Re-emission Of Photons Research Articles

A low damage GaN-based light-emitting diode with textured/inclined sidewalls and an air–buffer layer

A GaN-based light-emitting diode (LED) with an air–buffer layer and textured sidewalls is fabricated and investigated. After the deposition of a 1-μm silicon dioxide (SiO 2) passivation layer on the whole device region except for the sidewall, a hot KOH etching solution is used to develop the desired textured sidewall. Many sidewalls with triangular-like shape are produced along the specific direction of 〈 1 1 ¯ 0 0 〉 , where photons introduced within multiple-quantum-well (MQW) exhibit more opportunities to scatter into the air. Based on the lateral etching, many triangular-shaped cones are formed on the patterned sapphire substrate (PSS). The device sidewall inclines with a 24° angle which results in an improved reemission of photons in the vertical/lateral direction. At 20 mA, as compared with the LED without KOH treatment, the studied device exhibits improved output power by 11.7% without deteriorating its electrical characteristics. This phenomenon can be attributed to the enhanced output light from vertical and lateral directions due to the formation of an air–buffer layer, textured and inclined sidewalls.

Interaction of an ultrashort electromagnetic pulse with a molecular hydrogen ion

Analytical expressions that describe the spectrum of reemission during the interaction of an ultrashort electromagnetic pulse with a molecular hydrogen ion are obtained. The role of the orientation of the axis of the molecular ion during this interaction is studied. It is shown that the spectra of reemitted photons strongly depend on the orientation of molecular targets.

Spectra induced by interactions of atoms with ultrashort electromagnetic pulses

An ultrashort pulse of an electromagnetic field incident on an atom shakes the atom and gives rise to various electron transitions in it. These processes are accompanied by the reemission of the incident ultrashort pulse. This paper studies the relation between the spectra of reemitted photons of an ultrashort pulse and the transitions of atomic electrons into particular states. The obtained partial reemission spectra can allow one to relate direction patterns to the probability of the excitation of an atom into different states.

Analytic Theory of the YORP Effect for Near-Spherical Objects

The YORP effect is produced when the surface of a small object in interplanetary space is heated by sunlight and reradiates the absorbed energy in thermal wavelengths. The absorbed, reflected, and emitted photons produce tiny torques on the small body that can change its spin rate and obliquity over planetary timescales. Previous theories of the YORP effect relied on numerical or seminumerical evaluation of the radiation torques. Here we develop an alternative approach and calculate the YORP torques analytically. Our theory is limited to near-spherical objects. While unsuitable for a precise determination of torques on elongated and/or highly irregular objects, the analytic theory helps to explain several general properties of the YORP torques that were identified in previous numerical works. For example, we demonstrate that the component of the YORP torque that affects the spin rate, s, can vanish for obliquity values ≈ 55° (and ≈ 125°). As discussed by Vokrouhlický and coworkers, this property of s is important for establishing the so-called Slivan states, which arise as evolutionary end states of spin vectors of small solar system bodies such as asteroids. We show that s (averaged over spin and orbit periods) is a second-order quantity in the small parameter that describes the deviation of the shape from an ideal sphere. We calculate s explicitly as polynomials of cos . These expressions show that the YORP torque arises from coupled deformations of the body's shape in topographic longitude and latitude. Moreover, by introducing a small phase lag to mimic the delay between the absorption and reemission of photons we demonstrate that s is insensitive to the exact value of the surface thermal conductivity. These and other analytic results described here provide a baseline for understanding the YORP effect on bodies with more complicated surface shapes and properties other than the ones considered here. We discuss applications of the analytic theory on near-spherical asteroids like 1998 KY26 and on more elongated and/or irregular objects like (1862) Apollo and (25143) Itokawa.

