Elisabeth Koroknay

Periodic Large‐Area Metallic Split‐Ring Resonator Metamaterial Fabrication Based on Shadow Nanosphere Lithography

Metamaterials have gained substantial attention because of their potential for negative permeability as well as negative refractive index in the optical frequency range[1,2]. Due to their unique electromagnetic properties, these nanostructures show promise for numerous applications such as perfect lenses and optical cloaking devices.

Electrically driven quantum dot single-photon source at 2 GHz excitation repetition rate with ultra-low emission time jitter

The influence of the bias voltage on emission properties of a red emitting InP/GaInP quantum dot based single-photon source was investigated. Under pulsed electrical excitation, we can influence the band bending of the p-i-n diode with the applied bias voltage and thus the charge carrier escape by quantum tunneling. This leads to control over the non-radiative decay channel and allows carrier escape times as low as 40 ps, effectively reducing the time jitter of the photon emission. We realized high excitation repetition rates of up to 2 GHz while autocorrelation measurements with g(2)(0)-values of 0.27 attest dominant single-photon emission.

High-power InP quantum dot based semiconductor disk laser exceeding 1.3 W

We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)0.52In0.48]0.5P0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of ∼654 nm with a slope efficiency of ηdiff=25.4 %.

Reducing vortex losses in superconducting microwave resonators with microsphere patterned antidot arrays

We experimentally investigate the vortex induced energy losses in niobium coplanar waveguide resonators with and without quasihexagonal arrays of nanoholes (antidots), where large-area antidot patterns have been fabricated using self-assembling microsphere lithography. We perform transmission spectroscopy experiments around 6.25 GHz in magnetic field cooling and zero field cooling procedures with perpendicular magnetic fields up to B = 27 mT at a temperature T = 4.2 K. We find that the introduction of antidot arrays into resonators reduces vortex induced losses by more than one order of magnitude.

Strong mode coupling in InP quantum dot-based GaInP microdisk cavity dimers

We report on strong mode coupling in closely spaced GaInP microdisk dimer structures including InP quantum dots as the active medium. Using electron beam lithography and a combination of dry- and wet-etch processes, dimers with inter-disk separations down to d < 100nm have been fabricated. Applying a photo-thermal heating scheme, we overcome the spectral mode detuning due to the size mismatch between the two disks forming the dimer. We observe signatures of mode coupling in the corresponding photoluminescence spectra with coupling energies of up to 0.66MeV. With the aid of a numerical analysis, we specify the geometrical and physical factors of the microdisk dimer precisely, and reproduce its spectrum with good agreement.

Low-density InP quantum dots embedded in Ga0.51In0.49P with high optical quality realized by a strain inducing layer

We present a method to reduce the intrinsically high InP quantum dot density embedded in a Ga0.51In0.49P barrier by introducing an InGaAs quantum dot seed layer. The additional strain reduces the total InP quantum dot density by around one order of magnitude from 2×1010 to 3×109 cm−2 but only ∼1% of the InP nanostructures seem to be optically active (107 cm−2). Therefore, microphotoluminescence measurements could be accomplished without masks. We found resolution-limited photoluminescence linewidths (ΔE<100 μeV), good signal-to-noise ratios (∼65), single-photon emission behavior [g(2)(τ=0)=0.3], and excitonic decay times of typically between 1 and 2 ns. Furthermore the structural quantum dot properties were investigated.

InP quantum dots for applications in laser devices and future solid-state quantum gates

Single and stacked layers of InP quantum dots in (AlxGa1−x)0.51In0.49P barriers were grown by metal-organic vapor-phase epitaxy for applications in semiconductor laser devices and optical gate structures. An in-plane laser structure with a single layer of InP QDs is presented, emitting at 1.942 eV and exhibiting low threshold current densities of 780 A/cm2 in electrically pulsed laser operation at 288 K for a 2000μm long device with uncoated facets. In order to realize an externally driven optical gate structure the stacking behavior of InP in (AlxGa1−x)0.51In0.49P barriers was investigated. To ensure the optical addressability of each quantum dot layer, a special double dot structure where the high energetic smaller sized quantum dot is situated above the low energetic larger sized dot, was produced. The coupling between these quantum dots can be adjusted by the thickness of the spacer layer. The structures are embedded in a ni-Schottky structure and the influence of an external electric field on the emission of the quantum dot ensemble is investigated.

