Hsiang Lin Liu

Infrared and Raman-scattering studies in single-crystalline GaN nanowires

Abstract Infrared and Raman-scattering studies of high-purity and -quality GaN nanowires are presented. The nanosize dependences of the peak shift and the broadening of the four first-order Raman modes agree with those calculated on the basis of the phonon confinement model. Additionally, the appearance of one Raman mode at ∼ 254 cm −1 is attributed to zone-boundary phonon activated by surface disorders and finite-size effects. Moreover, the Raman-scattering intensities of certain phonons show a different resonantly enhanced behavior, which can be used to verify the information on the electronic structures and the electron–phonon interaction in GaN nanowires.

Correlation of microwave dielectric properties and normal vibration modes of xBa(Mg1/3Ta2/3)O3–(1−x)Ba(Mg1/3Nb2/3)O3 ceramics: II. Infrared spectroscopy

The relationship between the microwave dielectric properties and the IR active phonons of xBa(Mg1/3Ta2/3)O3–(1−x)Ba(Mg1/3Nb2/3)O3 ceramics was investigated. The IR modes were assigned, and the origin of dielectric response was determined. Among the 15 prominent IR modes, we found that the normal vibrations of the O layers and that of the Ta/Nb layers are strongly correlated to the measured dispersion parameters, such as the resonant strength (4πρ) and the damping coefficient (γ). The frequency shifts of the normal modes of the O layers and that of the Ta/Nb layers explain the linear decrease of microwave dielectric constant (K) as x increases, while the width of these modes correlate with the Q×f value.

Optical properties of monolayer transition metal dichalcogenides probed by spectroscopic ellipsometry

Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical-vapor-deposited monolayer transition metal dichalcogenides (TMDs). The extraordinary large value of the refractive index in the visible frequency range is obtained. The absorption response shows a strong correlation between the magnitude of the exciton binding energy and band gap energy. Together with the observed giant spin-orbit splitting, these findings advance the fundamental understanding of their novel electronic structures and the development of monolayer TMDs-based optoelectronic and spintronic devices.

Nonlinear bio-photonic crystal effects revealed with multimodal nonlinear microscopy

Highly optically active nonlinear bio‐photonic crystalline and semicrystalline structures in living cells were studied by a novel multimodal nonlinear microscopy. Numerous biological structures, including stacked membranes and aligned protein structures are highly organized on a nanoscale and have been found to exhibit strong optical activities through second‐harmonic generation (SHG) interactions, behaving similarly to man‐made nonlinear photonic crystals. The microscopic technology used in this study is based on a combination of different imaging modes including SHG, third‐harmonic generation, and multiphoton‐induced fluorescence. With no energy release during harmonic generation processes, the nonlinear‐photonic‐crystal‐like SHG activity is useful for investigating the dynamics of structure–function relationships at subcellular levels and is ideal for studying living cells, as minimal or no preparation is required.

1.2- to 2.2-μm Tunable Raman Soliton Source Based on a Cr : Forsterite Laser and a Photonic-Crystal Fiber

A 1.2- to 2.2-mum tunable femtosecond light source based on the soliton-self-frequency-shift effect of high-power Cr: forsterite laser pulses propagating inside a highly nonlinear photonic crystal fiber is reported. The demonstrated soliton self-frequency shift is higher than 42% of the pump laser frequency, corresponding to a record 910-nm wavelength tuning range. Due to the advantages of simplicity, easy tunability, high-temperature stability, and low cost of this new femtosecond light source, it accordingly, could be widely applicable for many applications.

Charge Dynamics and Electronic Structures of Monolayer MoS2 Films Grown by Chemical Vapor Deposition

THz absorption and spectroscopic ellipsometry were used to investigate the charge dynamics and electronic structures of chemical-vapor-deposited monolayer MoS2 films. THz conductivity displays a coherent response of itinerant charge carriers at zero frequency. Drude plasma frequency (~7 THz) decreases with decreasing temperature while carrier relaxation time (~26 fs) is almost temperature independent. The absorption spectrum of monolayer MoS2 shows a direct 1.95 eV band gap and charge transfer excitations that are ~0.2 eV higher than those of the bulk counterpart. The ground-state exciton binding energy is found to be about 0.48 eV.

Far-infrared, Raman spectroscopy, and microwave dielectric properties of La(Mg0.5Ti(0.5−x)Snx)O3 ceramics

La(Mg0.5Ti(0.5−x)Snx)O3 perovskite ceramics with composition (x=0.0−0.5) are prepared by the solid state reaction method. The ceramics are characterized by x-ray diffraction, far-infrared reflectance, Raman spectroscopy, and microwave dielectric properties. The symmetry of ceramics is monoclinic with P21/n space group. Intrinsic dielectric constant and loss are estimated by fitting reflectance to the four-parameter semiquantum model. Transverse optic phonon mode strengths and average phonon damping are calculated. The modes corresponding to B-site ordering are identified in Raman spectra and the A1g mode of La(MgTi)0.5O3 is analyzed by assuming two merging modes. The variation of long-range order is correlated with full width half maximum of the A1g mode. Microwave measurements are carried out in the frequency range of 8–10 GHz. The dielectric constant (e′) is found to gradually decrease from 28.4 to 19.7 with an increase in tin concentration, whereas the temperature coefficient of resonant frequency (τf)...

Preparation and characterization of carbon nanotubes encapsulated GaN nanowires

A novel two-step catalytic reaction is developed to synthesize gallium nitride nanowires encapsulated inside carbon nanotubes (GaN@CNT). The nanowires are prepared from the reaction of gallium metal and ammonium using metals or metal alloys as a catalyst. After the formation of the nanowires, carbon nanotubes are subsequently grown along the nanowires by chemical vapor deposition of methane. The structural and optical properties of pure GaN nanowires and GaN@CNT are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results show that GaN nanowires are indeed encapsulated inside carbon nanotubes. The field emission studies show that the turn-on field of GaN@CNT is higher than that of carbon nanotubes, but substantially lower than that of pure GaN nanowires. This work provides a wide route toward the preparation and applications of new one-dimensional semiconductor nanostructures.

Intrinsic dielectric and spectroscopic behavior of perovskite Ba(Ni1∕3Nb2∕3)O3–Ba(Zn1∕3Nb2∕3)O3 microwave dielectric ceramics

Ceramics of 0.35Ba(Ni1∕3Nb2∕3)O3–0.65Ba(Zn1∕3Nb2∕3)O3 were prepared by the mixed oxide route. The effect of the cooling rate (2 °C–240 °C∕h) after sintering on the microwave dielectric properties of the ceramics was examined. While the extrinsic factors, such as porosity and secondary phases, markedly influence the dielectric properties in the low-frequency regime, they have minimal effect on these properties in the high-frequency regime. The mechanisms involved in modifying the high-frequency dielectric properties of the materials were investigated by Fourier transform infrared and Raman spectroscopy, in conjunction with the Rietveld analysis of x-ray diffraction (XRD) spectra. A reduction in the cooling rate after sintering results in an increase in the high-frequency Q×f (product of dielectric Q value and measurement frequency) from 42 to 58 THz in the high-frequency regime (∼1.5 THz). Such behavior correlates very well with the increase in the B-site occupancy by Nb (deduced from the Rietveld analyses...

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.