Michael Martl

Influence of doping on the performance of terahertz quantum-cascade lasers

The authors present the effects of the doping concentration on the performance of a set of terahertz quantum-cascade lasers emitting around 2.75THz. The chosen design is based on the longitudinal-optical-phonon depopulation of the lower laser state. An identical structure is regrown varying the sheet density from 5.4×109to1.9×1010cm−2. A linear dependency of the threshold current density on the doping is observed. The applied field where lasing takes place is independent of the doping. The field is responsible for the alignment of the cascades and therefore the transport of the electrons through the structure.

Gain and losses in THz quantum cascade laser with metal-metal waveguide.

Coupling of broadband terahertz pulses into metal-metal terahertz quantum cascade lasers is presented. Mode matched terahertz transients are generated on the quantum cascade laser facet of subwavelength dimension. This method provides a full overlap of optical mode and active laser medium. A longitudinal optical-phonon depletion based active region design is investigated in a coupled cavity configuration. Modulation experiments reveal spectral gain and (broadband) losses. The observed gain shows high dynamic behavior when switching from loss to gain around threshold and is clamped at total laser losses.

Excitation of terahertz surface plasmon polaritons on etched groove gratings

The excitation of single and multiple-frequency surface plasmon polaritons (SPP) by a metallic groove grating is studied in the terahertz (THz) frequency range. The efficiency of such gratings for the excitation of SPPs is explored with respect to the grating profile. Numerical simulations provide information on how to design such gratings. Groove gratings were fabricated by anisotropic etching of semiconductor surfaces. Reflection and SPP excitation measurements were done using THz time-domain spectroscopy. The results demonstrate the potential of multisection groove gratings for efficient excitation of broadband THz SPPs.

Terahertz waveguide emitter with subwavelength confinement

The generation of broadband terahertz pulses on the facet of waveguides is presented as an alternative to widely used coupling techniques. Dielectric loaded subwavelength waveguide structures with lateral confinement are investigated with respect to propagating modes and waveguide losses. The results show the terahertz waveguide emitter to be a promising tool for terahertz spectroscopy in the near field and for the probing of microstructured devices such as quantum cascade lasers.

Nonorthodox heterodyne electro-optic detection for terahertz optical systems

An electro-optic detector of terahertz electromagnetic waves with boosted performance is presented. The detector utilizes a heterodyne detection technique to achieve orders of magnitude better responsivity compared to a standard detector setup. We prove theoretically as well as demonstrate experimentally the feasibility of this technique for electro-optic detection.

Terahertz Quantum Cascade Devices: From Intersubband Transition to Microcavity Laser

In this review, we report on the study of terahertz (THz) intersubband (ISB) transitions and on the optical devices based on them. We use time-resolved THz spectroscopy to examine ISB optical transitions in semiconductor quantum wells and quantum cascade lasers (QCLs). From these measurements, we obtain important information on the carrier relaxation, scattering mechanisms, and the gain. The waveguide losses are studied directly on the QCL devices and we show the main loss mechanism in the double-metal waveguides. Finally, we demonstrate THz-QCLs with low-mode volume optical cavity.

Coupled cavity terahertz quantum cascade lasers with integrated emission monitoring

We demonstrate the on-chip generation and detection of terahertz radiation in coupled cavity systems using a single semiconductor heterostructure. Multiple sections of a terahertz quantum cascade laser structure in a double-metal waveguide are optically coupled and operate either as a laser or an integrated emission monitor. A detailed analysis of the photon-assisted carrier transport in the active region below threshold reveals the detection mechanism for photons emitted by the very same structure above threshold. Configurations with a single laser cavity and two coupled laser cavities are studied. It is shown that the integrated detector can be used for spatial sensing of the light intensity within a coupled cavity.

Multi-cavity terahertz quantum cascade lasers

In this contribution, we demonstrate the detection capability of the quantum cascade laser (QCL) active region and show the flexibility of a multi-section THz QCL. The laser waveguide is split into two sections. The first section is intended as a detector element, capable of monitoring THz radiation in the laser cavity. The remaining section is driven independently, so different operation modes can be set. The electrical decoupling is achieved by a 3 μm wide gap between individual sections, preserving sufficient optical coupling.

THz time domain spectroscopy of coupled cavity THz quantum cascade lasers with metal-metal waveguide

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

New generation of the electro-optic terahertz detectors

Since the first demonstration of the electro-optic detection of coherent free space electromagnetic waves in 1996, this technique became a golden standard for the time-domain spectroscopy in the terahertz and mid-infrared frequency range. The detection principle is based on the Pockels effect leading to a terahertz (THz) electric field induced birefringence in the non-linear optical crystal. This birefringence is sensed by polarized near-infrared (NIR) light co-propagating with the THz wave. For a given non-linear crystal the detector sensitivity depends on the effective interaction length between THz and NIR waves. Therefore, the velocity mismatch between these waves is one of the essential problems of the electro-optic detection technique. It can be partially overcome by selecting a convenient combination of electro-optic (EO) crystal material and wavelength of the NIR probe light. A more general solution to this problem, independent on the crystal material and probe wavelength, has been recently introduced by us [1].