Low-frequency Photons Research Articles

An asymptotic preserving unified gas kinetic scheme for frequency-dependent radiative transfer equations

This paper presents an extension of previous work (Sun et al., 2015 [22]) of the unified gas kinetic scheme (UGKS) for the gray radiative transfer equations to the frequency-dependent (multi-group) radiative transfer system. Different from the gray radiative transfer equations, where the optical opacity is only a function of local material temperature, the simulation of frequency-dependent radiative transfer is associated with additional difficulties from the frequency-dependent opacity. For the multiple frequency radiation, the opacity depends on both the spatial location and the frequency. For example, the opacity is typically a decreasing function of frequency. At the same spatial region the transport physics can be optically thick for the low frequency photons, and optically thin for high frequency ones. Therefore, the optical thickness is not a simple function of space location. In this paper, the UGKS for frequency-dependent radiative system is developed. The UGKS is a finite volume method and the transport physics is modeled according to the ratio of the cell size to the photon's frequency-dependent mean free path. When the cell size is much larger than the photon's mean free path, a diffusion solution for such a frequency radiation will be obtained. On the other hand, when the cell size is much smaller than the photon's mean free path, a free transport mechanism will be recovered. In the regime between the above two limits, with the variation of the ratio between the local cell size and photon's mean free path, the UGKS provides a smooth transition in the physical and frequency space to capture the corresponding transport physics accurately. The seemingly straightforward extension of the UGKS from the gray to multiple frequency radiation system is due to its intrinsic consistent multiple scale transport modeling, but it still involves lots of work to properly discretize the multiple groups in order to design an asymptotic preserving (AP) scheme in all regimes. The current scheme is tested in a few frequency-dependent radiation problems, and the results are compared with the solutions from the well-defined implicit Monte Carlo (IMC) method. The UGKS is much more efficient than IMC, and the computational times of both schemes for all test cases are listed. The UGKS seems to be the first discrete ordinate method (DOM) for the accurate capturing of multiple frequency radiative transport physics from ballistic particle motion to the diffusive wave propagation.

GAMMA-RAY BURST SPECTRA AND SPECTRAL CORRELATIONS FROM SUB-PHOTOSPHERIC COMPTONIZATION

One of the most important unresolved issues in gamma-ray burst physics is the origin of the prompt gamma-ray spectrum. Its general non-thermal character and the softness in the X-ray band remain unexplained. We tackle these issues by performing Monte Carlo simulations of radiation-matter interactions in a scattering dominated photon-lepton plasma. The plasma -- initially in equilibrium -- is driven to non-equilibrium conditions by a sudden energy injection in the lepton population, mimicking the effect of a shock wave or the dissipation of magnetic energy. Equilibrium restoration occurs due to energy exchange between the photons and leptons. While the initial and final equilibrium spectra are thermal, the transitional photon spectra are characterized by non-thermal features such as power-law tails, high energy bumps, and multiple components. Such non-thermal features are observed at infinity if the dissipation occurs at small to moderate optical depths, and the spectrum is released before thermalization is complete. We model the synthetic spectra with a Band function and show that the resulting spectral parameters are similar to observations for a frequency range of 2-3 orders of magnitude around the peak. In addition, our model predicts correlations between the low-frequency photon index and the peak frequency as well as between the low- and high-frequency indices. We explore baryon and pair dominated fireballs and reach the conclusion that baryonic fireballs are a better model for explaining the observed features of gamma-ray burst spectra.

Thermal plasmonic interconnects in graphene

As one emerging plasmonic material, graphene can support surface plasmons at infrared and terahertz frequencies with unprecedented properties due to the strong interactions between graphene and low-frequency photons. Since graphene surface plasmons exist in the infrared and terahertz regime, they can be thermally pumped (excited) by the infrared evanescent waves emitted from an object. Here we show that thermal graphene plasmons can be efficiently excited and have monochromatic and tunable spectra, thus paving a way to harness thermal energy for graphene plasmonic devices. We further demonstrate that ``thermal information communication'' via graphene surface plasmons can be potentially realized by effectively harnessing thermal energy from various heat sources, e.g., the waste heat dissipated from nanoelectronic devices. These findings open up an avenue of thermal plasmonics based on graphene for different applications ranging from infrared emission control, to information processing and communication, to energy harvesting.

Chromatic acclimation and population dynamics of green sulfur bacteria grown with spectrally tailored light.

