Yu-Chieh Wen

Measuring plasmon-resonance enhanced third-harmonic χ(3) of Ag nanoparticles

By coinciding the plasmon frequency with the third-harmonic frequency of the excitation light, the authors determined the plasmon-resonance enhanced optical third-harmonic-generation (THG) susceptibility of a polyvinylpyrrolidone-coated Ag nanoparticle with a 5–7nm diameter. With dispersed Ag nanoparticles on a quartz surface and through measuring the frequency dependent THG intensities, interface THG showed evident enhancement when the third harmonic of excitation matched the Ag-nanoparticle’s plasmon-resonant frequency. According to the effective medium theory and by analyzing the interface THG under focused Gaussian beams, the ensemble-averaged χ(3)(3ω:ω,ω,ω) of a Ag nanoparticle can be estimated to be on the order of 2×10−11esu.

Ultrafast carrier thermalization in InN

Carrier thermalization dynamics in heavily doped n-type InN was investigated at room temperature with a femtosecond transient transmission measurement. The dependence of hot carrier cooling time on the total electron density indicates that the plasma screening of electron-LO phonon interactions is responsible for the reduction in energy-loss rate. Under low excitation, the carriers with different excess energies can be found to cool down with a fixed thermalization time of 1.4ps. Intensity dependent study suggests that this relatively slow carrier cooling time could be attributed to the screening effect by high-background-doping plasma rather than the hot phonon effect.

Generation of picosecond acoustic pulses using a p‐n junction with piezoelectric effects

We demonstrate the generation of picosecond acoustic pulses using a piezoelectric-semiconductor-based p‐n junction structure. This p‐n junction picosecond ultrasonic experiment confirms that the piezoelectric effect dominates the thermal expansion and deformation-potential coupling in the generation of picosecond acoustic pulses. The characteristics of the p‐n initiated acoustic pulses are determined by the width and the field strength inside the depletion region. Our study indicates the future possibility to electrically control the acoustic pulse characteristics if we could apply an external bias to modulate the depletion region width.

Probing Hydrophilic Interface of Solid/Liquid-Water by Nanoultrasonics

Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

Gap opening and orbital modification of superconducting FeSe above the structural distortion.

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a ~36 meV energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

Three-dimensional phononic nanocrystal composed of ordered quantum dots

We demonstrated a nanoscaled artificial phononic crystal composed of three-dimensionally ordered quantum dots (QDs) with functional acoustic properties. Femtosecond ultrasonic technique is used to investigate the lattice dynamics of this phononic nanocrystal. The measurement results indicate that three-dimensional ordering and uniformity of the QDs are important factors influencing the observed acoustic resonance at the forbidden bands. For well-arranged QDs, noticeable features of the phononic band gap and the associated phonon cavity mode can be found, while this nanocrystal also serves as an effective acoustic medium, or acoustic meta material, for low-frequency acoustic phonons.

Observation of femtosecond carrier thermalization time in indium nitride

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm−3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples.

Efficient generation of coherent acoustic phonons in (111) InGaAs∕GaAs multiple quantum wells through piezoelectric effects

The authors demonstrate the generation of coherent acoustic phonons in (111)-strained InGaAs∕GaAs multiple quantum wells through piezoelectric coupling by an optical pump-probe technique. Dependence of the induced strain amplitude on the photocarrier density reveals that coherent acoustic phonons can be efficiently excited by the piezoelectric effect with an unscreened built-in electric field. Under high fluence excitation, the generation of coherent acoustic phonons saturates. The experimental results are supported by a macroscopic theoretical model. No backward Raman signal and no zone-folded phonon modes were observed in this low impedance-mismatch and finite-period heterostructure.

Selectively probing vibrations in a plasmonic supracrystal

The coupling of plasmonic resonances with the multiple phonon modes of a plasmonic supracrystal is studied. Ultrafast optical pump-probe spectroscopy with variable wavelength allows the selective detection of the breathing mode, the interparticle vibrations, and the vibration of the whole structure. Thanks to this selectivity, the characterization of the bonding strength between nanoparticles in different directions of the supracrystal is possible. The observation of these vibrations could be useful for the realization of future phonon modulated photonic and plasmonic devices.

Femtosecond ultrasonic spectroscopy using a piezoelectric nanolayer: Hypersound attenuation in vitreous silica films

We report ultra-broadband ultrasonic spectroscopy with an impedance-matched piezoelectric nanolayer, which enables optical generation and detection of a 730-fs acoustic pulse (the width of ten lattice constants). The bandwidth improvement facilitates THz laser ultrasonics to bridge the spectral gap between inelastic light and x-ray scatterings (0.1-1 THz) in the studies of lattice dynamics. As a demonstration, this method is applied to measure sound attenuation α in a vitreous SiO2 thin film. Our results extend the existing low-frequency data obtained by ultrasonic-based and light scattering methods and also show a α∝ f2 behavior for frequencies f up to 650 GHz.