Temperature Dependence Of Optical Absorption Research Articles

Thermally induced effects on optical absorption in Ag-doped Bi12SiO20 single crystals

The spectral-temperature dependences of optical absorption of Bi12SiO20 crystals doped with Ag ions were measured in the visible range of photon energy hν. The temperature of the samples was varied in heating-cooling cycles in the interval T = 280 – 680 K. It was shown that the spectra fulfilled Urbach rule for hν > 2.1 eV at the forbidden zone width of 3.3 eV. The Urbach rule parameters, calculated within the frame of the electron-phonon interaction model, showed anomalous behavior in the interval T = 389 – 500 K. The observed peaks in the temperature dependences of optical absorption were attributed to the competition of thermally induced processes of color centers formation and destruction. The color centers were assumed in the form of Ag2+ – 2O- and Ag+ – O- complexes that are created due to thermal activation energy Ea1 = 0.1 eV and Ea2 = 0.145 eV, respectively. The spectrum of thermochromic effect characterizing non-reversible thermally induced weakening of optical absorption was detected and analyzed.

Exploring unusual temperature-dependent optical properties of graphite single crystal by spectroscopic ellipsometry

This study investigated the optical properties of graphite single crystal as a function of temperature between 4.5 and 500 K and within the spectral range of 0.73–6.42 eV through spectroscopic ellipsometry. At room temperature, the complex dielectric function of graphite exhibited broad spectra in the ultraviolet energy region. The Breit–Wigner–Fano (BWF) line shape analysis of the optical absorption spectrum demonstrated that two BWF line shapes were fitted with central energies at 4.85 ± 0.01 eV and 6.21 ± 0.03 eV. These two features were attributed to interference between an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Analysis of the temperature-dependent asymmetric BWF line shape indicated that the peak position shifted to lower energies under increasing temperatures, with the linewidth becoming narrow and the intensity increasing; this can be explained by the asymmetric factor 1/qBWF, which decreased under increasing temperatures. The temperature dependence of optical absorption for graphite is opposite to that of the interband absorptions observed in silicon. First-principles calculations verified the presence of two resonant conditions in the ultraviolet energy region. The lifetime of surface plasmons was expected to decrease with increased temperatures, leading to a decrease of the 1/qBWF. These results offer insight into quasiparticle band structures and collective excitations of graphite and provide valuable information for the technological development of graphite-based ultraviolet optoelectronic and photonic devices at different temperatures.

Anomalous optical switching and thermal hysteresis behaviors of VO2 films on glass substrate

In order to study the optical switching of vanadium dioxide (VO2) and its thermal hysteresis behavior, we fabricated VO2 films on glass substrates at various oxygen pressures. Only the VO2 films made at 5–10 mTorr displayed x-ray diffraction peaks and metal-insulator transition. Upon heating and cooling, remarkably, these samples exhibited significant optical switching behaviors with a clockwise thermal hysteresis in the infrared range of >850 nm or with a counterclockwise thermal hysteresis in the near visible range of 650–850 nm. The temperature dependence of optical absorption was explained by the O2p-V3d splitting depending on the structural transition of VO2.

Thermal vibrational disorder of a conjugated polymer in charge-transfer complex

Temperature dependences of optical absorption and Raman spectra of ground-state charge-transfer complex (CTC) formed in blends of a conjugated polymer, poly[2-methoxy-5-(2(')-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), and low-molecular-weight acceptor, 2,4,7-trinitrofluorenone, were studied. Upon cooling from 320 to 120 K, the polymer strongest Raman band shows a 0.5 cm(-1) low-frequency shift while it demonstrates a 0.5 cm(-1) high-frequency shift in the CTC. This behavior is explained by the temperature dependence of polymer-acceptor charge transfer in the CTC: The amount of transferred charge decreases by 25% upon cooling. At the same time, both the pristine polymer and CTC demonstrate a 0.05-0.1 eV redshift of the absorption edge. To account for these temperature shifts, we propose a model that relates temperature variations in the effective conjugation length with thermal torsion vibrations of the conjugated chains. Comparison of the model and experimental data reveals that the torsion rigidity of conjugated chains involved in the CTC is 30% stronger than that of the pristine ones. This enhanced rigidity increases the conjugation length of MEH-PPV chains in the CTC by 20%. However, the major contribution to the MEH-PPV absorption edge shift in the CTC is assigned to a local built-in electric field effect induced by the ground-state charge transfer.