Living organisms have to adjust to their surrounding in order to survive in stressful conditions. We study this mechanism in one of most primitive creatures – photosynthetic green sulfur bacteria. These bacteria absorb photons very efficiently using the chlorosome antenna complexes and perform photosynthesis in extreme low-light environments. How the chlorosomes in green sulfur bacteria are acclimated to the stressful light conditions, for instance, if the spectrum of light is not optimal for absorption, is unknown. Studying Chlorobaculum tepidum cultures with far-red to near-infrared light-emitting diodes, we found that these bacteria react to changes in energy flow by regulating the amount of light-absorbing pigments and the size of the chlorosomes. Surprisingly, our results indicate that the bacteria can survive in near-infrared lights capturing low-frequency photons by the intermediate units of the light-harvesting complex. The latter strategy may be used by the species recently found near hydrothermal vents in the Pacific Ocean.

Enhanced thermal photon and dilepton production in strongly coupled $ \mathcal{N} $ = 4 SYM plasma in strong magnetic field

We calculate the DC conductivity tensor of strongly coupled N=4 super-Yang-Mills (SYM) plasma in a presence of a strong external magnetic field B>>T^2 by using its gravity dual and employing both the RG flow approach and membrane paradigm which give the same results. We find that, since the magnetic field B induces anisotropy in the plasma, different components of the DC conductivity tensor have different magnitudes depending on whether its components are in the direction of the magnetic field B. In particular, we find that a component of the DC conductivity tensor in the direction of the magnetic field B increases linearly with B while the other components (which are not in the direction of the magnetic field B) are independent of it. These results are consistent with the lattice computations of the DC conductivity tensor of the QCD plasma in an external magnetic field B. Using the DC conductivity tensor, we calculate the soft or low-frequency thermal photon and dilepton production rates of the strongly coupled N=4 SYM plasma in the presence of the strong external magnetic field B>>T^2. We find that the strong magnetic field B enhances both the thermal photon and dilepton production rates of the strongly coupled N=4 SYM plasma in a qualitative agreement with the experimentally observed enhancements at the heavy-ion collision experiments.

Control of naturally coupled piezoelectric and photovoltaic properties for multi-type energy scavengers

In this paper, we present a simple, low-cost and flexible hybrid cell that converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using piezoelectric zinc oxide (ZnO) in conjunction with organic solar cell design. Since the hybrid cell is designed by coupled piezoelectric and photoconductive properties of ZnO, this is a naturally hybrid architecture without crosstalk and an additional assembling process to create multi-type energy scavengers, thus differing from a simple integration of two different energy generators. It is demonstrated that the behavior of a piezoelectric output is controlled from alternating current (AC) type to direct current (DC)-like type by tailoring mechanical straining processes both in the dark and under light illumination. Based on such controllability of output modes, it is shown that the performance of the hybrid cell is synergistically enhanced by integrating the contribution made by a piezoelectric generator with a solar cell under a normal indoor level of illumination. Our approach clearly demonstrates the potential of the hybrid approach for scavenging multi-type energies whenever and wherever they are available. Furthermore, this work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible to produce a promising multi-functional power generator that could be embedded in flexible architectures.

Correlated photon fluctuations at threshold of an interband cascade laser

We report the observation of correlated photon fluctuations over a narrow range of current at threshold of an interband cascade laser using single-detector photon noise measurements. The correlated photon noise is manifested by large fluctuations in the low-frequency photon noise spectral density at certain discrete frequencies which are sensitive to the laser gain characteristics. We observe the concurrent emergence and growth of the lasing mode over the same current range indicating that the correlated photon noise provides evidence of the occurrence of a change in the photon fluctuation statistics and the onset of first-order coherence in the laser emission at threshold.

Frequency downconversion for a quantum network

By using a parametric downconversion process with a strong signal field injection, we demonstrate coherent frequency downconversion from a pump photon to an idler photon. Contrary to a common misunderstanding, we show that the process can be free of quantum noise. With an interference experiment, we demonstrate that the coherence is preserved in the conversion process. This may lead to a high-fidelity quantum state transfer from a high-frequency photon to a low-frequency photon and connects a missing link in a quantum network. With this scheme of coherent frequency downconversion of photons, we propose a method of single-photon WDM.

Interband cascade laser photon noise

We report measurements of the low-frequency photon noise and the relative intensity noise of an interband cascade laser as a function of laser current at 30 and 100 K. Away from threshold, the laser primarily exhibits a frequency-independent photon noise spectral density in agreement with theory. At threshold, the observed photon noise spectral density exhibits large fluctuations at closely spaced discrete frequencies. Thermal effects at 100 K result in a large increase in the photon noise above threshold.

