1. Introduction

The terahertz (THz) region of the electromagnetic spectrum covers an important frequency range between electronics and optics [1, 2]. Over the past decade a number of important scientific and technological applications have emerged in this region, such as low-energy spectroscopy of materials [3], biomedical imaging [4] and security applications [5, 6]. Driven by these technological and scientific applications, much effort has been directed towards the development of THz sources and detectors with the appropriate strengths like ease-of-use, high signal to noise ratio (SNR), broad bandwidth and large peak electric fields [7–18]. Among the variety of THz sources, amplified laser systems provide high enough peak powers such that air plasma generation and detection may be employed, providing bandwidths beyond 100 THz and electric fields into the MV/cm range [19]. On the other hand, photoconductive antennas have demonstrated being very practical in regards to generation of sub-picosecond THz pulses with high SNR when using ultrafast Ti:sapphire oscillator systems [7,8,11–16,18,20]. Achieving broadband emission from antennas has shown capabilities of up to 30 THz when combined with electro-optic sampling for detection and low-temperature grown GaAs (LT-GaAs) as the antenna substrate [12]. LT-GaAs is known for providing broader bandwidths due to the faster carrier recombination for both emission [12, 14, 16] and detection [18,20,14]. In this context, semi-insulating GaAs (SI-GaAs), which is more easily accessible and less expensive, but shows longer carrier lifetimes, is usually disregarded for broadband spectroscopy applications.

In order to combine the possibility of larger spectral bandwidths and/or higher THz fields with the practicality and high SNR of photoconductive antennas, the development of new antenna geometries and system designs continues to be an active area of research. In particular, the interdigitated photoconductive antenna structure [13], Fig. 1(a, b), holds much promise for achieving both of these goals. In these antenna structures, two finger-like electrodes interweave together to create a large array of THz antenna sources, constructively interfering in the far field. This structure combines the advantages of a large aperture for optical excitation and small electrode gaps. The large area allows illumination by higher power and lower repetition rate optical sources and strongly reduces diffraction of the generated THz pulses, thus avoiding the need for silicon lenses. The small electrode spacing results in faster screening of the applied electric field by the depleted photocarriers, and thereby provides a broader bandwidth THz emitter [15, 26]. On a practical note, the smaller electrode gap allows for the application of a lower bias voltage, achievable with standard function generators, whilst still providing electric fields near to breakdown across the electrodes. This lower bias voltage in turn allows for higher frequency modulations of the antenna to improve the high SNR. This design allows for significantly higher peak THz electric fields compared to dipole antennas or large-aperture antennas.

 

Fig. 1 Side view (a) and top view (b) schematic of the interdigitated photoconductive antenna structure. The first two protective gold stripes in (b) are not shown to illustrate the underlying interdigitated structure. (c) Schematic of the THz-TDS setup for generating broadband THz radiation.

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In this letter, broadband THz emission, up to 20 THz, is measured from an interdigitated photoconductive antenna fabricated on a SI-GaAs wafer, optically excited by a high power and low repetition rate Ti:sapphire oscillator. The emission is investigated as a function of incident power and incident pulse width. A transmission measurement through a 75 µm, Teflon (PTFE) sample is performed to demonstrate spectroscopy up to 17 THz, and compared to a Fourier transform infrared spectrometer (FTIR) transmission measurement of the same sample. This is, to the best of our knowledge, the highest generated bandwidth reported for a SI-GaAs based photoconductive source.

2. Experiment

Figure 1(c) shows the THz Time Domain Spectroscopy (TDS) setup used to achieve broadband capabilities. A 45 fs, 4 MHz repetition rate pulse train centered at a wavelength of 800 nm from a Ti:sapphire oscillator system (FemtoSource XL650) is aligned through a nonlinear fiber to chirp the initial 45 fs pulse and then compress it down to 15 fs (FemtoSource XS). The compressed pulses are split into a generation and a reference path, where the generation pulse excites an interdigitated photoconductive antenna with up to 27.5 nJ/pulse. The antenna is fabricated by patterning metallic electrodes, with 3 µm width and 3 µm spacing, on a SI-GaAs wafer. 500 nm of Si0₂ provides a transparent, insulating coating before a top layer of gold is deposited to protect every second electrode gap, ensuring the generation of only in-phase radiation [13,15].

