harvard.edu/HRC/mcp/mcp.html MCP fabrication Disadvantages of Photomultiplers as sensors • Need expensive and fiddly high vacuum equipment • Expensive • Fragile • Bulky Photoconductors • As well as liberating electrons from the surface of materials, we can excite mobile electrons inside materials • The most useful class of materials to do this are semiconductors • The mobile electrons can be measured as a current proportional to the intensity of the incident radiation • Need to understand semiconductors…. Photoelecric effect with Energy Bands Evac Evac Ec Ef Ev Ef Metal Semiconductor Band gap: Eg=Ec-Ev Photoconductivity e To amplifier Ec Evac Ef Ev Semiconductor Photoconductors • Eg (~1 eV) can be made smaller than metal work functions f (~5 eV) • Only photons with Energy E=hf>Eg are detected • This puts a lower limit on the frequency detected • Broadly speaking, metals work with UV, semiconductors with optical Band gap Engineering • Semiconductors can be made with a band gap tailored for a particular frequency, depending on the application. • Wide band gap semiconductors good for UV light • III-V semiconductors promising new materials Example: A GaN based UV detector This is a photoconductor 5m Response Function of UV detector Choose the material for the photon energy required. •Band-Gap adjustable by adding Al from 3.4 to 6.2 eV •Band gap is direct (= efficient) •Material is robust Stimulated emission E2 - E1 = hf E2 E1 Two identical photons Same - frequency - direction - phase - polarisation Lasers • LASER - acronym for – Light Amplification by Stimulated Emission of Radiation – produce high intensity power at a single frequency (i.e. monochromatic) Laser Globe Principles of Lasers •Usually have more atoms in low(est) energy levels •Atomic systems can be pumped so that more atoms are in a higher energy level. • Requires input of energy • Called Population Inversion: achieved via • Electric discharge • Optically • Direct current Population inversion Lots of atoms in this level Energy N2 N1 Few atoms in this level Want N2 - N1 to be as large as possible Population Inversion (3 level System) E2 (pump state), t2 Pump light ts >t2 E1 (metastablestate), ts hfo Laser output hf E1 (Ground state) Light Amplification Light amplified by passing light through a medium with a population inversion. • Leads to stimulated emission Laser Laser Requires a cavity enclosed by two mirrors. • Provides amplification • Improves spectral purity • Initiated by “spontaneous emission” Laser Cavity Cavity possess modes • Analagous to standing waves on a string • Correspond to specific wavelengths/frequencies • These are amplified Spectral output Properties of Laser Light. • Can be monochromatic • Coherent •Very intense •Short pulses can be produced Types of Lasers Large range of wavelengths available: • Ammonia (microwave) MASER • CO2 (far infrared) • Semiconductor (near-infrared, visible) • Helium-Neon (visible) • ArF – excimer (ultraviolet) • Soft x-ray (free-electron, experimental) Optical Fibre Sensors • • • • • • • • Non-Electrical Explosion-Proof (Often) Non-contact Light, small, snakey => “Remotable” Easy(ish) to install Immune to most EM noise Solid-State (no moving parts) Multiplexing/distributed sensors. Applications • • • • • • • Lots of Temp, Pressure, Chemistry Automated production lines/processes Automotive (T,P,Ch,Flow) Avionic (T,P,Disp,rotn,strain,liquid level) Climate control (T,P,Flow) Appliances (T,P) Environmental (Disp, T,P) Optical Fibre Principles Cladding: glass or Polymer Core: glass, silica, sapphire TIR keeps light in fibre Different sorts of cladding: graded index, single index, step index. Optical Fibre Principles • • • • Snell’s Law: n1sin1=n2sin2 crit = arcsin(n2/n1) Cladding reduces entry angle Only some angles (modes) allowed Optical Fibre Modes Phase and Intensity Modulation methods • Optical fibre sensors fall into two types: – Intensity modulation uses the change in the amount of light that reaches a detector, say by breaking a fibre. – Phase Modulation uses the interference between two beams to detect tiny differences in path length, e.g. by thermal expansion. Intensity modulated sensors: • Axial displacement: 1/r2 sensitivity • Radial Displacement Microbending (1) Microbending – Bent fibers lose energy – (Incident angle changes to less than critical angle) Microbending (2): Microbending – “Jaws” close a bit, less transmission – Give jaws period of light to enhance effect • Applications: – Strain gauge – Traffic counting More Intensity modulated sensors Frustrated Total Internal Reflection: – Evanescent wave bridges small gap and so light propagates – As the fibers move (say car passes), the gap increases and light is reflected Evanescent Field Decay @514nm More Intensity modulated sensors Frustrated Total Internal Reflection: Chemical sensing – Evanescent wave extends into