We describe the characteristics of a sensitive photoconductive detector that simultaneously measures orthogonal electric field components of electromagnetic transients with bandwidths up to 30 THz. The device consists of an As+ implanted GaAs photoconducting region at the centre of a pair of perpendicular bow-tie antennas. The performance is illustrated by studies of optical rectification in GaSe, retardation in a birefringent polymer film and THz emission from impulsively excited optical phonons in GaN.
©2008 Optical Society of America
The development of techniques for the coherent generation and detection of few cycle, THz bandwidth electromagnetic transients has revolutionized exploitation of the far-infrared part of the electromagnetic spectrum over the last 20 years. The technique of time domain THz spectroscopy (TDTS), which embodies these developments, has a wide range of applications including dielectric characterization of materials, time resolved studies of electronic processes in condensed matter, pharmaceutical quality control and imaging of biological tissue . The recent extension of few cycle techniques into the mid-infrared  allows an even greater range of electronic and vibrational excitations and thus potential applications to be explored. Measurement of the polarization state of THz transients is important in many applications of TDTS including the Hall effect , magneto-optic Kerr effect , Faraday effect , ellipsometry , pulse propagation in disordered media , control of quantum processes  and polarization sensitive imaging . Of particular relevance to the life sciences is the potential for studying vibrational circular dichroism (VCD) in biopolymers, which are chiral and have macromolecular conformational modes at THz frequencies . VCD measurements are currently limited to frequencies above 20 THz. In the pharmaceutical context there is also interest in distinguishing between enantiomers which can have very different physiological effects, as exemplified by those of thalidomide.
Investigation of many of the above topics by TDTS requires the ability to rapidly modulate the polarization and to detect the change in the instantaneous electric field vector with high sensitivity over a wide bandwidth. Preliminary attempts at satisfying the first of these requirements have been made using optical rectification  and photoconducting sources . Attempts at the second have been reported at frequencies near 1 THz using a polarizing beam splitter and separate photoconductive or electro-optic detection of orthogonal field components [3, 9]. More conveniently, integrated photoconducting devices with two orthogonal gaps  or a single gap with three 120° spaced electrodes  have recently been described. This approach offers reduced system complexity, the possibility of near field probing and the requirement that each detector samples at the same optical delay is automatically satisfied. Here, we describe an alternative high dynamic range design based on a single photoconducting gap surrounded by four 90° spaced electrodes and illustrate the device’s ultrabroadband capabilities with studies of optical rectification in GaSe, the retardation produced by a biaxial polymer film and emission by impulsively excited phonons in GaN. We also compare its dynamic range with conventional photoconductive dipole receivers and comment on its potential application to the measurement of VCD.
Photoconductive (PC) THz receivers are based on planar metal antennas bridged by a high resistivity semiconductor with a short carrier lifetime. When the semiconductor is gated by a femtosecond optical pulse the electric field component of a THz bandwidth transient incident during the electron-hole plasma lifetime drives a current between the arms. This current can be measured as a function of the delay between the signal and gating pulses to map out the temporal profile of the incident electric field. It has recently been shown that this technique is effective at frequencies up to at least 40 THz with short enough gating pulses , and that the dynamic range is comparable to or better than that achieved by electro-optic sampling . Our device consists of a 1.5 µm thick As+ implanted GaAs (As:GaAs) layer grown on a semi-insulating GaAs substrate with an intervening 100 nm of AlAs. The AlAs layer electrically isolates carriers photo-generated in the substrate from the contacts and is effective in reducing noise. A triple implant with doses of 5.4×1014 cm-2 at 2 MeV, 3.1×1014 cm-2 at 1 MeV and 1.5×1014 cm-2 at 0.45 MeV was used to obtain an approximately uniform vacancy concentration profile in the GaAs layer. Subsequent proximity annealing at 500 °C for 20 minutes in a N2 atmosphere resulted in a dark resistivity of 200 kΩcm. The carrier lifetime measured in an 800 nm pump - THz probe experiment was 0.31±0.03 ps. Two perpendicular, Ti/Au, bow-tie antennas, Fig. 1(a), were used to contact a 50 µm square photoconducting region. The relatively large gap of the ‘quadrant receiver’ facilitates coupling to the THz and gating beams using reflecting optics. The antennas are affective below a few THz whilst above 10 THz they serve only as contacts. Each antenna is connected to separate current amplifiers and lock-in detectors referenced to the 2.2 kHz chopping frequency of the THz beam, thus allowing simultaneous measurement of the signal currents, Ix and Iy.
