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PH507 Astrophysics Dr Mark Price 1 Lecture 3: Infra-red and sub-millimetre detection techniques. The Golay-Cell Invented in 1947 by M. J. E. Golay, the Golay cell consists of an absorbing film placed inside a pressure cell. E-M radiation falling onto the film will be absorbed, and the film will heat up and heat the gas inside the cell. This gas will then expand, and the pressure in the cell will increase. At the back of the cell is a very thin, half silvered, membrane. As the pressure increases the membrane will flex outwards, this tiny movement is detected by shining a laser onto the film and measuring its deflection. Below is a very simplified schematic of a cell. Its major advantages are: 1) That it is responsive to all radiation falling onto the absorbing film. Spectral sensitivity is tuned by selecting the front window material appropriately. 2) Doesn’t require cooling. Operates at room temperature. Thus cheap and easy to use. However it does suffer from several disadvantages 1) The half-silvered membrane is extremely fragile, and the cell must be handled with care. 2) It has a slow response, due to the thermal response in warming and cooling the gas within the cell. This restricts their maximum speed of useful operation to ~20 Hz. PH507 Astrophysics Dr Mark Price 2 A commercially available Golay-Cell. Operating temperature = 300K Wavelength range (polyethylene window) = 20 to 1000 μm Optical responsivity at 15Hz ≥ 100 kV/W Optical N.E.P at 15Hz : < 100 x 10-12 Watt.Hz-1/2 Undergoing somewhat of a revival due to it’s ability to operate at room temperatures. NASA are developing a silicon micromachined tunnelling infrared detector based on the principle of the original Golay cell, but which uses changes in quantum tunnelling, instead of the deflection of a light beam, to measure pressure changes within the cell. PH507 Astrophysics Dr Mark Price 3 The bolometer Basic description. The bolometer, in principle, is a very simple detector. It effectively consists of a small (<1mm3) crystal of very pure semiconductor (normally germanium) whose heat capacity is very well known, and which has a very well characterised current-voltage characteristic. The bolometer is attached to a heat sink via a weak thermal link. When incident radiation of any frequency is incident upon the crystal it absorbs the radiation and warms up. As the crystal changes temperature, it’s resistance changes (where R is the slope of the I-V characteristic). Therefore if we supply a constant current, I, of, say, 10 nA (typical) and measure the voltage, V, across the device. Any change in V corresponds to a change in the resistance of the device (via Ohms law). Therefore if we measure the change in voltage, we can calculate the change in temperature experience by the device, and thus the amount of energy absorbed, Q, (and hence the incident photon energy) as we know the heat capacity, C, of the device. A schematic diagram of a typical bolometer set-up is show below A bolometer has several advantages and disadvantages. PH507 Astrophysics Dr Mark Price 4 Advantages: 1) Ease of operation. It is a much simpler device to operate than a heterodyne receiver. Only requiring a fixed current source and careful calibration. This is illustrated below with typical bolometer read-out circuit. 2) It is sensitive to all incident radiation, and thus can be used where no other device could be. This is particularly true in the sub-millimetre. 3) Growing ultra-pure semiconductors is a mature technology perfected from the semi-conductor industry and therefore pure germanium crystals are reasonably easy to source. Disadvantages: 1) It is sensitive to all incident radiation. This may sound contrary to what was stated above, but for spectral line work, or narrow band photometry we want a detector that is only sensitive to the frequencies we are interested in. 2) Slow frequency of operation (typically 10 Hz). As the bolometer is only weakly connected to it’s heat sink, it takes a certain length of time for the bolometer to dissipate any absorbed energy. This greatly restricts the speed of the devices. A stronger coupling to the heat-sink increases the speed, but reduces the sensitivity of the device. 3) Cooling requirement. The most sensitive bolometers operate at temperatures of milli-kelvin (SCUBA=100 mK, SCUBA-2=50 mK). This greatly increases the cost of operation and the complexity of the overall instrument. PH507 Astrophysics Dr Mark Price 5 Q: Why do we cool bolometers to such low temperatures? A: To reduce noise, and increase sensitivity. There are several, contaminating noise sources present in a bolometer. 