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تسجيل مستخدم جديدTerahertz frequency combs exploiting an on-chip solution processed graphene-quantum cascade laser coupled-cavity architecture
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F. P. Mezzapesa
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The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain spectra and with a full compensation of the group velocity dispersion, at Terahertz (THz) frequencies, is a fundamental need for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCS) in the far-infrared. In a THz QCL four-wave mixing, driven by the intrinsic third-order susceptibility of the intersubband gain medium, self-lock the optical modes in phase, allowing stable comb operation, albeit over a restricted dynamic range (~ 20% of the laser operational range). Here, we engineer miniaturized THz FCSs comprising a heterogeneous THz QCL integrated with a tightly-coupled on-chip solution-processed graphene saturable-absorber reflector that preserves phase-coherence between lasing modes even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes to be demonstrated, over more than 55% of the laser operational range. Furthermore, stable injection-locking is showed, paving the way to impact a number of key applications, including high-precision tuneable broadband-spectroscopy and quantum-metrology.
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Quantum cascade lasers are proving to be instrumental in the development of compact frequency comb sources at mid-infrared and terahertz frequencies. Here we demonstrate a heterogeneous terahertz quantum cascade laser with two active regions spaced e
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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-tempera
ture (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.
Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multi-heterodyne spectroscop
y using two terahertz quantum cascade laser combs. With just 100 $mu$s of integration time, we achieve peak signal-to-noise ratios exceeding 60 dB and a spectral coverage greater than 250 GHz centered at 2.8 THz. Even with room-temperature detectors we are able to achieve peak signal-to-noise ratios of 50 dB, and as a proof-of-principle we use these combs to measure the broadband transmission spectrum of etalon samples. Finally, we show that with proper signal processing, it is possible to extend the multi-heterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode, greatly expanding the range of quantum cascade lasers that could be suitable for these techniques.
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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.
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