arXiv subjects

Savvas Germanis

Publications and source records attributed to Savvas Germanis.

4 recordsLinked to original sources

arXiv2026

Observation of Electrically Tunable Chirality Inversion in a Slow-Light Waveguide

We identify chiral inversion points in slow-light, glide-plane-symmetric, photonic-crystal waveguides, defined as fixed locations where the local optical chirality changes sign over a narrow wavelength range. We experimentally access this behaviour using a waveguide-embedded InAs/InGaAs quantum dot. The slow-light spectral region is determined from time-integrated and time-resolved photoluminescence, and the dot exciton is electrically tuned across the slow-light bandwidth via the quantum-confined Stark effect. As the emission wavelength is swept through the slow-light region, the directional emission contrast shows a strong wavelength dependence and a sign reversal, consistent with the identified chiral inversion point. Numerical simulations attribute the switching primarily to the pronounced spectral variation of the local optical chirality for emitters displaced from the waveguide center. These results demonstrate on-demand electrical switching of chiral light-matter coupling in nanophotonic waveguides and enable tunable chiral interfaces for integrated quantum photonic devices.

physics.optics↗

arXiv2025

Waveguide Excitation and Spin Pumping of Chirally Coupled Quantum Dots

We report on an integrated semiconductor chip where a single quantum dot (QD) is excited in-plane via a photonic-crystal waveguide through its nearest p-shell optical transition. The chirality of the waveguide mode is exploited to achieve both directional absorption and directional emission, resulting in a substantial enhancement in directional contrast, as measured for the Zeeman components of the waveguide-coupled QD. This remote excitation scheme enables high directionality (greater than or equal to 0.95) across approximately 56% of the waveguide area, with significant overlap with the Purcell-enhanced region, where the electric field intensity profile is near its peak. In contrast, local excitation methods using an out-of-plane excitation beam focused directly over the area of the QD achieve only approximately 25% overlap. This enhancement increases the likelihood of locating Purcell-enhanced QDs in regions that support high directionality, enabling the experimental demonstration of a six-fold enhancement in the decay rate of a QD with greater than 90% directionality. The remote p-shell excitation protocol establishes a new benchmark for waveguide quantum optics in terms of the combination of Purcell enhancement and high directionality, thereby paving the way for on-chip excitation of spin-based solid-state quantum technologies in regimes of high beta-factor.

cond-mat.mes-hall↗

arXiv2024

Collecting single photons from a cavity-coupled quantum dot using an adiabatic tapered fiber

We demonstrate efficient in-plane optical fiber collection of single photon emission from quantum dots embedded in photonic crystal cavities. This was achieved via adiabatic coupling between a tapered optical fiber and a tapered on-chip photonic waveguide coupled to the photonic crystal cavity. The collection efficiency of a dot in a photonic crystal cavity was measured to be 5 times greater via the tapered optical fiber compared to collection by a microscope objective lens above the cavity. The single photon source was also characterized by second order photon correlations measurements giving g(2)(0)=0.17 under non-resonant excitation. Numerical calculations demonstrate that the collection efficiency could be further increased by improving the dot-cavity coupling and by increasing the overlap length of the tapered fiber with the on-chip waveguide. An adiabatic coupling of near unity is predicted for an overlap length of 5 microns.

physics.optics↗

arXiv2014

Extraction of absorption coefficients from GaN nanowires grown on opaque substrates

We demonstrate a new method to measure absorption coefficients in any family of nanowires, provided they are grown on a substrate having considerable difference in permittivity with the nanowire-air matrix. In the case of high crystal quality, strain-free GaN nanowires, grown on Si (111) substrates with a density of ~1010 cm-2, the extracted absorption coefficients do not exhibit any enhancement compared to bulk GaN values, unlike relevant claims in the literature. This may be attributed to the relatively small diameters, short heights, and high densities of our nanowire arrays.

physics.optics↗