WO2006072762A1

WO2006072762A1 - Polarization sensitive electromagnetic radiation detector

Info

Publication number: WO2006072762A1
Application number: PCT/GB2005/004938
Authority: WO
Inventors: Michael Benjamin Johnston
Original assignee: Isis Innovation Limited
Priority date: 2005-01-05
Filing date: 2005-12-19
Publication date: 2006-07-13

Abstract

A three terminal photoconductive switch or gate used to detect terahertz radiation. Three metallic terminals are disposed on a gallium arsenide photoconductive substrate, with the terminals being arranged such that a first pair measures an electric field in one direction and a second pair measures an electric field in an preferably orthogonal direction. The device is illuminated by a gating laser pulse which creates charge carriers in the photoconductive substrate, these charges carriers being redistributed on the application of electromagnetic radiation to be detected. The signals across the two pairs of terminals are representative of the time dependent electric field strength in two preferably orthogonal directions and the detector is thus useful for detecting elliptically or circularly polarised electromagnetic radiation. The detector is particularly suitable for detecting terahertz radiation and may be used in time domain spectroscopy and other terahertz applications.

Description

POLARIZATION SENSITIVE ELECTROMAGNETIC RADIATION DETECTOR

The present invention relates to an electromagnetic radiation detector, and in particular to a detector which can measure the strength, phase and polarisation of the electric field component of electromagnetic radiation which is incident upon it. It is particularly suitable for detecting radiation in the terahertz range.

In recent years considerable interest has developed in the use of electromagnetic radiation in the terahertz range. The terahertz range refers to radiation in the range from about 10¹¹ to 10¹³ Hertz, namely from the microwave to the mid-infrared region. Current applications include molecular spectroscopy in the mid and far infrared regions, imaging in the far infrared region and modelling of radar systems.

Particularly for use in imaging and spectroscopy applications, terahertz radiation generators have been developed which can produce pulses of electromagnetic radiation at terahertz frequencies which are so short that only one optical cycle is included in the pulse. Such pulses are useful in the field of time domain spectroscopy where a sample is exposed to a short pulse of EM radiation (for example one or less than one cycle) and the reflected or transmitted electric field is detected as a function of time. Fourier transformation of the output gives the frequency dependent behaviour of the sample. The short duration of the incident EM pulse corresponds to a large frequency bandwidth and thus, in essence, the behaviour of the sample at many different frequencies is measured simultaneously. Time domain spectroscopy is therefore very useful for monitoring dynamic physical processes in which the composition of the sample can be changing quickly.

Furthermore, while microwave and infrared spectroscopy have long been used for the analysis of molecular and condensed matter systems, a considerable amount of information about these systems lies in the terahertz region. For example, the motion (torsion, libration, vibration and rotation) of many molecules occurs at terahertz frequencies, and intermolecular interaction can also be observed using terahertz radiation. Additionally the characteristic energy of many quasiparticles in condensed matter systems fall into the THz range, making the technology very useful for characterising semiconductor and superconductor materials and devices. Terahertz radiation has until recently been difficult to generate and detect. Terahertz generators which rely on the emission of terahertz radiation when a semiconductor is illuminated with a sub-picosecond laser pulse have been developed, as described, for _

example, in WO 03/014823. A corresponding arrangement can be used to detect terahertz radiation, such as described in US 2003/0127673 and US 2002/0067480, and in the published paper by Auston, Cheung & Smith, Appl. Phys. Lett. 45:284, and which are incorporated herein by reference. Such detectors comprise a photoconductive gate or switch consisting of two metal contacts on a photoconductive semiconductor wafer with a small gap between them. When a short, usually infrared, laser pulse (sometimes referred to as the probe pulse) illuminates the gap, charge carrier are generated which makes the region between the contacts conductive for a short period of time. During this short conductive phase, any external electric field, such as the electric field component of an incident pulse of terahertz radiation, exerts a force on the carriers, causing them to redistribute. This redistribution of charges causes a current to flow between the metal contacts. Normally the incident electromagnetic pulse being measured and the (much shorter) probe laser pulse are shone on the detector co-linearly so that the switch becomes conductive for a period after the laser pulse arrives. Thus the integral of the current flowing between the contacts can be recorded using a lock-in amplifier. The current is integrated for a period between when the pulse arrives (i.e. the switch becomes conductive) and the time at which optically generated carriers in the switch recombine or become trapped at defects. Therefore the signal recorded with the LIA is an integral of current, the signal is then differentiated to give the THz pulse (representing electric field versus time). Such two-terminal photoconductive switches are currently used for performing time-domain spectroscopy with linearly polarised terahertz electromagnetic pulses in the frequency range 10GHz to 20THz.

