arXiv subjects

Masahiro Nomura

Publications and source records attributed to Masahiro Nomura.

69 records · Page 4Linked to original sources

arXiv2020

Self-synchronization of thermal phonons at equilibrium

Self-synchronization is a ubiquitous phenomenon in nature, in which oscillators are collectively locked in frequency and phase through mutual interactions. While self-synchronization requires the forced excitation of at least one of the oscillators, we demonstrate that this mechanism spontaneously appears due to the activation from thermal fluctuations. By performing molecular dynamic simulations, we demonstrate the self-synchronization of thermal phonons in a platform supporting doped silicon resonators. We find that thermal phonons are spontaneously converging to the same frequency and phase. In addition, the dependencies to intrinsic frequency difference and coupling strength agree well with the Kuramoto model predictions. More interestingly, we find that a balance between energy dissipation resulting from phonon-phonon scattering and potential energy between oscillators is required to maintain synchronization. Finally, a wavelet transform approach corroborates the generation of coherent thermal phonons in the collective state of oscillators. Our study provides a new perspective on self-synchronization and on the relationship between fluctuations and coherence.

cond-mat.mes-hall↗

arXiv2020

A generalized decay law for particle- and wave-like thermal phonons

Our direct atomic simulations reveal that a thermally activated phonon mode involves a large population of elastic wavepackets. These excitations are characterized by a wide distribution of lifetimes and coherence times expressing particle- and wave-like natures. In agreement with direct simulations, our theoretical derivation yields a generalized law for the decay of the phonon number taking into account coherent effects. Before the conventional exponential decay due to phonon-phonon scattering, this law introduces a delay proportional to the square of the coherence time. This additional regime leads to a moderate increase in relaxation times and thermal conductivity. This work opens new horizons in the understanding of the origin and the treatment of thermal phonons.

cond-mat.mes-hall↗

arXiv2019

A Simple Heuristic for Bayesian Optimization with A Low Budget

The aim of black-box optimization is to optimize an objective function within the constraints of a given evaluation budget. In this problem, it is generally assumed that the computational cost for evaluating a point is large; thus, it is important to search efficiently with as low budget as possible. Bayesian optimization is an efficient method for black-box optimization and provides exploration-exploitation trade-off by constructing a surrogate model that considers uncertainty of the objective function. However, because Bayesian optimization should construct the surrogate model for the entire search space, it does not exhibit good performance when points are not sampled sufficiently. In this study, we develop a heuristic method refining the search space for Bayesian optimization when the available evaluation budget is low. The proposed method refines a promising region by dividing the original region so that Bayesian optimization can be executed with the promising region as the initial search space. We confirm that Bayesian optimization with the proposed method outperforms Bayesian optimization alone and shows equal or better performance to two search-space division algorithms through experiments on the benchmark functions and the hyperparameter optimization of machine learning algorithms.

stat.ML↗

arXiv2019

Surface phonon-polaritons enhance thermal conduction in SiN nanomembranes

Surface phonon-polaritons can carry energy on the surface of dielectric films and thus are expected to contribute to heat conduction. However, the contribution of surface phonon-polaritons (SPhPs) to thermal transport has not been experimentally demonstrated yet. In this work, we experimentally measure the effective in-plane thermal conductivity of amorphous silicon nitride membrane and show that it can indeed be increased by SPhPs significantly when the membrane thickness scales down. In particular, by heating up a thin membrane (<100 nm) from 300 to 800 K, the thermal conductivity increases twice due to SPhPs contribution.

cond-mat.mes-hall↗

arXiv2019

Enhanced thermal sensitivity of MEMS bolometers integrated with two-dimensional phononic crystals

We have fabricated two-dimensional phononic crystal (PnC) structures on GaAs doubly-clamped microelectromechanical system (MEMS) beam resonators to modulate their thermal properties. Owing to the reduction in the thermal conductance of the MEMS beams by introducing the PnC structures, the MEMS bolometers with the PnC structures show 2-3 times larger thermal sensitivities than the unpatterned reference sample. Furthermore, since the heat capacitance of the MEMS beams is also reduced by introducing the PnCs, the thermal decay time of the patterned MEMS beams is increased only by about 30-40 %, demonstrating the effectiveness of the PnCs for enhancing the thermal sensitivities of bolometers without significantly deteriorating their operation bandwidths.

