ULTRAQCL

A paper on ultrafast switch-on dynamics of THz QCLs has been published in Nature Communications by the group at the University of Leeds. Frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent (...)

A paper on ultrafast THz detectors based on 3D meta-atoms has been published in Optica by the group at University of Paris South. Quantum-well infrared photodetectors (QWIPs) have proved to be an excellent technology as ultrafast THz receivers, given their intrinsic picosecond-range response. However, with research focused on diffraction-limited QWIP structures (λ∕2), RC constants cannot be reduced indefinitely, and detection speeds are bound to eventually meet an upper limit. The key to an ultra-fast response with no intrinsic upper limit even at tens of gigahertz (GHz) is an aggressive reduction in device size, below the diffraction limit. Here we demonstrate sub-wavelength (λ∕10) THz QWIP detectors based on a 3D split-ring geometry, (...)

A paper on the generation of Multi-THz electric fields with simultaneously high fields and high repetition rates has been published in Optics Letters by the group at University of Regensburg. We have demonstrated a compact source of energetic and phase-locked multi-terahertz pulses at a repetition rate of 190 kHz. Difference frequency mixing of the fundamental output of an Yb:KGW amplifier with the idler of an optical parametric amplifier in GaSe and LiGaS2 crystals yields a passively phase-locked train of waveforms tunable between 12 and 42 THz. Pulse energies of up to 0.16 μJ and peak electric fields of 13 MV/cm are achieved. The scalable scheme opens the door to strong-field terahertz optics at unprecedented repetition rates. The (...)

In the terahertz (THz) frequency range, a semiconductor-based technology platform for ultra-short pulse generation has yet to be realized. A limiting factor is the negative and positive chromatic dispersion of the gain of quantum cascade lasers (QCLs). Here, a novel dispersion compensation scheme based on an off-resonance Gires-Tournois Interferomenter is applied to a THz QCL, greatly reducing the THz pulse duration to a few optical cycles, with stable pulses as short as 4ps. The publication has been chosen as a cover image for Laser and Photonics Review (click here) The full publication can be accessed here and the submitted version (...)

A paper on Echo-less photoconductive switches as high resolution detectors has been published in Applied Physics Letters by the group at CNRS-LPA. Photoconductive (PC) switches are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limits the spectroscopic resolution achievable from their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes. This is realized (...)

A paper on evidence of Fermi level pinning at the Dirac point in epitaxial multilayer graphene has has been published in Physical Review B by the group at CNRS-LPA. The authors investigated the temperature-dependent conductivity of epitaxial multilayer graphene using THz time domain spectroscopy and show evidence that the Fermi level in quasi-neutral graphene layers is pinned at the Dirac point by mid-gap states. The authors demonstrated that the scattering mechanisms result from the interplay between mid-gap states, which dominate in the vicinity of the Dirac point, and short-range potentials that govern at higher energies (>8 meV). These results highlight the potential of multilayer epitaxial graphene for probing low-energy Dirac (...)

A paper on self modulation of QCLs has been published in Scientific Reports by the group at CNR. The authors show a compact architecture to induce both frequency and amplitude self-modulation in a THz QCL. By engineering a microwave avalanche oscillator into the laser cavity, which provides a 10 GHz self-modulation of the bias current and output power, we demonstrate multimode laser emission centered around 3 THz, with distinct multiple sidebands. The resulting microwave amplitude and frequency self-modulation of THz QCLs opens up intriguing perspectives, for engineering integrated self-mode-locked THz lasers, with impact in fields such as nano- and ultrafast photonics and optical metrology. The full publication can be accessed (...)

The CNRS-LPA partner has demonstrated, in a new publication in Optics Express, a simple method to engineer the beam profile of a Terahertz QCLs. The far-field emission metal-metal QCLs is controlled in directionality and form through planar horn-type shape structures, whilst conserving a broad spectral response. The structures produce a gradual change in the high modal confinement of the waveguides and permit an improved far-field emission profile and resulting in a four-fold increase in the emitted output power. Full details can be found here

A new type of photoconductive antenna has been realised that eliminates the ’echo’ signals that typically originate from substrate reflections. These new devices will be of use to the time resolved study of the QCL emission. Full details of the work can be found here

The ULTRAQCL project has started! The kick-off meeting for the ULTRAQCL project took place at the Centre Culturel Irlandais in Paris on 29th Nov 2015

A paper on generating short pulses from QCLs has been published in Optica by the group at CNRS-LPA. The authors bring new insights into the QCL modelocking and show, contrary to popular believe, that the ‘active microwave’ modulation is the key parameter that determines the pulse duration rather than the QCL bandwidth or ultrafast dynamics. This will now impact the future direction of the ULTRAQCL project. The full publication can be accessed here

Presentation

In Focus

Ultrafast switch-on dynamics of frequency tuneable THz Quantum Cascade Lasers

Ultrafast detection of THz emission using 3D meta-atoms

Phase-locked multi-terahertz electric fields with high fields and repetion rates

A dispersion compensated THz Quantum Cascade Laser

Monolithic Echo-less Photoconductive detectors for Terahertz Time-domain Spectroscopy

Fermi level pinning at the Dirac point in epitaxial multilayer graphene

Frequency and amplitude modulation of ultra-compact terahertz quantum cascade lasers using an integrated avalanche diode oscillator

Improving the output beam of Terahertz QCLs

New types of photoconductive antennas

Kick-off meeting

Publication on short pulse generation from THz QCLs

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