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We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging sub-nanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.
© 2015 Optical Society of America
1. Introduction
Terahertz time-domain spectroscopy (TDS) [1] is a well-established technique in which picosecond pulses of terahertz (THz) radiation are used to characterize the object of interest. The popularity of this method stems from its high signal-to-noise ratio (SNR) and dynamical range, although the average THz power typically does not exceed the microwatt level. Another unique feature of TDS is its coherent nature of detection, which allows retrieving not only the amplitude but also the phase of the measured THz electric field from a single time-domain scan. In the past years, merging THz-TDS with quantum cascade lasers (QCLs) [2] has led to significant progress of coherent radiation sources in the THz electromagnetic spectrum. QCLs are unipolar semiconductor lasers that rely on an optical transition between two confined states in a heterostructure. Major achievements of this versatile combination include the characterization of quantum cascade (QC) gain media [3,4], the coherent sampling of actively mode-locked THz QCLs [5,6] and phase-locking of free-space THz combs to QCLs [7]. Furthermore, the recent development of ultra-broadband gain media [8,9] and the demonstration of frequency combs [9,10] in the THz region strengthens the potential of TDS for the field of remote sensing and imaging, as identified in [11]. In standard THz-TDS systems picosecond THz pulses are generated by the conversion of near-infrared femtosecond (fs) laser pulses from optical to THz frequencies [12]. Due to the low efficiency of this conversion process, exceeding the microwatt level of average THz power requires bulky and high-cost amplified laser systems [13]. Therefore, the development of practical and efficient amplifiers, capable of boosting the intensity of broadband THz seed pulses, is highly desirable. The compact size and high design freedom of its optical gain spectrum make THz QCLs a promising candidate for the realization of such amplifiers. This would offer an interesting extension to standard THz-TDS systems that could remarkably improve its capabilities in terms of spectral coverage and SNR. The conventional approaches of realizing semiconductor optical amplifiers in the near-infrared (NIR) spectral range are to utilize laser diodes as traveling wave or Fabry-Pérot amplifiers. The former concept requires an opened optical cavity with intensity facet reflectivity below 10−3 to prevent the device from reaching the lasing threshold [14]. There have been various attempts to decrease the mirror reflectivities of QCL cavities, ranging from the deposition of dielectric anti-reflection coatings [15–17] to integrated antenna structures [18]. However, all of them are intrinsically narrowband and therefore do not satisfy the requirements for broadband THz amplifiers. The Fabry-Pérot amplifier is based on a device that employs a standard Fabry-Pérot optical cavity that is operated slightly below threshold, but the achievable single-pass gain and gain bandwidth are limited to low values.
An elegant alternative, demonstrated by Jukam et al. in [19], is to use ultrafast gain switching [20] to circumvent clamping of the optical gain. Utilizing this method, the bare cavity gain that is present during the non-steady state build-up of the laser field can be exploited. Within this period of time, the gain can significantly exceed the total losses and therefore large amplification values can be achieved. Additionally, if the intensity of the injected THz seed pulse is sufficiently high, it leads to injection seeding of the QCL with the result of phase-locking the QCL emission to the fs laser [21]. This enables coherent detection of the QCL emission and thus allows using QCLs as powerful sources for TDS systems [11].
In this letter, we present an integrated THz source with a gain switched broadband QCL amplifier that employs a multi-stack active region [8]. It is composed of three different QC active region designs in order to provide broadband optical gain. A metal-metal waveguide is used to fully exploit the potential of THz QCLs regarding maximum operating temperature and to ensure a uniform mode confinement over the entire lasing range. The device relies on a coupled cavity geometry that guarantees efficient coupling of the internally generated THz seed pulses into the QC amplifier section and further enables the independent control of THz generation and amplification [22]. Compared to previous work presented in [23–25], this allows us to achieve a larger amplification bandwidth and the subsequent coherent detection additionally enables using heterogeneous THz QCLs as broadband sources for TDS experiments.
2. Quantum cascade amplifier setup
The presented broadband THz amplifier consists of three main building blocks and its experimental setup is sketched in Fig. 1(a). The first block is a THz-TDS system using a mode-locked Ti:Sapphire laser (~80 fs pulse duration, 790 nm central wavelength, and 80 MHz repetition rate) for the generation of weak THz seed pulses and to detect the radiation emanating from the QC amplifier. The THz emission is collected and focused onto an electro-optic detector with a 1 mm thick (110) ZnTe crystal by a pair of 50 mm off-axis parabolic mirrors. The coherent nature of the electro-optic sampling technique allows measuring the phase-resolved THz electric field and thus provides temporal and spectral information in a single time-domain scan. An essential feature of our TDS system, especially if acquiring time-domain traces with durations of hundreds of picoseconds, is the elimination of parasitic THz pulses that arise from reflections within the detection crystal. We use a broadband metallic antireflection coating on the ZnTe detection crystal [26].
