Magneto-optical spectroscopy of carbon nanotubes

We review our recent optical experiments on single-walled carbon nanotubes in high magnetic fields. The data revealed magnetic-field-induced optical anisotropy as well as broadening, splittings, and shifts of interband absorption and photoluminescence peaks. Quantitative comparison with theoretical predictions based on the Aharonov–Bohm effect is presented.     

Electronic structure and dynamics of optically excited single-wall carbon nanotubes

We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron–spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for E₁₁^S transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron–phonon (e–ph) coupling in metallic tubes is characterized by a very small e–ph mass-enhancement of 0.0004. Ultrafast electron–electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs. PACS 78.47.+p; 81.07.De; 78.67.Ch; 87.64.Ni     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO, SiC and Boron-Nitride nanotubes and sheets. Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds. The excitation spectrum of the system in the presence of local electronic interactions has been found using mean field approach. We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter. The absorption spectra exhibits the remarkable peaks, mainly owing to the divergence behavior of density of states and excitonic effects. Also we compare optical absorption frequency behavior of BeO, SiC and Boron-Nitride nanotubes with each other. Furthermore we investigate the optical properties of BeO and SiC sheets. A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO,SiC and Boron-Nitride nanotubes and sheets.Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds.The excitation spectrum of the system in the presence of local electronic interactions has been found using mean Seld approach.We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter.The absorption spectra exhibits the remarkable peaks,mainly owing to the divergence behavior of density of states and excitonic effects.Also we compare optical absorption frequency behavior of BeO,SiC and Boron-Nitride nanotubes with each other.Furthermore we investigate the optical properties of BeO and SiC sheets.A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Ultra-fast optical spectroscopy of micelle-suspended single-walled carbon nanotubes

We present results of wavelength-dependent ultra-fast pump–probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks and, consequently, the pump–probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs) we observe subpicosecond decays, as has been seen in previous pump–probe studies. We ascribe these ultra-fast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultra-fast carrier relaxation: fast (0.3–1.2 ps) and slow (5–20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to interband carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H⁺ ions surrounding the nanotubes. PACS 78.47.+p; 78.67.Ch; 73.22.-f     

Effect of a random deformation-potential field on interband optical transitions

We have calculated the correlation function for the random field caused by the deformation potential due to spherical clusters randomly distributed throughout the sample. We evaluate the characteristic energy which determines the rate at which the interband optical absorption coefficient decreases as the frequency decreases in the neighborhood of the optical tail.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 3–6, February, 1984.     

Development of an optical time scale

A time standard based on the use of an optical oscillation period of a frequencystable He–Ne laser as a time scale is first described. We obtained highly frequency-stable oscillations in the SHF range that were locked to the oscillations of a He–Ne laser stabilized to an absorption resonance in methane at 3.39 m. A direct comparison of frequency stabilities of a rubidium standard and He–Ne/CH₄ laser has been made. The absolute measurement of the frequency of the He–Ne/CH₄ laser we performed gave a new value of frequency.This work was reported at the 3rd Frequency Standards and Metrology Symposium     

Computation of the oscillator strength and absorption coefficients for the intersubband transitions of the spherical quantum dot

The electronic structure and optical properties of one-electron Quantum Dot (QD) with and without an on-center impurity were investigated by assuming a spherically symmetric confining potential of finite depth. The energy eigenvalues and the state functions of QD were calculated by using a combination of Quantum Genetic Algorithm (QGA) and Hartree–Fock Roothan (HFR) method. We have calculated the binding energy for the states 1s,1p,1d,1f, oscillator strengths, the linear and third-order nonlinear optical absorption coefficients as a function of the incident photon energy and incident optical intensity for the 1s–1p, 1p–1d and 1d–1f transitions. The existence of the impurity has great influence on the optical absorption spectra and the oscillator strengths. Also we found that the magnitudes of the total absorption coefficients of the spherical QD increase for transitions between higher states.     

