E H Li

K Maussang

J Palomo

J.-M Manceau

R Colombelli

I Sagnes

L H Li

E H Linfield

A G Davies

J Mangeney

J Tignon

S S Dhillon

Monolithic Echo-less Photoconductive Switches as a High-Resolution Detector for Terahertz Time-domain Spectroscopy

Monolithic echo-less photoconductive switches as a high-resolution detector for terahertz time-domain spectroscopy

Introduction. Advances in terahertz (THz) frequency spectroscopy have resulted in a wide variety of applications from materials science [START_REF] Tonouchi | [END_REF][2][3] to sensing 4,[START_REF] Mittleman | Sensing with THz Radiation[END_REF] , owing to rapid developments in technologies and systems. With the emergence of ultrafast lasers, a popular technique is THz time-domain spectroscopy (TDS). This approach provides a broadband spectroscopic capability and overcomes many of the technical difficulties traditionally associated with research in the THz frequency range, such as the requirement for cryogenic cooling of the sources and detectors. Whilst other spectroscopic methods are conducted directly in the frequency domain, THz-TDS provides direct access to the time-resolved electric field, using femtosecond laser pulses for the generation and detection of single cycle THz pulses. Although a wide range of methods can be used to generate and detect such THz pulses, one of the most common uses photoconductive switches, [START_REF] Uhd Jepsen | [END_REF]7 which have the advantage of a large spectral bandwidth and a high signalto-noise ratio. Indeed, most commercial systems exploit such switches for both THz generation and detection. In a typical experimental arrangement, metallic contacts are fabricated on top of a semiconductor substrate (such as GaAs) via standard photolithography, and through interband excitation using an ultrafast laser (e.g. a Ti:Sapphire laser with ~810 nm), photocarriers are generated. When operated as an emitter, the photocarriers are accelerated by a static electric field applied between the electrodes [START_REF] Uhd Jepsen | [END_REF]7 , and the corresponding time-varying current generates an electromagnetic pulse in the THz frequency range. When operated as a detector, the incident THz electrical field accelerates the photocarriers, which when collected by the electrodes, generate a current detected that is proportional to the THz electric field. This detection mechanism requires the carrier lifetime to be shorter than the THz pulse duration (typically < 1 ps), which is satisfied by using active layers based on low-temperature-grown GaAs (LT-GaAs) 8,9,10 . However, for both emitter and detector switches, owing to the finite thickness of the substrate and to the refractive index contrast between the air and the semiconductor, the original pulse is reflected back and forth in the device substrate. This results in the generation of 'echoes' i.e. successive THz pulses separated by the round-trip time between two interfaces, leading to Fabry-Pérot (FP) resonances in the spectrum. For the GaAs based switches used here, this can correspond to a modulation of the spectral amplitude of up to 50% 12 and can greatly affect the possibility to resolve spectral absorption features. In the case of two closely spaced and weak absorption lines, i.e. weaker modulation than the FP resonances, these are difficult to observe 12 . In practice, the time-domain scans are truncated to just before the first echo, revealing the absorption but limiting the spectral resolution (proportional to the duration of the scan). For strong absorption features, i.e. greater spectral modulation than the FP resonances, although absorptions can be resolved, the clarity of the spectrum is degraded. Consequently, the echoes and hence the FP resonances limits the resolution and precision of THz-TDS. Although the use of silicon hemispherical lenses on the switches can reduce the effect of these echoes, these require post-processing alignment on the photoconductive switch [START_REF] Nuss | Millimeter and Submillimeter Wave Spectroscopy of Solids[END_REF] . Furthermore, owing to the large area of interdigitated photoconductive (iPC) switches used here, silicon lenses are not adapted as the THz spot size is large. (The THz emission from an iPC switch does not act as point source). A monolithic scheme for iPC switches is therefore desirable.

In previous work, we demonstrated an iPC switch geometry for THz emission that contained a buried metal plane, which eliminated emission of such echoes 12 . (Electro-optic sampling using ZnTe crystals was used for the pulse detection). A metal layer was placed below a 10-µm-thick GaAs layer, realizing a subwavelength THz bandpass filter. As the THz generated from the surface comprises wavelengths greater than 70 µm, this geometry prevents the propagation of the generated THz pulse into the sample, and hence eliminates the echo. The distance between the surface of the emitter and the buried metal plane, d, has two critical roles: i) it determines the spectral band-width in which echo suppression is effective 12 and ii) avoids bias field distortion from the buried metal plan when the distance d is larger than the electrode separation, 12 which here was 4 µm. As mentioned above, to apply this concept to THz detection, LT-GaAs is required for short carrier lifetimes and low dark currents. However, the use of thick layers of this material are prohibited owing to a reduction in material quality, with layer thicknesses typically limited to 2 µm. In this work, we develop a sample design and growth methodology that solves this issue, and we demonstrate experimentally that the resulting iPC switch permits echo-less detection in THz-TDS systems.

