Excitonic molecule in semiconductors by two-photon absorption

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect.     

Stabilized high-temperature hexagonal phase in copper halide nanocrystals

Low-temperature (T = 4.2–77 K) absorption spectra of CuCl and CuBr nanocrystals in photochromic glass matrixes are studied. A fine structure of exciton absorption bands (Z₃ band for CuCl and Z₁₂ band for CuBr) is discovered and studied as a function of nanocrystal size. It is suggested that the high-energy part of the absorption band is due to the high-temperature hexagonal β phase being stabilized in very small samples; a transition to the stable cubic phase with increasing nanocrystal size is demonstrated.     

Transmission Spectra of HgTe-Based Quantum Wells and Films in the Far-Infrared Range

Strained 80-nm-thick HgTe films belong to a new class of materials referred to as three-dimensional topological insulators (i.e., they have a bulk band gap and spin-nondegenerate surface states). Though there are a number of studies devoted to analysis of the properties of surface states using both transport and magnetooptical techniques in the THz range, the information about direct optical transitions between bulk and surface bands in these systems has not been reported. This study is devoted to the analysis of transmission and reflection spectra of HgTe films of different thicknesses in the far-infrared range recorded in a wide temperature range in order to detect the above interband transitions. A peculiarity at 15 meV, which is sensitive to a change in the temperature, is observed in spectra of both types. Detailed analysis of the data obtained revealed that this feature is related to absorption by HgTe optical phonons, while the interband optical transitions are suppressed.

     

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi₂O₅ glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.     

Distribution of electron density of states in allowed bands and interband absorption in amorphous semiconductors

Different types of electronic transitions and the corresponding spectral characteristics of the absorption coefficient of amorphous semiconductors, where the energy of absorbed photons exceeds the mobility band gap, have been investigated. Partial spectral characteristics of the absorption coefficient and, correspondingly, the distributions of the electron density of the states involved in these optical transitions are obtained for the case where the electron densities of states in the allowed bands and the tails of these bands change with energy according to the power and exponential laws, respectively. The conditions for the occurrence of a peak in the spectral characteristic of the interband absorption coefficient are determined.     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.     

IR absorption and Raman spectra of single crystals of stable germanium isotopes

The Raman and IR absorption spectra of single crystals of germanium isotopes ⁷²Ge, ⁷³Ge, ⁷⁴Ge, and ⁷⁶Ge in the region of phonon absorption and interband electronic transitions are studied at room temperature. The dependence of the Raman peak position on the atomic mass has the form ν ~ M^–1/2. The shifts of the phonon absorption peaks of individual isotopes with respect to germanium of natural isotopic composition ^natGe are determined. With increasing average atomic mass of germanium, these peaks shift to longer wavelengths. In the region of interband electronic transitions, the intrinsic absorption edge of ⁷⁶Ge is observed to shift by 1 meV to higher energies with respect to Ge of natural isotopic composition. For isotopes with atomic masses close to that of natural germanium (⁷²Ge,⁷³Ge, ⁷⁴Ge), we found no significant difference in the band gap width at room temperature.     

Thermally induced optical bistability in CdHgTe

We report optical bistability in a room temperature, uncoated Cd_0.185Hg_0.815Te etalon using a cw CO₂ laser at 10.6 μm. The results are consistent with a band gap resonant, dispersive nonlinearity induced by a thermal shift of the band gap energy through interband and free carrier absorption. A refractive index temperature coefficient, dn/dT ～ ? 1 × 10^-3K^-1 was measured, and bistability was observed at incident powers of 20 mW.     

Indirect and direct optical transitions in the self-absorption spectra of SbI3 single crystals

The long-wave self-absorption edge of SbI₃ single crystals in the 293–95 ° K temperature range and their reflection spectrum inside the self-absorption band at room temperature were investigated. The absorption associated with indirect and direct interband transitions was isolated. The principal parameters characterizing indirect and direct optical transitions were determined.     

Etude de l'absorption optique du vanadium en couches minces interpretation par la theorie des bandes d'energie

Optical absorption of vanadium thin films has been determinated from 0.32 to 5.5 eV from reflectance and transmittance data. The films have been deposited in ultra high vacuum and the measurements have been realised in situ. Between 1 to 5.5 eV, we note a large absorption band which is independant of the photons angle of incidence and polarisation; with an important maximum and a shoulder respectively at 3.1 and 2 eV. We explain this absorption band by direct interband transitions deduced from theorical bands calculated by Yasui et al. From 0.32 to 1 eV, the observed absorption is principally due to intraband transitions.     

The reflection spectra of cuprous halides at low temperature in the 4.5 to 30 eV range and related band structures

In a previous paper, we proposed a band scheme for C u C l established with the help of the reflection spectra measured at low temperature up to 30 eV. Now we are extending this study to CuBr and CuI where the optical data are obtained in the same way. Some experimental features are characterized, through the whole sequence, by their thermal behaviour and by the evolution of their structure in relation to the increasing spin orbit separation from CuCl to CuI. This study also shows the similarity of the band schemes in the three halides and the interest of optical measurements, at low temperature and in a wide energy range, for band scheme determination.     