Radiative equilibrium in Monte Carlo radiative transfer using frequency distribution adjustment

The Monte Carlo method is a powerful tool for performing radiative equilibrium calculations, even in complex geometries. The main drawback of the standard Monte Carlo radiative equilibrium methods is that they require iteration, which makes them numerically very demanding. Bjorkman and Wood recently proposed a frequency distribution adjustment scheme, which allows radiative equilibrium Monte Carlo calculations to be performed without iteration, by choosing the frequency of each re-emitted photon such that it corrects for the incorrect spectrum of the previously re-emitted photons. Though, the method appears to yield correct results, we argue that its theoretical basis is not completely transparent, and that it is not completely clear whether this technique is an exact rigorous method, or whether it is just a good and convenient approximation. We critically study the general problem of how an already sampled distribution can be adjusted to a new distribution by adding data points sampled from an adjustment distribution. We show that this adjustment is not always possible, and that it depends on the shape of the original and desired distributions, as well as on the relative number of data points that can be added. Applying this theorem to radiative equilibrium Monte Carlo calculations, we provide a firm theoretical basis for the frequency distribution adjustment method of Bjorkman and Wood, and we demonstrate that this method provides the correct frequency distribution through the additional requirement of radiative equilibrium. We discuss the advantages and limitations of this approach, and show that it can easily be combined with the presence of additional heating sources and the concept of photon weighting. However, the method may fail if small dust grains are included, or if the absorption rate is estimated from the mean intensity of the radiation field.

Solid state quantum memory using complete absorption and re-emission of photons by tailored and externally controlled inhomogeneous absorption profiles

We present a scheme for storage and retrieval of the quantum state of a single-photon or a weak classical or non-classical optical wave-packet, in a solid-state medium. We describe how the spectral absorption profile of a material such as a rare-earth-ion-doped crystal can be manipulated to efficiently absorb and subsequently reemit a wave-packet, in a photon echo-like process. We discuss how the scheme can be realised experimentally and suggest candidate materials for the storage medium.

Effect of anisotropic velocity distribution on the linear polarization of coronal lines

The effect of an anisotropic velocity field distribution of scattering ions on the polarization parameters of a spectral line emitted by resonance scattering is considered. The anisotropy of the velocity field distribution can be interpreted in terms of the ion-cyclotron effect that is believed to influence some heavy ions in the solar corona. We present a theoretical study of the Stokes parameters of a spectral line emitted by atoms or ions in the presence of a bi-Maxwellian velocity field distribution. It is found from test calculations that such a distribution measurably changes the polarization properties of the O VI D 2 coronal line. Consequently, measurements of the linear polarization of this line may serve as a new diagnostic of a possible bi-Maxwellian velocity distribution. As a preliminary application, the obtained theoretical results are used to interpret the polarization parameters of the O VI D 2 coronal line (λ1031.92) measured using SUMER/SoHO observations. The obtained results are compatible with SUMER's observations for more reasonable solar wind parameters than for an isotropic velocity field distribution of the scattering ions. Thus, the outflow velocity field vector of the emitting ions is less inclined with respect to the polar axis of the Sun (the minimal value of the polar angle η is 8° for an anisotropic velocity distribution versus 17° for an isotropic velocity field distribution). These results are obtained assuming that the re-emitted photons come from a small area in the center of the coronal polar hole, with zero magnetic field. Since SUMER/SoHO observations integrate over the line of sight, the results of the current analysis must be considered preliminary pending computations including an integration along the line of sight.

A manifestation of the magnetopolaron effect in reflection and absorption of light by a three-level system in a quantum well

