Photon Absorption Process Research Articles

Suppression of magnetic subbands in semiconductor superlattices driven by a laser field

The effect of strong laser radiation on magnetic subbands in semiconductor superlattices is investigated. Due to the presence of a magnetic field perpendicular to the growth direction, nonlinear effects such as band suppression and electron localization become relevant at relatively lower intensities and for any polarization perpendicular to the magnetic field. Electron quasienergies and density of states are calculated in the Kramers–Henneberger approximation, whose validity is discussed. The conditions under which collapse of magnetic subbands and quenching of N -photon emission or absorption processes occur are discussed. We conclude that at laser frequencies close to cyclotronic frequency and intensities typical of c.w. lasers, magnetic subbands become flat, magnetotunneling is inhibited and multiphotonic processes dominate optical absorption.

Pn-Pair Coupling in the(γ,pn)Reaction at 72 MeV

J. R. M. Annand, G. I. Crawford, V. Holliday, J. C. McGeorge, and G. J. Miller** Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland (Received 27 May 1999) The 16O g,pn reaction was measured at Eg 72 MeV with resolution sufficient to distinguish the low-lying states of the residual 14N nucleus. Cross sections averaged over the acceptance of the detector system were determined for each of the resolved states. The relative population of residual states indicates that proton-neutron pairs coupled to J ,T 0, 1 play a minor role in the photon absorption process compared to 1, 0 pairs and that both L 0 and L 2 pairs participate in the reaction.

A Quantum Mechanical/Molecular Mechanical Approach to Relaxation Dynamics: Calculation of the Optical Properties of Solvated Bacteriochlorophyll-a

We have applied both classical and mixed quantum mechanical/molecular mechanical (QM/MM) techniques to the calculation of electronic−vibrational coupling. In order to assess these approaches, we compare results to the steady state absorption and emission spectra of solvated bacteriochlorophyll-a (BChl-a) at room temperature. We find that the method chosen for the calculation of the S0−S1 energy gap significantly affects the calculated spectra. Mixed QM/MM approaches perform substantially better than the purely classical approach, and where an ab initio method is used for calculating the S0−S1 energy gap, the predicted Stokes shift (related to the reorganization energy), and the spectral absorption width are within 5% of the experimental values. We find that the decay of the transition energy correlation function occurs largely over two time scales. Most of the decorrelation occurs in less than 5 fs. This is less than the time taken for the process of photon absorption, indicating that the optical spectrum...

Hot free-electron absorption in nonparabolic III–V semiconductors at mid-infrared wavelengths

A quantum mechanical model based on the second order perturbation theory was constructed for the calculation of the free-electron absorption coefficient in nonparabolic III–V semiconductors. The implemented model allows for the calculation of the absorption changes when the free-electron gas temperature differs from the lattice temperature. Several mechanisms, which assist in the photon absorption process, were taken into account. At the considered lattice and electron temperature range and doping concentrations, the most important scattering mechanisms are impurity scattering, thermal and hot longitudinal optical phonon scattering, and finally acoustic phonon scattering. For all the interaction potentials we included the effect of screening by the conduction electrons. The model was developed in a fully consistent nonparabolic way. The electron dispersion relation as well as the interaction probabilities feature nonparabolic effects. Computations are performed for GaAs, InAs, and InSb at different mid-IR wavelengths, doping concentrations, and lattice and electron temperatures. Our nonparabolic hot-electron model is validated with experimental results available in the literature. It turns out that our model is much more accurate and consistent than other more relaxed models. The competition between different hot free-electron absorption mechanisms is discussed.

Cyclotron-Stark-phonon resonances in semiconductor superlattices under terahertz irradiation

The influence of strong terahertz irradiation on quantum transport in semiconductor superlattices under parallel magnetic and dc electric fields is studied within a quantum-kinetic approach. The consideration is focused on field strengths at which Wannier-Stark and Landau quantization occur. The current density comprises two different contributions. One is due to scattering-induced delocalization of carriers. The other one is negative, and results mainly from photon absorption processes. Within a microscopic as well as a simple relaxation-time approach, the electric- and magnetic-field dependencies of combined cyclotron-Stark-phonon and Stark-photon resonances are studied.

A new approach of digital signal processing in gamma- and X-ray spectroscopy

A new approach of digital signal processing in gamma- and X-ray spectroscopy which overcomes limitations of state-of-the-art systems is described. The basic technical equipment consists of an energy resolving detector with a preamplifier followed by a fast sampling analog digital converter. The basic idea of the approach is the numerical decomposition of the measured signal into distributions caused by single photon absorption processes using standard pulses. The working principle and potential of the approach is demonstrated by the example of preamplifier pulses with large overlap on time scale.

