High resolution spectroscopy of Rb in magnetic field by far-detuning electromagnetically induced transparency

We have presented a high resolution spectroscopy of Rb in magnetic field by far-detuning electromagnetically induced transparency (EIT). The EIT spectrum in the $\Xi$-type configuration is usually companied by a double resonance optical pumping (DROP) due to the strong optical coupling between the two upper states, leading to the spectral lines seriously deformed and widely broadened for complex relaxation processes in DROP. Here we demonstrate a high resolution spectroscopy by far-detuning EIT for $^{87}\rm{Rb}$ $\rm{5S_{1/2}\rightarrow5P_{3/2}\rightarrow5D_{5/2}}$ in magnetic fields. The method of far-detuning eliminates the relaxation in DROP to the most extent and decreases the spectral linewidth from more than 20 MHz down to its natural linewidth limit (6 MHz). The deformation of the spectral lines also disappears and the observed spectra are well in accordance with the theoretical calculation. Our work shows that far-detuning EIT is a reliable high resolution spectroscopic method when the relaxation in DROP cannot be neglected, especially for the case of transition to low excited states.     

Photoinduced changes of reflectivity in single crystals of YBa2Cu3O6.5 (ortho II)

We report measurements of the photoinduced change in reflectivity of an untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay rate of the transient change in reflectivity is found to decrease rapidly with decreasing temperature and, below T(c), with decreasing laser intensity. We interpret the decay as a process of thermalization of antinodal quasiparticles, with a rate determined by inelastic scattering of quasiparticle pairs.     

Polarized infrared reflectivity study in charge density wave conductor Tl0.3MoO3

Polarized infrared reflectivity measurements between 300 and 10 K have been carried out on charge density waves (CDW) conductor blue bronze Tl_0.3MoO₃. Three important features are observed: (i) A bump at 1155 cm⁻¹ in the reflectivity spectra of Tl_0.3MoO₃ at 300 K is a precursor of the Peierls gap due to optical excitations across a pseudogap, and this kind of Peierls-like gap opens gradually with decreasing temperature from 180 to 160 K. (ii) The three sharp modes as “triplet” of infrared reflectivity between 800 and 1000 cm⁻¹ of Tl_0.3MoO₃ along [1 0 2] axis show red shift compared to K_0.3MoO₃ and Rb_0.3MoO₃, which is assigned to the increase of the distance of Mo-O bond with the substitution of thallium ions. (iii) Two peaks at about 514 and 644 cm⁻¹ in the far-infrared reflectivity spectra of Tl_0.3MoO₃ along [1 0 2] direction are suggested to be the electronic transitions from the valence band to the midgap state and from occupied midgap state to the conduction band, respectively.     

Carrier relaxation time divergence in single and double layer cuprates

We report the transient optical pump-probe reflectivity measurements on single and double layer cuprate single crystals and thin films of ten different stoichiometries. We find that with sufficiently low fluence the relaxation time ( of all samples exhibits a power law divergence with temperature . Further, the divergence has an onset temperature above the superconducting transition temperature for all superconducting samples. Possible causes of this divergence are discussed.Received: 30 June 2003, Published online: 23 December 2003PACS: 74.78.Bz High-T _c films - 74.72.-h Cuprate superconductors (high-T _c and insulating parent compounds) - 74.25.Gz Optical propertiesM.L. Schneider: Present address: NIST, mc 816.01, 325 Broadway, Boulder, CO 80305-3328, USA     

Ultrafast temperature relaxation evolution in Au film under femtosecond laser pulses irradiation

Ultrafast temperature relaxation processes in Au film including two temperature relaxation and thermal diffusion relaxation with femtosecond laser pulse excitation were investigated numerically by Finite Element Method (FEM). With the temperature dependent thermal parameters, the full 2D temperature field evolution in picosecond and nanosecond domains were obtained. It is proposed that the heat transfer depth can be alternatively localized or enhanced by the distinct temperature relaxation mechanisms. Moreover, the effect of laser parameters and Au film thickness and surface reflectivity on the two temperature relaxation time were analysed.     

