1. Introduction

Ultrashort THz pulses (0.1–10 THz) are an active area of research with potential application in both in fundamental research [1, 2] and industrial applications. Nearly all kinds of plastic materials, polystyrene, paper, and clothes are transparent to THz radiation and show distinctive contrast in THz imaging. For example, THz technology allows the investigation of composite materials in terms of delaminations, inhomogeneities, and form deviations. Also many biological molecules like drugs or explosives show discrete absorption lines in this frequency range. With spectroscopic THz imaging these drugs and explosives can be detected even inside a closed envelope [3].

Using fs-pulses for the excitation it is possible to generate single cycle ultrashort THz pulses covering more than one order of magnitude in the frequency range. These pulses can be detected coherently by probing the THz pulses with fs-pulses. Here, the amplitude and phase of the electric field can be detected time resolved using electro-optic sampling or photoconductive antennas [4–6].

For applications like THz imaging a high number of electric field traces have to be taken [7]. In addition to the two lateral dimensions the THz pulses have to be scanned in time, which slows down the measurement process. Typical THz ultrashort pulse imaging systems having only one detection channel therefore require scan times from minutes up to hours. To achieve a fast scan speed several methods have been presented.

An excellent review of different THz imaging methods can be found in [8]. The use of special scanning techniques can decrease the scan time up to two orders of magnitudes, but they are limited to particular materials and applications [9]. By using a circular delay stage it is theoretically possible to measure several hundred electric field traces per second [10]. For asynchronous optical sampling the mechanical delay line is replaced by two fs-lasers synchronized to two frequencies with a small difference frequency Δν allowing data acquisition rates up to 0.25 s per pulse [11, 12]. Other methods try to accelerate the spatial acquisition: 2D electro-optic sampling and 1D single shot electro-optic sampling with a CCD camera [14–16] have been demonstrated. But due to the lack of balanced detection and lock-in amplifiers for CCD cameras expensive low repetition laser amplifier systems with high fs-pulse energies are required. Although a modulation algorithm using difference images was introduced [17–19], real lock-in techniques are not available for CCD cameras. Thus, the signal to noise ratio is worse than with a single channel detection by a photoconductive antenna or by balanced electro-optic detection. Recently, we demonstrated multichannel balanced electro-optic detection with 8 detection channels in combination with a multichannel lock-in amplifier to achieve a high signal to noise ratio together with a high scan speed [20].

In this paper we present 16-channel THz ultrashort pulse detection by a photoconductive antenna array. In combination with a microlens array the optical excitation is nearly an order of magnitude more efficient than with the excitation by a line focus as proposed in [23, 24]. This method provides an excellent signal/noise ratio while keeping the scan time short. In addition the use of the microlens array makes the setup less sensitive to adjustment, which makes THz ultrashort pulses applicable to real world applications. Recently, we have also demonstrated the use of a microlens coupled interdigital photoconductive switch, which generates up to two orders of magnitude higher THz output power than a conventional surface emitter [21].

2. Experimental setup

The multichannel THz detection system is schematically shown in Fig. 1. It is based on a THz surface emitter and a low temperature grown (lt)-GaAs photoconductive switch array for detection.

The excitation is realized by an amplified fs-laser, which has already been presented and discussed in [22]. It is based on fs-oscillator in combination with a fiber amplifier and a compressor. It delivers an output power of about 10W at a repetition rate of 74 MHz, a pulse length of 100 f s and a center wavelength of 1060 nm. Due to the large bandgap of 1.45 eV (855 nm) it is not possible to excite photocarriers in the lt-GaAs, the standard material for photoconductive switches, at the wavelength of 1060 nm (1.17 eV). On the other hand the use of materials with a lower bandgap as e. g. lt—In _0.3 Ga _0.7 As for photoconductive switches has been reported only a few times. Therefore about 10% of the fs-pulse energy is frequency doubled to a wavelength of 532 nm by a thin BBO crystal as proposed in [22]. The wavelengths are separated by a dichroitic mirror into a emitter beam (1060 nm) and a gate beam (530 nm).