Modeling of intersubband and free-carrier absorption coefficients in heavily doped conduction-band quantum-well structures

Theoretical modelings of the transition energy for intersubband absorptions, and the intersubband and free-carrier absorption coefficients in heavily doped conduction-band anisotropic semiconductor quantum-well (QW) structures are presented. The transition matrix elements for photon absorption and emission, which are not identical due to the different many-body effects involved in the photon absorption and emission processes, are rigorously derived. We also show that the linewidth broadening effect caused by various scattering processes gives a considerable increase in resonance energy, which explains the relatively large parallel-mode transition energy which cannot be inferred from previous modeling studies. In addition, theoretical modeling of free-carrier absorption in anisotropic semiconductor QW structures is presented for the first time. The calculated results are compared with the experimental values for /spl delta/-doped Si QW's.

Fiber Bragg Grating Growth Simulation Using an Inverse Scattering Matrix Approach

ABSTRACTFiber Bragg gratings are playing an important role in the field of optical telecommunications and sensors. We have simulated the growth of gratings, longitudinally written in the core of germanosilicate optical fibers by an Ar+ laser. Following a Kramer-Kronig model, absorption in the Germanium Oxygen deficient centers band (∼240 nm) is considered to be responsible for the fiber photosensitivity and a two photon absorption process models the dynamics of the refractive index variation.The spatial propagation of light along the grating is described by the coupled wave theory and is calculated by using an inverse scattering matrix technique. The predicted physical properties of the gratings, reflectivity and temporal dispersion of propagating light as a function of wavelength and input power, show that a sustained growth is possible. A good agreement with previous simulations and experiments is found.

Photoinduced phonon fluorescence in GaAs/Al xGa 1− xAs quantum wells

Phonon fluorescence in semiconductor quantum wells induced by two radiation fields is investigated here. The rate of change of the interface LO-phonon population due to the scattering of confined electrons in the two laser fields is calculated using first-order perturbation theory from which the amplification coefficient is derived. It was found, by considering the full photon absorption and emission processes of laser fields that for a semiconductor quantum well at arbitrary temperature, interface LO phonons propagating parallel to the direction of polarization of the external fields may be amplified over a relatively broad band of phonon wavenumber. An application is made for a GaAs/AlGaAs quantum well system.

Dispersion pre-compensation of 15 femtosecond optical pulses for high-numerical-aperture objectives.

The excitation efficiency in two-photon absorption (TPA) microscopy depends strongly - owing to the square dependence of the TPA fluorescence on the excitation intensity - on the temporal width of the excitation pulse. Because of their inherently large frequency bandwidth, ultrashort optical pulses tend to broaden substantially because of dispersion from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of microscope objectives are investigated. It is shown that the induced dispersion can be pre-compensated in all cases for pulses as short as 15 fs. Because of the excellent agreement between the results from theoretical modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well as for even shorter pulses, can be inferred. Since for TPA imaging the background due to single photon absorption processes and scattering is independent of the pulse width, proper dispersion pre-compensation - which minimizes the pulse duration at the focal point and hence maximizes the excitation efficiency - provides optimal image contrast in TPA microscopy.

Dense electron beam radiation in a dielectric medium

We have studied both the spectral intensity and the energy of radiation produced by an electron beam in dielectric medium. Since the device starts up from a photon vacuum the investigation is based on a quantum electrodynamics approach. We have assumed that the electron beam longitudinal size is indefinite and its energy spread is of Gaussian form. In such a case the radiation process has a cutoff with respect to the electron density ${\ensuremath{\rho}}_{0}.$ If the electron density is lower than some characteristic density ${\ensuremath{\rho}}_{\mathrm{ch}}$ then the radiation will be mainly spontaneous and its emission line will have Gaussian shape. If the electron beam is dense, ${\ensuremath{\rho}}_{0}>{\ensuremath{\rho}}_{\mathrm{ch}},$ then a drastic change of line form takes place. Namely, one observes a high and narrow peak (or superradiation effect) on its right-hand side and radiation energy saturation on its left-hand side. Both of these phenomena are due to the stimulated processes of photon absorption and emission. We have evaluated the level of radiation fluctuations in the peak vicinity. Numerical estimations of both obtained effects are presented.

