260 research outputs found
Multistability at arbitrary low optical intensities in a metallo-dielectric layered structure
We show that a nonlinear metallo-dielectric layered slab of subwavelength
thickness and very small average dielectric permittivity displays optical
multistable behavior at arbitrary low optical intensities. This is due to the
fact that, in the presence of the small linear permittivity, one of the
multiple electromagnetic slab states exists no matter how small is the
transmitted optical intensity. We prove that multiple states at ultra-low
optical intensities can be reached only by simultaneously operating on the
incident optical intensity and incidence angle. By performing full wave
simulations, we prove that the predicted phenomenology is feasible and very
robust.Comment: 4 pages, 4 figure
Layer-Resolved Ultrafast XUV Measurement of Hole Transport in a Ni-TiO2-Si Photoanode
Metal-oxide-semiconductor junctions are central to most electronic and
optoelectronic devices. Here, the element-specificity of broadband extreme
ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and
recombination kinetics in each layer of a Ni-TiO2-Si junction. After
photoexcitation of silicon, holes are inferred to transport from Si to Ni
ballistically in ~100 fs, resulting in spectral shifts in the Ni M2,3 XUV edge
that are characteristic of holes and the absence of holes initially in TiO2.
Meanwhile, the electrons are observed to remain on Si. After picoseconds, the
transient hole population on Ni is observed to back-diffuse through the TiO2,
shifting the Ti spectrum to higher oxidation state, followed by electron-hole
recombination at the Si-TiO2 interface and in the Si bulk. Electrical
properties, such as the hole diffusion constant in TiO2 and the initial hole
mobility in Si, are fit from these transient spectra and match well with values
reported previously
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Accurate and efficient radiation transport in optically thick media -- by means of the Symbolic Implicit Monte Carlo method in the difference formulation
The equations of radiation transport for thermal photons are notoriously difficult to solve in thick media without resorting to asymptotic approximations such as the diffusion limit. One source of this difficulty is that in thick, absorbing media thermal emission is almost completely balanced by strong absorption. In a previous publication [SB03], the photon transport equation was written in terms of the deviation of the specific intensity from the local equilibrium field. We called the new form of the equations the difference formulation. The difference formulation is rigorously equivalent to the original transport equation. It is particularly advantageous in thick media, where the radiation field approaches local equilibrium and the deviations from the Planck distribution are small. The difference formulation for photon transport also clarifies the diffusion limit. In this paper, the transport equation is solved by the Symbolic Implicit Monte Carlo (SIMC) method and a comparison is made between the standard formulation and the difference formulation. The SIMC method is easily adapted to the derivative source terms of the difference formulation, and a remarkable reduction in noise is obtained when the difference formulation is applied to problems involving thick media
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Piecewise linear discretization of Symbolic Implicit Monte Carlo radiation transport in the difference formulation
We describe a Monte Carlo solution for time dependent photon transport, in the difference formulation with the material in local thermodynamic equilibrium (LTE), that is piecewise linear in its treatment of the material state variable. Our method employs a Galerkin solution for the material energy equation while using Symbolic Implicit Monte Carlo (SIMC) to solve the transport equation. In constructing the scheme, one has the freedom to choose between expanding the material temperature, or the equivalent black body radiation energy density at the material temperature, in terms of finite element basis functions. The former provides a linear treatment of the material energy while the latter provides a linear treatment of the radiative coupling between zones. Subject to the conditional use of a lumped material energy in the vicinity of strong gradients, possible with a linear treatment of the material energy, our approach provides a robust solution for time dependent transport of thermally emitted radiation that can address a wide range of problems. It produces accurate results in the diffusion limit
Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiOâ‚‚-Si photoanode
Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiOâ‚‚-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiOâ‚‚, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiOâ‚‚ interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiOâ‚‚ and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously
Narrowing of EIT resonance in a Doppler Broadened Medium
We derive an analytic expression for the linewidth of EIT resonance in a
Doppler broadened system. It is shown here that for relatively low intensity of
the driving field the EIT linewidth is proportional to the square root of
intensity and is independent of the Doppler width, similar to the laser induced
line narrowing effect by Feld and Javan. In the limit of high intensity we
recover the usual power broadening case where EIT linewidth is proportional to
the intensity and inversely proportional to the Doppler width.Comment: 4 pages, 2 figure
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