Perfect Mirror Research Articles

Adatom-induced local reconstructions in zigzag silicene nanoribbons: Spin semiconducting properties and large spin thermopowers

Using first-principles methods, we have investigated magnetic properties and thermospin effects of zigzag silicene nanoribbons (ZSiNRs) absorbed by a single Si atom. After a relaxation, a steady dumbbell-like structure is formed, which induces a weaker antiferromagnetic (AFM) coupling between two zigzag edges. Therefore, a band gap is opened, meanwhile the adsorbed ZSiNRs show a spin semiconducting property. A large spin thermopower and weak charge thermopower in adsorbed ZSiNR-based devices can be simultaneously achieved, which is attributed to a nearly perfect mirror symmetry of spin-up and spin-down transmission spectra relative to the Fermi level.

Dynamical Casimir effect withδ−δ′mirrors

We calculate the spectrum and the total rate of created particles for a real massless scalar field in $1+1$ dimensions, in the presence of a partially transparent moving mirror simulated by a Dirac $\delta-\delta^{\prime}$ point interaction. We show that, for this model, a partially reflecting mirror can produce a larger number of particles in comparison with a perfect one. In the limit of a perfect mirror, our formulas recover those found in the literature for the particle creation by a moving mirror with a Robin boundary condition.

Manipulation of solar-absorbing metamaterial coatings for implementing anti-frost and energy-saving transparent glass windows

This paper will present a concept to use matermaterial thin film deposited on a window glass to absorb tuned solar spectrum for anti-frost/anti-fog and energy-saving in cold weather, meanwhile the luminous transmittance of the windows can be maintained. In warmer weather, the opposite side of a rotatable window can be turned to face the sunlight to reflect the sunlight away with the perfect mirror function of the base ground layer of the metamaterial thin film. By adjusting base ground coating structure, perfect or non-perfect mirror can be formed for natural solar reflection or transmittance. The metal-dielectric-metal nano-scale metamaterial structures with periodic metal-dielectric interfaces, when shined with light, acquire surface plasmons thus trapping light at subwavelength scales. When the metamaterial film is coated on the outside of a glass window, it will lead to efficient solar radiation absorption, which can be used for anti-frost/anti-fog and energy-saving windows of transportation vehicles ...

Exact Fundamental Function for the One‐Dimensional Random Walk with a Perfect Mirror under the External Field

The exact analytical probability function is presented for the random walk with a perfect mirror under the constant external field in one dimension. Unlike the field‐free solution, the symmetry around the boundary is broken by the external field and the fundamental function is not given by a simple form. We prove the solution by mathematical induction method and numerical simulations. Monte Carlo simulations can be replaced by the function without statistical noise. Based on this function, we also obtain the solution for the continuum diffusion‐influenced reaction, which is shown to be superior to the known solution especially for the system with a strong external field.

Quantum critical transport at a continuous metal-insulator transition

In contrast to the first-order correlation-driven Mott metal-insulator transition, continuous disorder-driven transitions are intrinsically quantum critical. Here, we investigate transport quantum criticality in the Falicov-Kimball model, a representative of the latter class in the strong disorder category. Employing cluster-dynamical mean-field theory, we find clear and anomalous quantum critical scaling behavior manifesting as perfect mirror symmetry of scaling curves on both sides of the MIT. Surprisingly, we find that the beta function $\ensuremath{\beta}(g)$ scales as $log(g)$ deep into the bad-metallic phase as well, providing a sound unified basis for these findings. We argue that such strong localization quantum criticality may manifest in real three-dimensional systems where disorder effects are more important than electron-electron interactions.

Self-collimation-based photonic crystal Mach–Zehnder demultiplexer

A photonic crystal Mach–Zehnder demultiplexer (PC-MZDmux) with four output ports based on the self-collimation phenomenon in a two-dimensional (2D) PC is proposed and numerically studied using finite-difference time-domain simulations. The PC-MZDmux is composed of three Mach–Zehnder interferometers (MZIs) and each MZI consists of two 50:50 beam splitters and two perfect mirrors. Employed as the design parameters to achieve the demultiplexing functionality are the radius of phase control rods (PCRs) in the mirrors and the distance between the beam spitter and the mirror in the three MZIs. From spatial electric field distributions and transmission spectra, it is demonstrated that an incident self-collimated beam with four different frequencies can be demultiplexed to four output ports of the PC-MZDmux with proper design parameters. Our results indicate that this device design may constitute an efficient approach to light propagation manipulation and increase the application range of self-collimated beams.