physics.app-ph↗

arXiv2018

Quasi-ballistic heat conduction due to Lévy phonon flights in silicon nanowires

Future of silicon-based microelectronics relies on solving the heat dissipation problem. A solution may lie in a nanoscale phenomenon known as ballistic heat conduction, which implies heat conduction without heating the conductor. But, attempts to demonstrate this phenomenon experimentally are controversial and scarce whereas its mechanism in confined nanostructures is yet to be fully understood. Here, we experimentally demonstrate quasi-ballistic heat conduction in silicon nanowires (NWs). We show that the ballisticity is strongest in short NWs at low temperatures but weakens as the NW length or temperature is increased. Yet, even at room temperature, quasi-ballistic heat conduction remains visible in short NWs. To better understand this phenomenon, we probe directionality and lengths of phonon flights. Our experiments and simulations show that the quasi-ballistic phonon transport in NWs is the Lévy walk with short flights between the NW boundaries and long ballistic leaps along the NW.

cond-mat.mes-hall↗

arXiv2016

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures

Unlike classical heat diffusion at the macroscale, nanoscale heat transport can occur without energy dissipation because phonons can travel in straight lines for hundreds of nanometres. Despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a way to control the directionality of ballistic phonon transport using silicon thin-films with arrays of holes. First, we demonstrate the formation of directional heat fluxes in the passages between the holes. Next, we use these nanostructures as a directional source of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce a nanoscale thermal lens in which the phonons converge at a focal point, thus focusing heat into a spot of a few hundred nanometres. These results provide a basis for ray-like heat manipulations that enable nanoscale heat guiding, dissipation, localization, confinement and rectification.

cond-mat.mes-hall↗

arXiv2016

Heat conduction tuning using the wave nature of phonons

The world communicates to our senses of vision, hearing and touch in the language of waves, as the light, sound, and even heat essentially consist of microscopic vibrations of different media. The wave nature of light and sound has been extensively investigated over the past century and is now widely used in modern technology. But the wave nature of heat has been the subject of mostly theoretical studies, as its experimental demonstration, let alone practical use, remains challenging due to the extremely short wavelengths of these waves. Here we show a possibility to use the wave nature of heat for thermal conductivity tuning via spatial short-range order in phononic crystal nanostructures. Our experimental and theoretical results suggest that interference of thermal phonons occurs in strictly periodic nanostructures and slows the propagation of heat. This finding broadens the methodology of heat transfer engineering by expanding its territory to the wave nature of heat.

cond-mat.mes-hall↗

arXiv2016

Reduction of thermal conductance by coherent phonon scattering in two-dimensional phononic crystals of different lattice types

The impact of lattice type, period, porosity and thickness of two-dimensional silicon phononic crystals on the reduction of thermal conductance by coherent modification of phonon dispersion is investigated using the theory of elasticity and finite element method. Increase in the period and porosity of the phononic crystal affects the group velocity and phonon density of states and, as a consequence, reduces the in-plane thermal conductance of the structure as compared to unpatterned membrane. This reduction does not depend significantly on the lattice type and thickness of phononic crystals. Moreover, the reduction is strongly temperature dependent and strengthens as the temperature is increased.

cond-mat.mes-hall↗

arXiv2015

Crystal structure dependent thermal conductivity in two-dimensional phononic crystal nanostructures

Thermal phonon transport in square- and triangular-lattice Si phononic crystal (PnC) nanostructures with a period of 300 nm was investigated by measuring the thermal conductivity using micrometer-scale time-domain thermoreflectance. The placement of circular nanoholes has a strong influence on thermal conductivity when the periodicity is within the range of the thermal phonon mean free path. A staggered hole structure, i.e., a triangular lattice, has lower thermal conductivity, where the difference in thermal conductivity depends on the porosity of the structure. The largest difference in conductivity of approximately 20% was observed at a porosity of around 30%. This crystal structure dependent thermal conductivity can be understood by considering the local heat flux disorder created by a staggered hole structure. Numerical simulation using the Monte Carlo technique was also employed and also showed the lower thermal conductivity for a triangular lattice structure. Besides gaining a deeper understanding of nanoscale thermal phonon transport, this information would be useful in the design of highly efficient thermoelectric materials created by nanopatterning.

