Sascha Kalusniak

Visible-wavelength laser action of ZnCdO∕(Zn,Mg)O multiple quantum well structures

We report on laser action of ZnCdO∕ZnO quantum well structures up to room temperature under optical pumping. Prerequisite is a novel annealing step increasing the radiative efficiency of the low-temperature grown structures by more than one order of magnitude. The carrier states involved are localized making the lasing properties temperature robust. The longest wavelength reached so far is 490nm.

Vertical cavity surface emitting laser action of an all monolithic ZnO-based microcavity

We report on room temperature laser action of an all monolithic ZnO-based vertical cavity surface emitting laser (VCSEL) under optical pumping. The VCSEL structure consists of a 2λ microcavity containing eight ZnO/Zn0.92Mg0.08O quantum wells embedded in epitaxially grown Zn0.92Mg0.08O/Zn0.65Mg0.35O distributed Bragg reflectors (DBRs). As a prerequisite, design and growth of high reflectivity DBRs based on ZnO and (Zn,Mg)O for optical devices operating in the ultraviolet and blue-green spectral ranges are discussed.

Molecular beam epitaxy of n-Zn(Mg)O as a low-damping plasmonic material at telecommunication wavelengths

We demonstrate that Zn(Mg)O:Ga layers can be grown by molecular beam epitaxy in a two-dimensional mode with high structural perfection up to Ga mole fractions of about 6.5%. The doping efficiency is practically 100% so that free-carrier concentrations of almost 1021 cm−3 can be realized providing a zero-crossover wavelength of the real part of the dielectric function as short as 1.36 μm, while the plasmonic damping does not exceed 50 meV. Structural, electrical, and optical data consistently demonstrate a profound change of the Ga incorporation mode beyond concentrations of 1021 cm−3 attended by deterioration of the plasmonic features.

Strong Coupling between Surface Plasmon Polaritons and Molecular Vibrations

The hybridization of different quasi-particles is an extensively studied subject; both from a practical as well as fundamental point of view. The resultant hybrid excitations exhibit new properties that are not available by the isolated constituents.

Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers.

Longitudinal bulk plasmons in an n-doped ZnO layer system are studied by two-color femtosecond pump-probe spectroscopy in the midinfrared. The optical bulk plasmon resonance identified in linear reflectivity spectra undergoes a strong redshift and a limited broadening upon intraband excitation of electrons. The nonlinear changes of plasmon absorption decay on a time scale of 2 ps and originate from the intraband redistribution of electrons. Theoretical calculations explain the plasmon redshift by the transient increase of the ensemble-averaged electron mass and the concomitantly reduced plasma frequency in the hot electron plasma. The observed bulk plasmon nonlinearity holds strong potential for applications in plasmonics.

Demonstration of hyperbolic metamaterials at telecommunication wavelength using Ga-doped ZnO.

Hyperbolic metamaterials (HMMs) have attracted much attention because they allow for broadband enhancement of spontaneous emission and imaging below the diffraction limit. However, HMMs with traditional metals as metallic component are not suitable for applications in the infrared spectral range. Using Ga-doped ZnO, we demonstrate monolithic HMMs operating at infrared wavelengths. We identify the materials hyperbolic character by various optical measurements in combination with theoretical calculations. In particular, negative refraction of the extraordinary wave and propagation of light with wave vector values exceeding that of free-space are demonstrated in the entire telecommunication window. These findings reveal a considerable potential for creating novel functional elements at telecommunication wavelengths.

Strong exciton-photon coupling in a monolithic ZnO/(Zn,Mg)O multiple quantum well microcavity

We report on strong exciton-photon coupling in an epitaxially grown (Zn,Mg)O-based λ-microcavity (MC) containing four 3.5 nm wide ZnO quantum wells (QWs) as active layers. At 5 K, the observed Rabi splitting in absorption is 26 meV, while the inhomogeneous linewidth of A and B excitons in similar QWs without a MC is about 10 meV. The strong coupling regime (SCR) is lost between 150 K and 200 K due to additional homogeneous broadening. From transfer matrix calculations, we deduce that increasing the number of QWs per cavity length can extend the SCR up to room temperature.

Single crystalline Er2O3:sapphire films as potentially high-gain amplifiers at telecommunication wavelength

Single crystalline thin films of Er2O3, demonstrating efficient 1.5 μm luminescence of Er3+ at room temperature were grown on Al2O3 substrate by molecular beam epitaxy. The absorption coefficient at 1.536 μm was found to reach 270 cm−1 translating in a maximal possible gain of 1390 dBcm−1. In conjunction with the 10% higher refractive index as compared to Al2O3, this opens the possibility to use Er2O3:sapphire films as short-length waveguide amplifiers in telecommunication.

Negative refraction at telecommunication wavelengths through plasmon-photon hybridization

We demonstrate negative refraction at telecommunication wavelengths through plasmon-photon hybridization on a simple microcavity with metallic mirrors. Instead of using conventional metals, the plasmonic excitations are provided by a heavily doped semiconductor which enables us to tune them into resonance with the infrared photon modes of the cavity. In this way, the dispersion of the resultant hybrid cavity modes can be widely adjusted. In particular, negative dispersion and negative refraction at telecommunication wavelengths on an all-ZnO monolithical cavity are demonstrated.

Resonant interaction of molecular vibrations and surface plasmon polaritons: The weak coupling regime

Adjusting the free-electron concentration, the surface plasmon frequency of the semiconductor ZnOGa is tuned into resonance with the molecular vibrations of the n-alkane tetracontane. Closed molecular films deposited on the semiconductors surface in the monolayer regime generate distinct signatures in total-attenuated-reflection spectra at the frequencies of the symmetric and asymmetric stretching vibrations of the CH2 group. Their line shape undergoes profound changes from absorptive to dispersive and even anti-resonance behavior when moving along the surface- plasmon dispersion by the angel of incidence. We demonstrate that this line shape diversity results from a phase-sensitive perturbation of the surface-plasmon-polariton generation at the molecule/metal interface.