Photoemission of protons from4He in theΔ-resonance region

High resolution proton energy spectra from the4He(γ,p) reaction have been measured with tagged photons in the range Eγ = 130−525 MeV using the large Mainz NaI(T1) spectrometer atΘplab=37.1°. Three separate reaction channels were identified, viz.4He(γ,p)t two-body breakup,4He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.

Cellular automata simulation of the effects induced by an external electromagnetic field on microtubule formation dynamics

A cellular automaton-based model is presented to investigate the influence of an electromagnetic field on microtubule formation dynamics. The cellular space is assumed to represent a portion of the cytoplasm through which the electromagnetic field propagates. Tubulin dimers are seen as dipoles having two states of polarization, and the field is seen as a stream of photons. Both kinds of particles move all over the cellular space. Their interactions are represented as local elementary processes of photon absorption and emission. The effects of the ponderomotive force exerted by the field on the dimers and diffusion are taken into account. Simulation experiments show that a monochromatic electromagnetic beam with a bell-shaped transverse profile can give rise to filamentary regions where the dimer concentration is increased, thus creating a precondition for polymerization.

Determination of the Rotational and Vibrational Constants of Photolytically Generated CF Radicals by REMPI Measurements

The detailed spectroscopic study of transient free CF radicals with the resonance enhanced multiphoton ionization (REMPI) method is the aim of this paper. The CF radicals were generated from the gaseous (50—200 Pa) halocarbons CCl3F, CCl2F2, CClF3, 1,2-C2Cl2F4, C2ClF3, and C6F6by a combined infrared and UV/VIS laser photolysis. Because of the small absorption of the halocarbon molecules in the dye laser range used (328—485 nm) a preceding production of precursors (capable of absorbing dye laser photons) was necessary, which was achieved by IR multiphoton dissociation of the parent molecules with a TEA CO2laser pulse. In a subsequent absorption of UV/VIS dye laser photons the precursors were photolyzed yielding CF radicals. These could be spectroscopically investigated by their REMPI signals in a (2 + 1)-photon absorption process in the focussed radiation of the same dye laser pulse. About 20 vibrational bands were measured, rotationally resolved, and were assigned to a two-photon 3pπD2Πr←←X2Πrresonant transition followed by one-photon absorption for direct ionization of CF. Compared to the cold CF radicals, commonly extracted from a flow reactor, the laser-photolytically generated CF radicals were found to be vibrationally and rotationally extremely excited (v″ ≤ 12,J≤ 80.5). Using the wide rotational contour of the vibrational bands we could determine with greater precision than before the electronic, vibrational, and rotational constantsTe, ωe, ωexe, ωeye,Av,Bv, andDv, respectively, for the Rydberg state 3pπD2Πr(v′ = 0—6) and the electronic ground stateX2Πr(v″ = 0—12). Based on these constants we calculated the Morse and the Dunham potential curves for both states. A detailed study of missing and shifted rovibronic lines of theDstate yielded first information on the vibrational levelsv′ = 3, 4, and 5 of theB2Δrvalence state, which cannot be measured directly by REMPI. Additionally, we measured for the first time in the UV wavelength range a (1 + 1)-photon REMPI spectrum belonging to the vibrational bandsv′ = 0 ←v″ = 0 andv′ = 1 ←v″ = 0 of theA2Σ+←X2Πrtransition. Even in the range of high rotational quantum numbersJwe found no indication of Λ-type doubling of the rovibronic states within a resolution of 0.2 cm−1.

Photoemission of protons from 4He in the ⊿-resonance region

High resolution proton energy spectra from the 4He(γ,p) reaction have been measured with tagged photons in the range Eγ = 130–525 MeV using the large Mainz NaI(Tl) spectrometer at $[Theta _p^{lab} = 37.1^circ ]$ . Three separate reaction channels were identified, viz. 4He(γ,p)t two-body breakup, 4He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.

Photolysis of SiH4 by the Third Harmonic of a Nd:YAG Laser at 355 NM

The photolysis of silane (SiH4) was carried out using the third harmonic of a Nd: YAG laser at 355 nm, at a fixed SiH4 pressure of 350 Torr, varying the laser energy fluence in the range of 30–300 Jcm-2. The emission spectra indicates that the photofragments formed are SiH2, SiH, Si, H2, and H. The (A1B1-X1A1) transitions at 552.7 nm, 525.3 nm, 505.6 nm, and 484.7 nm of SiH2 are due to a two photon absorption process. The (A2Δ-X2π) transitions of SiH at 425.9 nm, 418 nm, 414.2 nm, 412.8 nm and 395.6 nm are due to a three photon absorption process. The brownish white deposit on the cell windows indicates the presence of amorphous silicon (a:Si-H). The two atomic lines of Si(4s1P0→ 3p21D2) at 288.1 nm, and (4s3Pj→ 3P3Pj) at 251.6 nm are observed. The atomic Si transitions are due to a three photon absorption. We observed seven transitions due to molecular hydrogen at wavelengths 577.5 nm, 565.5 nm, 534.4 nm, 542.5 nm, 471 nm, 461.7 nm, and 455.4 nm. These bands are due to a four photon absorption proc6ss. In addition to the molecular bands we also observed hydrogen atomic lines Hβ, Hγ and Hδ.

