1. Photomultipliers hf e e e e e e PE effect Secondary electron emission Electron multiplication

2. Photomultiplier tube Combines PE effect with electron multiplication to provide very high detection sensitivity Can detect single photons. -V hf e Anode Dynode

3. 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

4. http://hea-www.harvard.edu/HRC/mcp/mcp.html MCPs consist of arrays of tiny tubes Each tube is coated with a photomultiplying film The tubes are about 10 microns wide Microchannel plates

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

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

7. Disadvantages of Photomultiplers as sensors Need expensive and fiddly high vacuum equipment Expensive Fragile Bulky

8. 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….

9. Photoelecric effect with Energy Bands EfEf E vac Semiconductor Band gap: E g =E c -E v Metal EfEf E vac EcEc EvEv

10. Photoconductivity Semiconductor Ef Evac Ec Ev e To amplifier

11. Photoconductors E g (~1 eV) can be made smaller than metal work functions  (~5 eV) Only photons with Energy E=hf>E g are detected This puts a lower limit on the frequency detected Broadly speaking, metals work with UV, semiconductors with optical

12. 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

13. Example: A GaN based UV detector 5m5m This is a photoconductor

14. Response Function of UV detector

15. 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

16. Stimulated emission E2E2 E1E1 E 2 - E 1 = hf Two identical photons Same - frequency - direction - phase - polarisation

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

18. 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

19. Population inversion N2N2 N1N1 Energy Lots of atoms in this level Few atoms in this level Want N 2 - N 1 to be as large as possible

20. Population Inversion (3 level System) E2 (pump state), t 2 E1 (metastable- state), t s E1 (Ground state) Laser output hf Pump light hf o t s >t 2

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

22. Laser

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

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

25. Spectral output

26. Properties of Laser Light. Can be monochromatic Coherent Very intense Short pulses can be produced

27. Types of Lasers Large range of wavelengths available: Ammonia (microwave) MASER CO 2 (far infrared) Semiconductor (near-infrared, visible) Helium-Neon (visible) ArF – excimer (ultraviolet) Soft x-ray (free-electron, experimental)

28. 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.

29. 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)

30. 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.

34. 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

35. Optical Fibre Modes

36. 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.

37. Intensity modulated sensors: Axial displacement: 1/r 2 sensitivity Radial Displacement

38. Microbending (1) Microbending –Bent fibers lose energy –(Incident angle changes to less than critical angle)

39. Microbending (2): Microbending –“Jaws” close a bit, less transmission – Give jaws period of light to enhance effect Applications: – Strain gauge – Traffic counting

40. 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

41. 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

42. 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

43. Phase modulated sensors Bragg modulators: –Periodic changes in refractive index –Bragg wavelenght (λ b ) which satisfies λ b =2nD is reflected –Separation (D) of same order as than mode wavelength

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

45. Phase modulated sensors – distributed sensors

46. Temperature Sensors Reflected phosphorescent signal depends on Temperature Can use BBR, but need sapphire waveguides since silica/glass absorbs IR

47. 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.

48. 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.

49. NSOM/AFM Combined SEM - 70nm aperture Bent NSOM/AFM Probe 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.)

50. NSOM Setup Ideal for thin films or coatings which are several hundred nm thick on transparent substrates (e.g., a round, glass cover slip).

51. 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

52. Molecular Energies Classical Quantum Energy E0E0 E4E4 E3E3 E2E2 E1E1

53. Molecular Energy Levels Translation Nuclear Spin Electronic Spin Rotation Vibration Electronic Orbital Increasing Energy etc. Electronic orbital Vibrational E total + E orbital + E vibrational + E rotational +….. Rotational

54. 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? m M r k k = f (r)  = Mm/(M+m) Atomic mass concentrated at nucleus

55. Molecular Vibrations E vib =(n+1/2)hf  f =0.273eV/(1/2(h)) = 2.07x10 13 Hz To determine k we need μ μ=(Mm)/(M+m) =(1.008) 2 /2(1.008) amu =(0.504)1.66x10 -27 kg =0.837x10 -27 kg k= μ(2πf) 2 =576 N/m m M r K K = f (r)  = Mm/(M+m) Hydrogen molecules, H 2, have ground state vibrational energy of 0.273eV. Calculate force constant for the H 2 molecule ( mass of H is 1.008 amu)

56. Molecular Rotations Molecule can also rotate about its centre of mass v 1 =  R 1 ; v 2 =  R 2 L = M 1 v 1 R 1 + M 2 v 2 R 2 = (M 1 R 1 2 + M 2 R 2 2 )  = I  E KE = 1/2M 1 v 1 2 +1/2M 2 v 2 2 = 1/2I  2 R1R1 R2R2 M1M1 M2M2

57. Molecular Rotations Hence, E rot = L 2 /2I Now in fact L 2 is quantized and L 2 =l(l+1)h 2 /4  2 Hence E rot =l(l+1)(h 2 /4  2 )/2I Show that  E rot =(l+1) h 2 /4  2 /I. This is not equally spaced Typically  E rot =50meV (i.e for H 2 )

58. Populations of Energy Levels Depends on the relative size of kT and  E ΔE<<kT ΔE=kT ΔE>kT ΔEΔE (Virtually) all molecules in ground state States almost equally populated

59. Intensities of Transitions Quantum Mechanics predicts the degree to which any particular transition is allowed. Intensity also depends on the relative population of levels Strong absorption Weak emission Transition saturated hv 2hv hv

60. General Features of Spectroscopy Peak Height or intensity Frequency Lineshape or linewidth

61. 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. m M r K f vib = (K/  ) 1/2 K = f(r)  = Mm/(M+m)

62. 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.

63. Raman Microscope

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

65. Raman Spectroscopy and the Optical Measurement of Temperature Probability that a level is occupied is proportional to exp(  E/kT)