ABSTRACT. We discuss 2D-material heterostructures as nano-lego for light. In particular, we will show nano-optoelectronic devices that demonstrate the exciting properties of 2D polaritons, such as plasmon, phonon and exciton polaritons, which challenge the limits of quantum light-matter interactions. In addition, quantum confined status can be probed through intersubband transitions in few-layer semiconducting 2D materials. Device applications, such as detectors for infrared and THz light will also be discussed.

ABSTRACT. Scattering-type near-field optical microscopy (s-SNOM) can overcome the limits in spatial resolution of conventional THz imaging and spectroscopy techniques. In this talk we presents recent results and important applications of THz imaging and THz time domain spectroscopy with sub-20 nm spatial resolution based on the s-SNOM platform.

ABSTRACT. The feasiblity of detecting deeply subwalength vibrations with nanometer precision at terahertz frequencies is demonstrated. This result was achieved using a suspended silicon nitride membrane within a terahertz quantum cascade laser self-mixing apparatus. Besides representing a platform for the characterization of small displacements, this configuration can be exploited for the realization of optomechanical systems where lasers are coupled by a radiation pressure driven mechanical resonator.

ABSTRACT. We describe a novel near-field microscope, called terahertz (THz) scanning noise microscope (SNoiM), which maps ultrahigh frequency current fluctuation (15-30 THz) with nanoscale spatial resolution. The SNoiM is demonstrated to be a powerful and unique experimental tool for studying local charge-carrier dynamics in a variety of non-equilibrium systems.

ABSTRACT. GaAs/AlAs terahertz quantum-cascade lasers (QCLs) exhibit comparatively high wall-plug efficiencies. They are promising radiation sources for spectroscopic applications, if the output power and operating temperature reach sufficiently high values. We define a practical operating temperature T_po, for which the QCL emits an optical output power of at least 1 mW. For a 4.75-THz QCL, we obtain a maximum value for T_po of 72 K, which can be increased by using an additional back-facet mirror.

ABSTRACT. A phase-locking scheme is developed for monolithic surface-emitting terahertz quantum cascade lasers with hybrid second and fourth-order Bragg gratings that achieves large radiative efficiency, and symmetric, single-lobed radiation patterns with low far-field divergence. Peak output power of 1.88 W is measured for a dual-mode ∼ 3.3 THz laser with 10.6 ◦ × 9.2 ◦ beam divergence, and 1.34 W for a single-mode 3.24 THz laser with 4.6 ◦ × 4.2 ◦ divergence in pulsed mode of operation at ∼ 58 K.

ABSTRACT. We suggest and demonstrate experimentally a novel split-well direct-phonon (SWDP) scheme for THz-QCLs. As a result of this unique scheme, the lasers benefit from a more flexible design and an efficient isolation of laser levels from excited and continuum states. A clean three-level system, that is, most of the electrons reside in the three lowest subbands even at elevated temperatures, is achieved as indicated by an NDR behavior at room temperature.

ABSTRACT. The emission of a THz QCL with two optical transitions is studied. The population of the upper laser states, which correspond to 3.4 THz and 3.8 THz, are investigated for different operating temperatures and in a strong magnetic field, which influences the elastic scattering channel for the two upper laser states and changes the lasing frequency.

ABSTRACT. We present the first thermoelectrically (TEC) cooled terahertz quantum cascade laser (QCL). A high temperature 3-well THz QCL is mounted to a novel Peltier cooler and housing. The temperature- and time-dependent laser performance, the TEC characteristics, and the duty cycle are investigated.

ABSTRACT. We present THz QCLs which operate from 195 K to 210.5 K on a Peltier cooler, with a peak output power as high as 200 mW at 10 K. The design is based on two quantum wells per period and has been optimized with a nonequilibrium Green’s function model and processed using a dry etched Cu-Cu double-metal waveguide.

