Liang, Qijun
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Optimized seeded Bridgman growth and temperature dependent THz optical properties of LiInS2 crystals
High quality and large size nonlinear optical LiInS2 (LIS) crystals were successfully grown by the optimized seeded Bridgman method. And the optical properties and carrier dynamics of LIS in the THz range were investigated by THz time-domain spectroscopy (THz-TDS) in the temperature range of 20–300 K for the first time. The refractive index and absorption coefficients present obvious temperature dependent behavior. A distinct absorption peak was observed at around 1.68 THz, which can be attributed to the vibration of In–S bonds revealed by Raman spectroscopy analysis. And the carrier concentration, mobility and scattering rate also show obvious temperature dependent behavior. These results will provide further understanding of the optical properties of LIS crystals and a perspective for future applications of LIS crystals in THz technology.
Temperature dependence of free carriers and optical phonons in LiInSe2 in the terahertz frequency regime
We investigate free carriers dynamics and anharmonic decay of optical phonons in LiInSe2 by using low temperature THz time-domain spectroscopy. The quantitatively determined refractive index and absorption coefficient are significantly influenced by temperature. By fitting the evaluated dielectric functions with a Drude-Lorentz model, the temperature dependencies of the plasmon frequency, scattering rates of free carriers, activation energies of carriers, and frequencies and linewidths of optical phonons are investigated. The contributions of three- and four-phonon processes to the temperature dependent optical phonon frequency and linewidth variations are quantitatively determined.
Optical properties of LiInSe2 in the THz frequency regime
We investigate the optical properties of LiInSe2 (LISe) by THz time-domain spectroscopy (THz-TDS). The experimental results indicate that LISe has excellent transparency in the THz frequency region. We determine quantitatively the birefringence of LISe. The refractive index and absorption coefficient along the three dielectric axes of LISe are reported in the THz frequency region. The lattice vibrations of LISe are analyzed on the basis of the THz-TDS measurement in combination with infrared and Raman spectra. The origin of the low-frequency vibrational modes below 3 THz as interatomic bond-bending modes is revealed.
Catalytically Doped Semiconductors for Chemical Gas Sensing : Aerogel-Like Aluminum-Containing Zinc Oxide Materials Prepared in the Gas Phase
Atmospheric contamination with organic compounds is undesired in industry and in society because of odor nuisance or potential toxicity. Resistive gas sensors made of semiconducting metal oxides are effective in the detection of gases even at low concentration. Major drawbacks are low selectivity and missing sensitivity toward a targeted compound. Acetaldehyde is selected due to its high relevance in chemical industry and its toxic character. Considering the similarity between gas-sensing and heterogeneous catalysis (surface reactions, activity, selectivity), it is tempting to transfer concepts. A question of importance is how doping and the resulting change in electronic properties of a metal-oxide support with semiconducting properties alters reactivity of the surfaces and the functionality in gas-sensing and in heterogeneous catalysis. A gas-phase synthesis method is employed for aerogel-like zinc oxide materials with a defined content of aluminum (n-doping), which were then used for the assembly of gas sensors. It is shown that only Al-doped ZnO represents an effective sensor material that is sensitive down to very low concentrations (<350 ppb). The advance in properties relates to a catalytic effect for the doped semiconductor nanomaterial.
Publisher's Note : Temperature dependence of free carriers and optical phonons in LiInSe2 in the terahertz frequency regime [Phys. Rev. B 92, 144303 (2015)]
Optical properties and birefringence in LiInS2 in the terahertz frequency range
The birefringence of LiInS2 (LIS) crystals in the THz frequency region is investigated by THz time-domain spectroscopy (THz-TDS). The experimental results indicate that LIS has large birefringence and low absorption in the THz frequency region. The optical properties of LIS are quantitatively determined. A sharp absorption caused by a TO-phonon resonance is observed at around 1.70 THz when the Z-axis is parallel to the polarization of the incident THz wave. A temporal separation of the transmitted THz pulses with different polarization components is realized by changing the orientation of the LIS crystal with respect to the polarization of the incident THz pulses. By controlling the relative phase and amplitude of the temporally separated THz pulses, THz polarization pulse shaping caused by birefringence in LIS crystal is demonstrated.
Terahertz Time-Domain Spectroscopy : Characterization of Nonlinear Crystals, Nanowires, 2D Gratings,Organic Liquids, and Polystyrene Particles
Different materials including nonlinear crystals, nanowires, 2D aluminum gratings, organic liquids, and polystyrene particles are characterized using THz time-domain spectroscopy based on asynchronous optical sampling. Generated THz radiation by surface-field screening in semiconductors and optical rectification in nonlinear crystal are demonstrated in this dissertation. Significant birefringence and optical phonons are observed in the investigated nonlinear crystals. The temperature dependence of free carriers and anharmonic decay of optical phonons in LiInSe2 is investigated using low-temperature THz time-domain spectroscopy. The complex conductivity of TiO2 nanowires in the THz frequency region is quantitatively determined, and the influences of doping as well as post-annealing conditions on the complex conductivity are presented. Surface plasmon resonances in the THz frequency range are observed in 2D aluminum gratings fabricated on silicon substrates with dfferent lattice constants and shapes, which agree well with the theoretical interpretation. The quantitative determination of the temperature dependent refractive indices and absorption coefficients of several organic liquids in the THz frequency range are demonstrated by performing transmission THz time-domain spectroscopy measurements. The extinction cofficients of polystyrene particles in the THz frequency range are calculated.
Origin of potential errors in the quantitative determination of terahertz optical properties in time-domain terahertz spectroscopy
We demonstrate theoretically and experimentally how changes of a terahertz (THz) beam induced by the sample affect the accuracy of the determination of THz dielectric properties in THz time-domain transmission spectroscopy (TDTS). We apply a Gaussian beam and the ABCD matrix formalism to describe the propagation of the THz beam in a focused beam setup. The insertion of the sample induces a focus displacement which is absent in the reference measurement without a sample. We show how the focus displacement can be corrected. The THz optical properties after focus displacement correction reported in this Letter are in quantitative agreement with those obtained using collimated beam THz–TDTS in previous work.
Nanoarchitecture Effects on Persistent Room Temperature Photoconductivity and Thermal Conductivity in Ceramic Semiconductors: Mesoporous, Yolk–Shell, and Hollow ZnO Spheres
A gas-phase approach is applied to synthesize a set of spherical particles with mesoporous, yolk−shell, or hollow character. A special arrangement of the ZnO lattice results in a polar character of the particle shell, and this facilitates effective separation of electrons and holes on different sides of the interface.