Linear Pulse Compansion using Co-propagating Space-Time Modulation

This paper presents a pulse compansion, i.e. compression or expansion, technique based on co-propagating space-time modulation. An engineered asymmetric space-time modulated medium, co-propagating with a pulse compands the pulse continuously and at a constant rate. The space-time medium locally modifies the velocity of different sections of the pulse in order to shape the pulse as it propagates. There is no theoretical limit on the compansion factor with the proposed system. Moreover, it can be designed to transform the pulse shape and its modulation linearly, without any distortion. Therefore the proposed technique can be used for up or down-conversion of modulated pulses, with extreme conversion ratios. The presented compansion technique is linear with respect to the input wave and therefore can be used to perform compansion or frequency conversion on multiple pulses simultaneously

Electromagnetic Chirality

This paper presents a first-principle and global perspective of electromagnetic chirality. It follows for this purpose a bottom-up construction, from the description of chiral particles or metaparticles (microscopic scale), through the electromagnetic theory of chiral media (macroscopic scale), to the establishment advanced properties and design principles of chiral materials and metamaterials. It preliminarily highlights the three fundamental concepts related to chirality -- mirror asymmetry, polarization rotation and magnetodielectric coupling -- and points out the nontrivial interdependencies existing between them. The first part (chiral particles) presents metamaterial as the most promising technology for chirality, compares two representative particles involving magnetoelectric coupling, namely the planar Omega particle and the twisted Omega or helix particle, and shows that only the latter is chiral, and finally links the response of microscopic particles to that of the medium formed by arranging them according to a subwavelength lattice structure. The second part (electromagnetic theory) infers from the previous microscopic study the chiral constitutive relations as a subset of the most general bianisotropic relations, derives parity conditions for the chiral parameters, computes the chiral eigenstates as circularly polarized waves, and finally shows that the circular birefringence of these states leads to polarization rotation. The third part (properties and design) introduces an explicit formulation of chirality based on spatial frequency dispersion or nonlocality, analyzes the temporal frequency dispersion or nonlocality of chiral media, and finally provides guidelines to design a practical chiral metamaterial

Generalized Brewster Effect using Bianisotropic Metasurfaces

We show that a properly designed bianisotropic metasurface placed at the interface between two arbitrary different media, or coating a dielectric medium exposed to the air, provides Brewster (reflectionless) transmission at arbitrary angles and for both the TM and TE polarizations. We present a rigorous derivation of the corresponding surface susceptibility tensors based on the Generalized Sheet Transition Conditions (GSTCs), and demonstrate the system with planar microwave metasurfaces designed for polarization-independent and azimuth-independent operations. Moreover, we reveal that such a system leads to the concept of effective refractive media with engineerable impedance. The proposed bianisotropic metasurfaces provide deeply subwavelength matching solutions for initially mismatched media, and alternatively lead to the possibility of on-demand manipulation of the conventional Fresnel coefficients. The reported generalized Brewster effect represents a fundamental advance in optical technology, where it may both improve the performance of conventional components and enable the development of novel devices

Heat Evacuation from Active Raman Media Heat Evacuation from Active Raman Media Using Quasi-PT Symmetry Coupling

We propose to use frequency-selective quasi parity-time symmetry to reduce the heat generated by coherent anti-Stokes Raman scattering (CARS) reversed cycles in active Raman media with phase mismatch. This is accomplished using a coupled-waveguide structure, which includes the active Raman waveguide (RW), where the Stokes signal undergoes amplification via stimulated Stokes Raman scattering (SSRS), and a dissipative waveguide (DW), which is tuned to the anti-Stokes wavelength so as to evacuate the corresponding anti-Stokes photons from the RW by coupling. The DW introduces optical loss that partially offsets the growth of the anti-Stokes signal in the RW and hence suppress the reversed CARS cycles that would otherwise result into heat generation in the RW. It is shown that the frequency-selective quasi parity-time symmetry provided by the DW can reduce the heat in active Raman media by a very factor of up to five when the CARS phase mismatch is compensated for by the optimum level of coupling between the RW and the DW

