Transcript optical_sensors_4oct

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
5m
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
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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
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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
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Snell’s Law: n1sin1=n2sin2
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
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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)h2/42
• Hence Erot=l(l+1)(h2/42)/2I
• Show that DErot=(l+1) h2/42/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)