Specific features of the reflection of infrared radiation by crystalline dielectrics in a magnetic field

Magnetic-field-induced variations in the reflection spectra R(λ) of the crystalline dielectrics Al₂O₃, LiF, and MgO in the infrared band (λ = 2.5–25 μm) are investigated. It is found that the reflection spectra exhibit specific features in the neighborhood of wavelengths corresponding to the excitation of optical phonon modes in the above-mentioned crystals and that a magnetic field causes an appreciable variation in the reflectivity at these wavelengths. To qualitatively describe the effect of a magnetic field on the reflection of light, the magnetoreflection spectra ΔR/R are investigated. The spectra ΔR/R exhibit sharp peaks in the neighborhood of wavelengths at which the materials under investigation are characterized by minimal reflectivity. The values of ΔR/R for p-polarized infrared radiation in a magnetic field of about 12 kOe amount to about 0.5% for Al₂O₃ at λ ≈ 9.6 μm, 7% for LiF at λ ≈ 11.1 μm, and 0.07% for MgO at λ ≈ 11.7 μm.     

Two mutually-repelling self-avoiding walks: A Monte Carlo study

We consider two self-avoiding walks (SAWs) on a square lattice, beginning simultaneously from two arbitrary nearest neighbours. The walks cross neither themselves nor each other, and grow simultaneously. Using Monte Carlo technique we study the variation, with the length N of the walks, of the average end-to-end distance 〈R_N〉 of each walk and of their average separation 〈S_N〉. We find 〈R_N²〉∞N^2ν, 〈S_N²〉∞N^2λ, where ν ≈ λ ≈ 0.75 for two ordinary SAWs and ν ≈ λ ≈ 0.67 for two growing SAWs in two dimensions. For two directed SAWs, we find 〈R_N∥/⊥²〉 ∞ N^2ν∥/⊥ and 〈S_N∥/⊥² 〉 ∞ N^2λ_∥/⊥, where ν_∥ ≈ 1.00, ν_⊥ ≈ 0.56, λ_∥ ≈ 0.50 and λ_⊥ ≈ 0.59 in two dimensions. We thus find an indirect excluded volume effect on one directed SAW, due to the other.     

Coloring of silica glass with silver nanoparticles

The possibility of doping of surface layers of optical fused silica with silver nanogranules upon heating of a thin Ag film on the silica surface by a CO₂ laser beam (P ≈ 30 W, λ = 10.6 μm) is established. The effect of exposure time on the doped layer structure has been investigated. The absorption band of the colloidal solution of Ag in silica has been studied. It is shown that this band is homogeneously broadened and its peak (at 420 nm) corresponds to the small volume filling factor (q < 0.1) at an average granule radius of about 3 nm. Based on the measurements of the radial distributions of the reflectance and refractive index over the doped region at λ = 633 nm, it is revealed that the doped layer is an area-irregular asymmetric step waveguide.     

Intersubband absorption with difference-frequency generation in GaAs asymmetric quantum wells

An asymmetric quantum well(AQW) is designed to emit terahertz(THz) waves by using difference frequency generation(DFG) with the structure of GaAs/Al 0.2 Ga 0.8 As/Al 0.5 Ga 0.5 As.The characteristics of absorption coefficients are analysed under the parabolic and non-parabolic energy-band conditions in detail.We find that the absorption coefficients vary with the two pump optical intensities,and they reach the maxima when the pump wavelengths are given as λ p1 = 9.70 μm and λ p2 = 10.64 μm,respectively.Compared with non-parabolic conditions,the total absorption coefficient under parabolic conditions shows a blue shift,which is due to the increase in the energy difference between the ground and excited states.By adjusting the two pump optical intensities,the wave vector phase-matching condition inside the AQW is satisfied.     

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature T _c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, Δ_Γ = 5.1–6.5 meV, Δ_L = 3.8–4.8 meV, and Δ_S = 0.9–1.9 meV (at T ? T _c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ～12, and ～20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming Δ_Γ and Δ_L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ_ΓL ≈ λ_LΓ ? λ_SΓ, λ_SL.     

