Optical Transducers Transducer converts non-electrical domain data to electrical domain data (charge, resistance, voltage, current etc.) Thermal transducers: • respond to incident energy rate • relatively flat spectral response curves (determined by window and coating) • generally slow (milliseconds or slower) • usually single channel Photon transducers: • respond to incident photon rate • highly variable spectral response (determined by photosensitive material) • respond quickly (microseconds or faster) • single or multichannel (1-D or 2-D) CEM 835 page 4-1 Responsivity R(λ) and Sensitivity Q(λ): R( λ ) = X(λ) Φ(λ ) Q(λ) = dX( λ) dΦ(λ) where X(λ) is output signal (voltage, current, charge) Φ(λ) is incident flux (W) Plot of R(λ) or Q(λ) versus λ is called the spectral response CEM 835 page 4-2 Thermal detectors: Thermocouple: - based on thermoelectric potential when two dissimilar metals are in contact - junction attached to blackened disc of known area but small heat capacity (0.8-40 µm) - output is nV-µV range (limited sensitivity) - Q(λ) constant over modest temperature range (10-10-10-7 W) - moderate responsivity R(λ) 5-25 V·W-1 - junctions with different sensitivities are available - response time limited by capacitance of wires to ms - multiple junction thermocouples called thermopiles CEM 835 page 4-3 Thermistor bolometer • blackened semiconductor with narrow band-gap (0.8-40 µm) • radiation excites electron-hole pairs which decrease resistance • decrease in resistance is compared with unirradiated bolometer • difference is amplified - Q(λ) constant 10-6-10-1 W • high responsivity R(λ) 1000 V·W-1 • response time 1-10 ms Pyroelectric detector • based on a piezoelectric material - eg DTGS • non-centrosymmetric crystal has permanent dipole moment across unit cell - acts like a capacitor • when irradiated crystal expands slightly, capacitance decreases, current flows • high responsivity R(λ) up to 104 V·W-1 • Q(λ) constant 10-6-10-1 W • response time <10 ms CEM 835 page 4-4 Photon detectors: Photon detectors are based on • photoconductive materials (MCT transducer) • photovoltaic cells (Si, Se photocell) • photoemissive materials (PMT's, phototubes) • semiconductor pn junctions (photodiodes) CEM 835 page 4-5 Photoconductive cell: • semiconductor (CdS, PbS, PbSe, InSb, InAs, HgCdTe, or PbSnTe) behaves like resistor • in series with constant voltage source and load resistor - voltage across load resistor used to measure the resistance of the semiconductor • incident radiation causes band-gap excitation and lowers the resistance of the semiconductor • most sensitive in near IR (PbS) • sometimes cooling is necessary to reduce thermal band-gap excitation Photovoltaic cells: • thin layer of crystalline semiconductor (Se, Si, Cu2O, HgCdTe) sandwiched between two different metal electrodes • no bias but irradiation causes formation of electron hole-pair formation CEM 835 page 4-6 • electron migrates one way, holes migrate in opposite direction • if resistance of external circuitry is small, microamps produced • high sensitivity in near IR to UV (102-106 V·W-1) - eg Fe-Se-Ag 300-700 nm R(λ) peaking near 550 nm Phototubes: • two electrodes enclosed in glass or silica envelope • bias (70-180 V) is applied between two electrodes • cathode is a photoemissive material (Cs3Sb, NaO, AgOCs) emits photoelectrons • current collected by anode - photoemission only if hν > surface workfunction (1-5 eV) CEM 835 page 4-7 CEM 835 page 4-8 Total rate and current are r ap = η⋅ r cp = η⋅ Φ ph ⋅K(λ ) { photon flux (s -1 ) i ap = η⋅ e ⋅ r cp = η⋅ e ⋅ Φ ph ⋅ hυ ⋅ R( λ) 2 3 1 424 3 1 −1 Φ = radient A⋅W flux (J ⋅s -1 ) where η - anode collection efficiency (0-1) - design and bias dependent K(λ) - number of electrons produced per incident photon photocathode quantum yield (0 to ~0.5) R(λ) - responsivity (A·W-1) Φph - incident flux (s-1) e - electron charge (C) • High sensitivity (10-3-10-1 A·W-1) • If iap >0.1 mA, secondary electrons and Bremsstrahlung are ejected from the anode when the photoelectrons are incident • Dark currents (typically 10-12-10-14 A) caused by - thermionic emission - field ionization (high bias) - ohmic resistance CEM 835 page 4-9 Photomultiplier tubes (PMT's): • similar to phototubes - photoemissive cathode and anode • multiple secondary electron emissive dynodes (MgO, GaP) - each dynode is biased ~100 V more positive than previous to accelerate electrons from dynode to dynode - gain per dynode, g, is typically 2-5 - total gain m = g n is 10 6-108 • charge pulse at anode is few ns wide