Evolution of low-frequency features in the CMB spectrum due to stimulated Compton scattering and Doppler broadening

We discuss a new solution of the Kompaneets-equation for physical situations in which low frequency photons, forming relatively narrow spectral details, are Compton scattered in an isotropic, infinite medium with an intense ambient blackbody field that is very close to full thermodynamic equilibrium with the free electrons. In this situation the background-induced stimulated Compton scattering slows down the motion of photons toward higher frequencies by a factor of 3 in comparison with the solution that only takes into account Doppler-broadening and boosting. This new solution is important for detailed computations of cosmic microwave background spectral distortions arising due to uncompensated atomic transitions of hydrogen and helium in the early Universe. In addition we derive another analytic solution that only includes the background-induced stimulated Compton scattering and is valid for power-law ambient radiation fields. This solution might have interesting applications for radio lines arising inside of bright extra-galactic radio source, where according to our estimates line shifts because of background-induced stimulated scattering could be amplified and even exceed the line broadening due to the Doppler-effect.

Observation of Photon-Assisted Tunneling in Optical Lattices

We have observed tunneling suppression and photon-assisted tunneling of Bose-Einstein condensates in an optical lattice subjected to a constant force plus a sinusoidal shaking. For a sufficiently large constant force, the ground energy levels of the lattice are shifted out of resonance and tunneling is suppressed; when the shaking is switched on, the levels are coupled by low-frequency photons and tunneling resumes. Our results agree well with theoretical predictions and demonstrate the usefulness of optical lattices for studying solid-state phenomena.

Scattering of low-frequency radiation by a gyrating electron

The scattering of electromagnetic radiation by the particle gyrating in an external magnetic field is considered. Particular attention is paid to the low-frequency case, when the frequencies of incident radiation are much less than the electron gyrofrequency. The spectral and polarization features of the scattering cross-section are analysed in detail. It is found that the scattering transfers the low-frequency photons to high harmonics of the gyrofrequency, into the range of the synchrotron emission of the electron. The total scattering cross-section appears much larger than that for the particle at rest. The problem studied is directly applicable to the radio wave scattering in the magnetosphere of a pulsar. The particles acquire relativistic rotational energies as a result of resonant absorption of the high-frequency radio waves and concurrently scatter the low-frequency radio waves, which are still below the resonance. It is shown that the scattering can affect the radio intensity and polarization at the lowest frequencies and can compete with the resonant absorption in contributing to the low-frequency turnover in the pulsar spectrum. Moreover, the scattering can be an efficient mechanism of the pulsar high-energy emission, in addition to the synchrotron re-emission of the particles. Other astrophysical applications of the scattering by gyrating particles are pointed out as well.

Low-frequency-field-induced spontaneous-emission interference in a two-level atom placed in an anisotropic photonic crystal

We investigate the spontaneous-emission properties of a two-level atom embedded in a three-dimensional anisotropic photonic crystal. In addition to the modified density of states, the atom is driven by a coherent intense low-frequency field (LFF), which creates additional multiphoton decay channels with the exchange of two low-frequency photons and one spontaneous photon during an atomic transition. Due to the low frequency of the applied field, the various transition pathways may interfere with each other and thus give rise to a modified system dynamics. We find that even if all the atomic (bare and induced) transition frequencies are in the conducting band of the photonic crystal, there still may exist a photon–atom bound state in coexistence with propagating modes. The system also allows us to generate narrow lines in the spontaneous-emission spectrum. This spectrum is a function of the distance of the observer from the atom due to the band gap in the photonic crystal. The system properties depend on three characteristic frequencies, which are influenced by quantum interference effects. Thus these results can be attributed to a combination of interference and band-gap effects.

Up-Converted Emission in a Series of Phenylazomethine Dendrimers with a Porphyrin Core

The nonlinear optical and time-resolved properties of a series of phenylazomethine-porphyrin dendrimers are reported. The linear optical properties were also investigated, and the efficiency of the energy transfer process was obtained. Measurements were also carried out with the basic building-block molecules. The process of frequency up-converted emission was observed in these porphyrin dendrimers. The mechanism for this effect is investigated and related to the process of "hot-band" absorption in the phenylazomethine-porphyrin system. Time-resolved measurements also suggested efficient intramolecular vibrational energy redistribution in these systems. These properties suggest that the porphyrin dendrimers may also have applications in light harvesting of low-frequency photons, as well as in sensors.