Fig. 1 Realization of the broadband QCL based THz amplifier. (a) Schematic of the experimental setup. A Ti:Sapphire based TDS setup is used to generate phase-locked THz seed pulses in the emitter section of a coupled cavity QCL device and to coherently detect the THz electric field emerging from the QC amplifier section. Synchronized sub-nanosecond RF pulses, generated from a part of the fs laser beam, are injected into the QC amplifier section to achieve amplification of the THz seed by ultrafast gain switching. (b) Sketch of the coupled cavity metal-metal QCL device with attached silicon lens. (c) Time trace of a typical RF pulse measured at the output of the power amplifier across a 50 Ω load.
The second building block is a coupled cavity QCL device that was processed into 100 µm wide metal-metal laser ridges. For the definition of the two electrically insulated, but optically coupled, sections we used an anisotropic reactive ion etching process yielding almost vertical side walls [27]. The short section (39 µm long) is used as an integrated photoconductive emitter to generate coherent broadband THz pulses, which are subsequently injected into the QC amplifier section (1920 µm long). The QCL active region is a GaAs/Al0.15Ga0.85As heterostructure that consists of a stack with three different QC structures designed for emission frequencies at 2.3, 2.7 and 3.0 THz. A detailed investigation of this multi-stack gain medium is presented in [28]. The active region exhibits a lasing bandwidth of ~1 THz and THz-TDS gain measurements revealed a spectral gain full width at half-maximum (FWHM) of 1.1 THz centered at 2.65 THz. A hyper-hemispherical silicon lens was attached to the cleaved QC amplifier output facet. This significantly improves the THz collection efficiency and effectively removes the parasitic air-side pulses, which would distort the electro-optically sampled THz time-domain traces otherwise (more details in [29]).
Figure 1(b) shows a sketch of the QC amplifier device with attached silicon lens. The QC amplifier was indium soldered to a copper sub-mount and placed into a liquid helium continuous-flow cryostat. All presented results were obtained at a stabilized temperature of 10 K. In order to deliver the radio frequency signal to the QC amplifier section, one end of the top metal contact was wire bonded to a 50 Ω microstrip line that was connected to a high-frequency coaxial feeding line.
The remaining third building block is formed by a radio frequency (RF) setup that generates RF pulses with sufficient power to switch the gain of the QC amplifier. Every 12.5 nanoseconds (ns), defined by the repetition rate of the fs laser, a phase-locked THz seed pulse is injected into the QC amplifier that is synchronously driven above threshold by an RF pulse. This places the amplifier section into a non-equilibrium state, in which the gain is unclamped and large amplification factors can be achieved [19]. The RF pulses are initially generated by illuminating a fast GaAs photodetector with a fraction of the near-infrared laser beam. These pulses are subsequently fed into a limiting amplifier that converts the photodiode output into rectangular shaped pulses and allows the adjustment of the RF pulse amplitude. As a last stage, a GaN power amplifier is used to increase the RF power up to a maximum value of + 31 dBm. Figure 1(c) shows the shape of such a RF pulse, measured with a 20 GHz sampling oscilloscope across a 50 Ω load. It exhibits a rise time of ~150 ps and a FWHM pulse width of 650 ps. The amplified RF pulse train is amplitude modulated with a 50 Ω switch at 10 kHz and combined with the conventional quasi-DC bias in a bias-T. The f/2f double modulation scheme of the QC amplifier (10 kHz, 10% duty cycle) and the THz emitter (20 kHz, 20% duty cycle) permits the lock-in detection of the THz electric field emitted from the QC amplifier. A more detailed description of this method can be found in [30]. Apart from having the usual optical delay line to probe the THz electric field at different instances of time (probe delay), a second optical delay line is used to optimize the relative timing between the THz seed and the RF pulse (RF delay).
3. Results and discussion
Figure 2(a) shows the measured THz electric field amplitude for different operating modes of the QC amplifier section. The three employed modes OFF, QCL and AMP are depicted in the light-current-voltage characteristic of the QC amplifier section (see inset). In the OFF mode, the amplifier section is operated slightly below the lasing threshold at a current density of 360 A/cm2. This can be considered as the reference mode and is used to quantify the amplification factor of the gain switched amplifier at a later point. The QCL mode represents the standard laser operation with the QC section driven at maximum THz output power (J = 630 A/cm2), whereas the AMP mode corresponds to the actual amplification mode, in which RF switching pulses with a power of + 31 dBm are used to boost the gain of the QC amplifier section. From the impedance mismatch between the QCL and the RF feeding line, we estimate a RF power coupling efficiency of ~40%. This drives the amplifier section in the AMP mode slightly below the misalignment point of the structure.