A 20 V (peak to peak) square wave, with a 10 kHz modulation and 50% duty cycle, corresponding to an electric field of approximately 67 kV/cm, is applied between the electrodes. The emitted THz waves are collected in a reflection geometry to avoid propagation of the THz pulses through the GaAs dispersive medium and focused onto a 20 µm ZnTe crystal, spatially overlapped with the reference pulse. Electro-optical sampling is used to measure the electric field of the THz pulse. All measurements are performed in a nitrogen environment.

The temporal plot shown in Fig. 2(a) is the raw data measured by the TDS and is converted into the frequency domain via a fast-Fourier-transform, Fig. 2(b). The temporal waveform shows a main subpicosecond transient followed by long lasting oscillations, a signature of narrow linewidth absorptions. The FWHM of the main positive pulse is 160 fs. The spectra maximise at approximately 1.5 THz and show distinct features corresponding to TO-phonon absorptions of ZnTe at 5.1 THz and GaAs at 8.0 THz. One also observes the phonon emission peak at 8.7 THz corresponding to the LO phonon oscillations in GaAs.

 

Fig. 2 (a) Temporal plot from the THz-TDS with 27.5 nJ incident onto the antenna. Inset: Zoomed-in view of the pulse. (b) FFT amplitude as a function of frequency, showing a bandwidth approaching 20 THz.

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The emission from the antenna illuminated with 15 fs pulses is first studied with a pulse energy varying from 2.5 nJ to 27.5 nJ (corresponding to carrier densities from 5.6 10¹⁶ cm⁻³ to 1.2 10¹⁷ cm⁻³), with the reference beam power on the optical balance fixed. Figure 3(a) plots the Fourier transform amplitude, as a function of frequency, extracted from the temporal data. The data reveals a systematic increase in signal and bandwidth as the pulse energy onto the antenna is increased. The measurable bandwidth for the 2.5 nJ pulse only extends as far as 12 THz, whereas the 27.5 nJ pulse suggests a measureable bandwidth approaching 20 THz. This enhancement of the bandwidth is attributed to a faster Coulomb screening of the bias field as the number of generated photocarriers increases with incident optical power. In Fig. 3(b), we plot the THz bandwidth dependence of the antenna as a function of photoexcitation pulse duration. The pulse width is increased by adding a positive chirp to the pulse train before it enters the compression module. Monitoring the pulse width via a simultaneous autocorrelation ensures the correct pulse widths can be established and are maintained throughout the measurement. From the optimized 15 fs pulse there is a decrease in measurable bandwidth as the pulse width is increased up to 35 fs. The longer pulses result in a reduced bandwidth due to a slower rise time of photo-induced charge carriers in the GaAs substrate.

 

Fig. 3 (a) Fourier transform amplitude of the broadband pulses, as a function of frequency, measured with varying input power onto the interdigitated antenna. The pulse width is maintained at 15 fs with an ultrafast beamsplitter in the generation path, used to modulate the power onto the antenna. (b) Fourier transform amplitude, as a function of frequency, for varying pulse widths. The pulse energy is maintained at 27.5 nJ.

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The electric field amplitude is calculated using the method of Planken et. al. [21]. A peak THz electric field of 1 kV/cm is obtained for an applied bias of 85 V under an optical excitation of 27.5 nJ, with a linear increase in emission as the bias field is increased (not shown). For bias fields of 20 V peak-to-peak, we observe fields on the order of several hundred V/cm. This illustrates the ability to achieve large bandwidth and high peak electric field from interdigitated antennas with sub 40 fs high power oscillator systems.

The GaAs and ZnTe phonon lines already exhibit a broadband spectroscopy capability towards 10 THz, to confirm the output beyond this frequency we performed a transmission measurement through a 75 um, Teflon (PTFE) sample. Reference and sample measurements are taken and the resultant absorption spectrum is plotted in Fig. 4 (red). The phonon lines at 5.1 THz and 8.0 THz are highlighted in grey. Other than the small CF₂ twisting mode at 6.1 THz [22–24], the overall absorption of the PTFE is extremely low and uniform up to 14 THz. Above 14 THz, we observe a broad absorption from the PTFE, with two distinguishable peaks at approximately 15.1 THz and 15.6 THz, in good agreement with previous works [12, 22].