The absorption and reflection of light by a quantum well are investigated in the case of two closely spaced levels of electronic excitations in the well. The dependences of the dimensionless absorptance Open image in new window and reflectance ℛ on the frequency ωl of the exciting light are calculated. The overall sequence of processes involving absorption and reemission of photons is taken into account. This is beyond the scope of the perturbation theory for the photon-electron coupling constant. It is shown that the perturbation theory is inapplicable when the reciprocal radiative lifetimes of excitations are comparable to the reciprocal nonradiative lifetimes. In this case, the nontrivial dependences Open image in new window and ℛ(ωl) are obtained. The total reflection and the total transparency points are determined. The relationships derived are used to analyze the special case of two excitation levels that are formed in the quantum well in a strong magnetic field H normal to its plane due to the Johnson-Larsen magnetopolaron effect. The reciprocal radiative lifetimes of electron-hole pairs are calculated far from and in the vicinity of the magnetophonon resonance. It is found that these lifetimes are proportional to H in the range far from the resonance and depend strongly on the difference H-Hres in the vicinity of the resonance. The dependences of the coefficients Open image in new window and ℛ on the magnetic field H at different frequencies of the exciting light are deduced.

Effects of absorption and reemission of photons in large scintillation counters on the quantities measured by an observing phototube

The absorption and reemission process occurring in scintillation counters of extended size plays an important role in determining the time distribution of the photons detected by a single phototube observing the scintillator, as well as the relevant detection probability. In this paper, a methodology to evaluate both these quantities is outlined, with specific reference to spherical scintillator geometry. Besides providing useful analytical formulations, this work is mainly intended to give a thorough insight to this complex phenomenon, showing how each physical step is reflected in the corresponding mathematical description.

Time-Resolved Reflectance Spectroscopy Applied to the Nondestructive Monitoring of the Internal Optical Properties in Apples

Time-resolved reflectance has been used for the nondestructive measurement of optical properties in apples. The technique is based on the detection of the temporal dispersion of a short laser pulse injected into the probed medium. The time distribution of re-emitted photons interpreted with a solution of the diffusion equation yields the mean values of the absorption and reduced scattering coefficients of the medium. The proposed technique proved useful for the measurement of the absorption and scattering spectra of different varieties of apples, revealing the spectral shape of chlorophyll. No major variations were observed in the experimental data when the fruit was peeled, showing that the optical properties measured were those of the pulp. With this technique the change in chlorophyll absorption during storage and ripening could be followed. Finally, a compact prototype working at few selected wavelengths was designed and constructed, demonstrating potentialities of the technique for industrial applications.

Propagation of exciton polaritons in inhomogeneous semiconductor films

A theoretical study of the propagation of short light pulses in semiconductor slabs containing excitons has been performed using the scattering-state technique and the steepest-descent method. These methods have allowed us to calculate numerically and to describe analytically the temporary behavior of the dielectric polarization inside the films as well as the time-resolved optical transmission spectra. The inhomogeneous broadening of exciton resonances has been taken into account within a local model. The appearance of a grating of the dielectric polarization in a semiconductor film illuminated by a short light pulse is predicted. This grating, which is due to interfering exciton polaritons, moves backward with respect to the light-propagation direction. We have shown that reabsorption and re-emission of photons by excitons in quantum wells and semiconductor films have a crucial effect on the coherent time-resolved spectra of these structures. The time-resolved transmission decay rate is found to depend on the thickness of semiconductor films with inhomogeneously broadened exciton resonances, which is not the case for a purely homogeneously broadened exciton. This fact, which follows from the energy dependence of the polariton damping in the former case, has also been explained in terms of multiple reabsorption--re-emission of photons by excitons.

