Optically-driven photoconductive switches are one of the predominant sources currently used in terahertz imaging systems. However, owing to their low average powers, only raster-based images can be taken, resulting in slow acquisition times. In this work, we show that placing a photoconductive switch within a cavity, we are able to generate absolute THz powers of 181 µW. The cavity is based on a metal-insulator-metal structure that permits an enhancement of the average power by almost one order of magnitude whilst conserving a broadband response. We demonstrate real-time imaging using this source, with the broadband spectrum permitting to eliminate diffraction artefacts.
Fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair using quantum cascade lasers (QCLs) as a gain medium. However, no demonstrations of harmonic modelocking have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at harmonics of the cavity’s fundamental round trip frequency. Here, using time resolved THz techniques, we show for the first time harmonic injection, and active and passive mode-locking where THz QCLs are modulated at harmonics of the round-trip frequency. Furthermore, using the unique ultrafast nature of our approach, we show that passive or self-starting harmonic modelocking originates from the QCL self-generating a harmonic microwave modulation. The latter auto-modulates the gain and loss in the system, spontaneously forcing the QCL to operate up to its 15th harmonic and opening up prospects of passive THz short pulse generation.
Reaching high average powers and room temperature operation for THz sources has become the key challenge for the uptake of THz applications that require real-time imaging. In this work, we show that by placing a photoconductive switch within a quasi-resonant cavity based on a metal-insulator-metal geometry, we are able to generate, at room temperature, average THz powers greater than of 200 µW, with the frequency of the THz emission centred at 1.5THz, specifications ideally adapted to NDT. We demonstrate proof-of-principle real-time THz imaging.
Recently, modelocked THz QCLs have been shown to generate 4ps pulses using monolithically integrated Gires-Tournois Interferometer (GTI) dispersion compensation schemes. However, Fourier limit pulse trains were not achieved to date that is vital to realize shorter pulses. Here we show a Fourier-limited pulse train of 3.4ps obtained from an active-modelocked QCL by exactly matching the spectral bandwidth to that of an appropriate GTI. This is despite a spectral bandwidth that is much lower than previous demonstrations.
A QCL based on a multiple stack hybrid active region was used and processed into a metal/metal waveguide. The emission frequency is centred at 3THz with a free-running bandwidth of ~0.1THz. The GTI is fabricated by etching a sub-wavelength air gap through the active region at one end of the QCL ridge. Electromagnetic simulations were performed to optimize the GTI size resulting in a 66.2µm long GTI compensating for the dispersion over a range of 0.3THz around 3.15THz. The pulse measurements are based on coherent sampling of the electric-field using electro-optic detection.
A stable 3.4ps pulse train was obtained by actively modelock the THz QCL with a microwave modulation. Each pulse shows a spectrum with 0.13THz FWHM, exactly at the Fourier-transform limit. In the frequency domain, lasing action occurs only at the off-resonance condition of the GTI as this appoints the dispersion compensated region as the most favourable range for modelocked emission.
In this work we attained the Fourier transform limit permitting the shortest demonstrated stable pulse train from a modelocked THz QCL.
Despite their ultrafast dynamics, modelocking of THz QCLs has been shown recently as a robust method to generate short THz pulses with pulse trains as short as ~4ps at a central frequency of ~2.5THz. This opens up applications of QCLs to synthesizer controlled frequency combs as well as operation of QCLs in more exotic modelocked regimes. In this work we demonstrate for the first time harmonic active modelocking of QCLs where pulses are generated at a harmonic of the round trip frequency. This is achieved by active modelocking at the second harmonic of the QCL fundamental round trip frequency, generating a short pulse train with two pulses per round-trip. This realisation potentially allows for higher average power from modelocked QCLs, modulated at multiple harmonics of the fundamental frequency, and is ideally adapted to the QCLs ultrafast dynamics with modulation beyond 60 GHz attainable.
As applications such as heterodyne spectroscopy require only single mode operation, the selection, suppression and tuning of individual lasing modes in THz QCLs has received considerable attention over the last decade. By periodically patterning the QCL in one- or two dimensions (e.g. distributed feedback (DFB) or photonic crystal lasers), single mode emission can be enforced. An alternative approach which requires no modification of the QCL waveguide is based on injection seeding technique with tunable narrowband THz seeds. Using this technique, we will show how the same QCL can be operated in both multi-mode and single mode regimes.
On the other hand, short pulses allow for time-resolved measurements and the generation of frequency combs. As the duration of a pulse is limited by its spectral bandwidth, a multimode operation of the QCL is highly desirable. By addition of a microwave modulation at the round-trip frequency, where the spacing and phase of the QCL modes is consequently fixed, results in active modelocking. This leads to laser emission of a train of THz pulses separated by the round-trip frequency. Coupled to coherent detection and a novel application of dispersion compensation, we demonstrate the generation of a stable 4 ps train pulse train. This opens up the possibility to reach sub-picosecond pulses and potentially the single cycle regime.
To conclude, we will show two methods to control the THz QCL emission from single mode regimes to the generation of short THz pulses.
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