Spontaneous emission properties of a two-level atom with an intense low-frequency field in a three-dimensional photonic crystal

We have investigated spontaneous emissions from a two-level atom embedded in a p hotonic crystal with three-dimensional dispersion relation as well as interacted with a coherent intense low-frequency field .Due to this field, additional deca y channels with exchange of one low-frequency photon during an atomic transition are created, resulting in the spontaneously induced quantum interference. The s pontaneous emission can be suppressed significantly because of the combinational influences of the quantum interference effect and the band edge effect. The pop ulation in the upper levels depends on the relative position of the upper level from the band edge. The frequency of the low-frequency field and the relative in tensity of the coupling strength for the transitions can affect spontaneous emis sion from the atom.

Spontaneous-emission suppression on arbitrary atomic transitions.

We propose a very simple scheme to slow down the usual exponential decay of upper state population in an atomic two-level system considerably. The scheme uses an additional possibly intense field with frequency lower than the total decay width of the atomic transition. This allows for additional decay channels with the exchange of one or more low-frequency photons during an atomic transition. These channels may then interfere with each other. The intensity and frequency of the low-frequency field are shown to act as two control parameters modifying duration and amount of the population trapping. An extension of the scheme to include transitions to more than one lower state is straightforward.

Compton scattering in strong magnetic fields in the laboratory frame

The Compton scattering cross-section in strong magnetic fields in the laboratory frame is derived, which gives Herold's result in the electron rest frame but cannot be recovered vice versa from the latter by relativistic transformations. For the scattering of a low-frequency photon with electrons, this cross-section has a simple form which can also be reduced to Herold's nonrelativistic limit but cannot be recovered either from the latter. Compared with previous works, this cross-section is directly applicable in astrophysical calculations for both relativistic and nonrelativistic cases and may lead to some revisions to past astrophysical calculations based upon Herold's expressions.

Accretion Disk Boundary Layers around Neutron Stars: X‐Ray Production in Low‐Mass X‐Ray Binaries

The boundary layer where the accretion disk meets the star is expected to be the dominant source of high-energy radiation in low-mass X-ray binaries which contain weakly magnetized accreting neutron stars. We present Newtonian solutions for the structure of the boundary layer in such a system. We find that the main portion of the boundary layer gas is hot (108 K), has low density, and is radially and vertically extended. It will emit a large luminosity in X-rays, mainly produced by Comptonization of soft photons which pass through the hot gas. The gas is generally optically thick to scattering but optically thin to absorption. Energy is transported by viscosity from the rapidly rotating outer part of the boundary layer to the slowly rotating inner part, and this has the important effect of concentrating the energy dissipation in the dense, optically thick (to Thomson scattering) zone close to the stellar surface. Advection of energy also plays an important role in the energy balance. We explore the dependence of the boundary layer structure on the mass accretion rate and rotation rate of the star. We also examine the effects of changes in the ? viscosity parameter and the viscosity prescription. Radiation pressure is the dominant source of pressure in the boundary layer. The radiation flux in the boundary layer is a substantial fraction of the Eddington limiting flux even for luminosities well below (~0.01 times) the Eddington luminosity LEdd for spherically symmetric accretion. At luminosities near LEdd, the boundary layer expands radially and has a radial extent larger than 1 stellar radius. This radial expansion increases the surface area of the boundary layer and allows it to radiate a larger total luminosity. Based on the temperatures and optical depths which characterize the boundary layer, we expect that Comptonization will produce a power-law spectrum at low source luminosities. At high luminosities the scattering optical depth is quite large, and Comptonization of low-frequency bremsstrahlung photons will produce a quasi-Planckian spectrum in the dense region where most of the energy is released. This spectrum will be altered by Comptonization as the radiation propagates through the lower density outer boundary layer.

Limitations due to the spontaneous emission of low-frequency photons on the applicability of the Newtonian description of particle motion in a gas

It is shown that the spontaneous emission of low-frequency photons in collision of atoms and molecules in a gas is very rapidly, in several free-transit intervals, radically changes the trajectories and the order of the collisions of the particles as compared with the predictions of the equations of classical mechanics.

Quantum photon emission from a moving mirror in the nonperturbative regime

We consider the coupling of the electromagnetic vacuum field with an oscillating perfectly-reflecting mirror in the nonrelativistic approximation. As a consequence of the frequency modulation associated to the motion of the mirror, low frequency photons are generated. We calculate the photon emission rate by following a nonperturbative approach, in which the coupling between the field sidebands is taken into account. We show that the usual perturbation theory fails to account correctly for the contribution of TM-polarized vacuum fluctuations that propagate along directions nearly parallel to the plane surface of the mirror. As a result of the modification of the field eigenfunctions, the resonance frequency for photon emission is shifted from its unperturbed value.