A passive two-band sensor of sunlight-excited plant fluorescence

We have designed and built a passive remote sensor of sunlight-excited chlorophyll fluorescence (U.S. Patent No. 5,567,947, Oct. 22, 1996) which provides for the real-time, in situ sensing of photosynthetic activity in plants. This sensor, which operates as a Fraunhofer line discriminator, detects light at the cores of the lines comprising the atmospheric oxygen A and B bands, centered at 762 and 688 nm, respectively. These bands also correspond to wavelengths in the far-red and red chlorophyll fluorescence bands. The sensor is based on an induced fluorescence approach; as light collected from fluorescing plants is passed through a low-pressure cell containing oxygen, the oxygen absorbs the energy and subsequently reemits photons which are then detected by a photomultiplier tube. Since the oxygen in the cell absorbs light at the same wavelengths that have been strongly absorbed by the oxygen in the atmosphere, the response to incident sunlight is minimal. This mode of measurement is limited to target plants sufficiently close in range that the plants’ fluorescence is not itself appreciably absorbed by atmospheric oxygen (∼200 m). In vivo measurements of fluorescence in the 760 and 690 nm bands of vegetation in full sunlight are also presented. Measurements of plant fluorescence at the single-plant canopy level were obtained from greenhouse-grown bean plants subjected to a range of nitrogen treatments. The ratio of the fluorescence obtained from the two measurement bands showed statistically significant variation with respect to nitrogen treatments.

Resonances and dichroism in the scattering of an electron in an intense laser field by the Coulomb potential

We use a new type of Hamiltonian representing the electron-proton interaction in an intense laser field for an ab initio calculation of the differential scattering cross section. We give the diagrams of the results of calculations of the shapes and widths of the resonances in the cross section of electron scattering by the “field-dressed” Coulomb potential. The resonances emerge because of re-emission of photons by the electron. We also give the angular distribution of the scattered electrons as a function of circular dichroism for different values of the laser field strength and frequency.

Theory of phase-sensitive measurement of photon-assisted tunneling through a quantum dot

Recent double-slit interference experiments have demonstrated the possibility of probing the phase of the complex transmission coefficient of a quantum dot via the Aharonov-Bohm effect. We propose an extension of these experiments: an ac voltage imposed on the side gate with the concomitant photonic sidebands leads to additional structure both in the amplitude and in the phase of the Aharonov-Bohm signal. Observation of these effects would be a definitive proof of coherent absorption and reemission of photons from the ac source.

Chemistry of excited states: Its impact on plasma wall interactions in fusion devices

Differently from molecular hydrogen, energetic hydrogen escaping from plasma is directly implanted into plasma facing materials depositing its energy to excite electrons, to displace lattice atoms and to excite phonons and finally thermalized in the target. This makes subsequent migration of the thermalized hydrogen in the target very complex. Electrons and photons escaping from plasmas are superposed to influence the hydrogen migration, too. Simultaneously, various particles are emitted from the surface, including reflected, sputtered, and re-emitted particles, secondary electrons and photons. Those emitted particles are not necessarily equilibrated thermodynamically with the temperature of the target surface but are often in higher excited states and some are even ionized. Hence the surface exhibits various chemical reactions with impurities and/or chemical change. Enhanced erosion of carbon materials by implanted hydrogen producing methane, the chemical sputtering of carbon, is one of the most well-known examples. Even atomic hydrogen emission from carbon is observed at such a low temperature as 1000°C. The super permeation of hydrogen in metals is also caused by impinging of energetic hydrogen. Re-emitted hydrogen molecules are not necessarily in thermal equilibrium at the surface and bring hyperthermal energy released upon recombination. Then the recombination model based on the surface equilibrium should be modified. And the hydrogen recombination process may not be the rate limiting process for hydrogen re-emission. Taking into account the present knowledge of surface physics and chemistry, the kinetics and mechanisms of those phenomena given by energetic hydrogen injection which are important in fusion devices are discussed.

Evidence for photon recycling in InP

An examination of low-temperature photoluminescence from chemically thinned InP illustrates the effect of multiple absorption and reemission of photons in bulk liquid-encapsulated Czochralski grown material. Luminescence spectra show that such photon recycling dramatically increases the nonequilibrium carrier density in the material and causes excess carrier distribution to penetrate tens of micrometers beneath the sample surface, an order of magnitude more than a diffusion length. Nonequilibrium carriers also penetrate deeper with increasing excitation levels as a consequence of more efficient radiative recombination. Although these effects have not been widely recognized, they have important consequences in the interpretation of luminescence spectra and the design of electronic and optical devices based on InP that are sensitive to minority-carrier diffusion lengths.

