Terahertz phonon polariton imaging

In this article, we primarily review the time-resolved imaging of THz phonon polariton, which is generated by femtosecond laser in ferroelectric crystal. We pay more attention to the imaging in thin crystal, which can be used as an integration platform for terahertz-optics or terahertz-electrics. The imaging techniques, which can get quantitatively in-focus time-resolved images, are introduced in more detail. They have made enormous progress in recent years, and are powerful tools for the research of phonon polariton, optics, and THz wave. We also briefly introduce the generation principle and general propagation properties of THz phonon polariton.     

Phonon‐polariton entrapment in homogenous surface phonon cavities

Surface phonon cavities that are homogenous in both mechanical and dielectric properties are reported. The cavities are formed by the placement of a defect of a single domain within periodic domain inversion of single crystal piezoelectric lithium niobate that exhibits surface phononic bandgap through the phonon‐polariton coupling. Surface cavity resonances are observed within the bandgap, which manifest in entrapment of phonon‐polariton within the defect. In addition to demonstrating that the observed resonances are non‐radiative and decoupled to bulk radiation, which is critical for high Q cavities, it is also shown the possibility to tune the surface cavity resonance spectra simply by varying the defect width. Such an ability to excite surface cavity resonance that is non‐radiative with simultaneous localization of the electric field together with the advantage of a cavity that is physically formed from a completely monolithic and uniform material offers unique opportunities for widespread applications for example in actuation, detection, and phonon lasing that can be fully integrated with other physical systems such as quantum acoustics, photonics, and microfluidics.

     

Study of phonon polaritons in GaAs/GaPAs superlattices using methods of IR reflection spectroscopy

We studied phonon-polariton modes in long-period GaAs/GaPAs semiconductor superlattices using the methods of frustrated total internal reflection and IR spectroscopy in the region of residual rays. The excitation of waveguide, real, and virtual surface and volume phonon polariton modes was observed experimentally. Their dispersion relations were calculated and compared with experimental data, and the excitation nature was identified with the aid of the spectra of permittivity and a normal component of the wave vector in heterostructure layers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 460–466, July–August, 1999.     

Surface plasmon polariton coupling between a metal nanowire and a metal helix

The transmittance and steady-state electrical field distribution of a silver nanowire–helix system are investigated using the finite-difference time-domain method. In the nanowire–helix system, surface plasmon polaritons are coupled into the helix or squeezed into the space between the nanowire and helix. The transmittance strongly depends on the topologic shapes of the helix, especially the pitch height. Thus, the nanowire–helix system enables the detection of the displacement associated with helical deformation.     

Annealing and surface conduction on Hydrogen peroxide treated bulk melt-grown,single crystal ZnO

We report on the studies carried out on hydrogen peroxide treated melt-grown, bulk single crystal ZnO samples. Results show the existence of two shallow donors in the as-received ZnO samples with energy levels (37.8±0.3) meV that has been suggested as Zn_i related and possibly H-complex related and (54.5±0.9) meV, which has been assigned to an Al-related donor. Annealing studies performed on the hydrogen peroxide treated samples reveal the existence of a conductive channel in the samples in which new energy levels have been observed, Zn vacancies, related to the Group I elements, X_Zn. The surface donor volume concentration of the conductive channel was calculated from a theory developed by Look (2007) [1]. Results indicate an increase in the surface volume concentration with increasing annealing temperature from 60×10¹⁷ cm⁻³ at 200 °C to 4.37×10¹⁸ cm^-3 at 800 °C.     

Polar optical phonon states and dispersive spectra of wurtzite ZnO nanocrystals embedded in zinc-blende MgO matrix

Based on the macroscopic dielectric continuum model and Loudon’s uniaxial crystal model, the polar optical phonon modes of a quasi-0-dimensional (Q0D) wurtzite spherical nanocrystal embedded in zinc-blende dielectric matrix are derived and studied. It is found that there are two types of polar phonon modes, i.e. interface optical (IO) phonon modes and the quasi-confined (QC) phonon modes coexisting in Q0D wurtzite ZnO nanocrystal embedded in zinc-blende MgO matrix. Via solving Laplace equations under spheroidal and spherical coordinates, the unified and analytical phonon states and dispersive equations of IO and QC modes are derived. Numerical calculations on a wurtzite/zinc-blende ZnO/MgO nanocrystal are performed. The frequency ranges of the IO and QC phonon modes of the ZnO/MgO nanocrystals are analyzed and discussed. It is found that the IO modes only exist in one frequency range, while QC modes may appear in three frequency ranges. The dispersive frequencies of IO and QC modes are the discrete functions of orbital quantum numbers l and azimuthal quantum numbers m. Moreover, a pair of given l and m corresponds to one IO mode, but to more than one branches of QC. The analytical phonon states and dispersive equations obtained here are quite useful for further investigating Raman spectra of phonons and other relative properties of wurtzite/zinc-blende Q0D nanocrystal structures.     

