THz roadmap

Future directions in THz quantum cascade laser research has been presented in "The 2017 terahertz science and technology roadmap" issue in Journal of Physics D: Applied Physics. This highlights the impact of QCLs to the THz domain and future directions related to modelocking. The full publication can be found here

Multi-Watt high-power THz frequency quantum cascade lasers

Multi-Watt high-power terahertz (THz) frequency quantum cascade lasers are demonstrated, based on a single, epitaxially grown, 24-μm-thick active region embedded into a surface-plasmon waveguide. The devices emit in pulsed mode at a frequency of ∼4.4 THz and have a maximum operating temperature of 132 K. The maximum measurable emitted powers from a single facet are ∼2.4 W at 10 K and ∼1.8 W at 77 K. The full publication can be found here

Bow-Tie Cavity for Terahertz Radiation

We report on the development, testing, and performance analysis of a bow-tie resonant cavity for terahertz (THz) radiation, injected with a continuous-wave 2.55 THz quantum cascade laser. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F = 123, and a high quality factor Q. The resonator performances and the relevant parameters are theoretically predicted and discussed, and a comparison among simulated and experimental spectra is given. The full publication can be found (...)

QCL-based frequency metrology from the mid-infrared to the THz range: a review

Quantum cascade lasers (QCLs) are becoming a key tool for plenty of applications, from the mid-infrared (mid-IR) to the THz range. Progress in related areas, such as the development of ultra-low-loss crystalline microresonators, optical frequency standards, and optical fiber networks for time and frequency dissemination, is paving the way for unprecedented applications in many fields. For most demanding applications, a thorough control of QCLs emission must be achieved. In the last few years, QCLs’ unique spectral features have been unveiled, while multifrequency QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity (...)

Heterogeneous, terahertz quantum cascade lasers with thermally tunable frequency comb operation

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. Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning (...)

Advanced GaAs/AlGaAs ICP-DRIE etching for optoelectronic applications

We investigate the parameter optimization for micron-scale etching by Inductive Coupled Plasma - Deep Reactive Ion Etching (ICP-DRIE) of GaAs/AlGaAs semiconductor heterostructures. Although dry etching approaches have been reported in the literature using a broad variety of plasma etch tools, there is still need to meet the majority of microsystems dry etching requirements. The desired etch characteristics for GaAs/AlGaAs heterostructures can be achieved by controlling the various process parameters with good reliability, high selectivity, and – simultaneously – high etch rates and sidewall verticality. Etch rates from 1 to over 5.5μm/min have been obtained. The selectivity with optical photoresist varies from 2.3 to 16. The presented (...)

Waveguided Approach for Difference Frequency Generation of Broadly-Tunable Terahertz Radiation

The spectral features of stabilized telecom sources can be transferred to the THz range by difference frequency generation. In this paper we extensively discuss the role of Lithium Niobate (LN) channel-waveguides in the experimental accomplishment of a room-temperature continuous wave (CW) spectrometer, with μW-range power levels and a spectral coverage of up to 7.5 THz. To this purpose, and looking for further improvements, a thought characterization of specially-designed LN waveguides is presented, whilst discussing its nonlinear efficiency and its unprecedented capability to handle high optical power on the basis of a three-wave-mixing theoretical model. The full publication can be found (...)

Room-Temperature Frequency-Referenced Spectrometer up to 7.5 THz

High-precision frequency measurements of molecular transitions in the THz range are crucial but remain an open challenge. Here, room-temperature generation and detection of continuous-wave, broadly tunable, narrow-linewidth THz radiation are presented, and their application to high-resolution spectroscopy in the broad 1–7.5 THz spectral range is demonstrated. This result has been achieved by implementing a Cherenkov phase-matching scheme into a channel waveguide in a nonlinear crystal. This simple approach, entirely based on robust telecom technology, unprecedently merges in a single source an ultra-broad continuous-wave spectral coverage and a state-of-the-art accuracy in molecular-transition-center determination. The full publication (...)

Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale, to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources. The publication can be (...)

Broadband THz QCLs

We demonstrate a broadband, heterogeneous terahertz frequency quantum cascade laser by exploiting an active region design based on longitudinal optical (LO)-phonon-assisted interminiband transitions. We obtain continuous wave laser emission with a low threshold current densities of 120 A/cm2, a dynamic range of 3.1, and an emission spectrum spanning from 2.4 to 3.4 THz at 15 K. The full publication can be accessed here

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

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 (...)

Ultrafast detection of THz emission using 3D meta-atoms

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, (...)

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

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 (...)

A dispersion compensated THz Quantum Cascade Laser

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 (...)

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

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 (...)

Fermi level pinning at the Dirac point in epitaxial multilayer graphene

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 (...)

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

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 (...)

Improving the output beam of Terahertz QCLs

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

New types of photoconductive antennas

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

Kick-off meeting

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

Publication on short pulse generation from THz QCLs

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