ABSTRACT. For high-resolution molecular spectroscopy and metrology applications, infrared frequency combs need a full frequency stabilization of all the emitted modes. Here we demonstrate the full phase stabilization and independent control of the two comb degrees of freedom, offset and mode spacing, of quantum cascade lasers frequency combs. A technique enabling to monitor the obtained degree of coherence is also presented.

ABSTRACT. In this contribution we present our investigations of the gain recovery dynamics of a heterogeneous terahertz quantum cascade laser and their role in the stable operation of a broadband active region. We employ THz-pump/THz-probe time domain spectroscopy (TDS). The measurements reveal a strong coupling between the stacks that lead to the same gain recovery time for each stack although not all of them recover according to a simple exponential curve.

ABSTRACT. Here we demonstrate the heterodyne mixing of two unidirectional, 3rd order DFB QCLs with a Schottky diode mixer. The DFB’s are centered around 2.7 THz with a frequency difference of 6-8 GHz. After phase-locking, the stabilized beat note has a narrow linewidth of 0.3 Hz, producing a high dynamic range of 80 dB.

ABSTRACT. In laser technology harmonic modelocking is usually employed to attain higher repetition rates. Despite the favourable dynamics of THz QCLs, harmonic active modelocking hasn't been experimentally demonstrated yet. Here we present active modelocking experiments performed at the fundamental and second harmonic on a 6mm THz QCL. We will also show that in certain cases, QCLs can spontaneously attain second or higher harmonic emission without active modulation, i.e. self-starting harmonic behaviour

ABSTRACT. We observe continuous frequency tuning over a range of up to 40 GHz by near infrared optical excitation of terahertz quantum-cascade lasers. We developed a model which explains the effect by an optically excited electron-hole plasma. We further exploited the effect for frequency and power stabilization of QCLs.

ABSTRACT. THz transmission responses of THz-QCLs are studied. Increasing the incident field amplitude the response transfers from the small-signal (gain clamping) regime to a transparent (zero gain) regime.

ABSTRACT. We demonstrate the most compact terahertz multiheterodyne dual-comb spectroscopy using two quantum cascade lasers (QCLs) without a need of additional fast detectors. To prove the spectroscopic ability, we further show that the compact dual-comb system can be used to calibrate the relative humidity in the air and fast identify gases in the enviornment. Due to the small optical coupling aperture (150 μm), it is also potential to use the dual-comb technique for terahertz imaging.

ABSTRACT. We describe the experimental techniques, approaches, as well as results of recent measurements on nonlinear optical conversion using  different types of Dirac materials. For example, highly efficient THz harmonics generation up to the 7th order in a single layer graphene sample at ambient conditions has been demonstrated utilizing THz pulses with rather moderate electric field strength of only few 10 kV/cm.

ABSTRACT. We present graphene-on-polymide Terahertz frequency modulators, in which the optical power is modulated by the gate-controlled optical conductivity of a single layer graphene, placed on a double-metal polyimide waveguide. With the introduction of a suitable grating coupler, the modulator bandwidth can be tuned beyond the nl/4 mode. We demonstrate a gate-tuned reflectance modulation higher than 25%, with a tunable by design modulation peak of 90% at fixed frequencies in the 1.85-2.3 TH range.

ABSTRACT. Weyl fermions were recently shown to exist in the nonrelativistic condensed-matter setting as gapless quasiparticle states in topological metals. We calculate their bulk and surface conductivity tensors and determine the peculiar properties of both bulk and surface electromagnetic eigenmodes. We show how optical spectroscopy can be used to study topological properties of electron states. Moreover, unusual optical properties of these materials can be utilized in future optoelectronic devices.

ABSTRACT. Enabled by cryogenic “scanning plasmon interferometry”, we resolve the effect of phonon scattering and dielectric loss on graphene plasmons, thereby revealing fundamental limits for plasmon propagation at T<70K. Moreover, by directly resolving the enhanced propagation of hyperbolic phonon polaritons in isotopically pure boron nitride as a function of temperature, we can forecast the ultimate performance of hBN-encapsulated graphene for mid-infrared plasmonics at cryogenic temperatures.