A surface sensitive optical method for the evaluation of processed ZnO: exploitation of LO phonon interaction

We report on a surface property of bulk ZnO crystals and an optical method to evaluate it. Bulk ZnO crystals have a damaged surface layer due to chemomechanical polishing. We prepared the ZnO crystals by etching, and evaluated the improvement of the surface by high-resolution X-ray diffraction (XRD) and photoluminescence (PL). In PL measurements, the relative intensity of the first order longitudinal optical phonon replica of free exciton (FX-1LO) to second order process (FX-2LO) was compared. The relative intensity becomes weak with increasing etched depth and finally saturates at the etched depth of 5 μm. This result agrees well with XRD results.     

用衰减全反射(ATR)及拟合法对固体表面物理性质的研究

提出了一种利用光学中全反射衰减特性及拟合法对介质上蒸镀的金属薄膜物理性质进行研究的方法,并运用该方法对金属薄膜的厚度和复介电常量进行了精确测定,该方法是研究固体表面物理的一种有效的方法和技术.     

Room temperature soft chemical route for nanofibrous wurtzite ZnO thin film synthesis

Room temperature soft chemical deposition route has been utilized to grow thin films of ZnO on glass substrate. Annealing at 673 K removed zinc hydroxide phase and nanofibrous ZnO films with wurtzite crystal structure were obtained. Decrease in the room temperature electrical resistivity from 10⁷ to 10⁴ Ω cm was observed after annealing. The nanofibrous ZnO thin films were sensitive to the explosive liquefied petroleum gas (LPG) and the maximum response of 17% at 698 K under the exposure of 6500 ppm of LPG was obtained.     

Fabrication and characteristics of thermal crosslinking polyurethane integrated double-channel electro-optic modulator

The fabrication and characteristics of a new type of polymeric electro-optic modulator with double channels in attenuated total reflection (ATR) configuration, made of a thermally crosslinkable polyurethane, has been demonstrated in this paper. The fabrication procedure of proposed double-channel ATR modulator is extremely simple, it can be implemented by replacing a single electrode with two discrete electrodes in a single channel ATR modulator. Independent operation of both channels is demonstrated at a modulation speed of 500 MHz. The total measured insertion loss of the modulator is below 1.5 dB. There are no measurable cross talks due to the large horizontal separation of two channels and the small vertical separation of the electrode pair. A long-term thermal stability of the fabrication modulator at an elevated temperature, 100 °C, is obtained due to crosslinking polyurethane as the waveguide layer of the devices.     

纤锌矿氮化物量子阱中光学声子模的三元混晶效应

本文先比较了几种常用方法(修正的无规元素等位移模型、虚晶近似和简化相干势近似等)对纤锌矿三元混晶体声子频率的拟合结果,再选用与实验数据接近的拟合方法,结合介电连续和单轴晶体模型导出含纤锌矿三元混晶In_xGa_1－xN和Al_xGa_1－xN单量子阱各类光学声子模的色散关系,进一步分析了声子模随组分的变化. 结果表明,修正的无规元素等位移模型对单模性纤锌矿     

Surface polished of bulk methylammonium lead tribromide single crystal

Organic-inorganic halide perovskite materials have been demonstrated with wide applications in optoelectronics and ionization radiation detection. For bulk as-grown crystals, the existence of surface cracks and defects can significantly increase charges recombination and reduce the performance of the device. Herein, we polished the crystal surfaces with both mechanical and chemical mechanical methods at room temperature. After been chemical-mechanical polished, the crystal surface with root mean square roughness about 0.5 nm was obtained. Optical transmission and photoluminescence spectra indicate that chemical mechanical polishing technology can effectively reduce the density of crystal surface defects. The achieved low leakage current density on the surface and bulk crystal is 0.05 nA mm⁻² and 0.07 nA mm⁻², respectively. Furthermore, the current-voltage curve under visible photons and X-ray photons reveals that surface polishing treatment can suppress the charges recombination and increase the charges transportation.     

Back-propagating surface polaritons of a one-dimensional photonic crystal containing single negative metamaterials

The existence of the surface polaritons at the interface separating a semi-infinite uniform left-handed metamaterial and a one-dimensional photonic crystal composed of alternating layers of two kinds of single-negative materials is theoretically investigated. The dispersion characteristics of the surface polaritons are analyzed and demonstrated that in the presence of metamaterial, the surface polaritons are sensitive to light polarization, so that there exist only backward TM-polarized (or TE-polarized) kind of the surface polaritons depending on the ratio of the thicknesses of the two periodic stacking layers. The existence regions of the surface polariton modes are determined for both TM-polarized and TE-polarized surface polariton modes.     