The quadrant receiver was characterized differently at high (>9 THz) and low (<5 THz) frequencies. We first describe the high frequency case where 350 mW of light from a 12 fs Ti:sapphire oscillator with a centre wavelength of 810 nm were used to generate THz radiation by optical rectification in an 100 µm thick GaSe crystal. The pump beam was polarized at 45° to the horizontal and focused to a diameter of 30 µm (full width at half maximum) using a 50 mm working distance off-axis parabolic mirror (OAPM). Phase matching near 14 THz was achieved by rotating the GaSe crystal about a horizontal axis perpendicular to the pump beam so that the c-axis made an angle of θ=42° with the horizontal. A typical spectrum is shown in Fig. 1(b). The average THz beam power, measured with a thermopile, was 3.4 µW (all quoted powers are before chopping). The THz beam was collimated using a second OAPM and made collinear with a 40 mW gating beam by means of a silicon beam splitter. The two beams were then focused on the front side of the detector using a 76 mm working distance OAPM. This geometry avoids dispersion and attenuation of the THz pulse in the GaAs substrate. The gate beam had a diameter equal to the electrode separation and was positioned so as to minimize the resistance between each pair of electrodes. The THz beam had a diameter at the antenna of 250 µm and was positioned so as to maximize the signal in both arms for radiation polarized at 45° to the bow-tie axes.
Figure 1(c) shows a polar plot of the peak to peak THz emission signal from the GaSe crystal measured along two orthogonal axes. The time domain traces are similar to those shown in Fig. 2(a). The phase matching alternates between types I and II as the azimuthal angle, ϕ, between the pump polarization and the GaSe a-axis is incremented from zero in 30° steps. In consequence, the THz beam switches between ordinary (linearly polarized horizontally i.e. along x) and extraordinary (linearly polarized vertically i.e. along y). There is good agreement with the cos(3ϕ) and sin(3ϕ) variation of the effective nonlinear optical coefficient for the two types of phase matching, shown by the solid and dashed curves respectively. The approximately equal amplitudes of Ix and Iy is co-incidental since they have a different dependence on phase matching angle, θ, and at larger angle Iy is reduced relative to Ix because of the smaller THz transmission at the GaSe exit surface for s-polarized light.
With the THz beam blocked, the detector noise was 55 fA/√Hz when gated compared with a current amplifier noise of 12 fA/√Hz for 108 V/A gain. This is explained by the thermal noise of the photoconducting gap, which has an average illuminated resistance of 6 MΩ. The amplitude dynamic range with this source-detector arrangement is 5500/√Hz which is close to that obtained with an ultrabroadband device designed to measure only a single polarization component . The dynamic range could be improved by at least an order of magnitude by using higher receiver gating power (the signal is linear in power up to at least 300 mW) and by tighter focusing of the THz beam using a diamond immersion lens. Another important property is the field extinction ratio, which we define as the peak to peak signal for radiation polarized along one arm divided by the signal in the orthogonal arm. This was measured to be only 9 but this figure is a lower limit because polarizers for 14 THz were not available to us.
To explore the ability of the quadrant receiver to measure the phase difference between orthogonal fields we studied the optical properties of a piece of birefringent bi-axially stretched poly-ethylene terephthalate (PET) film of the type sometimes used in photocopiers. The 89 µm thick film had an isotropic amplitude transmission of 50% and in-plane optical axes making angles of 15° and 105° with the long axis of an A4 sheet. The average refractive index was 1.61 for radiation incident perpendicular to the film and the difference between the refractive indices for light polarized along the two in-plane optical axes was 0.06. The polarization of the THz beam was arranged to make an angle of 45° with the detector axes in the absence of the sample by choosing an azimuthal angle of 15° for the GaSe source. Figure 2(a) shows the x and y components of the transmitted transients when the long edge of the sheet is aligned at angles of 20° and 60° to the y-axis of the detector. The THz beam is linearly polarized for the first angle and circularly polarized for the second. The angular variation of the phase difference between orthogonal components is shown in detail in Fig. 2(b).
As an illustration of the sensitivity of the quadrant device, we now describe a study of weak THz emission from an optically excited 1.5 µm thick, n-type (1018 cm-3), c-plane GaN epilayer grown by chemical vapor deposition on a c-plane sapphire substrate. The p-polarized pump beam was incident at 45° to the GaN  axis and the THz emission, shown in Fig. 3(a), was detected in the specular direction. The y antenna arm was aligned perpendicular to the plane of incidence and parallel to the GaN  axis. The data acquisition time was 20 minutes and the root mean square noise was 11 fA. Emission at discrete frequencies arises from coherent polar optical phonons excited by impulsive stimulated Raman scattering . Infrared and Raman active GaN phonons of A1(LO) and E1(TO) symmetry at 22.2±0.2 THz and 16.9±0.3 THz are apparent in the p-polarized emission spectrum shown in Fig. 3(b). The peak at 13.5±0.2 THz is probably associated with the sapphire substrate. The absence of an s-polarized signal shows that the phonons have dipole moments in the plane of incidence.