1) Thermal noise. Caused by the random, thermal motions of electrons in the lattice. 2) Shot noise. Caused by the quantum nature (ie. single electrons) travelling in the lattice/connecting wires. 3) Photon noise. Caused by the emission, and absorption of photons by the crystal from/into it’s surroundings. 4) 1/f noise. Caused by slow changes in the system. For example, changes in the amplifier gain, slow drifts in the heat sink temperature etc. PH507 Astrophysics Dr Mark Price 6 It can be shown, although it’s non-trivial, that the sensitivity of the bolometer, called it’s Noise Equivalent Power (NEP), is given by: NEP=4 ( A S kT 5 B) Watts/Hz0.5 Where A is the detector’s absorbing area (m2), S is Stefan’s constant (5.67 x 10-8 W m-2 K-4), k is Boltzmann’s constant (1.3805 x 10-23 J K-1), T is the physical temperature of the bolometer (K) and B is the detection bandwidth of the system (Hz). NEP can be thought of as the power that would give rise to a signal-to-noise ratio=1. [Q: Does everyone know what a signal-to-noise ratio is?] For example, a detector with A=1 mm2 , B=1 Hz and T=300 K has an NEP of: NEP300K = 5.5 x 10-12 Watts/Hz0.5 Meaning that a power of 5.5 x 10-12 Watts would be detected with a signal-tonoise ratio of 1, in 1 second with a bandwidth of 1 hertz. However, the same system cooled down from 300K to 3K would have an NEP of: NEP3K = 5.5 x 10-17 Watts/Hz0.5 A factor of 100,000 lower! Thus is can be clearly seen that the sensitivity of the detector is a very strong function of temperature, in fact: Detector sensitivity T 5 2 PH507 Astrophysics Dr Mark Price 7 The picture below is of a typical, commercially available liquid helium dewar. PH507 Astrophysics Dr Mark Price 8 The photoconductor Review of band-gap theory. As the next two devices we shall look at, the photoconductor and the CCD, require a simple knowledge of semiconductors, we need to review band-gap theory. [Q: How much band-gap theory do you know?] All materials can be broken down into three main electrical conduction classes. Insulators (nylon, diamond, paper etc.), metals (copper, silver, gold etc.) and semiconductors (germanium, silicon, indium antimonide etc.). The difference between them is caused by the size of the energy gap between the valence band, and the conduction band. The valence band can be thought of as the region where electrons are bound ‘in orbit’ around the nucleus of an atom. The conduction band is where the electrons are “free” and can move within the lattice of the material. In a conductor the conduction band and valence band overlap, and electrons can freely move within the lattice. In an insulator, the energy gap is very large (5.4 eV for diamond) and electrons (under normal conditions) cannot ‘escape’ from the valence band into the conduction band. However, in a semiconductor, the energy gap is sufficiently small that a small number of electrons can move into the conduction band. PH507 Astrophysics Dr Mark Price 9 The figure above shows the number of conduction electrons per cubic centimetre for various semiconductors, semimetals and metals. It is the energy gap in semiconductors that is of interest to us, and provides the physical mechanism by which we can use these materials as detectors. PH507 Astrophysics Dr Mark Price 10 The table below shows the energy gap of various materials (in eV) and the corresponding photon frequency and wavelength. Material Diamond Silicon (pure) Germanium (pure) InSb Ge:Be Ge:Ga Energy gap (eV) Photon wavelength (microns) Photon frequency (Hz) 5.4 1.17 0.79 0.23 0.0238 0.0108 0.23 1.06 1.57 5.2 (near infra-red) 52 (mid infra-red) 114 (far–infrared /submm) 1.303 x 1015 2.828 x 1014 1.909 x 1014 5.765 x 1013 5.765 x 1012 2.630 x 1012 The most important thing to notice is that certain semi-conductors have energy gaps which correspond to the energy of infra-red photons. This means they can be used as detectors of radiation via the mechanism of photoconductivity. Q: What is photoconductivity? Photoconductivity is the process where an electron is ‘assisted’ to escape into the conduction band by absorbing the energy of an incident photon. As illustrated in the diagram below. PH507 Astrophysics Dr Mark Price 11 Therefore if a photon arrives with an energy = hf which is greater, or equal, to the band-gap energy of the material, an electron can be escape from the valence band, into the conduction band and be detected as an increase in electrical current in the device. This is referred to as intrinsic photoconductivity. InSb