Pressure dependence of elastic properties of wurtzite ZnO crystal

We present the pressure dependence of elastic properties of the wurtzite phase of ZnO undergoing wurtzite to the rocksalt phase transition. A simple Landau theory is developed to describe the structural phase transition between wurtzite to rocksalt phases observed in ZnO. We have defined the necessary order parameter of the wurtzite to the rocksalt phase transition. We present a detailed analysis of the pressure dependence of the elastic and shear constants of the wurtzite phase of ZnO. The theoretical predictions are in the line with experimental results.     

The ZnO non-polar (10 0) surface: an X-ray structural investigation

We performed a structural analysis of the non-polar ZnO (10 0) surface by means of grazing incidence X-ray diffraction. The analysis was conducted on ten rods, smooth surface domains, though of small coherent width, having been obtained after several Ar⁺ sputtering–annealing cycles. The surface derived from the bulk structure exposes one ZnO dimer per unit cell, parallel to the [001] axis. All the existing models, derived from ab initio calculations or low-energy electron diffraction (LEED) analyses, consist in a surface dimer whose O and Zn atoms are shifted inwards, with the O pointing outwards with respect to Zn. Whereas the LEED studies conclude on a dimer distance greater than in the bulk, the theoretical studies agree on a dimer contracted by amounts ranging from 5.5 to 7.5%. This contraction is interpreted as a result of the strong ionicity of ZnO, and is associated with a moderate dimer rotation. The latter, however, is found between 2.3 and 11.4°. Despite the discrepancies between the models, the Zn atom is always found shifted downwards by more than 0.25 Å. This is unambiguously rejected by our data, which show that the Zn atom keeps very close to its bulk position. It is displaced downwards by ΔZ_Zn=−0.06±0.02 Å, and it moves along [001] towards O by ΔX_Zn=0.05±0.02 Å. We denote a trend for the O atom to be displaced downwards too, with a concomitant displacement towards Zn. The faint X-ray scattering of O prevents us from assessing its position with accuracy. Depending on the choice of position for Zn in the error bar range, the buckling is evaluated as between −6.5 and 3°, or between −4 and 0.5°. The dimer distance is evaluated equal to 1.90 Å, with a deviation equal either to 0.06 or 0.11 Å.     

Structural and optical characteristics of spin-coated ZnO thin films

Zinc oxide (ZnO) thin films were deposited onto glass substrates by spin-coating method, from a precursor solution containing zinc acetate, ethanol and ammonium hydroxide. After deposition, the films were heated at a temperature of 100 °C in order to remove unwanted materials. Finally, the films were annealed at 500 °C for complete oxidation. X-ray diffraction showed that ZnO films were polycrystalline and have a hexagonal (wurtzite) structure. The crystallites are preferentially oriented with (0 0 2) planes parallel to the substrate surface. The films have a high transparency (more than 75%) in the spectral range from 450 nm to 1300 nm. The analysis of absorption spectra shows the direct nature of band-to-band transitions. The optical bandgap energy ranges between 3.15 eV and 3.25 eV.Some correlations between the processing parameters (spinning speed, temperature of post deposition heat treatment) and structure and optical characteristics of the respective thin films were established.     

基于闪锌矿晶体中受激电磁耦子散射产生可调谐太赫兹波的理论研究

基于受激电磁耦子散射原理,采用已报道的利用非线性光学参量振荡方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,以GaAs,GaP, InP,ZnTe晶体为代表,计算分析了在闪锌矿晶体中参量振荡产生太赫兹波的吸收、增益特性,对输出THz波的调谐特性给出了详尽分析.分析太赫兹波高效耦合输出的腔型结构,并与掺氧化镁铌酸锂晶体组成的太赫兹波参量振荡器做对比. 关键词： 太赫兹波 太赫兹波参量振荡 电磁耦子 闪锌矿晶体     

Quantitative analysis of surface donors in ZnO

At low temperatures, typically up to 30 K or even higher, the electrical properties of bulk ZnO samples are nearly always dominated by a conductive near-surface region. Here we show that a single, low-temperature Hall-effect measurement, say at 20 K, and a reasonable assumption regarding the upper limit of the surface compensation ratio, yields a value of surface donor concentration N_D,surf accurate to within about a factor two. Examples are given for bulk materials grown by the vapor-phase, melt, and hydrothermal processes.