The Experts below are selected from a list of 300 Experts worldwide ranked by ideXlab platform

Fumio Koyama - One of the best experts on this subject based on the ideXlab platform.

  • AThermal and widely tunable VCSEL with bimorph micromachined mirror
    Optics express, 2014
    Co-Authors: Masanori Nakahama, Takahiro Sakaguchi, Akihiro Matustani, Fumio Koyama
    Abstract:

    We demonstrate an aThermal and electrostatically-tunable 850 nm-band MEMS VCSEL for the first time. The Thermal Wavelength drift is compensated by the Thermal actuation of a cantilever-suspended mirror with a bimorph effect. At the same time, the resonant Wavelength can be continuously tuned by electro-static force as a voltage is applied in the cantilever structure. A continuous Wavelength tuning of 10 nm is obtained with a low Thermal Wavelength drift, which is 10 times smaller than that of conventional VCSELs. Our aThermal and tunable VCSELs enable us to reduce the channel spacing in course Wavelength division multiplexing optical interconnects even under uncooled operations.

  • AThermalization and on-chip multi-Wavelength integration of VCSELs employing Thermally actuated micromachined mirrors
    Applied Physics Letters, 2014
    Co-Authors: Masanori Nakahama, Takahiro Sakaguchi, Akihiro Matsutani, Fumio Koyama
    Abstract:

    An aThermal multi-Wavelength vertical cavity surface emitting laser (VCSEL) array is demonstrated using a Thermally actuated cantilever structure with different cantilever lengths. The cavity length of each VCSEL is precisely controlled via the deflection of the cantilever due to difference in the lattice constant of GaAlAs layers. Also, the Thermally induced actuation of the cantilever reduces the cavity length as the ambient temperature increases, which compensates the Thermal Wavelength drift of the VCSEL. The Wavelength drift could be reduced within ±0.017 nm/K, which is 4 times smaller than that of conventional VCSELs. The proposed multi-Wavelength VCSEL array enables four Wavelength channels with 2.5 nm spacing under uncooled operations.

  • Tuning Characteristics of Monolithic MEMS VCSELs With Oxide Anti-Reflection Layer
    IEEE Photonics Technology Letters, 2013
    Co-Authors: Masanori Nakahama, Takahiro Sakaguchi, Hayato Sano, Shunya Inoue, Akihiro Matsutani, Fumio Koyama
    Abstract:

    We demonstrate a micromachined tunable vertical-cavity surface-emitting laser with a monolithically formed anti-reflection (AR) layer for highly efficient electro-Thermal Wavelength tuning. The AR layer is formed by the lateral oxidation of an aluminum-rich AlGaAs layer and no extra process is needed. We fabricated a micromachined GaAs vertical-cavity surface-emitting laser and measured its Wavelength tuning characteristic. The measurement result shows that the oxide layer functions as an AR layer for linear and efficient tuning characteristics. A large negative temperature dependence of Wavelength from -0.24 to -2.0 nm/K was demonstrated using a Thermally actuated cantilever for efficient electro-Thermal Wavelength tuning. An electro-Thermal tuning efficiency of -2.3 to -4.8 nm/mW was obtained.

  • Electro-Thermal Wavelength tuning of 1.2 /spl mu/m GaInAs/GaAs vertical cavity surface emitting laser array
    2005 IEEE LEOS Annual Meeting Conference Proceedings, 2005
    Co-Authors: Yasuhiro Uchiyama, Akihiro Matsutani, Takashi Kondo, Takeda Kazutaka, T. Uchida, Tomoyuki Miyamoto, Fumio Koyama
    Abstract:

    This paper demonstrates an electro-Thermally tunable VCSEL array with an additional tuning electrode. The VCSEL array achieves continuous Wavelength tuning of approximately 3 nm with a single-tuning-contact. The deviation in output power is within 1 dB. The power consumption for Wavelength tuning is as low as 46 mW. Fast tuning response is expected in the proposed tunable VCSEL because of the small Thermal capacity.