 

Fig. 4 Absorption, as a function of frequency, for a 75 µm sample of PTFE tape. Measurements performed using the broadband THz-TDS (red) and room-temperature FTIR (black) are shown. The phonon resonances within the THz-TDS are highlighted in grey. The absorption peaks of PTFE at 6.1, 15.1, 15.6 and 16.6 THz are readily observed. The FTIR trace is offset in the y-axis for clarity.

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Further confirmation of these results is performed using a Fourier transform infrared spectrometer (FTIR) from Bruker. A room temperature measurement of the same 75 µm PTFE sample is taken between 1.5 THz and 18 THz, Fig. 4 (black). The spectral resolutions for these measurements are 15 GHz and 20 GHz for the FT-IR and THz-TDS, respectively. Here we resolve several absorption peaks, which correspond well with the features observed in the THz-TDS absorption data. We note that the split of the 15.1 and 15.6 THz peaks in the THz-TDS spectra, as compared to the FTIR spectra, are not a result of poor resolution in the FTIR measurement – 0.5 cm⁻¹ or 15 GHz for the FTIR and 20 GHz for the TDS measurement. Instead, the observed effect could be a result of differences between FTIR and THz-TDS techniques previously discussed [27], e.g. incoherent, continuous excitation versus coherent, sub-picosecond excitation, and is under further investigation.

3. Calculation

We present numerical simulations to illustrate qualitatively that THz broadband generation is achievable by using a long carrier lifetime material such as SI-GaAs excited by ultrashort optical pulses. Using a Drude-Lorentz model, described in references [8] and [15], we calculate the generated THz field for interdigitated antennas fabricated on SI-GaAs and, for the sake of comparison, on LT-GaAs. This phenomenological model, takes in pulse energy, scattering rate, carrier lifetime, applied field and pulse duration as input parameters. The small electrode spacing in the interdigitated structure is taken into account with a capacitive term within the electrode gap [25]. For the pulse energy, we used 27.5nJ, which in combination with the antenna geometry (a quarter of the substrate area is illuminated) gives a carrier density of 1.2 10¹⁷cm⁻³. For the scattering rate, we have assumed an average scattering time of 70 fs for both SI-GaAs and LT-GaAs in agreement with [8] and [28]. For the SI-GaAs and LT-GaAs substrates, we use carrier lifetimes of 100 ps and 800 fs, respectively. For the applied electric field, we used 67 kV/cm. This sets the carrier velocities close to the saturation velocity of 10⁷ cm/s for both LT-GaAs and SI-GaAs. In Fig. 5, we plot the idealized output from SI-GaAs and LT-GaAs interdigitated antennas for varying incident pulse durations from 15 fs to 35 fs. This simulation does not include the limitations owing to the electro-optic detection, i.e. losses from the detection crystal (such as phonons) and phase mismatch.

 

Fig. 5 Normalised Drude-Lorentz simulations of the idealized frequency output for interdigitated antennas fabricated on GaAs. The incident pulse width is varied from 100 fs (green) to 50 fs (red), 15 fs (blue) and 10 fs (black). The solid lines represent SI-GaAs (recombination time = 100 ps) and the dashed lines correspond to low temperature grown GaAs (recombination time = 800 fs).

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The dashed lines show the broad response of the LT-GaAs based antenna, for incident pulses of 15 fs and 10 fs, demonstrating their use in broadband THz generation. The solid lines show the response from SI-GaAs, first with incident pulses of 100 fs and 50 fs, where the bandwidth at one percent of the peak signal is limited to approximately 6 THz and 10 THz respectively. Applying the 15 fs and 10 fs pulses to the SI-GaAs reveals a previously unreported broadband capability of over 30 THz and 40 THz respectively. With broadband THz spectroscopy becoming more widely available and commercial, the use of cheaper and more accessible substrates for antenna fabrication as well as practical and versatile geometries as provided by the interdigitated design will be of interest.

4. Conclusion

In conclusion, we have studied the THz emission from an interdigitated photoconductive antenna fabricated on SI-GaAs under excitation from a high power Ti:sapphire oscillator. We observe a THz bandwidth of 20 THz with peak electric fields exceeding 1 kV/cm. High frequency resonances in a PTFE sample are in good agreement with FTIR measurements, confirming the spectroscopic capability of the emitted radiation up to 17 THz. This study provides insight into the development of broad bandwidth, high peak electric field THz sources with high SNR using SI-GaAs photoconductive antennas and Ti:sapphire oscillator systems. Drude-Lorentz modelling of the interdigitated structure is also in good agreement with the broad bandwidths obtained.