Also known in the art is US6476596, in which a THz detector is disclosed which is sensitive to one polarisation, based on a pair of multiple quantum well detectors rotated by some relative angle.

It is an object of the invention to provide an improved detector, in one example usable for detecting and measuring terahertz radiation, which allows, for example, more detailed time-domain spectroscopic studies of molecular structures.

The present invention provides an electromagnetic radiation detector which is polarisation sensitive. It achieves this by measuring the strength of the electric field component of an incident pulse of electromagnetic radiation in two different directions (preferably orthogonal directions). This allows, for example, elliptically or circularly polarised pulses to be measured, and also the detection of the rotation of linearly polarised electromagnetic pulses. Such measurements are useful, for example, in circular dichroism spectroscopy to identify the chirality (or "handedness") of molecules. This is achieved by measuring the difference in a sample's reaction to left-handed and right-handed circularly polarised radiation.

In more detail the invention provides a polarisation sensitive electromagnetic radiation detector for measuring the strength of the electric field component of electromagnetic radiation incident thereon, the detector comprising: a photoconductive material in which electric charge carriers are generated under stimulating illumination; at least three electrically conductive terminals for detecting the flow of charge caused by the redistribution in said photoconductive material of said electric charge carriers, a first pair of said terminals being disposed separated from each other by a first gap in a first direction, and a second pair of said terminals being disposed separated from each other by a second gap extending in a second direction, whereby the electric field component of electromagnetic radiation incident on the detector causes redistribution of said electric charge carriers in said photoconductive material and thus generates currents between said first and second pair of terminals which are representative of the strength of two components of said electric field in said first and second directions thereby providing simultaneous measurement of said two components.

Preferably the first and second directions are substantially orthogonal to each other.

Preferably there are three of the electrically conductive terminals, one of which is common to both the first pair and the second pair and may, for example, be earthed. Preferably the electrically conductive terminals are disposed on the surface of the photoconductive material, and each may be shaped to taper to a tip, the relative disposition of the tips defining the preferably orthogonal directions in which the electric field component is measured.

The photoconductive material may be a semiconductor, for example gallium arsenide, preferably low- temperature grown or ion implanted gallium arsenide (in order to improve the signal to noise ratio of the detector). The general construction of the device and the materials used may be as described in the earlier two-terminal switches as detailed in, for example, US 2003/0127673 and US 2002/0067480, and Kono, Applied Physics Letters - August 13, 2001 ~ Volume 79, Issue 7, pp. 898- 900.

Preferably the stimulating illumination is co-linear with the incident electromagnetic radiation, preferably in the form of a short laser beam pulse in the infrared region. The pulse timing of the stimulating illumination may be adjustable so that different parts of the waveform of the incident electromagnetic radiation are sampled.

The terminals may be connected to lock-in amplifiers which can detect the signal and eliminate noise, and the output (which represents the integral of the electric field over the sample time period of the incident electromagnetic pulse) can be differentiated to produce a signal representative of the strength and phase of the electromagnetic radiation.

The detector is particularly suitable for the detection of terahertz radiation.