  • Micromachined tunable filters using stress control of multilayer semiconductor mirrors
    Integrated Optics: Devices Materials and Technologies VII, 2003
    Co-Authors: Fumio Koyama, Takeru Amano
    Abstract:

    We proposed and demonstrated a micromachined filter with a strain control layer, which gives us novel functions including temperature insensitive operation, Thermal Wavelength tuning, Wavelength trimming and 2-D multi-Wavelength integration. In this paper, we present the design and the fabrication of micromachined Thermally tunable filters with a low tuning voltage. In our micromachined micromachined filter, an air gap is formed between GaAlAs/GaAs DBRs with an upper DBR mirror freely suspended above the substrate by a cantilever structure. A novelty in our devices is to add a GaAs or GaAlAs Thermal strain control layer on the upper DBR. We can freely control the temperature dependence of the proposed MEMS cavity. Either temperature insensitive operation or wide Wavelength tuning induced by temperature change can be realized. Also, we fabricated a micromachined Thermally tunable filter with a heating element. There are two electrodes integrated on the top p-type doped strain control layer of this filter for heating the cantilever. When a voltage is applied between the two electrodes resulting in heating, the micromachined cantilever moves due to Thermal strain. The proposed structure enables Thermal Wavelength tuning either for red shift or blue shift. The amount of Wavelength tuning is controlled by the length and the Thermal capacity of the cantilever. We can expect much lower tuning voltage than conventional electrostatic force tuning scheme. We measured the tuning characteristics of fabricated filters with changing an applied voltage between two electrodes. We could obtain blue-shift Wavelength tuning of over 50 nm with an applied voltage of 6 V.

Kamran Behnia - One of the best experts on this subject based on the ideXlab platform.

  • Heavy Nondegenerate Electrons in Doped Strontium Titanate
    Physical Review X, 2020
    Co-Authors: Clément Collignon, Phillipe Bourges, Benoît Fauqué, Kamran Behnia
    Abstract:

    Room-temperature metallicity of lightly doped SrTiO3 is puzzling, because the combination of mobility and the effective mass would imply a mean-free path below the Mott-Ioffe-Regel limit and a scattering time shorter than the Planckian time (τP=ℏ/kBT). We present a study of electric resistivity, Seebeck coefficient, and inelastic neutron scattering extended to very high temperatures, which deepens the puzzle. Metallic resistivity persists up to 900 K and is accompanied by a large Seebeck coefficient whose magnitude (as well as its temperature and doping dependence) indicates that carriers are becoming heavier with rising temperature. Combining this with neutron scattering data, we find that between 500 and 900 K the Bohr radius and the electron Wavelength become comparable to each other and twice the lattice parameter. According to our results, between 100 and 500 K, metallicity is partially driven by temperature-induced amplification of the carrier mass. We contrast this mass amplification of nondegenerate electrons with the better-known case of heavy degenerate electrons. Above 500 K, the mean-free path continues to shrink with warming in spite of becoming shorter than both the interatomic distance and the Thermal Wavelength of the electrons. The latter saturates to twice the lattice parameter. Available theories of polaronic quasiparticles do not provide satisfactory explanation for our observations.

  • Thermal Transport and Phonon Hydrodynamics in Strontium Titanate.
    Physical review letters, 2018
    Co-Authors: V. Martelli, Julio Antonio Larrea Jiménez, Mucio A. Continentino, Elisa Baggio-saitovitch, Kamran Behnia
    Abstract:

    We present a study of Thermal conductivity, κ, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow. In undoped SrTiO_{3}, κ evolves faster than cubic with temperature below its peak and in a narrow temperature window. Such behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones. The effect disappears in the presence of dopants. In SrTi_{1-x}Nb_{x}O_{3}, a significant reduction in lattice Thermal conductivity starts below the temperature at which the average inter-dopant distance and the Thermal Wavelength of acoustic phonons become comparable. In the high-temperature regime, Thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time (τ_{p}=(ℏ/k_{B}T)).

Austin Jerome Minnich - One of the best experts on this subject based on the ideXlab platform.

