CHEM 524

CHEM 524 Course Outline (Part 8)--2005

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V. Detectors - Two main types Thermal and Photon

A. Characterization

Responsitivity (R = X/F--signal size) vs. Sensitivity (Q = dX/dF --change of signal with change of flux)

Linearity (region Q = const.),
Dynamic Range (magnitude of Q variation measurable),
Stability (time for const. R, Q),
Degradation (long term stability),
Hysterisis (power dependent change in R,Q)

Timing:

Rise time (10-90% full response) and
Time constant [t = (2pf_c)^-1 f_c--> R = 0.7 R_max]

Signal to noise ratio:

Noise equivalent Power (NEP is level to obtain S/N = 1)
Detectivity (D*= DA_d^1/2 (Df)^1/2,D = (NEP)^-1)
Quantum efficiency (k(l) = # electron / # photon)

B. Thermal (energy) detectors -- increase in temperature creates electrical response (table)

1. Typically light irradiate blackened plate, heating it, causing response in the sensor coupled to it

Expect them to be slow and modest sensitivity, must heat and accommodate detector heat capacity

Thermocouple, thermopile (voltage vs. T),
Thermistor bolometer (resistance vs. T),
Pneumatic Golay (pressure vs. T)—can tune to specific gas absorbance (dedicated sensor)

2. Pyroelectric -- e.g. TGS -- responds to dT/dt, change in T – standard FTIR detector

relatively fast time constant, integrates flux,

flat response with wavelength, relatively inexpensive,

small chip size, can be made into arrays

C. Photon Detectors -- quantum response to # photons above threshold (table)

--D* will be limited by background radiation from room temperature windows, optics

--power respose will fall off in uv compared to IR, more energy per photon

1. Photo multiplier -- current source based on photo-electric effect

Photo cathode -- P-E effect -- modest quantum efficiency --source of spectral response--see curves
Multiplier -- gives internal gain / results in low light level sensitive --based on dynode chain, each with secondary electron emit
Dark current -- main source of noise at high temperature
Shot noise-- proportional to F^1/2-- more light better S/N
Can be pulse-counted -- best S/N at low light level

2. Variants: Channeltron, micro channel plate, intensifier

3. Photodiodes -- photo-voltaic (P-V) -- excite e^- to conduction band, act as current source

Sensitivity: quantum efficiency high but no internal gain, need significan amplification (I -->V)
Spectrum depends on material, Si-vis to 1.1 µ, Ge to 1.8 µ, InSb (near IR to 5 µ), MCT (Hg_xCd_{1- x}Te, mid IR, varies with x, normally to 12 µ)--see response curves or more recent Judson chart
Time response depends on material (see handout)

4. Photoconductors -- dominate IR market -- effectively variable resistance of semi-conductor, operate down to band gap, needs bias voltage, together act as voltage source

PbS, PbSe, --near IR, cheap, room temperature, slow
InAs (to 3+ µ), InSb (to 5.5 µ) -- fast response, but lower noise as P-V
MCT -- Hg_x(Cd)_1-xTe -- variable spectral range, depend on x, vis to mid IR,

8-20 m limit are available, narrow band vs. wide band
liquid N₂ cool needed for nearly BLIP limited D*

PbSeTe – nearly the same sort of properties as MCT
Doped Ge—dopants (e.g. Au, As, Cu) vary Ge band gap—

typically need more cooling, more bias (can be fast)

D. Multichannel Detectors—growth area--Check out links to Multichannel Detector companies (below)

Film, Vidicon, Diode Array (1-D, Si based, PDA)
CCD - Si based, 2D, high quantum yield –dominate field

Can be “square” ~1024x1024 pixels, 26 m size or smaller with e.g. 1300x100 for spectra
Speed and sensitivity can tradeoff, back-thinned have high quantum efficiency

or back illuminated and specially coated for UV or other spectral performance

Cooling can reduce dark current approx. to digitization level

Intensifier can increase sensitivity (not generally needed with top CCD), useful for PDA
Latest are “focal plane array detectors” of InSb or MCT where pixels are IR detectors, but array sizes range form 64x64 up to 256x256 (more?)
Room temperature arrays are available made from GaAs/AlGaAs materials with a Quantum Well IR photodetector (QWIP) array technology