cond-mat.mes-hall↗

arXiv2010

Zero-cell photonic crystal nanocavity laser with quantum dot gain

We demonstrate laser oscillation in a hexagonal-lattice photonic crystal nanocavity using an InGaAs quantum dot gain material by optical pumping at 5 K. The cavity comprises a defect created by shifting several air holes in a two-dimensional photonic crystal slab structure without removing any air holes to achieve both small mode volume and a high cavity quality factor. The measured cavity quality factors and estimated mode volume for the nanocavity are ~33,000 and ~0.004 um^3. The laser threshold is compared between the zero-cell and L3-type nanocavity lasers, and the zero-cell nanolasers are found to have small thresholds of about one-third of the L3-type nanolasers. This result suggests that a higher Purcell factor of the zero-cell nanolaser is reflected as a smaller laser threshold.

cond-mat.other↗

arXiv2010

Lasing oscillation in a three-dimensional photonic crystal nanocavity with a complete bandgap

We demonstrate lasing oscillation in a three-dimensional photonic crystal nanocavity. The laser is realized by coupling a cavity mode, which is localized in a complete photonic bandgap and exhibits the highest quality factor of ~38,500, with high-quality semiconductor quantum dots. We show a systematic change in the laser characteristics, including the threshold and the spontaneous emission coupling factor by controlling the crystal size, which consequently changes the strength of photon confinement in the third dimension. This opens up many interesting possibilities for realizing future ultimate light sources and three-dimensional integrated photonic circuits and for more fundamental studies of physics in the field of cavity quantum electrodynamics.

quant-ph↗

arXiv2010

Circularly-Polarized Light Emission from Semiconductor Planar Chiral Photonic Crystal

We proposed and demonstrated a scheme of surface emitting circularly polarized light source by introducing strong imbalance between left- and right-circularly polarized vacuum fields in an on-waveguide chiral grating structure. We observed circularly polarized spontaneous emission from InAs quantum dots embedded in the wave guide region of a GaAs-based structure. Obtained degree of polarization reaches as large as 25% at room temperature. Numerical calculation visualizes spatial profiles of the modification of vacuum field modes inside the structure with strong circular anisotropy.

cond-mat.mes-hall↗

arXiv2009

Investigation of the spectral triplet in strongly coupled quantum dot-nanocavity system

We experimentally investigated the excitation power dependence of a strongly coupled quantum dot (QD)-photonic crystal nanocavity system by photoluminescence (PL) measurements. At a low-excitation power regime, we observed vacuum Rabi doublet emission at QD-cavity resonance condition. With increasing excitation power, in addition to the doublet, a third emission peak appeared. This observed spectral change is unexpected from conventional atomic cavity quantum electrodynamics. The observations can be attributed to featured pumping processes in the semiconductor QD-cavity system.

cond-mat.other↗

arXiv2009

Photonic crystal nanocavity laser with a single quantum dot gain

We demonstrate a photonic crystal nanocavity laser essentially driven by a self-assembled InAs/GaAs single quantum dot gain. The investigated nanocavities contain only 0.4 quantum dots on an average; an ultra-low density quantum dot sample (1.5 x 108 cm-2) is used so that a single quantum dot can be isolated from the surrounding quantum dots. Laser oscillation begins at a pump power of 42 nW under resonant condition, while the far-detuning conditions require ~145 nW for lasing. This spectral detuning dependence of laser threshold indicates substantial contribution of the single quantum dot to the total gain. Moreover, photon correlation measurements show a distinct transition from anti-bunching to Poissonian via bunching with the increase of the excitation power, which is also an evidence of laser oscillation with the single quantum dot.

cond-mat.other↗