  • Semiconductor-based Multilayer Selective Solar Absorber for Unconcentrated Solar Thermal Energy Conversion
    Scientific Reports, 2017
    Co-Authors: Nathan H. Thomas, Zhen Chen, Shanhui Fan, Austin Jerome Minnich
    Abstract:

    Solar Thermal energy conversion has attracted substantial renewed interest due to its applications in industrial heating, air conditioning, and electricity generation. Achieving stagnation temperatures exceeding 200 °C, pertinent to these technologies, with unconcentrated sunlight requires spectrally selective absorbers with exceptionally low emissivity in the Thermal Wavelength range and high visible absorptivity for the solar spectrum. In this Communication, we report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the bandgap energy to achieve a measured absorptance of 76% at solar Wavelengths and a low emittance of approximately 5% at Thermal Wavelengths. In field tests, we obtain a peak temperature of 225 °C, comparable to that achieved with state-of-the-art selective surfaces. With straightforward optimization to improve solar absorption, our work shows the potential for unconcentrated solar Thermal systems to reach stagnation temperatures exceeding 300 °C, thereby eliminating the need for solar concentrators for mid-temperature solar applications such as supplying process heat.

Karl Joulain - One of the best experts on this subject based on the ideXlab platform.

  • VO2-based radiative Thermal transistor with a semi-transparent base
    Journal of Quantitative Spectroscopy and Radiative Transfer, 2018
    Co-Authors: Hugo Prod'homme, Younès Ezzahri, Jose Ordonez-miranda, Jérémie Drevillon, Karl Joulain
    Abstract:

    We study a radiative Thermal transistor analogous to an electronic one made of a VO2 base placed between two silica semi-infinite plates playing the roles of the transistor collector and emitter. The fact that VO2 exhibits an insulator to metal transition is exploited to modulate and/or amplify heat fluxes between the emitter and the collector, by applying a Thermal current on the VO2 base. We extend the work of precedent studies considering the case where the base can be semi-transparent so that heat can be exchanged directly between the collector and the emitter. Both near and far field cases are considered leading to 4 typical regimes resulting from the fact that the emitter-base and base-collector separation distances can be larger or smaller than the Thermal Wavelength for a VO2 layer opaque or semi-transparent. Thermal currents variations with the base temperatures are calculated and analyzed. It is found that the transistor can operate in an amplification mode as already stated in [1] or in a switching mode as seen in [2]. An optimum configuration for the base thickness and separation distance maximizing the Thermal transistor modulation factor is found.

  • VO 2 -based radiative Thermal transistor in the static regime
    arXiv: Optics, 2017
    Co-Authors: Hugo Prod'homme, Younès Ezzahri, Jose Ordonez-miranda, Jérémie Drevillon, Karl Joulain
    Abstract:

    We study a near-field radiative Thermal transistor analogous to an electronic one made of a VO 2 base placed between two silica semi-infinite plates playing the roles of the transistor collector and emitter. The fact that VO 2 exhibits an insulator to metal transition is exploited to modulate and/or amplify heat fluxes between the emitter and the collector, by applying a Thermal current on the VO 2 base. We study the transistor behavior in 4 typical regimes where the emitter-base and base-collector separation distances can be larger or smaller than the Thermal Wavelength, and in which the VO 2 layer can be opaque or transparent. Thermal currents variations with the base temperatures are calculated and analyzed. An optimum configuration for base thickness and separation distance maximizing the Thermal transistor modulation factor is found.

  • Thermal Emission by a SubWavelength Aperture
    Journal of Quantitative Spectroscopy & Radiative Transfer, 2016
    Co-Authors: Karl Joulain, Younès Ezzahri, Remi Carminati
    Abstract:

    We calculate, by means of fluctuational electrodynamics, the Thermal emission of an aperture filled by vacuum or a material at temperature T. We show that Thermal emission is very different whether the aperture size is large or small compared to the Thermal Wavelength. SubWavelength apertures filled with vacuum (subWavelength blackbody) have their Thermal emission strongly decreased compared to classical blackbodies. A simple expression of their emissivity can be calculated and their total emittance scales as T 8 instead of T 4 for large apertures. Thermal emission of disk of materials with a size comparable to the Wavelength is also discussed. It is shown in particular that emissivity of such a disk is increased when the material can support surface waves such as phonon polaritons.