Comparison of magnetoreflection from CoFe-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> multilayer and granular films in the infrared spectral region

The paper reports on a study of light reflection and of the magnetorefractive effect in CoFe-Al₂O₃ multilayer and granular films in the IR region at λ = 2.5–25 μm. These films exhibit a noticeable tunneling magnetoresistance of 5–8%. It has been found that the spectra of the magnetorefractive effect have features at λ ≈ 6.7 and ≈8.1 μm originating from phonon mode excitation in the dielectric matrix. It has been established that the magnitude of the magnetorefractive effect exceeds 2.5% for multilayer films, a value substantially larger than that for granular ones. Magnetoreflection has been analyzed for two film systems with roughly equal tunneling magnetoresistance. It has been shown that magnetoreflection for granular films can be described in a first approximation by the modified Hagen-Rubens relation, while the enhanced effect exhibited by multilayer films requires inclusion of interference of the magnetooptical response.     

Stimulated recombination from the defect level in doped lead telluride

The relaxation processes of the photoexcited carriers from the defect level in the band gap to the valence band states were investigated in Na and Tl doped p-type PbTe single crystals at T = 77 K. The observed photosignal oscillations were proved to be induced by stimulated recombination of photoexcited carriers from the defect level E_d ≈ 50 meV above the top of the valence band. Non-equilibrium carrier inversion population was produced by impulses of a TEA CO₂-laser. The observed stimulated recombination may presumably be used for designing IR semiconductor lasers operating in the wavelength range of λ ∼ 25 μm at T = 77 K.     

Image force effects on the short wavelength (UV) photoresponse of silicon Schottky barriers

An experimental study has been made of the short wavelength λ response of near-ideal silicon Schottky barrier photodetectors. It is shown that the major cause of reduced quantum efficiency (Q.E.) in this range of λ is the collection by the metal of majority carriers photogenerated within the image force maximum, in agreement with the theoretical predictions of Green. The bias-voltage dependence of the photocurrent in the near ultraviolet region is in good quantitative agreement with the image force model, with a Q.E. near unity for large reverse bias decreasing by approx. 15% at zero bias for λ = 0.37 μm, for example. The Q.E. under short-circuit conditions is relatively independent of λ for λ ? 0.45 μm, falling rapidly with decreasing λ from 95% at 0.45 μm to approx. 80% at 0.35 μm, again in good quantitative agreement with the above theory.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

The orange luminescence band in MgO crystals

Nominally pure MgO crystals exhibit a bright orange luminescence when excited with u v or visible laser light This luminescence has been characterized using a variety of measurements with pulsed and cw laser sources Its amplitude increases with temperature to about 400 K. while the lifetime of emission decreases from 270 μs at 290 K to 9 μs at 433 K This emission appears to originate in hole recombination with Fe²⁺ ions Hole trapping at V^?_OH and V^?_Al centers may occur as an intermediate step Bleaching with 514 5-nm Ar⁺ laser light is reversible upon exposure to u v radiation Present data are compared with those obtained previously on the thermoluminescence of irradiated MgO crystals.     

The effect of phase difference of subpicosecond IR laser pulses propagating in a GaAs/ZnTe periodic structure on the efficiency of generation of the sum and difference frequencies radiations

The results of theoretical research and numerical simulation of the nonlinear interaction process of mutually orthogonal and linearly polarized IR subpicosecond laser pulses with the one-dimensional periodic structure of the GaAs/ZnTe are presented. In particular, the interaction of such pulses with the equal amplitude electric field of 70.71 MV/m, with the duration of 333 fs at the central wavelength λ₀ = 10 μm, with the periodic structure of the GaAs and the ZnTe with the thickness of layers equal to λ₀/2n_GaAs = 1.527 μm and λ0/4n_ZnTe = 0.931 μm, respectively, and with the number of periods equal to 100 is considered. It is shown that the efficiency of generation of sum and difference frequencies is proportional to the phase difference of the interacting IR pump pulses.     