Rovibrational quenching of BH in ultracold 3He collisions

The interaction potential of a He-BH complex is investigated by the coupled-cluster single-double plus pertur-bative triples (CCSD (T)) method and an augmented correlation consistent polarized valence (aug-cc-pV)5Z basis set extended with a set of (3s3p2d1f1g) midbond functions. Using the five two-dimensional model potentials, the first three-dimensional interaction potential energy surface is constructed by interpolating along (r–r e ) by using a fourth-order polynomial. The cross sections for the rovibrational relaxation of BH in cold and ultracold collisions with 3 He atom are calculated based on the three-dimensional potential. The results show that the Δv =-1 transition is more efficient than the Δv =-2 transition, and that the process of relaxation takes place mainly between rotational energy levels with the same vibration state and the Δj =-1 transition is the most efficient. The zero temperature quenching rate coefficient is finite as predicted by Wigner’s law. The resonance is found to take place around 0.1–1 cm-1 translational energy, which gives rise to a step in the rate coefficients for temperatures around 0.1–1 K. The final rotational distributions in the state v = 0 resulting from the quenching of state (v = 1, j = 0) at three energies corresponding to the three different regimes are also given.     

Contribution of longitudinal polarization waves to the optical properties of Ag surface layers

Diffential reflectivity measurements of thin Ag films deposited on Al and Au are reported. The silver plasmon excitation peak is broadened and shifts to higher energies with decreasing thickness. This is interpreted by taking into account longitudinal polarization waves and a shorter effective relaxation time of the free electrons. Good quantitative agreement is obtained.     

Temperature-frequency dependence and mechanism of dielectric properties for γ-Y2Si2O7

This paper reports that single-phase γ-Y2Si2O7 is prepared via a sufficient blending and cold-pressed sintering technique from Y2O3 powder and SiO2 nanopowder. It studies the dielectric properties of γ-Y2Si2O7 as a function of the temperature and frequency. The γ-Y2Si2O7 exhibits low dielectric loss and non-Debye relaxation behaviour from 25 to 1400℃ in the range of 7.3-18 GHz. The mechanism for polarization relaxation of the as-prepared γ-Y2Si2O7 differing from that of SiO 2 is explained. Such particular dielectric properties could potentially make specific attraction for extensive practical applications.     

铁电体中极化长程涨落的光子关联谱实验研究

从极化团簇的随机涨落出发, 利用维纳过程模型, 推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式. 阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系. 通过对原有氦氖激光光子关联谱实验装置进行改进, 观测了BaTiO₃和0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程. 在BaTiO₃中发现极化团簇长程涨落在居里点之上4 K存在双弛豫现象, 此现象与其有序无序相变机理相联系. 在Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象. 利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间. 两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变, 并呈现临界慢化现象.     

Temperature-dependent far-infrared properties of Bi12GeO20 single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide (Bi12GeO20) single crystals are measured from room temperature down to 10 K. All the reflectivity spectra are fitted to a complex dielectric function ε(ω) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150 K. The temperature-dependent static dielectric constants are obtained from the Lyddane-Sachs-Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.     

Polariton thermalization in GaN microcavities in the strong light–matter coupling regime

We discuss the thermalization of the polariton population from a bulk GaN λ/2 microcavity at room temperature. Simultaneous optical measurements of reflectivity and photoluminescence (PL), as well as time resolved measurements, reveal strong light–matter coupling. Transfer matrix theory is used to calculate reflectivity, absorption, and transmission coefficients for the structure. The PL emission is found to be thermalized, despite its very short lifetime, suggesting the existence of very fast energy relaxation channels.     