iPC switch design. As for the echo-less switch for THz generation, 12 the device design exploits a wafer bonding process to realise a buried metal layer. To adapt to the use of high quality 2µm thick LT-GaAs for iPC switches, a multi-layer growth comprising GaAs and AlGaAs layers was realized to obtain a total switch thickness of 6µm. The sample was grown by molecular beam epitaxy (MBE) on a semi-insulating (SI) GaAs wafer. Figure 1(a) shows the layer structure before wafer bonding. From the substrate to the surface, it consists of a 250-nm-thick GaAs buffer layer, a 200-nm-thick Al0.5Ga0.5As etch stop layer, the 2-µm-thick LT-GaAs active layer, a 300-nm-thick Al0.3Ga0.7As electron barrier layer, and a 3.7-µm-thick undoped GaAs layer. (A 2µm thick LT-GaAs layer is used to absorb entirely the optical excitation laser pulse). An electron barrier layer prevents any photoexcited carriers from reaching the GaAs layer. This intermediate sample is annealed at 540°C for 15 minutes to increase the resistivity 13 . After annealing, a 300nm-thick layer of gold was evaporated onto the surface, and the wafer was Au-Au bonded onto a host SI-GaAs substrate. The original substrate and the AlGaAs etch stop were then removed with selective etching using citric and hydrofluoric acid, respectively. This leaves the LT-GaAs active region exposed, with a metal plane 6 µm below the surface, as shown in Figure 1(b). An interdigitated switch geometry 14 was then fabricated onto the LT-GaAs surface, with a design similar to that of reference 12, as shown in Figure 1(c). The individual electrodes were 4 µm wide, and each was separated by 4 µm. As for a standard iPC, a second set of metallic fingers with a periodicity double that of the first, and isolated from the interdigitated electrodes by a 300 nm thick layer of SiO2, was then fabricated. 14 The total active surface area of the switch was 500 µm × 500 µm. An optical pump -THz probe experiment 15 of the bare LT-GaAs active region showed a carrier lifetime of ~400 fs (see Figure 1(d)). This confirms that the ultrafast properties of the LT-GaAs are preserved despite the buried growth of the layer 13 .

Fig. 2 Detected time-domain trace of the THz electric field with a LT-GaAs-based buried metal switch for scan time of 400ps. The THz pulse was generated with a 10µm thick GaAs-based buried metal switch. The top inset shows an enhanced view of the THz pulse and the bottom inset shows a view of the field oscillations owing to residual water absorption.

