Alexander Benz

Phased-array sources based on nonlinear metamaterial nanocavities

Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5 μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

Terahertz microcavity quantum-cascade lasers

We demonstrate circular-shaped microcavity quantum-cascade lasers emitting in the THz region between 3.0 and 3.8 THz. The band structure design of the GaAs∕Al0.15Ga0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900A∕cm2 at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K.

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.

Second harmonic generation from metamaterials strongly coupled to intersubband transitions in quantum wells

We theoretically analyze the second harmonic generation capacity of two-dimensional periodic metamaterials comprising sub-wavelength resonators strongly coupled to intersubband transitions in quantum wells (QWs) at mid-infrared frequencies. The metamaterial is designed to support a fundamental resonance at ∼30 THz and an orthogonally polarized resonance at the second harmonic frequency (∼60 THz), while the asymmetric quantum well structure is designed to provide a large second order susceptibility. Upon continuous wave illumination at the fundamental frequency we observe second harmonic signals in both the forward and backward directions, with the forward efficiency being larger. We calculate the overall second harmonic conversion efficiency of the forward wave to be ∼1.3 × 10−2 W/W2—a remarkably large value, given the deep sub-wavelength dimensions of the QW structure (about 1/15th of the free space wavelength of 10 μm). The results shown in this Letter provide a strategy for designing easily fabricated ...

Strong coupling in the sub-wavelength limit using metamaterial nanocavities

The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared (~10 μm) to the near-infrared (~1.5 μm). Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10−4 (λ/n)3 from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process.

Multi-Colour Nanowire Photonic Crystal Laser Pixels

Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form “super-pixels”. Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.

Active photonic crystal terahertz laser

We present the design and the realization of active photonic crystal (PhC) semiconductor lasers. The PhC consists of semiconductor nanostructure pillars which provide gain at a quantized transition energy. The vertical layer sequence is that of a terahertz quantum cascade laser. Thereby, the artificial crystal itself provides the optical gain and the lateral confinement. The cavities do not rely on a central defect, the lasing is observed in flat-band regions at high symmetry points. The experimental results are in excellent agreement with the finite-difference time-domain simulations. For the vertical confinement a double-metal waveguide is used. The lasers are showing a stable single-mode emission under all driving conditions. Varying the period of the PhC allows to tune the frequency by 400 GHz.

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.

Terahertz quantum cascade lasers based on type II InGaAs/GaAsSb/InP

We report the demonstration of a terahertz quantum cascade laser based on the In0.53Ga0.47As/GaAs0.51Sb0.49 type II material system. The combination of low effective electron masses and a moderate conduction band offset makes this material system highly suitable for such devices. The active region is a three-well phonon depopulation design and laser ridges have been processed in a double-metal waveguide configuration. The devices exhibit a threshold current density of 2 kA/cm2, provide peak optical powers of 1.8 mW, and operate up to 102 K. Emission frequencies are in the range between 3.6 and 4.2 THz.

An electrically driven terahertz metamaterial diffractive modulator with more than 20 dB of dynamic range

We design and experimentally demonstrate a switchable diffraction grating for terahertz modulation based on planar active metamaterials, where a Schottky gate structure is implemented to tune the metamaterial resonances in real-time via the application of an external voltage bias. The diffraction grating is formed by grouping the active split-ring resonators into an array of independent columns with alternate columns biased. We observe off-axis diffraction over a wide frequency band in contrast to the narrow-band resonances, which permits operation of the device as a relatively high-speed, wide-bandwidth, high-contrast modulator, with more than 20 dB of dynamic range.