Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

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We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

The importance of the Fano resonance concept is recognized across multiple fields of physics. In this Review, Fano resonance is explored in the context of optics, with particular emphasis on dielectric nanostructures and metasurfaces. Rapid progress in photonics and nanotechnology brings many examples of resonant optical phenomena associated with the physics of Fano resonances, with applications in optical switching and sensing. For successful design of photonic devices, it is important to gain deep insight into different resonant phenomena and understand their connection. Here, we review a broad range of resonant electromagnetic effects by using two effective coupled oscillators, including the Fano resonance, electromagnetically induced transparency, Kerker and Borrmann effects, and parity–time symmetry breaking. We discuss how to introduce the Fano parameter for describing a transition between two seemingly different spectroscopic signatures associated with asymmetric Fano and symmetric Lorentzian shapes. We also review the recent results on Fano resonances in dielectric nanostructures and metasurfaces.

Fano resonances in photonics

We report the frequency-dependent optical constants, n(ν) and α(ν), or, equivalently, the complex permittivity e(ω) = e‘(ω) − ie‘‘(ω), over the frequency range from 2 to 50 cm-1 for water, methanol, ethanol, 1-propanol, and liquid ammonia. These spectra have been measured with femtosecond terahertz pulse transmission spectroscopy. These liquids exhibit multiple-Debye behavior, making their frequency-dependent dielectric constants valuable benchmarks for molecular dynamics simulations and other theoretical treatments of liquids.

Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy

We have investigated the dynamical properties of the coherent anharmonic phonons generated in Bi under high density excitation. The time-resolved reflectivity in the intensely photoexcited Bi film is modulated by the coherent A(1g) phonon oscillation with a time-dependent oscillation period. As the pump power density is increased, the line shape of the A(1g) mode in the Fourier transformed spectra becomes asymmetric, and the redshift of the phonon frequency is observed. Analysis of the transient redshift with a wavelet transform reveals that the frequency of the A(1g) mode depends on the squared amplitude of the oscillation, which is attributed to an anharmonicity of the lattice potential.

Dynamics of Coherent Anharmonic Phonons in Bismuth Using High Density Photoexcitation

Interference of impulsively excited coherent phonons in semimetals has been studied by using a double‐pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double‐pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro‐optic sampling.

Optical control of coherent optical phonons in bismuth films

Depolarized Raman spectra below 250 cm−1 in water and heavy water were measured and analyzed from 373 K to the supercooled region The spectral feature of the central component below 20 cm−1 is stressed in the present work The spectra below 250 cm−1 in water and heavy water are interpreted as a superposition of one Debye‐type relaxation mode and two damped harmonic oscillators The damped harmonic oscillators (broadbands around 60 cm−1 and 190 cm−1) are interpreted as the restricted translational modes Analyzing the temperature dependence of the relaxation mode, the reciprocal relaxation time τ−1 in heavy water changes linearly with τ−1 ∝ (T − 240 K) in the whole temperature range On the other hand, the temperature dependence of the reciprocal relaxation time in water above 303 K deviates from a straight line which holds below 298 K as τ−1 ∝ (T − 225 K)

Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy

Coherent phonon spectra of bismuth measured at temperatures from 10 to 295 K by a femtosecond pump-probe reflectivity technique were compared with the corresponding Raman-scattering spectra. The decay rate and frequency show almost the same behavior for the coherent and incoherent phonons. This result indicates that the decay process of the coherent optical phonons is dominated by the anharmonic decay (energy relaxation) and that the contribution of the pure dephasing is negligibly small.

Dynamics of coherent phonons in bismuth generated by ultrashort laser pulses

Local crystallographic orientations of semiconductors have been determined by a scanning Raman microprobe combined with polarization measurements. We present a method of Raman scattering determination of crystallographic orientations in silicon crystals which allows rapid determination with an accuracy of ±2°. It is found that surface morphology affects the Raman polarization analysis. For laser‐annealed silicon layers capped with silicon nitride films, unpolarized scattered light is superimposed on polarized scattered light, because incident light beams entering rough surfaces are directed into random orientations and produce the unpolarized scattered light. The degree of the surface roughness is estimated by assuming that the rough surface consists of periodically arranged spherical segments.

Kohji Mizoguchi

Papers

Dynamics of Coherent Anharmonic Phonons in Bismuth Using High Density Photoexcitation

Optical control of coherent optical phonons in bismuth films

Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy

Dynamics of coherent phonons in bismuth generated by ultrashort laser pulses

Determination of crystallographic orientations in silicon films by Raman‐microprobe polarization measurements