The invention will be further described by way of example with reference to the accompanying drawings in which:

Figure 1 schematically illustrates the detector structure in accordance with one embodiment of the present invention;

Figure 2 schematically illustrates the use of the detector in an apparatus for time domain spectroscopy;

Figure 3 illustrates the time relationship between the probe pulse, the conductivity of the photoconductive substrate and the terahertz pulse being detected; Figure 4 schematically illustrates the use of the detector for detecting linearly polarised electromagnetic radiation;

Figure 5 schematically illustrates the use of the detector for detecting elliptically polarised electromagnetic radiation; Figures 6(a) to (m) schematically illustrate a variety of possible electrode configurations which can be used in embodiments of the invention;

Figure 7 illustrates experimental results of measuring terahertz radiation with an embodiment of the invention;

Figure 8 shows an alternative arrangement for the detector; and Figures 9 and 10 illustrate the results of using the detector of Figure 8 to detect a linearly polarized THz transient. As shown schematically in Figure 1 the detector comprises a semiconductor substrate 1 on which three metallic terminals 3, 5 and 7 are disposed. One of the three terminals, 5, forms a common terminal which is earthed. As shown the three terminals are generally triangular in shape, tapering to tips, 3a, 5a and 7a. A line joining tips 3a and 5a is at right angles to a line joining tips 5a and 7a. Thus terminals 3 and 5 measure a current in a direction preferably orthogonal to that measured by terminals 5 and 7. The distance between the tips of the terminals may be about 10 microns. Terminals 3 and 7 are connected to lock-in amplifiers 9 and 11 for measuring the signals from the terminals.

The semiconductor substrate is preferably low-temperature-grown gallium arsenide and the terminals 3, 5 and 7 may be titanium gold. However the substrate may, for example, be or include layers of AlGaAs, intrinsic GaAs, ion-implanted GaAs, InAs, InGaAs, InP, silicon on sapphire, or any semiconducting materials.

In use a probe laser pulse in the infrared range (for example a 4nJ, lOfs laser pulse from a mode-locked Ti: Sapphire laser with a central wavelength of 800nm) is used to illuminate the circular region 13 of the detector. This region is arranged to just overlap the tips of the electrical contacts. Thus it may have a diameter of, for example, 10 to 100 microns.

The incident terahertz radiation to be measured is arranged to illuminate a larger region 15 of the detector. In the overlap region between the probe laser pulse and the terahertz radiation to be measured, charge carriers will be created in the semiconductor substrate, and their redistribution under the influence of the electric field component of the terahertz electromagnetic radiation causes currents to flow between the tips 3a, 5a and 7a which are amplified/integrated.

Figure 8 shows an alternative arrangement for the detector. The substrate was formed by implanting semi-insulating InP (100) using 2.0 MeV and 0.8 Mev Fe⁺ ions with doses of 1.0 x 10 cm and 2.5 x 10 cm^" respectively. These multi-energy implants give an approximately uniform density of vacancies to a depth of 1 micron resulting in a carrier lifetime of about 130fs. The substrate was then annealed at 500°C for 30 minutes under a PH₃ atmosphere. Then three chromium-gold contacts were defined using standard photolithography and lift-off techniques and deposited to a thickness of 20/250 nm using a thermal evaporator. As shown the ground electrode 5' is formed at its tip with two orthogonal edges 5 a' and 5b', which face the end edges of the two other terminals 3' and 7' which are connected to the lock-in amplifiers. Thus two parallel-sided gaps 80a and 80b are formed, extending at right angles to each other.

The device sensitivity can be improved by improving the signal-to-noise ratio. This can be achieved by optimising the substrate, for example, by using low temperature grown GaAs instead of ion implanted InP, or by optimising the ion implantation of InP, GaAs, InGaAs, or InGaAsP for a high resistivity, short carrier lifetime and high mobility. Also, the growth conditions for the semiconductor materials can be altered to optimise disorder and doping in the material. Other optimisations can be made to the device, for example by:

1. Optimising the contacts, hi the above Schottky contacts are used, but Ohmics can be used and/or the contacts can be alloyed. 2. The electrode geometry can be varied.