CEM 835 page 4-10 Can measure average current traveling between the cathode and anode i ap = m ⋅η⋅Φ⋅ R( λ) Φ = Φ ph ⋅ h ⋅υ = m ⋅e ⋅η⋅Φ ph ⋅K(λ ) • R(λ) or K(λ) are functions of photocathode material • very high sensitivity (10-105 A·W-1) • Alternatively, the rate of charge pulses can be counted, a technique called photon counting. In this case, rate is r ap = η⋅Φ ph ⋅ K(λ) = η⋅Φ⋅ K(λ) h ⋅υ • same equation as for a phototube but each pulse contains m electrons not just single electron - detection limit lower - PMT's best in low incident flux conditions - but max iap ~ 1 µA so max Φ is also lower • Dark currents in PMT's result from similar processes to phototube - thermionic emission associated with the photocathode can be significant (multiplied by dynodes) 10-11-10-7 A - cooling the PMT (0 to -60 °C) helps. CEM 835 page 4-11 Photodiode: • contains a reverse-bias semiconductor pn junction - p-type semiconductor has excess holes (eg B-doped Si) - n-type semiconductor has excess electrons (eg P-doped Si) • under reverse bias, depletion layer formed (resistivity of depletion layer is very high) CEM 835 page 4-12 • under irradiation, electron-hole pairs created that move under bias (holes → p-type, electrons → n-type) - momentary current is produced - ns or sub-ns • spectral response of a typical photodiode depends on band-gap of semiconductors used (typically near IR into near UV) • R(λ) less than PMT (no internal gain) but Q(λ) constant over 6-7 orders of magnitude • poor sensitivity (10-2-1 A·W-1) • can be made very small, ideal for use in multichannel devices Multichannel detectors: Photographic film: Photodiode arrays: • based on pn-photodiodes constructed by semiconductor chip techniques • regions of p-type Si deposited onto n-type Si crystal • distance between elements is typically 25 or 50 µm (up to 4096 elements per array) • usually operated in the- depletion layer is formed around each p-type islands • upon irradiation (integration time) bias is turned off and electrons and holes are created in depletion regions - holes migrate to the p-type islands and accumulate (max ~106) CEM 835 page 4-13 CEM 835 page 4-14 • during read-out period, each p-type region is interrogated (1) capacitor connected to each island is charged by 5 V supply (2) each island is connected in turn to it's capacitor and charge flows out of the capacitor to neutralize the positive charge accumulated on each island (3) current needed to return the capacitor to 5 V value is measured (total time ~25 µs) (4) once 5 V reverse-bias has been reestablished, holes are neutralized and system is ready for integration step • thermal excitation of electron-hole pairs creates difficulties with long integration times - array is often cooled • dynamic range 2-4 orders of magnitude - at detection limit thermal excitation dominates - at LOL end, charge storage becomes non-linear • sensitivity can be increased by coupling diode array with microchannel plate (MCP) CEM 835 page 4-15 MCP: CEM 835 page 4-16 Charge transfer devices: • photodiode arrays are inferior to PMT's in respect to sensitivity, dynamic range and signal-to-noise ratio • charge transfer devices approach the performance of PMT's • each pixel is metal oxide semiconductor • negative bias is applied to each electrode, a potential well collects photogenerated holes - more than 106 holes bleed onto adjacent pixels • charge accumulated during the integration time can be integrated in two ways - charge-injection device (CID) - charge-coupled device (CCD) CEM 835 page 4-17 Charge-injection device (CID) (two electrode per pixel): (1) during integration, one electrode (B) more negative than the other (A) - all photogenerated holes are accumulated under B (2) voltage applied to A is removed and the surface charge measured on A (3) potential on electrode B is switched to a positive potential, causing the holes to migrate to electrode A (4) charge under A is remeasured and the signal is the difference between the two measurements (5) positive voltage applied to electrode A to repel accumulated holes and return system to initial state CEM 835 page 4-18 Charge-coupled devices (CCD): (three electrodes per pixel) • substrate is p-type Si so electrons (not holes) are accumulated • each pixel contains three electrodes • following the integration period, a three-phase voltage transfers electrons in a step-wise manner along row Normalized Voltage 1.0 0.5 0.0 1 2 3 -0.5 -1.0 150 200 250 300 350 Time (a.u.) • readout process is destructive CEM 835 page 4-19 400 450 CEM 835 page 4-20