We now discuss the low frequency behavior, which was characterized using a planar GaAs photoconducting source pumped by 775 nm, 80 fs pulses at an average power of 100 mW. Transient photocurrents in the high electric field region between parallel, 50 µm spaced electrodes were excited at the edge of the positive metal track over a 5 µm×100 µm line focus region . In this arrangement, the THz beam, of average power 7 µW, was guided to the rear of the detector using a pairs of OAPMs and silicon substrate lenses. The THz beam diameter at the antenna was 200 µm at 0.5 THz. The 40 mW gating beam was focused to a diameter of 50 µm and incident from the front side. The spectral sensitivity is compared with that of conventional 10 µm and 50 µm long dipole receivers  made from the same material in Fig. 4(a). This data was acquired by rapid delay scanning and the noise is determined by the resolution of the 8 bit signal averager rather than the detectors as would be the case when directly reading the 16 bit lock-in amplifier. The dipole devices had a 5 µm wide photoconducting gap and were gated by 22 mW of 775 nm light focused to 5 µm. The illuminated noise of 220 fA/√Hz of the dipole receivers is 4 times higher than that of the quadrant detector noise of 55 fA/√Hz so that the dynamic ranges have a similar value of up to ~105/√Hz near 1THz. This compares with ~100 reported in Ref.  and ~1000 in Ref. .
The output of the photoconductive source is predominantly linearly polarized along x but 7% of the power arrives at the detector in the orthogonal polarization due to some quadrupolar emission from the source and a change in polarization on reflection at the 90° OAPMs. Two polarizers were therefore placed immediately before the receiver for extinction measurements. Each consisted of 5 µm wide, 10 µm pitch gold strips on a high resistivity silicon substrate. Tests with a 10 µm long dipole receiver showed that this reduces the peak field along y to less than 0.3% of that along x. Figure 4(b) shows the signals obtained with this receiver for incident polarization at 45° to the antenna axis which lies along x. The polarizers were aligned for maximum transmission of either x or y polarized radiation, denoted by Ex, Ey respectively. The signal for Ey is 150 times smaller than that for Ex which demonstrates the low sensitivity of the dipole receiver to radiation polarized perpendicular to the dipole axis. The signal shape for Ey is the second derivative of that for Ex because of the differentiating action of each polarizer. The amplitude is accurately predicted by the theory presented in Ref.  (dashed curve in Fig. 4(b)). The x and y arm signals from the quadrant receiver in response to radiation polarized along x are shown in Fig. 4(c). The field extinction ratio is 16, which is much smaller than expected from the amplitude of the cross-polarized component of the incident field but better than the ratio of 10 (100 in power) reported in Ref. . In this case, Iy is proportional to the first derivative of the x signal. This ‘cross-talk’ is unrelated to the leakage of y-polarized radiation through the polarizers and is probably due to capacitative coupling between orthogonal antenna arms. If we assume that the receiver current is proportional to the THz electric field then in general Iy∝Ey+αdEx/dt. Assuming an integral or differential relationship between signal and field doesn’t significantly change the discussion below. The coupling constant, α, is proportional to the capacitance and is sensitive to the optical alignment. It might be reduced by decreasing the bow-tie angle and by etching trenches in the GaAs between the antenna arms.
The quadrant detector could be used to analyze the circular dichroism of a material. In an experiment in which the polarization is modulated between left and right hand circular, the spectra of the differential signals δIx(ω), δIy(ω) are proportional to El(ω)-Er(ω)-αω(El(ω)+Er(ω)) and El(ω)+Er(ω)+αω(El(ω)-Er(ω)) respectively. For small α, δIxδIy∝(Pr-Pl)(1-α 2 ω 2)+4αωP̄ where Pl and Pr are the transmitted powers and Pl-Pr is likely to be very small compared with the mean power, P̄. The circular dichroism is therefore proportional to the product of orthogonal differential signal spectra if absorption is weak and, more importantly, if α is negligible. However, even if reducing cross-talk proves difficult, the quadrant detector could still be used to measure a small rotation, δθ, of the plane of polarization of linearly polarized light in response to a perturbation because here δIx-δIy∝δθ[1-αω] and the cross-talk has a relatively small effect. The sensitivity is governed by the source noise for signals of a few nA or more. With a signal of 20 nA, which is the peak signal without chopping or polarizers, the noise is 180 fA/√Hz which is consistent with the fractional intensity noise of the argon ion pumped femtosecond oscillator. This means that changes in polarization angle as small as 10-4 radians could be detected near 1THz.
In summary, we have shown that photoconductive detection can be extended to the simultaneous measurement of orthogonal electric fields in a single device at tens of THz and that it is possible to achieve similar dynamic range to conventional detection. This suggests the prospect of versatile few cycle, polarization-resolved spectroscopy based on single photoconductive devices and spanning the far and mid infrared if cross-talk can be sufficiently reduced and high quality polarizers and polarization modulated sources can be developed.
We thank the UK Engineering and Physical Science Research Council for financial support. AH also acknowledges Teraview Ltd for support of a studentship.
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