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Photomultipliers hf e PE effect e e Secondary electron emission e e e Electron 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 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 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 Microchannel plates • MCPs consist of arrays of tiny tubes • Each tube is coated with a photomultiplying film http: //hea-www. harvard. edu/HRC/mcp. html • The tubes are about 10 microns wide

http: //hea-www. harvard. edu/HRC/mcp. html MCP array structure http: //hea-www. harvard. edu/HRC/mcp. html MCP array structure

http: //hea-www. harvard. edu/HRC/mcp. html MCP fabrication http: //hea-www. harvard. edu/HRC/mcp. html MCP fabrication

Disadvantages of Photomultiplers as sensors • Need expensive and fiddly high vacuum equipment • 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 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 Ec Ef Ev Ef Metal Semiconductor Band gap: Photoelecric effect with Energy Bands Evac Ec Ef Ev Ef Metal Semiconductor Band gap: Eg=Ec-Ev

Photoconductivity Ec e To amplifier Evac Ef Ev Semiconductor Photoconductivity Ec e To amplifier Evac Ef Ev Semiconductor

Photoconductors • Eg (~1 e. V) can be made smaller than metal work functions Photoconductors • Eg (~1 e. V) can be made smaller than metal work functions f (~5 e. V) • 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 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 Ga. N based UV detector This is a photoconductor 5 m Example: A Ga. N based UV detector This is a photoconductor 5 m

Response Function of UV detector Response Function of UV detector

Choose the material for the photon energy required. • Band-Gap adjustable by adding Al Choose the material for the photon energy required. • Band-Gap adjustable by adding Al from 3. 4 to 6. 2 e. V • Band gap is direct (= efficient) • Material is robust

Stimulated emission E 2 - E 1 = hf E 2 E 1 Two Stimulated emission E 2 - E 1 = hf E 2 E 1 Two identical photons Same - frequency - direction - phase - polarisation

Lasers • LASER - acronym for – Light Amplification by Stimulated Emission of Radiation Lasers • LASER - acronym for – Light Amplification by Stimulated Emission of Radiation – produce high intensity power at a single frequency (i. e. monochromatic)

Principles of Lasers • Usually have more atoms in low(est) energy levels • Atomic 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 N 2 N 1 Few Population inversion Lots of atoms in this level Energy N 2 N 1 Few atoms in this level Want N 2 - N 1 to be as large as possible

Population Inversion (3 level System) E 2 (pump state), t 2 Pump light hfo Population Inversion (3 level System) E 2 (pump state), t 2 Pump light hfo ts >t 2 E 1 (metastable state), ts Laser output hf E 1 (Ground state)

Light Amplification Light amplified by passing light through a medium with a population inversion. Light Amplification Light amplified by passing light through a medium with a population inversion. • Leads to stimulated emission

Laser Laser

Laser Requires a cavity enclosed by two mirrors. • Provides amplification • Improves spectral Laser Requires a cavity enclosed by two mirrors. • Provides amplification • Improves spectral purity • Initiated by “spontaneous emission”

Laser Cavity possess modes • Analagous to standing waves on a string • Correspond Laser Cavity possess modes • Analagous to standing waves on a string • Correspond to specific wavelengths/frequencies • These are amplified

Spectral output Spectral output

Properties of Laser Light. • Can be monochromatic • Coherent • Very intense • 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 • CO Types of Lasers Large range of wavelengths available: • Ammonia (microwave) MASER • CO 2 (far infrared) • Semiconductor (near-infrared, visible) • Helium-Neon (visible) • Ar. F – excimer (ultraviolet) • Soft x-ray (free-electron, experimental)

Optical Fibre Sensors • • Non-Electrical Explosion-Proof (Often) Non-contact Light, small, snakey => “Remotable” 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, 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 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: n 1 sin 1=n 2 sin 2 • Optical Fibre Principles • Snell’s Law: n 1 sin 1=n 2 sin 2 • crit = arcsin(n 2/n 1) • Cladding reduces entry angle • Only some angles (modes) allowed

Optical Fibre Modes Optical Fibre Modes

Phase and Intensity Modulation methods • Optical fibre sensors fall into two types: – 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/r 2 sensitivity • Radial Displacement Intensity modulated sensors: • Axial displacement: 1/r 2 sensitivity • Radial Displacement

Microbending (1) Microbending – Bent fibers lose energy – (Incident angle changes to less 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 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 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 @514 nm

