Daniel Dietze

Terahertz meta-atoms coupled to a quantum well intersubband transition

We present a method of coupling free-space terahertz radiation to intersubband transitions in semiconductor quantum wells using an array of meta-atoms. Owing to the resonant nature of the interaction between metamaterial and incident light and the field enhancement in the vicinity of the metal structure, the coupling efficiency of this method is very high and the energy conversion ratio from in-plane to z field reaches values on the order of 50%. To identify the role of different aspects of this coupling, we have used a custom-made finite-difference time-domain code. The simulation results are supplemented by transmission measurements on modulation-doped GaAs/AlGaAs parabolic quantum wells which demonstrate efficient strong light-matter coupling between meta-atoms and intersubband transitions for normal incident electromagnetic waves.

Silver nanoisland enhanced Raman interaction in graphene

Graphene shows great potential for optoelectronic applications but suffers from rather weak interaction with light due its single-atomic thickness. Here, we report the enhanced interaction of graphene and light for Raman transitions using localized surface plasmons. The plasmons are generated in silver nanoislands that we fabricate by simple means of metal deposition on top of graphene. Despite the broad size distribution of the nanoislands, we find a 100-fold enhancement of the Raman signal. We provide an analytical model for the description of the optical properties and obtain the scattering cross section as well as enhancement factors for the Raman transitions. In addition, we investigate, both optically and electrically, the doping that is introduced by the nanoislands.

Polarization of terahertz radiation from laser generated plasma filaments

An analysis of the polarization of terahertz (THz) radiation from a laser-induced plasma source is presented. THz emission is achieved by mixing a laser pulse with its second harmonic after focusing through a β-BaB2O4 (β-BBO) crystal. Numerical calculations, based on the nonlinear four-wave mixing model and the microscopic polarization model, are compared with experimental results. The main focus lies on the study of the dependence of THz polarization on the polarization and relative phase of the incident fundamental and second-harmonic pulses. We show that the modulation of the fundamental pulse by the BBO crystal has to be taken into account in order to describe experimental observations. By including the finite extension of the plasma and considering cross- and self-phase modulation of the two-color pump pulse, we are able to explain the observed ellipticity of the THz pulse as well as the orientation of the polarization axis.

Resonant metamaterial detectors based on THz quantum-cascade structures

We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.

Dynamically phase-matched terahertz generation

We present the generation of intense terahertz pulses by optical rectification of 780 nm pulses in a large area gallium phosphide crystal. The velocity mismatch between optical and terahertz pulses thereby limits the bandwidth of the terahertz pulses. We show that this limitation can be overcome by a dynamic modification of the refractive index of the gallium phosphide crystal through generation of hot phonons. This is confirmed by excellent agreement between experimental results and model calculations.

Guided Modes in Layered Semiconductor Terahertz Structures

In this paper, we present a simple and robust method for calculating the dispersion relation of guided modes in layered media with conducting interfaces in the terahertz frequency range. The procedure is based on the transfer matrix method and utilizes a Nelder-Mead simplex algorithm for the solution of the resulting transcendental equations. By applying a semi-analytical approach, the algorithm shows very good convergence behavior even for structures containing metallic layers. Analytic forms of the transcendental equations for transverse electric and transverse magnetic polarization for systems with one to four boundaries are presented. Conducting interfaces can, for instance, be used to describe a 2-D electron gas or a planar metamaterial. We demonstrate the performance of this method by examples which are of current technological interest. These include surface waves bound to surface carriers on p-InAs, a parallel plate waveguide and plasma oscillations in a high electron mobility transistor structure.

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.

CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts

We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Efficient population transfer in modulation doped single quantum wells by intense few-cycle terahertz pulses

We demonstrate the direct observation of non-equilibrium intersubband dynamics in a modulation-doped multiple quantum well sample subject to intense few-cycle terahertz (THz) pulses. The transmission spectra show a distinct dependence on the incident THz field strength and contain signatures of a multitude of nonlinear effects that can be observed owing to the large THz-pulse bandwidth. We focus our attention on a case of transient nonlinear refractive index caused by the efficient transfer of electronic population from the ground state to higher-excited states of the quantum well sample. By comparing the experimental results with a one-dimensional finite-difference model going beyond the slowly varying envelope approximation, we prove that, depending on the pulse shape, the leading part of the intense pulse efficiently transfers electrons from the ground state to higher lying excited states. For weak electric fields and small-population transfer, the linear Lorentz model holds. For strong electric fields, up to 55 and 20% of the ground-state electrons are transferred to the first and second excited subbands, respectively, which could lead to the observation of the optical gain.

Ultrastrong coupling of intersubband plasmons and terahertz metamaterials

We report on the ultrastrong-coupling between localized plasmons of a planar terahertz metamaterial and intersubband plasmons in a modulation doped quantum well sample. Such a system exhibits the formation of a lower and an upper polariton branch when the metamaterial eigenfrequency is tuned close to resonance with the intersubband transition. We achieve a normalized polariton splitting of 22% and a polaritonic gap of 2.4% of the intersubband transition frequency. In addition to the usual geometrical scaling, we demonstrate the effective tuning of the metamaterial resonance by dry etching with a tuning range of more than 1 THz.