Temperature dependence of optical absorption for the visible region in transparent conducting oxides

We study theoretically the temperature dependence of the optical-absorption coefficient, for the visible region, in thin films of transparent conducting oxides by using the well-known Varshni approach relative to optical band-gap energy. Zero absorption is considered and an approximate formula for the coefficient of visible absorption is derived when photon energy is near the band-gap energy, that is, when absorption is negligible.

Temperature dependence of optical absorption in In1-xGaxAs solid solutions

Absorption spectra near the fundamental absoption edge of n-type of In 1-x Ga x As are studied. The temperature coefficient of the In 1-x Ga x As energy gap, dEg/dT, has been obtained and compared with calculated data. An exponential dependence of the absorption coefficient on photon energy has been found. The slope of the exponential absorption curve is discussed.

Photo-and thermally induced optical absorption and photoconductivity of sillenite crystals

A study is reported of photo-and thermally induced changes in the optical absorption and photo-conductivity spectra of Bi12SiO20 crystals in the 0.5–3.5 eV spectral interval, and of the temperature dependence of optical absorption for temperatures from 80 to 600 K. An analysis is made of the electron redistribution processes between the shallow and the deep donor and acceptor levels by invoking the configurational coordinate model.

Thermal defect formation in undoped and Cr-and Mn-doped Bi12SiO20 crystals

The thermally stimulated depolarization currents and the temperature dependences of optical absorption are investigated. Interaction processes between defects are observed. A model consisting of association-dissociation of quasidipoles in the form of donor-acceptor pairs with a distribution of activation energies and relaxation times is proposed.

Temperature Dependence of the Optical Absorption Peaks at around 1990 cm-1 in Electron-irradiated Si doped with Hydrogen

We studied the temperature dependences of optical absorption peaks around 1990 cm-1 in electron-irradiated Si doped with hydrogen. Contrary to a previous report, they are well separated even at room temperature.

Temperature dependence of optical absorption by the single-substitutional nitrogen donor in diamond

In contrast to virtually all natural diamonds, synthetic diamonds show a considerable, reversible, change in colour upon heating. This colour change from yellow to reddish brown in the type Ib diamonds is especially noticeable above 500 K and is attributed to the interaction of optical transitions at the substitutional nitrogen impurity centres with lattice vibrations. Optical absorption spectra of homogeneous regions in several crystals were measured as a function of temperature (77-773 K) in the wavelength interval 200-700 nm. The spectra are interpreted in terms of an analytical model for electronic transitions from a deep donor level to a continuous range of levels in the conduction band, where strong electron-phonon coupling is taken into account. Numerical evaluation based on this model yielded a Huang-Rhys factor, S=16+or-1, a phonon energy, h(cross) omega =0.15+or-0.02 eV and the temperature dependence of the nitrogen donor ionization energy. Upon a temperature rise to 750 K the latter showed a decrease of about 0.08 eV.

Temperature dependence of optical absorption induced by exciton resonances in GaAs/GaAlAs multi-quantum-well structures

Optical transmission of GaAs/GaAlAs MQW has been measured in the temperature interval 2–300 K. The absorption coefficient has been derived at different temperatures. Splitting of excitonic resonance peaks related to heavy and light hole-electron are clearly observed up to room temperature. The energy of excitonic peaks changes as a function of temperature just as the forbidden energy gap of GaAs does and takes up the values which could be calculated when the related well size was considered. The line-shape of absorption spectra is fitted by Gaussian functions and a temperature independent ratio of the oscillator strengths related to light and heavy hole excitons has been determined. The thermal broadening of the absorption peaks related to n = 1 exciton states is explained in the light of interaction with the thermal phonons (LO phonons in GaAs); the energy derived from the high temperature increase of the absorption peak halfwidth was found to be dependent upon the internal electric field.