CSDUST3: A radiation transport code for a dusty medium with 1-d planar, spherical or cylindrical geometry

The program solves the radiation transport problem in a dusty medium with one-dimensional planar, spherical or cylindrical geometry. It determines self-consistently the effects of multiple scattering, absorption, and re-emission of photons on the temperature of dust grains and the characteristics of the internal radiation field. The program can treat radiation field anisotropy, linear anisotropic scattering, and multi-grain components. The program output consists of the dust temperature distribution, flux spectrum, surface brightness at each frequency and the observed intensities (involving a convolution with a telescope beam pattern).

Excitation of Lamb waves by mode conversion on a periodic fluid/solid plate interface

For over 40 years, the Implicit Monte Carlo (IMC) method has been used to solve challenging problems in thermal radiative transfer. These problems typically contain regions that are optically thick and diffusive, as a consequence of the high degree of “pseudo-scattering” introduced to model the absorption and reemission of photons from a tightly-coupled, radiating material. IMC has several well-known features that could be improved: a) it can be prohibitively computationally expensive, b) it introduces statistical noise into the material and radiation temperatures, which may be problematic in multiphysics simulations, and c) under certain conditions, solutions can be nonphysical, in that they violate a maximum principle, where IMC-calculated temperatures can be greater than the maximum temperature used to drive the problem.We have developed a variant of IMC called iterative thermal emission IMC, which is designed to have a reduced parameter space in which the maximum principle is violated. ITE IMC is a more implicit version of IMC in that it uses the information obtained from a series of IMC photon histories to improve the estimate for the end of time step material temperature during a time step. A better estimate of the end of time step material temperature allows for a more implicit estimate of other temperature-dependent quantities: opacity, heat capacity, Fleck factor (probability that a photon absorbed during a time step is not reemitted) and the Planckian emission source.We have verified the ITE IMC method against 0-D and 1-D analytic solutions and problems from the literature. These results are compared with traditional IMC. We perform an infinite medium stability analysis of ITE IMC and show that it is slightly more numerically stable than traditional IMC. We find that significantly larger time steps can be used with ITE IMC without violating the maximum principle, especially in problems with non-linear material properties. The ITE IMC method does however yield solutions with larger variance because each sub-step uses a different Fleck factor (even at equilibrium).

Model for photon migration in turbid biological media.

Various characteristics of photon diffusion in turbid biological media are examined. Applications include the interpretation of data acquired with laser Doppler blood-flow monitors and the design of protocols for therapeutic excitation of tissue chromophores. Incident radiation is assumed to be applied at an interface between a turbid tissue and a transparent medium, and the reemission of photons from that interface is analyzed. Making use of a discrete lattice model, we derive an expression for the joint probability gamma(n, rho)d2 rho that a photon will be emitted in the infinitesimal area d2 rho centered at surface point rho = (x, y), having made n collisions with the tissue. Mathematical expressions are obtained for the intensity distribution of diffuse surface emission, the probability of photon absorption in the interior as a function of depth, and the mean path length of detected photons as a function of the distance between the site of the incident radiation and the location of the detector. We show that the depth dependence of the distribution of photon absorption events can be inferred from measured parameters of the surface emission profile. Results of relevant computer simulations are presented, and illustrative experimental data are shown to be in accord with the theory.

Imprisonment of resonance scattered radiation in the process of alkali atom optical pumping and polarized alkali ion sources

The dynamics of an optical pumping process in a 6Li atomic beam is considered. The influence of imprisonment of resonance scattered radiation is taken into account by Monte Carlo simulation. It is shown that imprisonment of reemitted photons restricts the intensity of the polarized lithium beam at the level 10 16 s − in the considered geometry. A scheme for the polarized negative alkali ion source is proposed. The imprisonment of the resonance scattered radiation is not essential in this scheme up to the current alkali atom density of 10 19 cm −2 s −1.