The pump beam is delayed with respect to the gate beam by an optical delay line, and the polarization is turned by a half wave plate for maximum absorption at the surface emitter. It is expanded by a telescope and focused by a cylindrical lens onto the surface emitter as a line. The surface emitter (p-doped InAs) is less efficient than a photoconductive switch, but the THz emission follows the excitation of the fs-laser beam shape. Therefore, it is possible to shape the emitted THz beam profile by shaping the exciting laser profile easily. In our case the surface emitter emits ultrashort THz pulses with an elliptical beam profile. These are collimated by an off-axis parabolic mirror with a NA of 0.26. Another off-axis parabolic mirror foccusses the THz pulses to the THz detector array.

 

Fig. 1. Schematic setup of the THz system for multichannel detection by photoconductive switches.

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Fig. 2. (a) Single photoconductive switch; (b) 16 channel photoconductive switch array.

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Fig. 3. Microlens array attached to the photoconductive switch array: (a) schematic, (b) photo.

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Fig. 4. Maximum THz amplitude for detection channel no. 9 of the array for different kinds of excitation: (full line, red) single focus by an aspheric lens, (dashed line, green) line focus by a cylindrical lens, (dotted line, blue) focus by a microlens.

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The gate beam is expanded about 12.5 times and only the inner 8 mm are taken for the detection to get a nearly homogeneous laser beam profile. The expanded beam is focused by a cylindrical lens with a focal length f=400mm to the detection array, which is shown in Fig. 2. It consists of 16 photoconductive antennas with a spacing of 500 µm, which is in the order of the detected THz wavelengths. Each photoconductive antenna is made of two electrodes with a distance of 40 µm. The electrodes have a gap with a length of 30 µm and width of 5 µm. Typically a single photoconductive antenna is excited by fs-pulses focused inside this gap. For the excitation of a whole array it is possible to focus the laser beam to a line as proposed in [23, 24]. Because only the laser power inside and near the gap is used for the THz detection, only about 5% of the laser power is used, while the rest of the laser power increases the photo current leading to screening effects and a higher noise level.

To increase the efficiency we have used a microlens array with 16 lenses to create one spot for each antenna. The fabrication of the microlens array has been described in [25]. It is attached to the photoconductive antenna as shown in Fig. 3. The microlenses have a radius of curvature of 799 µm and a diameter of 495 µm. The numerical aperture is about 0.18, which gives a minimum resolution of d_min=0.61 λ/NA≈3.6 µm. The lens pitch was designed to 500 µm to match the antenna pitch. The substrate thickness has been calculated that the focus position is at the backside of the substrate. The array was placed on the photoconductive antenna array, adjusted by a hexapod to achieve a submicron precision, and fixed with optical adhesive.

The resulting photocurrent of each photoconductive antenna is amplified by a transimpedance amplifier in combination with phase sensitive amplification by a multichannel lock-in amplifier.

3. Results and discussion

The efficiencies of the antenna excitation by a single spot of an aspheric lens, by a line focus of a cylindrical lens, and by a focus of one of the microlenses are compared. For the single spot excitation the telescope, the cylindrical lens and the microlens array have been removed from the setup in Fig. 1 and replaced by an aspheric lens with focal length of f_al=11.4 mm. For an excitation by a line focus only the microlens array has been removed from the setup in Fig. 1.

 

Fig. 5. Simultaneous measurement of 16 ultrashort THz pulses (each pulse was shifted in time for 2 ps for a better overview).

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The THz electric field traces have been measured time resolved and the maximum amplitudes have been recorded. Their dependence on excitation power for all three kinds of excitation are plotted (exemplarily) for the channel no. 9 in Fig. 4. For the line excitation and the excitation by the microlens array the incident laser power was measured after an aperture of 8 mm and divided by an factor of 16 to get the average excitation power per channel. The function

with the THz amplitude A_THz[a.u.], the excitation power P[mW], and the parameters A ₀,B ₀ has been fitted to the measurements.

For the excitation by an aspheric lens the THz signal increases for an excitation power up to 5 mW following equ. (1). Because GaAs has a 8 times higher absorption coefficient at 530 nm than at 800 nm, the saturation starts for smaller excitation powers at 530 nm than at 800 nm.