Optical verification of the valence band structure of cadmium arsenide

Optical absorption measurements were performed on thin single crystalline samples of Cd₃As₂ at temperatures of 300 K and 10 K. At low temperature the interband absorption coefficient shows clearly two steps due to direct transitions from the heavy hole and light hole valence bands to the conduction band. The absorption coefficient can be interpreted quantitatively in an isotropic inverted Kane band model with a modified heavy hole band with its maximum shifted from the Λ-point.     

Theory of interband optical absorption in a quasiclassical random field

Green's functions are used to study interband transitions in a quasiclassical (i.e. slowly varying in space) irregular field. It is assumed that the irregular field is random, i.e., the mean square of the fluctuation of the potential energy of an electron in such a field is much less than the width of the forbidden band. Explicit expressions are obtained for the coefficient of interband optical absorption for the case of an anisotropic irregular field and for the case when the irregular fields that act on electrons in the conduction and the valence band are different. As examples, we consider a fluctuation random field in ferroelectrics and a random field produced by long-wave acoustic lattice vibrations. The behavior of the tail of the optical absorption coefficient in ferroelectrics in the region of a phase transition is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 121–130, April, 1972.I am most grateful to V. L. Bonch-Bruevich for suggesting the subject and for his constant interest.     

Absorption and fluorescent properties of pyrylium compounds: II. Spectra and cross sections for absorption from the ground and excited states of the structurally rearranged form

The nonstationary absorption of solutions of 2,4,6-substituted pyrylium salt is studied by the pump-probe method. The solutions in methylene chloride are studied upon picosecond pumping and probing with different time delays. The solutions in methyl ethyl ketone are studied upon pumping and synchronous probing by nanosecond pulses. It is shown that excitation of the initially nonplanar molecule causes its structural rearrangement. As a result, substituent 2 enters into the pyrylium ring plane and two nonstationary conformers are formed that differ in the position of substituent 6. The motion upon structural rearrangement has a small amplitude, and localization of molecular orbitals changes only slightly. The temperature dependence of the nonstationary absorption is studied in the range from 293 to 223 K. The spectra and cross sections are measured for transitions from the S₀ and S₁ states. In addition, the activation energies and kinetic parameters of the rearrangement in the ground and excited states are determined. Experimental data are compared with theoretical calculations of the transitions from the S₁ state. It is shown that a partial flattening of conformer molecules results in the long-wavelength shift of the bands and an increase in the cross sections for transitions localized on a flat part of the molecule and does not change parameters of the band localized on the molecular part that did not change upon the rearrangement but remained nonplanar.     

Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.     

A new application of the diamond anvil cell: Measurements under uniaxial stress

We present optical absorption measurements on thin films of CuCl and CuBr at 100K under uniaxial stress up to ～3.0 GPa. The thin films were prepared by evaporating onto the inner face of one diamond of a diamond anvil cell. From the splitting of the excitons we were able to determine the shear deformation potential d = -0.69 ± 0.03 eV for CuBr and d = +0.45 ± 0.08 eV for CuCl. Our measurements indicate also that the spin-orbit splitting increases in both materials with increasing pressure. A new shear-induced phase is found for CuCl above 4.8 GPa.     

Some optical properties of amorphous and crystalline antimony trisulphide thin films

The change in optical properties accompanying the amorphous crystalline transition has been studied for antimony trisulphide thin films. The real and imaginary parts of the dielectric constant are found to be much lower for amorphous films at lower photon energies, possibly because of a large number of defect states which would mostly disturb the top of the valence band and the conduction band comprised, respectively, of chalcogen lone pair and antibonding states. The crystalline material shows some structure in the imaginary part of the dielectric constant that corresponds to interband transitions, and apparently the direct band edge is at 1.88 eV. In the edge region the power law absorption has been observed in the amorphous material from which the extrapolated optical gap has been found to be 1.7 eV.     

Optische Bestimmung der Nichtparabolizität des dritten Valenzbandes von Germanium

Measurements of the optical absorption of free holes in germanium produced by direct intraband transitions in the valence band have been made at temperatures of 300–390 °K. The experimental method of the photoabsorption was used which enables the absorption spectra to be determined at intrinsic material without an influence of the absorption edge due to interband transitions between the valence band and the conduction band. An analysis of the spectra gives an experimental determination of the shape of the non parabolic split-off valence band. By comparison withKane's theory a marked difference is found but good agreement with calculations fromFawcett is achieved. The temperature dependence of the shape of the split-off band is discussed concerning the optical absorption measurements on “hot holes” in high electric fields with respect of the non-equilibrium energy distribution of the carriers.