Picosecond time resolved reflectivity of direct gap semiconductors

The relaxation of optically excited hot carrier distributions in GaAs and CdSe has been measured by time resolved reflectivity measurements with sub-picosecond optical pulses from a mode-locked cw dye laser. A characteristic transient in the reflectivity has been interpreted by an analysis of the perturbed optical dielectric function. Estimated energy loss rates are 0.4 eV/ps for electrons and holes having mean energies in the range of 1 to 2 eV for GaAs at room temperature.     

腔增强吸收光谱技术中的腔镜反射率标定方法研究

腔增强吸收光谱技术具有实验装置相对简单、灵敏度高、环境适应性强等特点,是高灵敏吸收光谱技术的重要分支之一,在其应用过程中,腔镜反射率是影响其测量准确性的重要因素。利用2.0 μm可调谐二极管激光器作为光源搭建了一套腔增强吸收光谱测量系统,使用两片反射率为99.9%的高反镜作为腔镜,以CO₂气体在5 001.49 cm-1处的吸收谱线作为研究目标,对两种简单实用的腔镜反射率标定方法进行了对比研究。第一种标定方法利用已知程长多通池作为参考池,通过测量谐振腔和多通参考池的输出吸收信号,比较二者的吸收率推导出腔增强系统中的有效吸收路径,然后通过镜片反射率和有效吸收路径的关系对腔镜反射率进行标定;第二种标定方法根据理想气体状态方程得到气体分子数密度,并结合数据库中的谱线线强值,实现了对腔镜反射率进行标定。结果表明,方法一中积分腔与参考池测得信号的积分吸收面积之比为10.5,经过多次测量并计算得到积分腔的有效吸收路径与镜片的反射率分别为302.65 m和99.85%,得到大气中CO₂气体的浓度为0.037 3%,与实际大气CO₂的含量相符合,验证了此方法的准确性;该方法的优点是不受样品浓度影响,但因引入新的参考池,需要两池中气体的压强和温度都保持一致,此方法适用于开放式的腔体结构。方法二中测得大气中CO₂分子位于5 001.49 cm-1处吸收光谱,并结合大气中CO₂气体的分子数密度N为9.099×1015 molecule·cm-3,Hitran数据库中该条谱线线强为3.902×10-22 cm·molecule-1,计算得到镜片反射率约为99.84%;此方法优点是结构相较前一种方法更简单,但需要已知被测气体的分子数密度,因此在配置气体的过程中浓度、压力的误差会影响腔镜反射率的标定。由此可见两种镜片标定方法均可精确实现对腔镜反射率的标定,根据两种方法的特点,在实际应用中可选取相应适合的方法作为参考。     

ELECTRONIC AND OPTICAL PROPERTIES OF WURTZITE GaN:——A THEORETICAL APPROACH

The band structures of wurtzite GaN(α-GaN) are studied using the nearest and second-nearest neighbour semi-empirical tight-hinding method in sp³ s^* model. The calculated direct fundamental gap of α-GaN is 3.45 eV, which is in good agreement with the experimental data. The density of states and the imaginary pert of dielectric function (ε₂ (ω)) are evaluated to he in the regions - 10.0 - 12 eV and (1.0 - 10.0 eV, respectively. There are mainly three peaks at 6,4, 7,5, 8.4 eV, dominating the ε₂(ω) spectrum. The two components of the ε₂(ω) (i. e. ε_2xy(ω) and ε_2z(ω) ) are also calculated; and the real prat of dielectric function, reflectivity, absorption coefficient, and refractive index are all studied.     

Study on the transition from filamentary discharge to diffuse discharge by using a dielectric barrier surface discharge device