For the excitation by a line focus the detected THz signal is more than one order of magnitude smaller compared to the excitation by a single spot. With 5 mW excitation power only about 4% of the corresponding signal is achieved. Also the saturation of the slope starts for higher excitation powers than for the excitation by a single spot and the limit is smaller by an order of magnitude. This is due to the larger excited area on the chip. While only a small part of the excited area is used for the detection, the higher number of excited carriers leads to screening effetcs, which lowers the detectable photocurrent.

For the excitation with a spot produced by a microlens array the detected signal is still smaller than the one generated by the asphere, but nearly 7 times higher than with the line detection. At an excitation power of 5 mW over 40% of the THz amplitude in comparison to the excitation by a spot created by an asphere is achieved. With higher excitation power even more than 65% THz amplitude can be achieved. Nevertheless, the saturation of the slope starts not until higher excitation energies, which indicates that the excited area is still bigger than the excited area by the aspheric lens.

For a measurement with all 16 channels, a cylindrical silicon lens has been attached to the antenna. It focuses the THz radiation to the detection points and helps to prevent backreflection inside the GaAs-wafer. The results are shown in Fig. 5. The envelopes of the amplitudes show the convolution of the pump- and the gate laser beam profile. The system was adjusted that all amplitudes are in the full width half maximum distribution to achieve a similar signal/noise ratio.

 

Fig. 6. THz beam profile for a pump beam expansion of (red) M_E=0.5, (green) M_E=1, (blue) M_E=4, and (purple) M_E=10.

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For a first experimental application the pump beam has been expanded with different magnifications and the resulting THz beam has been measured by the 16 detection channels. The beam profiles for magnifications of M_E=0.5, M_E=1.0, M_E=4.0, and M_E=10.0 are plotted in Fig. 6. The FWHM (full width half maximum) of the beam profile increases with the magnification from M_E=0.5 to M_E=4.0, but for a magnification of M_E=10.0 it nearly keeps the same as for M_E=4.0. The reason is the limited capability of the parabolic mirrors to image electric field distributions outside the optical axis. Therefore, for further applications as THz imaging a more advanced quasioptical system as used in [20] is necessary.

The presented solution has a signal/noise ratio in the order of magnitude of a typical photoconductive anntenna, which is usually more than an order of magnitude higher than 1D or 2D electro-optic sampling with a CCD camera. In opposite to the multichannel balanced detection presented in [20] the marginal costs are lower and the antenna alignment in the THz setup is less sensitive and susceptible, which makes it attractive for a high number of detection channels.

4. Outlook

With the presented combination of photoconductive antennas and microlenses it is possible to develop THz ultrashort pulse detection arrays up to several hundred pixels. With 4 inch wafers of lt-GaAs and microlenses for example it is possible to build up up to 200 pixels in a line with a 500 µm spacing suitable for many THz imaging applications. If 15 mW are taken for every detection channel, about 3W of laser power is needed for the excitation, which can be easily generated by commercial available Ti:Sapphire fs-lasers. And even 2D arrays with more than a thousand detection channels are possible, which can be excited by fiber amplified fs-lasers with an average output power of more than a 100 W [26–28]. If the inital adjustment of the microlens array is done, the following adjustment inside the THz system is easy and reliable. While the initial costs for a microlens array are higher than an electro-optic sampling setup the marginal costs per channel are quite low.

In this work a surface emitter was used for the first experimental demonstration of multichannel THz detection by a microlens array coupled photoconductive antenna array. For real world application e.g. in THz imaging systems a more powerful and efficient THz emitter is required. A promising approach for the generation of high power THz pulses is the use of microstructured photoconductive switches as proposed in [21, 29] delivering several tens of microwatts average power. The size of such a microstructured emitter can be tailored to match the size of the detection array and due the lateral extend of the array the excitation intensity is below the damage threshold even for high excitation powers.

5. Conclusion

The combination of microoptics and optoelectronics offers new possibilities for the parallel detection of ultrashort THz pulses. 16-channel THz ultrashort pulse detection has been realized with an array of photoconductive antennas excited by a microlens array. A detected THz amplitude of 65% in comparison the standard excitation, the excitation by a focus of an aspheric lens, has been achieved leading to a reduction in scan time of more than an order of magnitude for applications like THz imaging. The presented solution is easily scalable to several hundred detection channels in a line and takes THz systems one step forward to real world application.