arXiv subjects

Tapio Niemi

Publications and source records attributed to Tapio Niemi.

7 recordsLinked to original sources

arXiv2026

Stimulated cooling in non-equilibrium Bose-Einstein condensate

We report on the experimental observation of stimulated cooling in the non-equilibrium Bose-Einstein condensate (BEC) of weakly interacting exciton-polaritons from approximately room temperature down to 20K. By resolving the condensate in energy-momentum space and performing interferometric measurements, we distinguish the condensate from thermalized particles yet occupying excited states macroscopically. In contrast to the analytical quantum theories of non-equilibrium BEC [Shishkov et al., Phys. Rev. Lett. 128, 065301 (2022)], we observe segmentation of the particle density along the excited states into two fractions both following Bose-Einstein distribution, albeit with different effective temperatures and chemical potentials. Our results indicate that the temperature of the weakly interacting Bose gas is universally set by the density-dependent chemical potential, revealing a defining property of non-equilibrium BECs. Finally, we demonstrate that the stimulated nature of the cooling process directly governs the emergence of quantum coherence of the condensate and shapes the dissipative properties of the excited states.

cond-mat.quant-gas↗

arXiv2025

Purcell-enhanced single-photon emission from InAs/GaAs quantum dots coupled to broadband cylindrical nanocavities

On-chip emitters that can generate single and entangled photons are essential building blocks for developing photonic quantum information processing technologies in a scalable fashion. Semiconductor quantum dots (QDs) are attractive candidates that emit high-quality quantum states of light on demand, however at a rate limited by their spontaneous radiative lifetime. In this study, we utilize the Purcell effect to demonstrate up to a 38-fold enhancement in the emission rate of InAs QDs by coupling them to metal-clad GaAs nanopillars. These cavities, featuring a sub-wavelength mode volume of 4.5x10-4 (λ/n)3 and low quality factor of 62, enable Purcell-enhanced single-photon emission across a large bandwidth of 15 nm. The broadband nature of the cavity eliminates the need for implementing tuning mechanisms typically required to achieve QD-cavity resonance, thus relaxing fabrication constraints. Ultimately, this QD-cavity architecture represents a significant stride towards developing solid-state quantum emitters generating near-ideal single-photon states at GHz-level repetition rates.

quant-ph↗

arXiv2025

Nonlinear Optical Microscopy of Semiconductor Metal-Nanocavities

We use second and third harmonic generation microscopy to investigate the nonlinear optical response of GaAs nanocavities embedded in a gold film and compare them to bare GaAs nanocavities. Our results reveal that the surrounding metallic environment significantly modifies both the intensity and spatial distribution of the nonlinear signals. When the harmonic wavelength is spectrally detuned from the nanocavity resonance, the effects due to the metallic environment start suppressing the SHG contrast. Numerical simulations confirm that at a 1060 nm pump wavelength, the SHG produced at 530 nm is suppressed due to the dominant plasmonic response of gold. Meanwhile, the THG produced at 353 nm, which coincides with the nanocavity resonance, enables high contrast imaging. Furthermore, by shifting the pump to 710 nm, aligning SHG at 356 nm with the nanocavity resonance, we recover strong SHG contrast, demonstrating a pathway to enhanced imaging of metal-semiconductor heterostructures.

physics.optics↗

arXiv2024

Ultrafast modulation of guided-mode resonance in a nonlinear silicon nitride grating

Nonlinear optical interactions in nanostructures are subjects of both fundamental research and practical applications. The optical Kerr effect, a third-order nonlinear optical phenomenon, induces anisotropic changes in the refractive index of a material under intense laser illumination. This effect is inherently weak, limiting its practical use in free-space nanophotonics. In this work, we demonstrate an enhancement of the optical Kerr effect by more than three orders of magnitude using guided-mode resonance in a dielectric diffraction grating with an overall thickness of less than 300 nm. Our study involves the design, fabrication, and measurement of the transmittance of a resonant silicon nitride crossed grating illuminated with short light pulses. We observe spectral resonance shifts with varying pulse power, leading to more than fifty percent modulation of the resonance magnitude and enabling transitions between `Off' and `On' states of transmission. We also demonstrate the capability of the grating for dynamic pulse shaping. The results presented here reveal promising directions for developing advanced all-optical devices based on free-space nanophotonics.

physics.optics↗

arXiv2022

High-Q guided-mode resonance of a crossed grating with near-flat dispersion

Guided-mode resonances in diffraction gratings are manifested as peaks (dips) in reflection (transmission) spectra. Smaller resonance line widths (higher Q-factors) ensure stronger light-matter interactions and are beneficial for field-dependent physical processes. However, strong angular and spectral dispersion are inherent to such high-Q resonances. We demonstrate that a class of high-Q resonant modes (Q-factor >1000) exhibiting extraordinarily weak dispersion can be excited in crossed gratings simultaneously with the modes with well-known nearly linear dispersion. Furthermore, we show that the polarization of the incoming light can be adjusted to engineer the dispersion of these modes, and strong to near-flat dispersion or vice-versa can be achieved by switching between two mutually orthogonal linear polarization states. We introduce a semi-analytical model to explain the underlying physics behind these observations and perform full-wave numerical simulations and experiments to support our theoretical conjecture. The results presented here will benefit all applications that rely on resonances in free-space-coupled geometries.

physics.optics↗

arXiv2014

Techniques and tools for measuring energy efficiency of scientific software applications

The scale of scientific High Performance Computing (HPC) and High Throughput Computing (HTC) has increased significantly in recent years, and is becoming sensitive to total energy use and cost. Energy-efficiency has thus become an important concern in scientific fields such as High Energy Physics (HEP). There has been a growing interest in utilizing alternate architectures, such as low power ARM processors, to replace traditional Intel x86 architectures. Nevertheless, even though such solutions have been successfully used in mobile applications with low I/O and memory demands, it is unclear if they are suitable and more energy-efficient in the scientific computing environment. Furthermore, there is a lack of tools and experience to derive and compare power consumption between the architectures for various workloads, and eventually to support software optimizations for energy efficiency. To that end, we have performed several physical and software-based measurements of workloads from HEP applications running on ARM and Intel architectures, and compare their power consumption and performance. We leverage several profiling tools (both in hardware and software) to extract different characteristics of the power use. We report the results of these measurements and the experience gained in developing a set of measurement techniques and profiling tools to accurately assess the power consumption for scientific workloads.

cs.DC↗

arXiv2012

GPU in Physics Computation: Case Geant4 Navigation

General purpose computing on graphic processing units (GPU) is a potential method of speeding up scientific computation with low cost and high energy efficiency. We experimented with the particle physics simulation toolkit Geant4 used at CERN to benchmark its geometry navigation functionality on a GPU. The goal was to find out whether Geant4 physics simulations could benefit from GPU acceleration and how difficult it is to modify Geant4 code to run in a GPU. We ported selected parts of Geant4 code to C99 & CUDA and implemented a simple gamma physics simulation utilizing this code to measure efficiency. The performance of the program was tested by running it on two different platforms: NVIDIA GeForce 470 GTX GPU and a 12-core AMD CPU system. Our conclusion was that GPUs can be a competitive alternate for multi-core computers but porting existing software in an efficient way is challenging.

physics.comp-ph↗