Two-channel perfect reconstruction (PR) quadrature mirror filter (QMF) bank design using logarithmic window function and spline function

In this work, two-channel perfect reconstruction quadrature mirror filter (QMF) bank has been proposed based on the prototype filter using windowing method. A novel window function based on logarithmic function along with the spline function is utilized for the design of prototype filter. The proposed window has a variable parameter ‘ $$\alpha $$ ’, which varies the peak side lobe level and rate of fall-off side lobe level which in turn affects the peak reconstruction error (PRE) and amplitude distortion ( $$e_{am}$$ ) of the QMF bank . The transition width of the prototype is controlled by the spline function using the parameter ‘ $$\mu $$ ’. The perfect reconstruction condition is satisfied by setting the cutoff frequency ( $$\omega _{c}$$ ) of the prototype low-pass filter at ‘ $$\pi /2$$ ’. The performance of the proposed design method has been evaluated in terms of mean square error in the pass band, mean square error in the stop band, first side lobe attenuation ( $$A_{1}$$ ), peak reconstruction error (PRE) and amplitude error ( $$e_{am}$$ ) for different values of ‘ $$\alpha $$ ’ and ‘ $$\mu $$ ’. The results are provided and compared with the existing methods.

Optimization of a T-shaped optical grating for specific applications

A detailed analysis of the optical reflectivity of a monolithic, T-shaped surface relief grating structure is carried out. It is shown that by changing the groove depths and widths, the frequency-dependent reflectivity of the diffraction grating can be greatly modified to obtain various specific optical elements. The basic T-shaped grating structure is optimized for three specific applications: a perfect mirror with a wide maximal reflection plateau, a bandpass filter, and a dichroic beam splitter. These specific mirrors could be used to steer the propagation of bichromatic laser fields, in situations where multilayer dielectric mirrors cannot be applied due to their worse thermomechanical properties. Colored maps are presented to show the reflection dependency on the variation of several critical structure parameters. To check the accuracy of the numerical results, four independent methods are used: finite-difference time-domain, finite-difference frequency-domain, method of lines, and rigorous coupled-wave analysis. The results of the independent numerical methods agree very well with each other indicating their correctness.

Boundary effects on radiative processes of two entangled atoms

We analyze radiative processes of a quantum system composed by two identical two-level atoms interacting with a massless scalar field prepared in the vacuum state in the presence of perfect reflecting flat mirrors. We consider that the atoms are prepared in a stationary maximally entangled state. We investigate the spontaneous transitions rates from the entangled states to the collective ground state induced by vacuum fluctuations. In the empty-space case, the spontaneous decay rates can be enhanced or inhibited depending on the specific entangled state and changes with the distance between the atoms. Next, we consider the presence of perfect mirrors and impose Dirichlet boundary conditions on such surfaces. In the presence of a single mirror the transition rate for the symmetric state undergoes a slight reduction, whereas for the antisymmetric state our results indicate a slightly enhancement. Finally, we investigate the effect of multiple reflections by two perfect mirrors on the transition rates.

The Voltage Boost Enabled by Luminescence Extraction in Solar Cells

Over the past few years, the application of the physical principle, i.e., “luminescence extraction,” has produced record voltages and efficiencies in photovoltaic cells. Luminescence extraction is the use of optical design, such as a back mirror or textured surfaces, to help internal photons escape out of the front surface of a solar cell. The principle of luminescence extraction is exemplified by the mantra “a good solar cell should also be a good LED.” Basic thermodynamics says that the voltage boost should be related to concentration ratio <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C </i> of a resource by Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C </i> }. In light trapping (i.e., when the solar cell is textured and has a perfect back mirror), the concentration ratio of photons <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> = {4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> }; therefore, one would expect a voltage boost of Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> } over a solar cell with no texture and zero back reflectivity, where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> is the refractive index. Nevertheless, there has been ambiguity over the voltage benefit to be expected from perfect luminescence extraction. Do we gain an open-circuit voltage boost of Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> }, Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> }, or Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> }? What is responsible for this voltage ambiguity Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) ln{4}≍36 mV? We show that different results come about, depending on whether the photovoltaic cell is optically thin or thick to its internal luminescence. In realistic intermediate cases of optical thickness, the voltage boost falls in between: ln{ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> } < ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kT</i> ) < ln{4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n </i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> }.