cladding – Change in refractive index of cladding will modify output intensity Disadvantages of intensity modulated sensors •Light losses can be interpreted as change in measured property −Bends in fibres −Connecting fibres −Couplers •Variation in source power Phase modulated sensors Bragg modulators: – Periodic changes in refractive index – Bragg wavelenght (λb) which satisfies λb=2nD is reflected – Separation (D) of same order as than mode wavelength Phase modulated sensors Period,D λb=2nD • Multimode fibre with broad input spectrum • Strain or heating changes n so reflected wavelength changes • Suitable for distributed sensing Phase modulated sensors – distributed sensors Temperature Sensors • Reflected phosphorescent signal depends on Temperature • Can use BBR, but need sapphire waveguides since silica/glass absorbs IR Phase modulated sensors Fabry-Perot etalons: – Two reflecting surfaces separated by a few wavelengths – Air gap forms part of etalon – Gap fills with hydrogen, changing refractive index of etalon and changing allowed transmitted frequencies. Digital switches and counters • Measure number of air particles in air or water gap by drop in intensity – Environmental monitoring • Detect thin film thickness in manufacturing – Quality control • Counting things – Production line, traffic. NSOM/AFM Combined •Optical resolution determined by Bent NSOM/AFM Probe diffraction limit (~λ) •Illuminating a sample with the "near-field" of a small light source. • Can construct optical images with resolution well beyond usual "diffraction limit", (typically ~50 nm.) SEM - 70nm aperture NSOM Setup Ideal for thin films or coatings which are several hundred nm thick on transparent substrates (e.g., a round, glass cover slip). Molecular Spectroscopy • Molecular Energy Levels – Vibrational Levels – Rotational levels • • • • Population of levels Intensities of transitions General features of spectroscopy An example: Raman Microscopy – Detection of art forgery – Local measurement of temperature Molecular Energies Energy Classical Quantum E4 E3 E2 E1 E0 Molecular Energy Levels Increasing Energy Translation Electronic orbital Vibrational Rotational Nuclear Spin Electronic Spin Rotation Vibration etc. Electronic Orbital Etotal + Eorbital + Evibrational + Erotational +….. Molecular Vibrations • Longitudinal Vibrations along molecular axis • E=(n+1/2)hf where f is the classical frequency of the oscillator • 1 f 2 k where k is the ‘spring constant • Energy Levels equally spaced • How can we estimate the spring constant? r k m M = Mm/(M+m) Atomic mass concentrated at nucleus k = f (r) Molecular Vibrations Hydrogen molecules, H2, have ground state vibrational energy of 0.273eV. Calculate force constant for the H2 molecule (mass of H is 1.008 amu) r • Evib=(n+1/2)hf f =0.273eV/(1/2(h)) = 2.07x1013 Hz • To determine k we need μ μ=(Mm)/(M+m) =(1.008)2/2(1.008) amu =(0.504)1.66x10-27kg =0.837x10-27kg • k= μ(2πf)2 =576 N/m K m M = Mm/(M+m) K = f (r) Molecular Rotations • Molecule can also rotate about its centre of mass • v1 = wR1 ; v2 = wR2 M1 • L = M1v1R1+ M2v2R2 = (M1R12+ M2R22)w = Iw • EKE = 1/2M1v12+1/2M2v22 = 1/2Iw2 M2 R1 R2 Molecular Rotations • Hence, Erot= L2/2I • Now in fact L2 is quantized and L2=l(l+1)h2/42 • Hence Erot=l(l+1)(h2/42)/2I • Show that DErot=(l+1) h2/42/I. This is not equally spaced • Typically DErot=50meV (i.e for H2) Populations of Energy Levels ΔE<<kT ΔE=kT ΔE>kT ΔE (Virtually) all molecules in ground state States almost equally populated • Depends on the relative size of kT and DE Intensities of Transitions • Quantum Mechanics predicts the degree to which any particular transition is allowed. • Intensity also depends on the relative population of levels hv Strong absorption hv Weak emission 2hv hv Transition saturated hv General Features of Spectroscopy • Peak Height or intensity • Frequency • Lineshape or linewidth Raman Spectroscopy • Raman measures the vibrational modes of a solid • The frequency of vibration depends on the atom masses and the forces between them. • Shorter bond lengths mean stronger forces. r K m M f vib= (K/)1/2 = Mm/(M+m) K = f(r) Raman Spectroscopy Cont... Laser In Sample Lens Monochromator CCD array •Incident photons typically undergo elastic scattering. •Small fraction undergo inelastic energy transferred to molecule. •Raman detects change in vibrational energy of a molecule. Raman Microscope 100 Detecting Art Forgery 80 YTI S NET NI • Ti-white became available only circa 1920. Pb white 60 40 • The Roberts painting shows clear evidence of Ti white but is dated 1899 20 Ti white 0 0 200 400 600 800 -1 WAVENUMBER (cm ) 200 150 YTI S NET NI 100 50 0 0 200 400 600 -1 WAVENUMBER (cm ) 800 Tom Roberts, ‘Track To The Harbour’ dated 1899 Raman Spectroscopy and the Optical Measurement of Temperature • Probability that a level is occupied is proportional to exp(DE/kT)