Observation of coherent phonon states in porous silicon films

The dynamics of differential transmission and reflectance spectra of porous silicon films was studied using the femtosecond excitation technique (τ≈50 fs, ?ω_pump=2.34 eV) with supercontinuum probing (?ω_probe=1.6–3.2 eV) and controlled time delay with a step of Δt=7 fs between the pump and probe pulses. A short-lived region of photoinduced bleaching was observed in the differential transmission spectra at wavelengths shorter than the pump wavelength. The excitation of coherent phonon states with a spectrum corresponding to nanocrystalline silicon with an admixture of a disordered phase was observed. The relaxation of electronic excitation was found to slow down in the spectral region where the amplitude of excited coherent vibrations was maximal.     

Saturated absorption spectroscopy: Elimination of crossover resonances with the use of a nanocell

It is demonstrated that the velocity-selective optical pumping/saturation resonances of the reduced absorption in a Rb vapor nanocell with thickness L = λ, 2λ, and 3λ (resonant wavelength λ = 780 nm) allow for the complete elimination of crossover (CO) resonances. We observe well-pronounced resonances corresponding to the F _g = 3 → F _e = 2, 3, and 4 hyperfine transitions of the ⁸⁵Rb D₂ line with line widths close to the natural width. A small CO resonance located midway between F _g = 3 → F _e = 3 and F _g = 3 → F _e = 4 transitions appears only for L ≥ 4λ. The D₂ line (λ = 852 nm) in a Cs nanocell exhibits a similar behavior. From the amplitude ratio of the CO and VSOP resonances, it is possible to determine the thickness of the column of alkali vapor in the range of 1–1000 μm. The absence of the CO resonances for nanocells with L ～ λ is attractive for the frequency reference application and for studying the transitions between the Zeeman sublevels in external magnetic fields.     

Absolute photoabsorption cross sections for H2O and D2O from λ 180–790 Å

Absolute photoabsorption cross sections for H₂O and D₂O have been measured photoelectrically from λλ 180 to 790 Å using synchrotron radiation. The cross sections increase smoothly with wavelength to ～λ610 Å, with both H₂O and D₂O displaying a broad absorption band extending above a nearly linear background from λλ 400 to 490 Å. The continuum has a maximum of ～ 22.5 Mb at λ 640 Å. Above λ 615 Å, superimposed on the continuum, a diffuse structure appears which is similar to the vibrational structure of the ²B₂ states of H₂O⁺ and D₂O⁺ as observed in photoelectron spectra. The structure is believed to arise from excitation of a 1b₂ electron to the vibrational levels of a Rydberg orbital with n¹ ≈ 2.64.     

X-ray spectroscopy of multiply-charged ions from laser plasmas

Original results are reported on the observation and identification of spectra of multiply-charged ions in the range of λ ≈ 1.5–15 Å, which corresponds to transitions with a range of principal quantum number n. The main part of the review consists of tables with about a thousand spectral lines, which have been mainly observed in laser-plasma radiation, as well as in the solar corona and other laboratory sources at an electron temperature T_e≈10⁷°K. The accuracy for the wavelengths (Δλ) is the following: Δλ is equal to ≈ 0.0005 Å for λ ≈ 2.5 Å and it is equal to ≈ 0.003 Å for λ ≈ 15 Å. The spectral lines are considered for the following transitions: 1-n type for [H]-like ions (Z = 11–16)and [He]-like ions (Z = 11–26); 2-n type for [Li]-like ions (Z = 19–26), [Be]-like ions (Z = 22–34) and [Ne]-like ions (Z = 26–42); 3-n type [Co]- and [Ni]-like ions (Z = 73). The line-list contains about four hundred wavelengths for multiply-charged iron L-ions (Fe(XVII)-Fe(XXIV)) and is presented with identification of some of the transitions. The wavelengths and intensities of satellites of the [H]-like ions and [He]-like ions, which are caused by transitions from the doubly-excited autoionization states 2l2l′ and 1s2l2L′ of [He]-like ions (Z = 11–16) and [Li]-like ions (Z = 11–26), respectively, are also considered.     