Discharge characteristics have been investigated in different gases under different pressures using a dielectric barrier surface discharge device. Electrical measurements and optical emission spectroscopy are used to study the discharge, and the results obtained show that the discharges in atmospheric pressure helium and in low-pressure air are diffuse, while that in high-pressure air is filamentary. With decreasing pressure, the discharge in air can transit from filamentary to diffuse one. The results also indicate that corona discharge around the stripe electrode is important for the diffuse discharge. The spectral intensity of N电介质 表面放电 扩散放电 发射光谱学dielectric barrier surface discharge, diffuse discharge, optical emission spectroscopyProject supported by the National Natural Science Foundation of China (Grant Nos 10575027 and 10647123), the National Science Foundation of Hebei Province, China (Grant No A2007000134), the Education Department of Hebei Province, China (Grant No 2006106),2006-10-24Discharge characteristics have been investigated in different gases under different pressures using a dielectric barrier surface discharge device. Electrical measurements and optical emission spectroscopy are used to study the discharge, and the results obtained show that the discharges in atmospheric pressure helium and in low-pressure air are diffuse, while that in high-pressure air is filamentary. With decreasing pressure, the discharge in air can transit from filamentary to diffuse one. The results also indicate that corona discharge around the stripe electrode is important for the diffuse discharge. The spectral intensity of N＋ （391.4nm） relative to N2 （337.1 nm） is measured during the transition from diffuse to filamentary discharge. It is shown that relative spectral intensity increases during the discharge transition. This phenomenon implies that the averaged electron energy in diffuse discharge is higher than that in the filamentary discharge.     

POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

Passively mode-locked Nd:YVO4 laser using semiconductor saturable absorption mirrors of interface states relaxation region

Semiconductor saturable absorber mirrors (SESAMs) with GaAs/air interface relaxation region have less non-saturable loss than those with low temperature grown In_0.25Ga_0.75As relaxation region. A thin layer of SiO₂ and a high reflectivity film of Si/(SiO₂/Si)₄ were coated on the SESAMs, respectively in order to improve the SESAM's threshold for damage. The passively continuous wave mode-locked lasers with two such SESAMs were demonstrated, and the SESAM with high reflectivity film of Si/(SiO₂/Si)₄ is proved to be helpful for high output power.     

Pyroelectric behavior and dielectric properties of linear copolysiloxane/eicosylamine superlattice

The pyroelectric behavior and dielectric properties of a superlattice containing a Langmuir-Blodgett (LB) film of alternately monomeric eicosylamine and linear copolysiloxane substituted with aromatic acid side chains have been investigated in the temperature range of C. The pyroelectric coefficient of the superlattice increases with temperature, exhibiting the highest coefficient of 6.2 ± 0.2 cm K-1 at 29 °C. The dielectric behavior of the superlattice is investigated by examining the dielectric loss and capacitance in the frequency range of 102 to 105 Hz. The maximum loss values for all temperatures in the range occur at a frequency ∼5000 Hz which correspond to a relaxation time of 3.18 × 10-5 s. The Cole-Cole plots obtained experimentally are found to be broad and fall inside the semicircle derived from the Debye relaxation theory. However, they agree fit with the semi-empirical Cole-Cole relaxation model. The parameter α obtained from the Cole-Cole relaxation model decreases with temperature indicating the rise in the internal degrees of freedom of the dipole in the superlattice.     

Observation of slowly decreasing molecular oscillations in ultrathin liquid films using X-ray reflectivity

We studied ~0.5 μm and 30–80 ? thick films of a normal dielectric liquid, tetrakis(2-ethylhexoxy)silane (TEHOS), at temperature range 228–286 K, deposited onto silicon (111) substrate with native oxide using X-ray reflectivity. TEHOS is spherical with size ~10 ?, non-polar, non-reactive, and non-entangling; TEHOS has been reported to show interfacial layering at room temperature and surface layering at 0.23 T_c (T_c≈ 950 K). For films m thick, the reflectivity data did not change significantly as a function of temperature; for films 30–80 ? thick, the reflectivity data did change. The data could be fitted with an electron density model composed of a minimum necessary number of Gaussians and a uniform density layer with error-function broadened interfaces. When the film thickness is 60–80 ? below 246 K, we found that the interface and the surface layering coexist but do not overlap. When the film thickness is 30–40 ? below 277 K, they overlap and the electron density profile shows slowly decreasing molecular oscillations at the air-liquid interface.