3. The noise on the detection electronics can be reduced, e.g. by putting a preamplifier very close to the detector.

Figure 2 illustrates a typical apparatus for conducting time domain spectroscopy which can use a detector in accordance with the present invention. As shown in

Figure 2 a pump laser 20 is used to pump a mode-locked Ti: Sapphire laser 22 which emits a pulse train of 4nJ lOfs laser pulses. These pulses are directed onto a terahertz source 24 via a delay mechanism at 26 which adjusts the path length of the laser beam. The terahertz radiation from the terahertz source 24 is directed onto a sample 28 and the transmitted terahertz radiation is directed onto an electro-optical terahertz detector 30, for example as described in Figure 1. As shown in Figure 2 the stimulating illumination for the detector 30 is split off from the pump laser pulse by a splitter 32 and is arranged to be incident co-linearly on the detector 30 with the terahertz radiation from the sample by means of mirrors 34 and 36. Thus the delay mechanism 26 adjusts the relative timing of the terahertz pulse and the stimulating pulse to the detector and this allows different parts of the waveform emerging from the sample to be sampled.

Figure 3 schematically illustrates the time relationship between the probe pulse, the conductivity of the photoconductive substrate and the terahertz pulse being detected. In Figure 3 A the lOfs pump pulse is timed so as to cause the conductivity σ(t) to rise close to the beginning of the terahertz pulse (i.e. Δt is small in Fig. 3A). Thus virtually the whole of the waveform is sampled, as shown by the hatched portion. In Figure 3B Δt is set so that the probe pulse is later and so the conductivity rises later and so only the tail of the waveform is sampled. In practice Δt is adjusted successively to sample the whole waveform.

In a typical run of the system shown in Figure 2 the lOfs pump laser pulses are produced in a train with a repetition frequency of 80 MHz, so that the detector is producing 80 million samples per second of the integrated output for the particular Δt set. Δt is changed slowly, say ten times a second, to sample the different parts of the waveform. The pump laser and lock-in amplifiers are turned on and off in synchronism at, say, 30kHz to provide the usual noise rejection.

Figure 4 schematically illustrates the detection of linearly polarised terahertz electromagnetic radiation. As explained above the terahertz radiation, which in this case is linearly polarized, causes the charge carriers in the photoconductive substrate to redistribute and thus a current to flow between the terminals 3, 5 and 7. Assuming that the electric field is oriented parallel to the line between the terminals 5 and 7, it will cause a current pulse J_y(t) as illustrated between those two terminals. As the electric field is preferably orthogonal to the line joining the terminals 3 and 5, no signal is seen on those terminals. Because the conductivity σ of the photoconductive substrate is substantially constant (once it has built up) over the period of the terahertz pulse, the output from terminals 5 and 7 is effectively an integral of the voltage E(t) over that time. Differentiating this signal gives the electric field strength as a function of time.

Figure 5 illustrates the corresponding situation for elliptically polarised terahertz radiation. Again, the illuminating laser pulse 50 is timed to sample a desired part of the terahertz radiation pulse 52. Because the terahertz radiation is elliptically polarised, its electric field vector rotates (and changes in magnitude) through the cycle. This creates phase-shifted currents as shown at 3b and 7b across the terminals 3 and 5 and 7 and 5. Differentiating these signals gives the respective orthogonal components of the electric field as a function of time, and thus the phase and magnitude of the electric field vector can be determined.

Although the detector above has been illustrated with three terminals, one being a common terminal, it is of course possible to use four terminals, without the need for a common terminal. Also, more terminals may be provided. For example, Figures 6(a) to (m) schematically illustrate a variety of possible electrode configurations which can be used in embodiments of the invention, three-terminal devices being illustrated in Figures 6(a), (c), (d), (f) and (i), the others being four-terminal. As can be seen the terminals can be triangular, linear, rectangular or circular or a combination may be used. The tips at the measurement point may be sharp, radiused or squared-off.

An array of detectors may be provided which could be used, for example, in imaging, for example as a raster. Where an array of detectors is provided, some terminals may be common to different detectors.

Figure 7 illustrates the results of using unoptimised Fe+-ion-implanted InP as a substrate and the electrode design shown in Figure 6(c) or (i) to measure three linearly polarised THz pulse trains: one horizontally polarised, one vertically polarised and one 45 degrees polarisation. The variation of electric field with time for the three different polarisations can be seen in the three traces.