More Intensity modulated sensors Frustrated Total Internal Reflection: Chemical sensing – Evanescent wave extends 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 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 Phase modulated sensors Bragg modulators: – Periodic changes in refractive index – Bragg wavelenght (λb) which satisfies λb=2 n. D is reflected – Separation (D) of same order as than mode wavelength

Phase modulated sensors Period, D λb=2 n. D • Multimode fibre with broad input Phase modulated sensors Period, D λb=2 n. D • Multimode fibre with broad input spectrum • Strain or heating changes n so reflected wavelength changes • Suitable for distributed sensing

Phase modulated sensors – distributed sensors Phase modulated sensors – distributed sensors

Temperature Sensors • Reflected phosphorescent signal depends on Temperature • Can use BBR, but 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 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 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 diffraction limit (~λ) • Illuminating a sample NSOM/AFM Combined • Optical resolution determined by 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. ) Bent NSOM/AFM Probe SEM - 70 nm aperture

NSOM Setup Ideal for thin films or coatings which are several hundred nm thick 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 • • 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 E 4 E 3 E 2 E 1 E Molecular Energies Energy Classical Quantum E 4 E 3 E 2 E 1 E 0

Molecular Energy Levels Increasing Energy Translation Electronic Vibrational orbital Rotational Nuclear Spin Electronic Spin Molecular Energy Levels Increasing Energy Translation Electronic Vibrational orbital Rotational Nuclear Spin Electronic Spin Rotation Vibration Electronic Orbital etc. Etotal + Eorbital + Evibrational + Erotational +…. .

Molecular Vibrations • Longitudinal Vibrations along molecular axis • E=(n+1/2)hf where f is the Molecular Vibrations • Longitudinal Vibrations along molecular axis • E=(n+1/2)hf where f is the classical frequency of the oscillator • 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, H 2, have ground state vibrational energy of 0. 273 Molecular Vibrations Hydrogen molecules, H 2, have ground state vibrational energy of 0. 273 e. V. Calculate force constant for the H 2 molecule (mass of H is 1. 008 amu) r • Evib=(n+1/2)hf f =0. 273 e. V/(1/2(h)) = 2. 07 x 1013 Hz K • To determine k we need μ μ=(Mm)/(M+m) =(1. 008)2/2(1. 008) amu m =(0. 504)1. 66 x 10 -27 kg =0. 837 x 10 -27 kg • k= μ(2πf)2 =576 N/m M = Mm/(M+m) K = f (r)

Molecular Rotations • Molecule can also rotate about its centre of mass • v Molecular Rotations • Molecule can also rotate about its centre of mass • v 1 = w. R 1 ; v 2 = w. R 2 M 1 • L = M 1 v 1 R 1+ M 2 v 2 R 2 = (M 1 R 12+ M 2 R 22)w = Iw • EKE = 1/2 M 1 v 12+1/2 M 2 v 22 = 1/2 Iw 2 M 2 R 1 R 2

Molecular Rotations • Hence, Erot= L 2/2 I • Now in fact L 2 Molecular Rotations • Hence, Erot= L 2/2 I • Now in fact L 2 is quantized and L 2=l(l+1)h 2/4 p 2 • Hence Erot=l(l+1)(h 2/4 p 2)/2 I • Show that DErot=(l+1) h 2/4 p 2/I. This is not equally spaced • Typically DErot=50 me. V (i. e for H 2)

Populations of Energy Levels ΔE<<k. T ΔE=k. T ΔE>k. T ΔE (Virtually) all molecules Populations of Energy Levels ΔE<k. T ΔE (Virtually) all molecules in ground state States almost equally populated • Depends on the relative size of k. T and DE

Intensities of Transitions • Quantum Mechanics predicts the degree to which any particular transition 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 2 hv hv Transition saturated hv

General Features of Spectroscopy • Peak Height or intensity • Frequency • Lineshape or General Features of Spectroscopy • Peak Height or intensity • Frequency • Lineshape or linewidth

Raman Spectroscopy • Raman measures the vibrational modes of a solid • The frequency 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 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 Raman Microscope

Detecting Art Forgery • Ti-white became available only circa 1920. Pb white Ti white Detecting Art Forgery • Ti-white became available only circa 1920. Pb white Ti white • The Roberts painting shows clear evidence of Ti white but is dated 1899 Tom Roberts, ‘Track To The Harbour’ dated 1899

Raman Spectroscopy and the Optical Measurement of Temperature • Probability that a level is Raman Spectroscopy and the Optical Measurement of Temperature • Probability that a level is occupied is proportional to exp(DE/k. T)