is an example of an intrinsic photoconductivity which is commercially available as an infra-red detector. PH507 Astrophysics Dr Mark Price 12 Extrinsic photoconductivity. In order to control the size of the bandgap, and effectively ‘tune’ the photoconductor, different dopants are deliberately added to the host crystal. The amount of dopant is very small, or the order of 1 in 1010 atoms! Dopants generally tend to be in Group-III, or Group-V of the periodic tables, and are thus tri-valent or penta-valent respectively. Germanium and silicon are in Group-IV and are quad-valent. By addition of a Group-III dopant to germanium we have (effectively) reduced the number of electrons/unit volume in the valence band, and the material is referred to as “p-type” (‘p’ = ‘p’ositive). Conversely, when we add a Group-V element, we have increased the number of electrons/unit-volume in the valence band, and the material is referred to as “n-type” (‘n’=’n’egative). From the table above, it can be seen that by deliberately adding gallium to germanium you can obtain a semiconductor with a bandgap of 0.0108 eV. But this is very different from pure germanium, so how does this work? Conduction (and subsequent current flow) is now obtained by exciting electrons from the dopant, gallium, into the conduction band. Thus by changing the dopant, and the concentration of the dopant, we can ‘tune’ the detector to be sensitive to a wavelength we are interested in. PH507 Astrophysics Dr Mark Price 13 The table below shows how the energy gap in germanium, silicon and gallium arsenide is affected by adding differing dopants. PH507 Astrophysics Dr Mark Price 14 Basic mode of operation. So, now we know the physics behind their operation, how would we use one in reality? Simply, we treat the photoconductor as a resistor in series with a capacitor and a ‘reset’ switch. As photons are absorbed by the photoconductor, electrons are excited into the conduction band and are swept out of the crystal by a bias voltage applied across the crystal. These electrons then go to charge up a capacitor, C. Thus the voltage across the capacitor tells us how charge has been collected, and therefore the intensity of the illuminating radiation. From simple circuit theory, the voltage across a capacitor in response to a step input (ie. suddenly shining photons onto our detector) is given by: V (t ) V0 [1 exp( t )] RC For t << RC, and by expanding exp(-t/ RC) to first order, the above equation becomes: V (t ) V0 [1 (1 V t It t )] 0 RC RC C and on differentiating simply becomes: dV I dT C Thus if the rate of change of voltage can be measured, then the current, I, can be calculated. This is exactly how photoconductors are operated in practise and signal currents of electron per second can be measured, making them very sensitive devices. PH507 Astrophysics Dr Mark Price 15 PH507 Astrophysics Dr Mark Price 16 The diagram below shows some representative signal ramps (effectively the ‘tail’ of the RC charge curve) taken from a detector flown on the ISO space mission. Of course, one of the problems with a space mission is that the spacecraft is constantly bombarded by the solar wind, and cosmic rays. These highly energetic particles with energies > 1 MeV can seriously compromise the sensitivity of photoconductors. How badly was seen very soon after the ISO space-craft was launched in 1995. From previous we saw that extrinsic photoconductivity is produced by exciting electrons into the conduction band from dopants within the main host lattice (normally germanium). However, a photon with a high enough energy (such as a gamma-ray) does not just excite electrons from the dopants, but from the host lattice itself! As shown in the next diagram. PH507 Astrophysics Dr Mark Price 17 This causes a lauch number of electrons to be liberated at once, and to be swept out of the crystal. This results in a sudden increase in voltage at the output. Below is an example of a real gamma-ray event as detected in the lab. PH507 Astrophysics Dr Mark Price 18 If we were simply to measure the slope of this ramp to measure dV/dt we would calculate an erroneous value for I due to the radiation induced discontinuity present in the ramp. The photoconductor described here is only one type of a range of infrared and submillimetre detectors. Others include BIBs (Blocked-Impurity-Band), NTDs (Neutron Transmutation Doped detectors), TEBs (Transition Edged Bolometers) etc. etc. In next week’s thrilling installment! CCDs, X-ray & exotic detectors, ground and space based observatories. Note: assignment #7 due back Wednesday 15th.