Conformation and conformational fluctuation of Bacillus stearothermophilus 5S ribosomal RNA

To investigate the conformational stability and fluctuation of the thermophilic 5S ribosomal RNA (rRNA), the thermal melting and hydrogen exchange processes of Bacillus stearothermophilus 5S rRNA were studied in Tris and other buffers including K + (or Na +) and/or Mg 2+, and the results were compared with those of other non-thermophilic 5S rRNA. The unfolding of two double helices was observed in differential optical absorption and c.d. melting curves in the absence of Mg 2+ or at low [Mg 2+]. At high [Mg 2+] the melting becomes apparently monophasic and temperature dependence of optical absorption is negative or almost zero, especially at 260 nm, at low temperatures < 40°C. Various physicochemical measurements show that under such conditions the 5S rRNA is in a highly folded and less fluctuating conformation that the other non-thermophilic 5S rRNA. Hydrogen exchange profiles demonstrated differences in the conformational fluctuations between B. stearothermophilus and the other 5S rRNA.

The temperature dependence of optical absorption near the successive phase transition points of RbNO3crystal

The temperature variation of the absorption spectrum in the energy range from 3.40 to 5.0 eV is measured at temperatures from 15 to 291 degrees C for the rubidium nitrate crystal. Anomalous behaviour in the absorption spectrum has been observed near the phase transition points. It is found from profile analyses of the spectra at various temperatures that the interaction between the electronic process and the lattice system in the crystal changes at the transition points.

Temperature Dependence of Optical Absorption near the Structural Phase Transition of Potassium Nitrate

AbstractThe electronic properties of a KNO3 crystal near the structural phase transition are investigated by observing ultraviolet absorption spectra. The spectra show an anomalous temperature dependence near the transition point Ttr. The anomalous behaviour of the exponential absorption tails in the spectra is observed and discussed phenomenologically in relation to anomalies in the dielectric constant and lattice constant of KNO3.

The temperature dependence of optical absorption near the absorption edge of Tl3 As Se3

The optical transmission of Tl3 As Se3 as a function of wavelength and temperature is reported for the 1.2 – 2.5 um spectral range, and at temperatures from 40 to 300K. The absorption edge for the material occurs at about 1.011 eV. The material also indicated direct transitions at about 1.015 eV and indirect transitions at about 1.001 eV. The direct and indirect transitions are found to be independent of temperature. Electrical measurements have identified the sample as p-type, with hole concentrations of 5 × 1015 and 6 × 1012 cm−3 at 395 and 300K, respectively. The sample seems to be a good insulator at all temperatures between 40 −300K.

Temperature Dependence of Optical Absorption near the Order-Disorder Phase Transition of Sodium Nitrate

The optical absorption spectrum of sodium nitrate, NaNO 3 , has been measured for the wavelength range from 6000 A to 2000 A at temperatures between 49.5°C and 298.6°C. The optical absorption of the two absorption bands shows anomalous temperature dependence near the order-disorder phase transition of NaNO 3 . Phenomenological relation between the energy gaps and the heat capacity has been confirmed. Low energy tails of the two absorption bands have shifted to lower energy side with increasing temperature, and they have been found to obey the Urbach rule within experimental error.

The influence of phonons on the optical properties and the conductivity of quasi-one-dimensional metals

The frequency dependent conductivity σ(ω) completely taking into account the interaction between electrons is studied. The shape and the temperature dependence of optical absorption near the frequency of a molecular phonon activated due to the interaction with electrons are found. For a system with attractive sign of the e- e backward scattering amplitude g 1<0 an absorption edge near the gap 2Δ in the electronic spectrum is studied. A low frequency conductivity is discussed. The properties under consideration depend essentially on the magnitudes of e- e interactions and are critical to the sign of g 1.

Temperature dependence of optical absorption of a solvated electron

The theory of the temperature shift in the absorption spectrum of a solvated electron is discussed. The theory is based on the concept of the interaction of an electron bound in a zero-range potential with long-wavelength classical vibrations of the medium. The theory is compared with data for the absorption spectra of e–s in methyl alcohol measured over a wide range of temperatures. Satisfactory quantitative agreement has been found.

Energy Levels of Chromium Ion Pairs in Ruby

Abstract : The lower energy levels of a number of transitions due to second, third, and fourth nearest neighbor chromium ion pairs in ruby are located by measuring the temperature dependence of optical absorption. Energy level diagrams are constructed, from which many of the observed lines are identified. The measurements are consistent with earlier results for the ground states of first, third,, and fourth nearest neighbor pairs. The ground-state levels of the second nearest neighbor pairs are describable by an isotropic exchange interaction. The ground-state coupling parameters for the first four nearest neighbor types are used to estimate the Neel and Curie-Weiss temperatures for Cr2O3. (Author)