  • Thermal emission by a subWavelength aperture
    Journal of Quantitative Spectroscopy and Radiative Transfer, 2016
    Co-Authors: Karl Joulain, Younès Ezzahri, Remi Carminati
    Abstract:

    International audienceWe calculate, by means of fluctuational electrodynamics, the Thermal emission of an aperture filled by vacuum or a material at temperature T. We show that Thermal emission is very different whether the aperture size is large or small compared to the Thermal Wavelength. SubWavelength apertures filled with vacuum (subWavelength blackbody) have their Thermal emission strongly decreased compared to classical blackbodies. A simple expression of their emissivity can be calculated and their total emittance scales as T 8 instead of T 4 for large apertures. Thermal emission of disk of materials with a size comparable to the Wavelength is also discussed. It is shown in particular that emissivity of such a disk is increased when the material can support surface waves such as phonon polaritons

  • Radiative Thermal Rectification between SiC and SiO2
    Optics Express, 2015
    Co-Authors: Karl Joulain, Younès Ezzahri, Jérémie Drevillon, Benoit Rousseau, Domingos De Sousa Meneses
    Abstract:

    By means of fluctuationnal electrodynamics, we calculate radiative heat flux between two pla-nar materials respectively made of SiC and SiO2. More specifically, we focus on a first (direct) situation where one of the two materials (for example SiC) is at ambient temperature whereas the second material is at a higher one, then we study a second (reverse) situation where the material temperatures are inverted. When the two fluxes corresponding to the two situations are different, the materials are said to exhibit a Thermal rectification, a property with potential applications in Thermal regulation. Rectification variations with temperature and separation distance are here reported. Calculations are performed using material optical data experimentally determined by Fourier transform emission spectrometry of heated materials between ambient temperature (around 300 K) and 1480 K. It is shown that rectification is much more important in the near-field domain, i.e. at separation distances smaller than the Thermal Wavelength. In addition, we see that the larger is the temperature difference, the larger is rectification. Large rectification is finally interpreted due to a weakening of the SiC surface polariton when temperature increases, a weakening which affects much less SiO2 resonances.

Thorsten Emig - One of the best experts on this subject based on the ideXlab platform.

  • Temperature distribution and heat radiation of patterned surfaces at short Wavelengths.
    Physical review. E, 2017
    Co-Authors: Thorsten Emig
    Abstract:

    We analyze the equilibrium spatial distribution of surface temperatures of patterned surfaces. The surface is exposed to a constant external heat flux and has a fixed internal temperature that is coupled to the outside heat fluxes by finite heat conductivity across the surface. It is assumed that the temperatures are sufficiently high so that the Thermal Wavelength (a few microns at room temperature) is short compared to all geometric length scales of the surface patterns. Hence the radiosity method can be employed. A recursive multiple scattering method is developed that enables rapid convergence to equilibrium temperatures. While the temperature distributions show distinct dependence on the detailed surface shapes (cuboids and cylinder are studied), we demonstrate robust universal relations between the mean and the standard deviation of the temperature distributions and quantities that characterize overall geometric features of the surface shape.

  • Temperature distribution and heat radiation of patterned surfaces at short Wavelengths.
    Physical Review E, 2017
    Co-Authors: Thorsten Emig
    Abstract:

    We analyze the equilibrium spatial distribution of surface temperatures of patterned surfaces. The surface is exposed to a constant external heat flux and has a fixed internal temperature that is coupled to the outside heat fluxes by finite heat conductivity across surface. It is assumed that the temperatures are sufficiently high so that the Thermal Wavelength (a few microns at room temperature) is short compared to all geometric length scales of the surface patterns. Hence the radiosity method can be employed. A recursive multiple scattering method is developed that enables rapid convergence to equilibrium temperatures. While the temperature distributions show distinct dependence on the detailed surface shapes (cuboids and cylinder are studied), we demonstrate robust universal relations between the mean and the standard deviation of the temperature distributions and quantities that characterize overall geometric features of the surface shape.

  • Temperature Distribution and Heat Radiation of Patterned Surfaces at Short Wave Lengths
    Physical Review E, 2017
    Co-Authors: Thorsten Emig
    Abstract:

    We analyze the equilibrium spatial distribution of surface temperatures of patterned surfaces. The surface is exposed to a constant external heat flux and has a fixed internal temperature that is coupled to the outside heat fluxes by finite heat conductivity across surface. It is assumed that the temperatures are sufficiently high so that the Thermal Wavelength (a few microns at room temperature) is short compared to all geometric length scales of the surface patterns. Hence the radiosity method can be employed. A recursive multiple scattering method is developed that enables rapid convergence to equilibrium temperatures. While the temperature distributions show distinct dependence on the detailed surface shapes (cuboids and cylinder are studied), we demonstrate robust universal relations between the mean and the standard deviation of the temperature distributions and quantities that characterize overall geometric features of the surface shape.