Two-Photon Absorption of Nonchain HF Laser Radiation in Germanium Single Crystals

Some results of experimental and numerical studies on the transmission of a nonchain high-frequency (HF) laser beam through germanium (Ge) single crystals of differing thickness and specific resistance are presented. Based on the experimental data for the HF(DF) lasing spectrum, the coefficient of two-photon absorption in germanium has been estimated as K₂ = 55 ± 10 cm/GW at λ = 2.8 μm. The results are in good agreement with theory. The developed experimental data-processing software has enabled the numerical study of the transmission of a high-power beam with λ = 2.6–3 μm through Ge at any laser-pulse-impact moment. It has been shown that thin germanium coatings can efficiently smoothen the distribution of energy over the beam aperture for high-power laser radiation with λ = 2.6–4 μm.     

XPS-investigation of intramolecular charge transfer in polynitro-aminobenzenes

It has been shown that the shake-up energy ΔE¹, which depends on the ionization potential (IP) of the neutral (poly)nitro-aminobenzenes, vanishes for IP≈ 8.4 eV. The λ values, which measure the contribution of intramolecular charge-transfer configurations in the molecules-in-molecules models correlate rather well with the difference E_II- E₁, i.e., the difference of the peak positions corresponding to lower and higher binding energies.     

Spontaneous periodic structures induced in thin AgCl-Ag films by the continuum in the case of optical rotation of the plane of polarization

The formation of spontaneous periodic structures in thin AgCl-Ag films subject to the continuum (λ≈450–700 nm) in the case of rotation of the plane of polarization with the help of crystalline quartz is studied. The periodic structures are formed due to the excitation of waveguide TE₀ modes in spontaneous Rayleigh scattering of radiation with a wavelength λ incident on the film. The structure periods are smaller than λ and vary in proportion with λ. After special treatment of the irradiated sample (fixing and deposition of an Al layer), periodic structures form a thin reflection hologram capable of reproducing the direction of linear polarization, the angle of optical rotation, and the color of the spectrum recorded. The study of diffraction patterns and small-angle scattering from small portions of a hologram, which were exposed to light with various wavelengths in the recording process, with the help of a probing UV beam (λ_p=337 nm), made it possible to reveal special features of the manifestation of Rayleigh and Wood anomalies associated with the domain structure of PS.     

Evaluation of XPS-data of oxide layers

XPS with variable take-off angle has been applied to the determination of the thickness of thin oxide layers (SiO₂ on Si). The information about D/λ_ox gained by such measurements is strongly influenced by surface roughness. This influence can be demonstrated when the parameters R and D/λ_ox are calculated from sets of the experimental results and for each set the corresponding pair of R and D/λ_ox is plotted in a diagram D/λ_ox = f(R). Having the true value of R it is possible to determine D/λ_ox. With the XPS-results of at least three different oxide layers and their ellipsometric thicknesses D_e one is able to calculate λ_Si, λ_ox/λ_Si and the difference ΔD between D_e and the XPS-thickness D.     

基于金银合金薄膜的近红外表面等离子体共振传感器研究

利用淀积在玻璃衬底上的金银合金薄膜作为表面等离子体共振(SPR)芯片, 构建了Kretschmann结构的近红外波长检测型SPR传感器. 采用不同浓度的葡萄糖水溶液测试了金银合金薄膜SPR传感器的折射率灵敏度. 实验结果表明随着入射角从7.5°增大到 9.5°, SPR吸收峰的半高峰宽从292.8 nm 减小到 131.4 nm, 共振波长从 1215 nm蓝移到 767.7 nm, 折射率灵敏度从35648.3 nm/RIU 减小到 9363.6 nm/RIU.在相同的初始共振波长(λ_R)下获得的金银合金薄膜SPR折射率灵敏度高于纯金膜(纯金膜在λ_R=1215 nm下的折射率灵敏度为29793.9 nm/RIU). 利用1 μmol/L的牛血清蛋白(BSA)水溶液测试了传感器对蛋白质吸附的响应.结果表明, BSA分子吸附使得金银合金薄膜SPR吸收峰红移了12.1 nm而纯金膜SPR吸收峰仅红移了9.5 nm. 实验结果还表明, 在相同λ_R下, 金银合金薄膜SPR吸收峰的半高峰宽大于纯金膜的半高峰宽, 因此其光谱分辨率比纯金膜SPR传感器低. 关键词： 金银合金薄膜 表面等离子体共振 波长检测型 高灵敏度