Cross section of supercooled238UF6 in multiphoton absorption induced by 16 micrometer Raman-laser radiation

Measurements of multiphoton absorption of 16 µm Raman-laser radiation in supercooled²³⁸UF₆ at 90 K were performed by using a pulsed Laval nozzle with an optical path length of 50 cm. The laser fluence was varied between 50 and 500 mJ/cm² for four frequencies in the range from 625 to 629 cm^–1. The energy absorbed by²³⁸UF₆ molecules was investigated as a function of laser frequency or fluence, and highly accurate results were obtained with the use of the nozzle whose optical path length is much greater than that of nozzles used before. The results indicated that the absorption cross section at the peak absorption frequency (627.8cm^–1) was proportional to the –1/3 power of the fluence.     

Pump–probe analysis of polaron decay in InAs/GaAs self-assembled quantum dots

We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T₁ polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.     

Mechanism for the optical absorption of cadmium sulfide

The optical transmission and reflection spectra of polycrystalline cadmium sulfide films heated in sulfur vapor have been measured at 77 and 300 °K. The absorption coefficient of the film is calculated for the range 2.25–3.5 eV. Analysis of the spectral dependence of the absorption coefficient shows indirect optical transitions of an electron from the upper valence band to the conduction band in the range 2.31–2.54 eV and direct substitutional transitions in the range 2.58–3.45 eV. Superimposed on this intrinsic absorption is selective absorption due to superstoichiometric atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 4, pp. 97–103, April, 1969.     

The effect of local electronic interaction on the optical properties of boron–nitride nanotubes

We investigate the behavior of optical absorption of boron–nitride nanotubes (6,0) in the context of Hubbard model at the paramagnetic sector. GW approximation has been implemented in order to make self-energy matrix of electronic system. Afterwards, the real and imaginary parts of transverse dielectric functions have been obtained using linear response theory. The results show that the frequency gap in the optical absorption decreases with Coulomb repulsion strength. Moreover the results show that the local Coulomb interaction leads to the appearance of the excitonic effects in the optical spectrum. Finally the effects of electronic concentration on the frequency behavior of imaginary part of dielectric function have been investigated.     

The effects of external optical feedback on the power penalty of DFB-LD modules for 2.5Gbs−1 optical transmission systems

We report the effects of external optical feedback on the power penalty of commercial distributed feedback laser diode (DFB-LD) modules for 2.5Gbs–1 optical transmission systems. External optical feedback presented to the DFB-LD modules causes the excitation of external cavity modes, resulting in increased relative intensity noise (RIN) and intensity noise ripples at low frequency region below 500MHz. For a 10–10 bit error rate (BER), the minimum power penalty is as much as 1.25dB for a feedback ratio of –8.8dB. An excess power penalty of 0.5dB per 3dB increase in the feedback ratio was also empirically obtained. We suggest that optical isolators in 2.5Gbs–1 DFB-LD modules used in conventional optical transmission systems or WDM systems must have a peak isolation ratio of better than 54.5dB, instead of the previously recommended 30dB, for negligible power penalty induced by external optical feedback.     

Interband optical absorption in disordered semiconductors including multiphonon effects. II

We examine the effect of multiphonon transitions on the frequency dependence of the absorption coefficient in the optical tail region in a semiconductor with a smooth Gaussian random field for the cases of high and low heat liberation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 39–45, March, 1980.I thank V. L. Bonch-Bruevich for suggesting this topic and constant interest in this work and B. Esser for valuable discussion.     