Fig. 2 (a) QC amplifier THz electric field output for the three operating modes OFF, QCL and AMP. The inset shows the light-current-voltage characteristic of the QC amplifier section, acquired under quasi-DC bias conditions (10 kHz, 10% duty cycle), and the employed operating modes. All measurements were performed at a heat-sink temperature of 10 K. (b) Temporal shape of the RF gain switching pulse with the relative timing to the THz pulse train.
Due to the high facet reflectivity of the metal-metal waveguide, after propagating through the QC amplifier section, a significant part of the THz seed pulse is reflected back into the cavity where it undergoes several circulations. This results in a train of THz pulses, separated by the cavity round-trip. From the cavity length of 1920 µm and the round-trip time of 48 ps we calculate a waveguide group refractive index of 3.78. This is in good agreement with the value that can be extracted from the beatnote frequency of a similar multi-stack device [9]. In the two conventional operating modes OFF and QCL, the combination of waveguide losses, facet reflectivity and dispersion yields a reduction of the THz amplitude and a temporal broadening of the circulating pulses as they propagate more often through the laser cavity. In particular, the rapid decline of the THz pulse amplitude in the OFF mode indicates a facet reflectivity of 0.7 and a propagation loss of 4.2 cm−1. The weak spontaneous emission that is present in this operating mode has a random phase, so it cannot be registered by the electro-optic detection. In the QCL mode, the pulse amplitude dynamics and its spectral content is determined by the clamped gain of the laser cavity. From the spectral amplitude ratio of the first two round-trip pulses we observe completely compensated waveguide losses within the QCL gain bandwidth. However, the formation of a stable pulse train is only permitted if the spectral content of consecutive round-trip pulses fully overlaps with the gain profile. The coherent laser emission that is present in the QCL mode also cannot be detected, since the carrier phase of the QC amplifier is not phase-locked to the fs laser. In case the QC amplifier section is operated in the AMP mode, the THz pulses experience more gain than losses, which leads to an increase of the electric field amplitude for consecutive round-trip pulses. This amplification regime lasts for about 150 ps, which corresponds to the rise time of the applied RF pulse. At later times, the gain is saturated and we observe stabilized THz pulse amplitudes for ~350 ps, until the gain is vanishing due to the RF pulse roll-off. The temporal shape of the pulses alters significantly as they undergo more passes through the gain medium, revealing the complex intersubband dynamics of the employed multi-stack active region. We observe the formation of an inner sub-pulse structure that does not originate from parasitic reflections of the NIR pump or THz beam in the setup. We tentatively attribute this to the successive gain build-up of the different QC stacks and the variable group velocity in the broad emission range. Finally, to underline the RF controlled gain of the QC amplifier, the RF pulse timing relative to the THz pulse train is shown in Fig. 2(b).
Figure 3 shows the temporal and spectral evolution of the THz seed on the basis of two selected round-trip pulse groups. The left panels display the phase-resolved electric field of the THz pulse passing through the QC amplifier 3 and 15 times, respectively. The operating modes OFF (red lines), QCL (orange lines) and AMP (blue lines) are plotted in identical colors as in Fig. 2(a). After three passes, as shown in Fig. 3(a), we observe for the AMP mode a considerable increase of the electric field amplitude and the extent of the electric field oscillations compared to the QCL and the OFF mode. The impact of the gain switching becomes further evident in the frequency domain and is displayed in Fig. 3(b). Only spectral components with positive gain are amplified, while the others experience losses and are diminished rapidly. In particular, this results in a significant increase of the THz signal between 2.15 and 2.75 THz and a shift of the center of mass towards ~2.5 THz. For longer interaction length with the gain medium, the differences between the operating modes become even more evident. Figure 3(c) shows the TDS time-domain traces for THz pulses propagating 15 times through the QC amplifier. The pulses in the OFF and QCL mode are attenuated severely, whereas the THz pulse amplitude in the gain switched system rises. Because the signal content in the OFF mode is approaching the noise level of the experiment, a reasonable comparison to the QCL and AMP mode is limited to 15 passes. From the corresponding spectra in Fig. 3(d), we see that the QCL based THz amplifier exhibits a symmetrically shaped spectrum with a −20 dB bandwidth of ~500 GHz around the center frequency of 2.47 THz. The amplified THz signal at the central frequency is enhanced by more than 30 and 50 dB with respect to the QCL and OFF mode.
