PHYS 352

Transducers for Optical Measurements

Optical Measurements detecting radiated energy in the electromagnetic

spectrum from infrared through visible light and UV X-ray and shorter λ: “nuclear” processes, covered

after reading week microwave and radio: not in this course

E = h ν and ν = c / λUV: λ = 200-400 nm; E = 6.2-3.1 eV

visible light: λ = 400-650 nm; E = 3.1-1.9 eVinfrared: λ from ~700 nm beyond 1 µm (all the

way out to 100 µm); E down to 0.015 eV

Two Categories of Light Detectors Thermal Detectors

incoming radiation heats something up and you detect the temperature rise and relate it to the radiation intensity

Photon Counters photon interactions “liberate” electrons and you

detect them

we're focusing on transducers, not other means of optical measurement (e.g. film)

Bolometers and Thermopiles incoming radiation absorbed by blackened elements, causing their

temperature to rise measure the change in resistance → bolometer radiation heats a pile of thermocouples connected in

series → thermopile rising temperature causes changing surface charges and

a current flow → pyroelectric thermal detectors are the most common radiation detectors

respond to broadband IR radiation cheap and easy to use

drawbacks affected by ambient temperature (thus, not terribly accurate) less sensitive than photon counters

uses: laser power meters, optical pyrometry, non-contact thermometry, IR satellite (weather) imaging

Thermistor Bolometer features broadband response (UV, visible, down to far IR

radiation, all can heat up the target in the detector) fairly fast response time if it’s a small detector e.g. Servo Corporation Model 1350 Thermistor IR detector

Actual Element Size: 1.0 mm x 1.0 mm, ± 10% Nominal Element Resistance: 2.7 megohms, ± 20 %, (BTL #1 material) Time Constant: 4.0 milliseconds, ± 20 % (1/e)

Responsivity: (600 K, 15 Hz): 500 volts/watt, open circuit N.E.P.(600 K, 15 Hz, 1 Hz BW): 6 × 10−10 watts/Hz1/2 (less window loss)

Wavelength Response: 2 to 20 microns

some typical characteristics 3 MΩ resistance

time constant: 4 to 30 ms sensitivity: 700-1200 V/W noise-equivalent power: few × 10−10 W/Hz1/2

Thermopile Example Melexis MLX90247

has an on-chip thermistor to determine ambient temperature

sensitivity 42 µV/K with about 10-15 mV output over the range −40 to 85 °C

time constant: 30 ms no supply voltage needed uses:

automotive HVAC ear thermometer

1 and 2: dissimilar metals forming thermocouple6: incoming radiation warms up 55: hot junction4: cold junction, thermally isolated, if possible3: cold junction compensation

Thermopile Uses

Thermopile Technology MEMS (microelectromechanical systems)

technology produces thermopiles with about 50 junctions (in series) in 2 x 2 mm2 area

hot junction in the centre, cold junction on the edges

Pyroelectric ferroelectric or piezoelectric materials can have

internal “permanent” electric polarization e.g. lithium tantalate crystals (LiTaO3), lead titanate

(PbTiO3), some polyvinyl fluoride plastic films caution: if piezoelectric, spurious signals from

vibrations these pyroelectric crystals acts as a charged capacitor changing temperature causes changing polarization in

these materials and hence a current flow fast response but only senses change in temperature

therefore, not so appropriate for steady temperature measurement

Pyroelectric IR Uses but can be used as an IR detector for

IR fire sensor, IR motion sensor (intruder alarm)

or can modulate or chop the signal gas analysis (e.g. emissions monitoring)

Servo Corp Model 1501Responsivity (10 Hz): 5,000 V/W NEP: 3.0 × 10−10 W/Hz1/2

Comparison of Sensitivity

from http://www.omega.com/literature/transactions/volume1/thermometers3.html

Photon Counters photoconductive detectors

increased conductivity due to excitation of electrons into the conduction band

somewhat similar to a thermistor – but, think population of the conduction band by photon interactions rather than thermodynamic population of the conduction band

thus, a direct response naturally, these detectors have temperature dependence

photoemissive detector more “quantum” in nature – detect the individual

electrons excited by the light

it's the band gap in the semiconductor that determines the spectral response (wavelength range for which the device is sensitive)

peak sensitivityCdS Eg = 2.1 eV 550 nm

CdSe Eg = 1.7 eV 680 nm

PbS Eg = 0.4 eV 2.9 µm

Si Eg = 1.1 eV 0.9 µm

Ge:Au-doped Eg = 0.1 eV 10 µm

Photoconductive Detectors

response time is relatively slow: ~tens ms

ambient temperature dependence

CdS Detector e.g. dark resistance 600 kΩ and illuminated resistance

of 9 kΩ (35 mW/m2 at 550 nm) this is the standard light sensor exhibits light history effect

hysteresis

different curves for different amounts of illumination

CdS Cell Light History Effect illuminate with 1 lux after being kept in darkness versus

illuminated at 1000 lux and then dropping the illumination to 1 lux

50% error!