Spectrophotometric Determination of Sm(III) Content in Hydrochloric Acid Solution

On the basis of the electronic absorption spectra of Sm³⁺ ions in aqueous solutions of hydrochloric and perchloric acids (1M) calibration plots of samarium content against the optical density of the solutions have been constructed for frequencies 47,800, 31,460, 29,140, 27,760, 26,740, 24,940, 24,020, 21,580, and 20,900 cm^–1 corresponding to the optical density maxima. The results of the calculation of errors in determining the metal concentration are presented. It has been shown that for hydrochloric acid solutions of samarium correct determination of its content is possible at frequencies 24,940, 21,580, and 20,900 cm^–1 in the concentration ranges 1.0–3.5, 2.5–5.0, and 0.08–5.00%, respectively. For perchloric acid solutions of samarium, correct determination of its content is possible at the 24,940 cm^–1 frequency in the concentration range from 1.0 to 3.5%.     

Interzone light absorption with consideration of multiphonon transitions in semiconductors with a Coulomb-like random field

The method of calculating the interzone light absorption coefficient in a semiconductor with Coulomb-like random field developed by Bonch-Bruevich is generalized by inclusion of multiphonon processes. A formula is obtained for the frequency dependence of the absorption coefficient in the optical tail range in the Condon approximation with neglect of frequency effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 64–67, July, 1981.In conclusion, the author expresses his gratitude to V. L. Bonch-Bruevich for his interest in the study.     

Spin resonance of irradiated luminescent LiH crystals

ESR line width and strength are reported for crystals exposed at room temperature to the unfiltered light of a SVD-120 mercury arc and to the electrons from a betatron operated at 8–10 MeV. There is a knee in the fall in the intensity and in the increase in the line width at 90 °–120 ° C (g = 2.001 ± ± 0.001) for crystals showing blue fluorescence. The relation of the ESR spectrum to the optical absorption and to the luminescence centers is discussed.We are indebted to M. D. Lemberberg for assistance in recording the optical absorption spectra of LiH.     

Lithographic positioning, areal density increase and fluid transport in rolled-up nanotubes

We study the formation process of rolled-up InAs/GaAs nanotubes (RUNTs) as a function of etching time and sacrificial layer thickness for tube diameters between 20 and 560 nm. Within this diameter range the roll-up velocity strongly depends on the sacrificial layer thickness but is independent of the tube diameter. We also find that the roll-up distance saturates with etching time for distances around 8–16 μm. Since we define the starting edge of the roll-up process by optical lithography, we are able to position individual RUNTs on a substrate surface with reasonable accuracy. We also show that the areal density of the tubes on a surface can be doubled if a two-fold stack of strained bilayers is selectively underetched. Finally, we record organic fluid transport within a RUNT in real time and we report intense red light emission from such filled-up nanotubes.     

Microscopic modeling of intersubband resonances in InAs/AlSb quantum wells

Linear absorption spectra from intersubband resonance in InAs/AlSb quantum wells are analyzed theoretically using the intersubband semiconductor Bloch equation approach. Our model goes beyond the Hartree–Fock approximation and treats particle–particle correlations under the second Born approximation. Electron–electron and longitudinal optical phonon scatterings from such a treatment describe intrinsic line broadening to the intersubband resonance. Electron subbands are determined self-consistently with a spurious-state-free 8-band k·p Hamiltonian under the envelope function approximation. To compare with experimental measurements, we also included line broadening due to electron-interface roughness scattering. Excellent agreement was achieved for temperature-dependent absorption spectra in the mid-infrared frequency range, after taking into careful account the interplay of material parameters, nonparabolicity in bandstructure, and many-body effects.     

Intersubband edge singularity in metallic nanotubes

The tunneling density of states of both the massless and massive (gapped) particles in metallic carbon nanotubes is known to have an anomalous energy dependence. This is the result of coupling to multiple low-energy bosonic excitation (plasmons). For both kinds of particles the ensuing effect is the suppression of the density of states by electron-electron interactions. We demonstrate that the optical absorption between gapless and gapped states is affected by the many-body effects in the opposite way. The absorption probability is enhanced compared with the noninteracting value and develops a power-law frequency dependence, A(ω) ∝ (ω - Δ)(-γ), where γ≈0.2 for typical nanotubes.