Figures 9 and 10 illustrate the results of using the detector of Figure 8 to detect a linearly polarized THz transient. Figure 9 shows the horizontal (solid) and vertical (dashed) components of the THz electric field plotted against time with the polarization plane of the THz radiation at 0°, 45° and 90°. The electric field is calculated by differentiating the two currents measured by the lock-in amplifiers. It can be seen that at 45° the horizontal and vertical components detected are substantially equal, as expected, and that at 0° and 90° only one component is present, the other being substantially zero. Figure 10 is a parametric representation plotting the horizontal and vertical components together for the three angles. The angle of polarization for the three waves is clearly observed.

Claims

1. A polarisation sensitive electromagnetic radiation detector for measuring the strength of the electric field component of electromagnetic radiation incident thereon, the detector comprising: a photoconductive material in which electric charge carriers are generated under stimulating illumination; at least three electrically conductive terminals for detecting current caused by the distribution in said photoconductive material of said electric charge carriers, a first pair of said terminals being disposed separated from each other by a first gap in a first direction, and a second pair of said terminals being disposed separated from each other by a second gap extending in a second direction, whereby the electric field component of electromagnetic radiation incident on the detector causes redistribution of said electric charge carriers in said photoconductive material and thus generates current between said first and second pair of terminals which are representative of the strength of two components of said electric field component thereby providing simultaneous measurement of said two components.

2. A polarisation sensitive electromagnetic radiation detector according to claim 1 wherein said first and second directions are mutually substantially orthogonal.

3. A polarisation sensitive electromagnetic radiation detector according to claim 1 or 2 wherein there are three of said electrically conductive terminals, one of said electrically conductive terminals being common to said first pair and said second pair.

4. A polarisation sensitive electromagnetic radiation detector according to claim 1, 2 or 3 wherein said electrically conductive terminals are disposed on the surface of said photoconductive material.

5. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein each of said electrically conductive terminals is shaped to taper to a tip, a line joining the tips of said first pair of terminals being substantially at right angles to a line joining the tips of said second pair of terminals.

6. A polarisation sensitive electromagnetic radiation detector according to any one of claims 1 to 4 wherein the tips of said terminals define two substantially parallel sided gaps extending substantially orthogonally to each other.

7. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein the photoconductive material is a semiconductor.

8. A polarisation sensitive electromagnetic radiation detector according to claim 7 wherein the photoconductive material is gallium arsenide.

9. A polarisation sensitive electromagnetic radiation detector according to claim 7 wherein the photoconductive material is low-temperature-grown gallium arsenide.

10. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein stimulating illumination is collinear with the THz electromagnetic radiation.

11. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein stimulating illumination comprises laser light of wavelength 780nm.

12. A polarisation sensitive electromagnetic radiation detector according to claim 11 wherein the timing of the stimulating illumination is adjustable to sample different parts of the waveform of the electromagnetic radiation.

13. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein the electromagnetic radiation is terahertz radiation.

14. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein the terminals are connected to the inputs of lock-in amplifiers.

15. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims, further comprising differentiators for differentiating the signals derived from the terminals to produce output signals representative of the strength of the electromagnetic radiation.

16. A polarisation sensitive electromagnetic radiation detector according to any one of the preceding claims wherein the output therefrom also represents the phase of the electromagnetic radiation.

17. A terahertz electromagnetic radiation imaging system comprising a plurality of polarisation sensitive electromagnetic radiation detectors according to any one of the preceding claims arranged as an array.

18. A terahertz electromagnetic radiation imaging system according to claim 17 wherein said polarisation sensitive electromagnetic radiation detectors are arranged as a raster.

19. A terahertz electromagnetic radiation imaging system according to claim 17 or 18 wherein adjacent ones of said polarisation sensitive electromagnetic radiation detectors comprise at least one common terminal.

20. A system for characterising birefiϊngent materials.

21. A system for characterising optically active materials.

22. A system according to claim 20 or 21 comprising a polarisation sensitive electromagnetic radiation detector according to any of claims 1 to 16.