Integrating Photonics with Luminescent Solar Concentrators: Optical Transport in the Presence of Photonic Mirrors

Luminescent solar concentrators downshift and concentrate the incident solar spectrum onto adjacent solar cells. In this paper we study the combination of highly performing luminescent nanocrystals with photonic structures to guide light to the edge of the concentrator. While one approach is to use a wavelength-selective Bragg mirror to reduce escape cone losses, we find that this also requires nearly perfect mirrors and luminophore quantum yield. The key issue is that light is trapped inside the concentrator in modes that inefficiently propagate toward the edge, leading to luminophore reabsorption and losses to imperfect mirrors. To overcome this difficulty, we use modeling to study the use of meta-mirrors that shift propagating photons to larger angles, minimizing interactions with the reabsorbing luminophores and mirrors. We find that this design favors mirrors with small changes in angle, separated from the luminophore–polymer layer by an air gap. This research indicates that a combination of photonic...

Spatial entanglement of nonvacuum Gaussian states

The vacuum state of a relativistic quantum field contains entanglement between regions separated by spacelike intervals. Such spatial entanglement can be revealed using an operational method introduced in Ann. Phys. 351, 112 (2014), Phys. Rev. D 91, 016005 (2014). In this approach, a cavity is instantaneously divided into halves by an introduction of an extra perfect mirror. Causal separation of the two regions of the cavity reveals nonlocal spatial correlations present in the field, which can be quantified by measuring particles generated in the process. We use this method to study spatial entanglement properties of nonvacuum Gaussian field states. In particular we show how to enhance the amount of harvested spatial entanglement by an appropriate choice of the initial state of the field in the cavity. We find a counterintuitive influence of the initial entanglement between cavity modes on the spatial entanglement which is revealed by dividing the cavity in half.

Dielectric nanoresonator based lossless optical perfect magnetic mirror with near-zero reflection phase

We report an all-dielectric lossless optical mirror for the realization of controllable reflection phase based on an array of isolated dielectric nanoresonators. This dielectric mirror is comprised of a cross-shaped amorphous silicon nanoresonator array that has been designed to achieve a 99.8% reflectivity and zero reflection phase at the wavelength of 0.99 μm. The measured results from the fabricated sample match the theoretical predictions with 99.5% reflectivity and near-zero degree reflection phase at 1 μm, which is very close to the targeted wavelength. This concept and approach pave the way for synthesizing lossless artificial reflecting electromagnetic boundaries with arbitrary phase response and hold great promise in applications ranging from nanocavities to nanowaveguides and nanoantennas.

Broadband Absorption Enhancement in Solar Cells with an Atomically Thin Active Layer

We numerically demonstrate broadband absorption enhancement in the visible in monolayer MoS2 placed on a photonic crystal slab, backed by a perfect electric conductor mirror. The absorption enhancement arises from interaction between the MoS2 and the guided resonances of the photonic crystal slab. Through consideration of light trapping theory and parametric studies of the design parameters, it is shown that the optimal photonic crystal slab has a grating period that is subwavelength to most of the frequency region of interest, has large air holes, and is thin. The optimal design features an average absorption of 51% at normal incidence in the visible above the band gap of MoS2 (400–690 nm) and is less than 100 nm thick. We also comment on the angular response of the optimal device.

Dynamical Casimir-Polder interaction between a chiral molecule and a surface

We develop a dynamical approach to study the Casimir-Polder force between a initially bare molecule and a magnetodielectric body at finite temperature. Switching on the interaction between the molecule and the field at a particular time, we study the resulting temporal evolution of the Casimir-Polder interaction. The dynamical self-dressing of the molecule and its population-induced dynamics are accounted for and discussed. In particular, we find that the Casimir-Polder force between a chiral molecule and a perfect mirror oscillates in time with a frequency related to the molecular transition frequency, and converges to the static result for large times.

Nonideal Effects Limit the Efficiency Gain for Angle-Restricted Solar Cells

Performance of a conventional solar cell at 1 sun is limited to ∼31%, defined by the Shockley–Queisser (SQ) limit. The thermodynamic limit can be improved to ∼41% by reducing the angle mismatch between incident light and emitted rays. This can be achieved either by increasing the range of incident angles (as in a concentrator cell; CPV) or by reducing the emission angle from the device (angle-restricted photovoltaics; APVs). The equivalence is exact in the radiative limit. However, the gains of angle-restricted cells may erode rapidly once nonradiative recombination and other optical losses are accounted for, suggesting the need for quantitative/careful design. Here, we report a detailed self-consistent absorption-emission-transport study to 1) define the limiting efficiency of APV scheme and 2) explain the efficiency loss due to practical constraints, such as Shockley–Read–Hall (SRH) and Auger recombination, and mirror reflectivity. Our analysis reflects a growing consensus in the field that any imperfection of the mirror quickly diminishes the advantages of angle restriction. We conclude that a 50-nm angle-restricted cell with moderate material quality, but with nearly perfect mirror and Lambertian trapping, may be ∼36–37% efficient.