Fig. 3 Temporal and spectral evolution of the THz round-trip pulses after 3 (a,b) and 15 (c,d) passes through the QC amplifier section. It displays the phase-resolved electric field amplitude (a,c) and the power spectral density (b,d) for the three selected modes OFF (red lines), QCL (orange lines) and AMP (blue lines). The spectra were calculated using a 9 ps long Hamming window.
In order to evaluate the maximum reached power amplification, we compare the single pass THz pulse in the OFF mode with the 15th pass signal in the AMP mode. In the OFF mode the QC section is not operated at the transparency point, which therefore leads to a slight overestimation of the amplification values. The spectra of the two considered THz pulses are shown in Fig. 4. The single pass pulse contains frequency components up to ~4 THz with maximum signal between 1 and 2 THz – a spectral shape observed previously for the integrated THz waveguide emitter [22]. In contrast, the spectral content of the THz pulse after 15 passes through the gain switched amplifier is strongly spectrally filtered by the QC gain medium. This clearly shows that we coherently detect the QCL emission and consequently demonstrates the use of multi-stack THz QCLs as broadband sources for TDS experiments. From the ratio of the spectra at the dominant frequency of 2.47 THz we estimate an amplification factor of 21 dB. In addition, from the difference between the two spectra in Fig. 4 (shaded area) we extract a ≥0 dB amplification bandwidth larger than 500 GHz.
Fig. 4 Normalized spectra of the sampled THz pulses in the OFF (AMP) mode after a single pass (15 passes) through the QC amplifier section. The shaded area represents the spectral region with net amplification.
Finally, to emphasize the potential of our QC amplifier system in the field of spectroscopy, high-resolution TDS spectra are shown in Fig. 5. They are obtained by Fourier transform of the first 400 ps long time traces in Fig. 2(a). This provides a frequency resolution of 2.5 GHz and is therefore similar to the resolution of commonly used commercial Fourier transform infrared spectrometers. The train of broadband THz pulses leads to the formation of a discrete spectrum with modes that are spaced by the free spectral range of the QC amplifier cavity (~21 GHz). We achieve amplification factors of more than 10 dB within a bandwidth of 540 GHz (localized between 2.14 and 2.68 THz) and ~30 dB for the central 150 GHz wide frequency range. At higher frequencies, the THz signal approaches the noise floor, and thus hinders exploiting the full gain bandwidth of the used heterogeneous active region [28]. We attribute this reduction of accessible amplification bandwidth to the non-trivial gain switching dynamics of the individual sections in the active region. The interplay of dynamic impedance of the QC sections in the highly non-equilibrium gain build-up regime appears to favor the 2.7 THz section. The transfer function of the used ZnTe detection crystal starts to roll-off at ~2.5 THz and therefore distorts the spectral shape of the measurements. However, this does not account for the fact that the frequency components of the 3.0 THz active region are missing entirely. Further studies will be necessary to shed more light on the origin of the reduced amplification bandwidth in the gain switched QC amplifier.
Fig. 5 High-resolution (2.5 GHz) intensity spectra, obtained from 400 ps long TDS traces with the QC amplifier driven in the OFF and AMP mode. The individual modes are spaced by the free spectral range of the QC amplifier waveguide (~21 GHz).
4. Conclusion
In conclusion, we have demonstrated a broadband QC based THz amplifier that employs an integrated source of coherent THz radiation. The amplifier utilizes a heterogeneous active region and its amplification process relies on ultrafast gain switching, which is initiated by a sub-nanosecond RF pulse. Using this approach, amplification over a bandwidth of more than 500 GHz and an amplification factor of 21 dB, centered at 2.47 THz, have been achieved. High-resolution TDS measurements of the generated THz pulse train revealed a discrete mode spectrum with a ~30 dB gain in the central 150 GHz wide spectral range. The presented results demonstrate that heterogeneous QC heterostructures can be efficiently used as powerful and broadband sources for TDS systems to boost the signal-to-noise ratio for frequencies above 2 THz.
Acknowledgments
The authors acknowledge partial financial support by the European Union via FET-Open grant TERACOMB ICT-296500, the Austrian Science Fund (FWF) in the Framework of the Doctoral School “Building Solids for Function” (W1243) and the Austrian Society for Microelectronics (GMe). Dominic Bachmann wants to thank Michael Krall, Christoph Deutsch and Martin Brandstetter for the support in QC amplifier device fabrication and for many fruitful discussions.
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