LASER INDUCED THERMAL LENS EFFECT IN RHODAMINE B — SIGNATURE OF RESONANT TWO PHOTON ABSORPTION

Measurement of thermal lensing signal as a function of laser power made in Rhodamine B solutions in methanol give clear evidence of two photon absorption process within certain concentration ranges when 488 nm Ar + laser beam is used as the pump source. Only one photon process is found to occur when 514 nm and 476 nm beams are used as the pump.

Third-order optical nonlinearity and all-optical switching in porous silicon

The third-order optical nonlinearity χ(3) of porous silicon has been measured using the Z-scan technique. Intensity dependent absorption was observed and attributed to a resonant two photon absorption process. The real and imaginary parts of χ(3) have been measured at 665 nm and found to be 7.5×10−9 esu and −1.9×10−9 esu, respectively. This constitutes a significant enhancement over crystalline silicon. All optical switching based on nonlinear absorption is demonstrated.

Fluorescence intensity and squeezing in a driven three-level atom: Ladder case.

A theoretical study is made of the steady-state intensity and squeezing properties of the fluorescent light from a three-level-atom system in a ladder configuration, which is subject to spontaneous-emission decay to the electromagnetic-field vacuum. Two cases are examined: in the equispaced (ES) level case, the two atomic transition frequencies are nearly equal and two photon absorption processes occur from a single coherent laser field that couples both transitions and has a small detuning from each; in the non- equispaced (NES) level case, the atomic transition frequencies are rather different and two photon absorption processes occur from two coherent laser fields, each coupled to a single transition and in near resonance with it. In both cases, a situation of small two-photon detuning occurs. Optical Bloch equations for the atomic density matrix in rotating frames are given and matrix expressions for determining the steady-state populations and coherences are obtained. Analytic expressions are also given for special cases. The fluorescent intensity is obtained from the populations of the intermediate and upper states, with the normally ordered variance (NOV) for quadrature components of the fluorescent field involving, in addition, the atomic coherences. The fluorescent intensities are shown graphically as a function of two-photon detuning for a variety of one-photon detunings and Rabi frequencies for the ES and NES cases. In both cases, the fluorescent intensities show the well known resonances from the upper state at zero two-photon detuning and from the intermediate state at zero one-photon detuning for the lower transition. However, further resonances are also found. The intermediate-state intensity displays a resonance at zero two-photon detuning. Also, the upper-state intensity shows a resonance at zero one-photon detuning for the upper transition, but only for the NES case; evidently, the transfer rate from upper to lower transition coherences destroys this resonance for the ES case. The time-averaged NOV is also shown graphically as a function of two-photon detuning for a variety of one-photon detunings and Rabi frequencies for the ES and NES cases. For the ES case, the quadrature frequency is chosen as the single laser frequency and for the NES case, it is chosen as either the average of the two laser frequencies or the lower transition laser frequency. Squeezing occurs near zero two-photon detuning for both ES and NES cases, though only for the quadrature frequency equal to the average laser frequency in the latter case. It also occurs near zero detuning for the lower transition for both cases, though only for the quadrature frequency equal to the lower transition laser frequency in the NES case. The squeezing minima show a splitting effect for large Rabi frequencies corresponding to two-photon or one-photon Rabi splitting of the dressed atom levels for the situation near two-photon or one-photon resonance, respectively. The large squeezing near two-photon resonance for moderate Rabi frequencies and large one-photon detuning corresponds to essentially pure three-level squeezing, while that near one-photon resonance for weak Rabi frequencies involves two-level squeezing.

The 16O(γ, p) reaction at Eγ = 60 and 72 MeV

Abstract Angular distributions within the range 25°–125° are presented for the 16O(γ, p) reaction populating low-lying states in 15N at Eγ = 60 and 72 MeV. A comparison of the data with self-consistent Hartree-Fock RPA calculations shows the importance of meson-exchange currents in the photon-absorption process.