Dispersion Characteristics of Accelerated Spacetime-Modulated Media

This paper opens up the field of nonuniform-velocity SpaceTime-Modulated (STM) metamaterials, with the canonical example of an STM metamaterial of constant proper acceleration or, equivalently, hyperbolic acceleration. Combining tools of General Relativity and Classical Electrodynamics, it derives the dispersion relation of this exotic medium and reports its fundamental physics, whose most striking feature is the bending of light in the direction opposite to the direction of the modulation

Extending the Brewster Effect to Arbitrary Angle and Polarization using Bianisotropic Metasurfaces

A bianisotropic metasurface design is proposed for extending the Brewster effect to arbitrary angles and polarizations. The metasurface is synthesized using the surface susceptibility tensor and Generalized Sheet Transition Conditions (GSTCs) synthesis method, and is demonstrated by GSTC-FDFD simulation. It is found that an extremely broad angular range (0-30) with near zero reflection may be obtained near the normal angle, which is of paramount interest in paraxial optics, while narrower angular range at more oblique angles may find applications in spatial filtering

Magnetless Reflective Gyrotropic Spatial Isolator Metasurface

We present the concept of a magnetless Reflective Gyrotropic Spatial Isolator (RGSI) metasurface. This is a birefringent metasurface that reflects vertically polarized incident waves into a horizontally polarized waves, and absorbs horizontally polarized incident waves, hence providing isolation between the two orthogonal polarization. We first synthesize the metasurface using surface susceptibility-based Generalized Sheet Transition Conditions~(GSTCs). We then propose a mirror-backed metaparticle implementation of this metasurface, where transistor-loaded resonators provide the desired magnetless nonreciprocal response. Finally, we demonstrate the metasurface by full-wave simulation results. The proposed RGSI metasurface may be used in various electromagnetic applications, and may also serve as a step towards more sophisticated magnetless nonreciprocal metasurface systems

Photon Transitions in Arbitrary Time-Varying Metamaterials

We present a general theory for calculating photon transitions in arbitrarily time-varying metamaterials. This theory circumvents the difficulties of conventional approaches in solving such a general problem by exploiting the eigenstates of time-dependent number operators. We demonstrate here the temporal evolution of these operators and the related transition probabilities for the cases of logistic and linear permittivity profiles. The theory is potentially extensible to arbitrary space-time modulations and may hence lead to multiple novel quantum effects and applications

Unidirectional Loop Metamaterials (ULM) as Magnetless Artificial Ferrimagnetic Materials: Principles and Applications

This paper presents an overview of Unidirectional Loop Metamaterial (ULM) structures and applications. Mimicking electron spin precession in ferrites using loops with unidirectional loads (typically transistors), the ULM exhibits all the fundamental properties of ferrite materials, and represents the only existing magnetless ferrimagnetic medium. We present here an extended explanation of ULM physics and unified description of its component and system applications

Mie Resonance Enhancement of Laser Cooling in Rare-Earth Doped Materials

Laser cooling of solids keeps attracting attention owing to abroad range of its applications that extends from cm-sized all-optical cryocoolers for airborne and space-based applications to cooling on nanoparticles for biological and mesoscopic physics. Laser cooling of nanoparticles is a challenging task. We propose to use Mie resonances to enhance anti-Stokes fluorescence laser cooling in rare-earth (RE) doped nanoparticles made of low-phonon glasses or crystals. As an example, we consider an Yb3+:YAG nanosphere pumped at the long wavelength tail of the Yb3+ absorption spectrum at 1030 nm. We show that if the radius of the nanosphere is adjusted to the pump wavelength in such a manner that the pump excites some of its Mie resonant modes, the cooling power density generated in the sample is considerably enhanced and the temperature of the sample is consequently considerably (~ 63%) decreased. This concept can be extended to nanoparticles of different shapes and made from different low-phonon RE doped materials suitable for laser cooling by anti-Stokes fluorescence