Multi-channel terahertz wavelength division demultiplexer with defects-coupled photonic crystal waveguide

Terahertz (THz) wavelength division demultiplexer based on a compact defects-coupled photonic crystal waveguide is proposed and demonstrated numerically. This device consists of an input waveguide that perpendicularly coupled with a series of defects cavities, each of which captures the resonance frequency from the input waveguide. Coupled-mode theory and finite element method are used to analyze the transmission properties of the structure. It is found that the transmission wavelength centered around 1 THz can be adjusted by changing the geometrical parameters of defects cavities, which equals to THz waves generated by optical methods such as difference frequency generation and optical rectification. Applications in this frequency range are urgently needed. Furthermore, the highest transmission efficiency of 0.94 can be achieved when a perfect wavelength-selective mirror is set in the output waveguide.

Quantum properties of an atom in a cavity constructed by topological insulators

We investigate the spontaneous decay of an atom in a cavity constructed by topological insulators. The method of Green’s function in stratified cavities has been used and expressions of atomic decay rates in two directions parallel and perpendicular to cavity mirrors are derived. It is shown that when an atom is embedded closely to a topological insulator with a strong axion field, the spontaneous decay of the dipole parallel to the surface will be suppressed due to the high anisotropy of the vacuum. The appearance of indirect quantum interference of two orthogonal dipoles representing the different decay channels of a Zeeman atom has also been revealed. It is found that when the cavity length is less than half the wave length in a vacuum—wherever the Zeeman atom is located in the cavity—spontaneous decay of the dipole parallel to the mirror is deeply suppressed while decay through the dipole perpendicular to the mirror is enhanced. As expected, strong quantum interference between the two decay channels is observed. In particular, when the axion field is strong enough, the maximum quantum interference can be generated between two orthogonal dipoles, which is consistent with the phenomenon that appears in cavities constructed by perfect mirrors. By taking loss of material into account, the results show that atomic decay through dissipation dominates when the atom is quite close to the mirror and leads to the destruction of quantum interference in a small region.

Atomic-Level Understanding of "Asymmetric Twins" in Boron Carbide.

Recent observations of planar defects in boron carbide have been shown to deviate from perfect mirror symmetry and are referred to as "asymmetric twins." Here, we demonstrate that these asymmetric twins are really phase boundaries that form in stoichiometric B(4)C (i.e., B(12)C(3)) but not in B(13)C(2). TEM observations and ab initio simulations have been coupled to show that these planar defects result from an interplay of stoichiometry, atomic positioning, icosahedral twinning, and structural hierarchy. The composition of icosahedra in B(4)C is B(11)C and translation of the carbon atom from a polar to equatorial site leads to a shift in bonding and a slight distortion of the lattice. No such distortion is observed in boron-rich B(13)C(2) because the icosahedra do not contain carbon. Implications for tailoring boron carbide with stoichiometry and extrapolations to other hierarchical crystalline materials are discussed.

Circularly polarised phosphorescent photoluminescence and electroluminescence of iridium complexes.

Nearly all the neutral iridium complexes widely used as dopants in PhOLEDs are racemic mixtures; however, this study observed that these complexes can be separated into stable optically active Λ and ∆ isomers and that their chirality is an intrinsic property. The circularly polarised phosphorescent photoluminescence (CPPPL) signals of Λ/Δ isomers are perfect mirror images with opposite polarisation and equal intensity exhibiting a “handedness” for the polarisation. For the first time, we applied the Λ/Δ iridium isomers as emitters in OLEDs, and the circularly polarised phosphorescent electroluminescence (CPPEL) spectra reveal completely positive or negative broad peaks consistent with the CPPPL spectra. The results demonstrate that the Λ/Δ isomers have potential application for 3D OLEDs because they can exhibit high efficiency and luminance, and 3D display technology based on circularly polarised light is the most comfortable for the eyes.