The phase boundary of superconducting niobium thin films with antidot arrays fabricated with microsphere photolithography

The experimental investigation of the Ic(B)–Tc(B) phase boundary of superconducting niobium films with large area quasihexagonal hole arrays is reported. The hole arrays were patterned with microsphere photolithography. We investigate the perforated niobium films by means of electrical directed current transport measurements close to the transition temperature Tc in perpendicularly applied magnetic fields. We find pronounced modulations of the critical current with applied magnetic field, which we interpret as a consequence of commensurable states between the Abrikosov vortex lattice and the quasihexagonal pinning array. Furthermore, we observe Little–Parks oscillations in the critical temperature versus magnetic field.

Fabrication and optical characterization of large scale membrane containing InP/AlGaInP quantum dots.

Single-photon sources with a high extraction efficiency are a prerequisite for applications in quantum communication and quantum computation schemes. One promising approach is the fabrication of a quantum dot containing membrane structure in combination with a solid immersion lens and a metal mirror. We have fabricated an 80 nm thin semiconductor membrane with incorporated InP quantum dots in an AlGaInP double hetero barrier via complete substrate removal. In addition, a gold layer was deposited on one side of the membrane acting as a mirror. The optical characterization shows in detail that the unique properties of the quantum dots are preserved in the membrane structure.

InP quantum dot based semiconductor disk laser emitting at 655 nm

Summary form only given. Semiconductor disk lasers (SDLs), also known as vertical-external surface-emitting-lasers (VECSELs) are an ideal choice to reach high output powers. Further advantageous characteristics [1] of this laser type, as e.g. superior beam quality with a radial symmetric TEM00 beam profile, are provided by the external cavity. A further big advantage of VECSELs is given by the possibility of bandgap engineering. By proper adjustment of the semiconductor material and their composition, many different wavelength areas can be covered. The use of quantum dot (QD) layers instead of quantum wells (QWs) should further lead, according to theory [2], to broader gain spectra as well as to lower laser thresholds accompanied by decreased temperature sensitivity.We present a continuous-wave VECSEL system, based on a RPG structure with multiple InP QD layers (see Fig. 1a), emitting around 655 nm. All samples of this study were fabricated by metal-organic vapor-phase epitaxy. The seven single InP QD layers are embedded in a separate confinement heterostructure (SCH) which consist of tensile strained (Al0.1Ga0.9)0.52In0.48P barriers and (Al0.55Ga0.45)0.52In0.48P cladding layers. Below the active region an Al0.45GaAs / AlAs distributed Bragg reflector (DBR) consisting of 55 λ/4 pairs to generate a reflectivity of R>99.9 % is fabricated. The QD characteristics in ensemble and micro-photoluminescence investigations indicate that really the QDs are contributing to this emission. Auto-correlation measurements on a sample with a single QD layer, proves that the luminescence consists of emission of individual QDs. Measurements of the standard laser parameters reveal maximum output powers of 1.4 W at a low emission wavelength ~ 654 nm with a slope efficiency of ηdiff = 25.4% (Fig. 1b). Laser characteristics like high output power at the mentioned wavelenth and the possibility of inserting optical intra-cavity elements for wavelength selection, frequency doubling and tuning, given by the external cavity, make the here introduced VECSEL a very well suited laser source for medical applications like photodynamic therapy (in the red sprctral range) or for scientific and bio-technological applications as coherent light source (frequency doubled to ultraviolet spectral range) for micro-photoluminescence of nitride structures and for luminescence microscopy on biological samples.