IR Photoconductive Sensors PbS or InSb are the most common HgCdTe for specialty applications (tunable band gap) uses:

thermal sensing and radiometry spectroscopy missile guidance emissions gas analysis

Quantum Well Infrared Photodetectors artificial “lattice” of two different layers of semiconductors with

different bandgaps photoexcitation of electrons between the ground state and the

first excited state subbands of multi-quantum wells designed for wider spectral sensitivity, long wavelength IR,

cheaper than HgCdTe, can fabricate focal plane arrays (i.e. cameras)

new technology

Photoemissive Detectors for low light levels, down to the single photon level the

mainstay is the photomultiplier tube photoelectric effect liberates electrons amplification of the signal by acceleration and secondary

electron emission 1012 or more gain is possible → translates to µA

PMT Characteristics spectral response depends on photocathode material quantum efficiency: the fraction of incident photons

that produce photoelectrons they're fast

transit time: ~10 ns transit time spread: ≤1 ns

sets the rise time decay time

set by output τ ~ RC dark current

thermal emission of electrons from photocathode

PMT Pros and Cons for large area coverage and single photon

sensitivity it's still unrivalled extremely fast time response bulky and delicate glass vacuum tube requires high voltage power supply may be affected by magnetic fields (thus,

orientation dependence is a possible concern)

collection of charges is statistical in nature large shot noise, even for higher light levels

the microchannel plate is an alternative configuration to the photomultiplier tube that still provides electron gain

can serve as an image intensifier gain 104 per plate, smaller gain than PMT, but less bulky

housing (still requires vacuum inside the plate though)

note: camcorder “night vision” is just illumination with IR and a near-IR sensitive CCD (all commercial electronics CCDs are made from silicon)

Microchannel Plate

how an MCP works

Photodiode

both photodiodes and photomultiplier tubes are modelled as constant current sources they are producing charge

let’s examine the equivalent circuit of a photodiode

Electrical Modelling

Shunt and Series Resistance Shunt (particularly relevant in photovoltaic mode)

it’s the junction resistance at zero voltage typical values are MΩ to GΩ describes how ideal of a current source it is determines the frequency response of the photodiode (along

with the junction capacitance) operated in photovoltaic mode

Series it’s just the resistance through the

undepleted semiconductor and the contact resistances

Leakage and Noise Currents with zero bias (photovoltaic mode)

the noise source is thermal noise generated by the shunt resistance (Johnson noise)

with reverse bias there is leakage current, also known as dark current the noise source is shot noise due to fluctuations of the dark

current plus the photocurrent

Junction Capacitance you have space charge across the junction (in the depletion region

hence you have junction capacitance

the larger the area your photodiode is, the greater the C

the larger the thickness of the depletion region, the smaller the C

thus, junction capacitance is also a function of reverse bias

RC time constant set by the junction capacitance• faster time response with more reverse bias

response time: RC time constant (junction capacitance) is reduced with

reverse bias because depletion region width increases transit time in depletion layer decreases with reverse

bias voltage because carrier drift velocity increases linearity range extended with reverse bias

Photodiode Reverse Bias

Other Important Characteristics sensitivity or responsivity (given in A/W) quantum efficiency

note how high it is compared with a photomultiplier tube

Can you deduce the equationof this line, from first principles?

remember the I-V curve as a light sensor (photon counter), you’re best operating in the

“3rd quadrant” or reverse biased in photovoltaic mode with zero bias, you will be operating in the

“4th quadrant” to have the cell deliver power to the circuit

Understanding Its Operation

Zero Bias Simple Circuit why is this a bad way to operate?

as the photocurrent increases, so does the voltage that appears across RL

which forward biases the diode which decreases the shunt resistance as a result, the output voltage does not increase linearly with

the photocurrent

p-i-n photodiode is for p-intrinsic-n larger intrinsic layer absorbs most of the photons

(charge carriers are produced mostly in the depletion region)

for fast response use small area (and thick depletion region) → thus small junction capacitance

suitable for applying reverse bias voltage fast response time

PIN Diode

C =εAd

Photodiode Uses remote control receiver camera light meters solar cells p-i-n photodiodes are very fast

optical communications optical sensor (CD and DVD)

Photodiode Pros and Cons linearity better than 1% over seven or eight orders of

magnitude, basically up to saturation (1 mW/mm2 HeNe laser)

response time: tens of ns, not bad (not as fast as PMT) depends on reverse bias voltage

low noise, compact size, low cost, rugged high quantum efficiency (around 80% at peak spectral

sensitivity) no high voltage required (however, moderate voltage bias

improves performance) small area no internal gain – no amplification

sensitivity is not like PMT (single photon)

gain comes from avalanche in high-field region provides sensitivity to weaker signals (approaching single

photon level) APDs suffer increased dark noise

also excess noise greater than shot noise due to gain fluctuations

higher voltage – fast response time also used in fibre-optic communications

Avalanche Photodiodes

CCDs – charge coupled devices each pixel is a photodiode

Boyle and Smith received 2009 Nobel Prize in Physics

Imaging