Photomultipliers

hf e ee

e ee

PE effect Secondary

electron

emission

Electron

multiplication

Photomultiplier tube

• Combines PE effect with electron multiplication to provide very high detection sensitivity

• Can detect single photons.

-V

hf e Anode

Dynode

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

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

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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

Ef

Evac

Semiconductor

Band gap: Eg=Ec-Ev

Metal

Ef

EvacEc

Ev

Photoconductivity

Semiconductor

Ef

EvacEc

Ev

e To amplifier

Photoconductors

• Eg (~1 eV) can be made smaller than metal work functions (~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

5m

This is a photoconductor

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

E2 - E1 = hf

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

N2

N1

Ene

rgy

Lots of atoms in this level

Few atoms in this level

Want N2 - N1 to be as large as possible

Population Inversion (3 level System)

E2 (pump state), t2

E1 (metastable- state), ts

E1 (Ground state)

Laser output

hf

Pump light

hfo

ts >t2

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 CavityCavity 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 fibreDifferent 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

• Multimode fibre with broad input spectrum• Strain or heating changes n so reflected wavelength changes• Suitable for distributed sensing

λb=2nD

Period,D

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

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.)

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

Classical Quantum

Ene

rgy

E0

E4

E3

E2

E1

Molecular Energy Levels

Translation

Nuclear Spin

Electronic Spin

Rotation

Vibration

Electronic Orbital

Incr

easi

ng

E

nerg

y

etc.

Electronic orbital Vibrational

Etotal + Eorbital + Evibrational + Erotational +…..

Rotational

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)k

f2

1

Atomic mass concentrated at nucleus

Molecular Vibrations

• 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

m M

r

K

K = f (r)

= Mm/(M+m)

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)

Molecular Rotations

• Molecule can also rotate about its centre of mass

• v1 = R1 ; v2 = R2

• L = M1v1R1+ M2v2R2

= (M1R12+ M2R2

2)

= I• EKE = 1/2M1v1

2+1/2M2v22

= 1/2I2

R1 R2

M1M2

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 Erot=(l+1) h2/42/I. This is not equally spaced

• Typically Erot=50meV (i.e for H2)

Populations of Energy Levels

• Depends on the relative size of kT and E

ΔE<<kT ΔE=kT ΔE>kT

ΔE

(Virtually) all molecules in ground

state

States almost equally populated

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 2hvhv hv 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.

m M

r

K

f vib= (K/)1/2

K = f(r)

= Mm/(M+m)

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

0 200 400 600 800

WAVENUMBER (cm-1)

0

20

40

60

80

100

INT

EN

SIT

Y Pb white

Ti white

0 200 400 600 800

WAVENUMBER (cm-1)

0

50

100

150

200

INT

EN

SIT

Y

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

Raman Spectroscopy and the Optical Measurement of Temperature

• Probability that a level is occupied is proportional to exp(E/kT)