Measured performances. The experimental arrangement used to detect the THz field is based on standard THz time domain spectroscopy [START_REF] Tonouchi | [END_REF] . An iPC emitter switch with a buried metal plane was used to avoid the emission of echoes, 12,14 biased at 4V (corresponding to an applied electric field of 10 kV/cm) and excited with an ultrafast (100 fs pulses) Ti:Sapphire oscillator centered at 810 nm. Using parabolic mirrors, the generated THz pulses were collected from the front surface of the GaAs active layer and focused onto the LT-GaAs detection iPC switch, simultaneously with a probe beam from the femtosecond laser (average power ~ 10 mW). A mechanical delay line was used to sample the THz ultrafast pulse as a function of time. The THz-TDS arrangement is placed in a dry-air purged chamber (<2% humidity) to reduce water absorption. The THz beam is modulated with a mechanical chopper for lock-in detection at 5 kHz. Figure 2 shows the time-domain trace of the detected THz field of the echo-less iPC switches. As shown, clearly no echo is visible on the time trace for the total time scan of 400 ps, limited by the length of the mechanical delay line. Oscillations visible in the time trace result from residual water vapour in the chamber. From the time trace 𝐸(𝑡), the spectral density 𝐸 ̃(𝜔) can be determined by FFT, as shown in Figure 3, with a broad spectral response from about 500 GHz to 3.5 THz, limited by the buried metal plane acting as a bandpass filter. 12 Coherent detection of the electrical field 𝐸(𝑡) provide both information on the spectral amplitude |𝐸 ̃(𝜔) | and the spectral phase arg(𝐸 ̃(𝜔)). The signal to noise ratio achieved is slightly lower than reference 12: this is a consequence of the use of two different detection schemes (electro-optic detection versus photoconductive switch), and lower modulation frequency resulting from the use of a mechanical chopper. Higher modulation frequency should be achievable by electrical modulation of the emitter and careful electromagnetic protection of the detector switch from emitter radiative pick-up noise. In order to illustrate the increase in spectral resolution achieved, one can compare the spectrum measured with different switches both with and without the buried metal plane, with specific absorption lines of residual water vapor in the chamber. To demonstrate the increase in spectral resolution, we focused on three characteristic rotational transitions of water 16,17 : 212-221 at 1.661007 THz, 101-212 at 1.669904 THz and 212-303 at 1.716769 THz corresponding to frequency separations of 8.9 GHz and 47 GHz, respectively. Measured spectra are shown in Figure 4. In the case of a standard iPC switch as a source (dotted blue curve), the three lines are undistinguishable, merged into a single broad peak as a result of a time window of only 10 ps owing to the first echo from the switch's substrate. 13 (A 200µm thick <110> ZnTe crystal placed on 2 mm thick host substrate was used for electro-optic detection of the electric field 18 ). In the case of a buried metal antenna as a THz source, the time window of 50 ps (limited to the echo from the ZnTe crystal for electro-optic THz detection 18 (dashed red curve)) permits the 212-303 line to be resolved from the 212-221 and 101-212 lines. However, the latter two lines are still unresolved. Finally, the use of a buried metal geometry both in emission and detection switches enables one to scan over a time window of 400 ps, resulting in a high spectral resolution (solid black curve), and allowing the 212-221 and 101-212 transitions to be resolved. (Note that with a strong absorption, greater than the amplitude modulation introduced by the FP oscillations, the features can be resolved but with less clarity). The fullwidth half maximum of the water lines in the latter case is typically 7 GHz. Moreover, since THz-TDS is a coherent detection of the electric field, one has direct access to the spectral phase. The dispersive profiles of these rotational lines are extracted from the FFT, as shown in Figure 5. The resolution is currently limited by the mechanical delay line but can be extended to times corresponding to the round-trip time of the Ti:Sapphire laser (76MHz). Conclusion. We have proposed and realized an echo-less LT-GaAs iPC switch for the coherent detection of THz radiation in TDS systems. This is based on a multi-layer growth with a buried LT-GaAs, which is wafer bonded to a gold coated host substrate to realize a monolithic echo-less iPC switch. Associated with an echo-less emitter, this provides a powerful echo-free THz TDS arrangement with high spectral resolution, limited only by the length of the temporal scan achievable. Two rotational lines of water vapour separated by 9 GHz have been experimentally resolved with such an iPC switch. Further work will concentrate on improving the cavity through the design of adapted antenna structures to engineer the spectral and intensity response of the iPC switches. This echo-less THz TDS system would also be relevant for coherent investigations of quantum cascade lasers that require high resolution temporal scans 19,20,21 .

Fig. 1

 1 Fig. 1 (a) LT-GaAs growth. The MBE growth sequence on a SI GaAs substrate consists of: i) a 250-nmthick GaAs buffer; ii) a 200-nm-thick AlGaAs (50%) etch stop layer; iii) 2-µm-thick LT-GaAs; iv) 300 nm of AlGaAs (30%); and, v) 3.7 µm of GaAs. (b) Pre-photolithography sample. The MBE grown sample is wafer bonded to a gold-coated host SI GaAs substrate. The substrate and the AlGaAs (50%) layer of the MBE grown wafer are removed, exposing the LT-GaAs active region with the echo-blocking metal plane 6 µm below the surface. (c) Schematic diagram of iPC switch, with interdigitated gold electrodes on top of the LT-GaAs layer consisting of 4-µm-wide electrodes, each separated by 4 µm. The THz pulse and the femtosecond pulse are focused on the front face of the switch, generating carriers in the LT-GaAs layer with the THz induced current collected from the switch contacts. (d) Experimental measurement of carrier lifetime in the LT-GaAs active layer (blue line). The red line is a numerical fit, showing a lifetime of ~400 fs.

Fig. 3

 3 Fig. 3 Spectral amplitude measured by computing the FFT of the time trace of Figure 2. Several absorption features are visible, owing to residual water vapor in chamber.

Fig. 4

 4 Fig. 4 Comparison of spectrum obtained with a standard iPC switch as emitter and a 200µm thick ZnTe crystal for detection (blue curve); a buried metal switch as emitter but a 200µm thick ZnTe crystal for detection (red curve); a buried metal switch both for emission and detection (black curve).

Fig. 5

 5 Fig. 5 Enhanced view of the spectral field between 1.65 THz and 1.74 THz. Several absorption lines of water vapour are clearly resolved: 212-221 at 1.661007 THz, 101-212 at 1.669904 THz, 212-303 at 1.716769 THz.

Acknowledgements. The authors acknowledge funding from European Union FET-Open grant ULTRAQCL 665158. Laboratoire Pierre Aigrain is a 'Unité Mixte de Recherche' associated to CNRS