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Register a new userHigh dynamic range, heterogeneous, terahertz quantum cascade lasers featuring thermally-tunable frequency comb operation over a broad current range
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Added by
Miriam Serena Vitiello Prof
Publication date
2019
fields
Physics
and research's language is
English
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We report on the engineering of broadband quantum cascade lasers (QCLs) emitting at Terahertz (THz) frequencies, which exploit a heterogeneous active region scheme and have a current density dynamic range (Jdr) of 3.2, significantly larger than the state of the art, over a 1.3 THz bandwidth. We demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers in continuous wave, with a maximum optical output power of 4 mW (0.73 mW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24), significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of 1.05 THz ans a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning coefficient ranging between 5 MHz/mA and 2.1 MHz/mA and a temperature-tuning coefficient of -4 MHz/K. The ability to tune the THz QCL combs over their full dynamic range by temperature and current paves the way for their use as powerful spectroscopy tool that can provide broad frequency coverage combined with high precision spectral accuracy.
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Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation from mid-IR devices are an important asset for applications in rotational molecular spectroscopy and sensing, beingthe only electrically pumped device able to operate in the 0.6-6 THz range without the need of bulky andexpensive liquid helium cooling. Here we present comb operation obtained by intra-cavity mixing of adistributed feedback laser at {lambda} = 6.5 {mu}m and a Fabry-Perot device at around {lambda} = 6.9 {mu}m. The resultingultra-broadband THz emission extends from 1.8 to 3.3 THz, with a total output power of 8 {mu}W at 78K.The THz emission has been characterized by multi-heterodyne detection with a primary frequencystandard referenced THz comb, obtained by optical rectification of near infrared pulses. The down-converted beatnotes, simultaneously acquired, confirm an equally spaced THz emission down to 1 MHzaccuracy. In the next future this setup can be used for Fourier transform based evaluation of the phaserelation among the emitted THz modes, paving the way to room-temperature, compact and field-deployable metrological grade THz frequency combs.
We present technology computer-aided design (TCAD) models for AlGaAs/InGaAs and AlGaN/GaN and silicon TeraFETs, plasmonic field effect transistors (FETs), for terahertz (THz) detection validated over a wide dynamic range. The modeling results are in good agreement with the experimental data for the AlGaAs/InGaAs heterostructure FETs (HFETs) and, to the low end of the dynamic range, with the analytical theory of the TeraFET detectors. The models incorporate the response saturation effect at high intensities of the THz radiation observed in experiments and reveal the physics of the response saturation associated with different mechanisms for different material systems. These mechanisms include the gate leakage, the velocity saturation and the avalanche effect.
We report a homogeneous quantum cascade laser (QCL) emitting at Terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz centered around 3.3 THz, a current density dynamic range of Jdr=1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ~ 200 uW of optical power per comb tooth, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of 500 Hz, is observed over a current density range of 240 A/cm2, and even across the negative differential resistance region. We further prove that the QCL frequency comb can be injection locked with moderate RF power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological application requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.
Terahertz sources based on intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically-pumped semiconductor sources capable of operating at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress in power output, that now exceeds 1mW in pulsed and 10 {mu}W in continuous-wave regime at room-temperature, knowledge of the major figure of merits of these devices for high precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. Here, by exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute frequency of individual emission lines of these sources with an uncertainty of 4 x 10-10. The unveiled emission LW (400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications requiring narrow-linewidth THz sources.
Hybrid systems consisting of a quantum emitter coupled to a mechanical oscillator are receiving increasing attention for fundamental science and potential applications in quantum technologies. In contrast to most of the presented works, in which the oscillator eigenfrequencies are irreversibly determined by the fabrication process, we present here a simple approach to obtain frequency-tunable mechanical resonators based on suspended nanomembranes. The method relies on a micromachined piezoelectric actuator, which we use both to drive resonant oscillations of a suspended Ga(Al)As membrane with embedded quantum dots and to fine tune their mechanical eigenfrequencies. Specifically, we excite oscillations with frequencies of at least 60 MHz by applying an AC voltage to the actuator and tune the eigenfrequencies by at least 25 times their linewidth by continuously varying the elastic stress state in the membranes through a DC voltage. The light emitted by optically excited quantum dots is used as sensitive local strain gauge to monitor the oscillation frequency and amplitude. We expect that our method has the potential to be applicable to other optomechanical systems based on dielectric and semiconductor membranes possibly operating in the quantum regime.
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