This PDF file contains the front matter associated with SPIE Proceedings Volume 9920, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

For 75 years it has been known that radiative heat transfer can exceed far-field blackbody rates when two bodies are separated by less than a thermal wavelength. Yet an open question has remained: what is the maximum achievable radiative transfer rate? Here we describe basic energy-conservation principles that answer this question, yielding upper bounds that depend on the temperatures, material susceptibilities, and separation distance, but which encompass all geometries. The simple structures studied to date fall far short of the bounds, offering the possibility for significant future enhancement, with ramifications for experimental studies as well as thermophotovoltaic applications.

We report our results on random lasing from rhodamine 6G based colloidal gain medium consisting of urchin-like TiO₂ structures. Multimode behaviour is observed even at low pump laser powers. Emission linewidth narrowing and lasing threshold are investigated. Coherent back scattering is used to obtain the disorder degree of the sample. This urchin based system is demonstrated to possess lower lasing thresholds and enhanced efficiency with multimode behaviour compared to a TiO₂ spherical particle system with same disorder degree. This work opens up a new avenue for low threshold, high efficiency lasing.

Transition metals (TM) doped ZnSe crystals were synthesized by vapor phase thermal diffusion method. The absorption spectra of TM²⁺-doped ZnSe crystal were measured. First-principles calculations of the electronic structure and optical properties of TM²⁺: ZnSe were performed. The results demonstrate that when Zn is replaced by a TM atom, impurity bands (IB) are created in the bandgap and additional absorption peak appears in infrared region. The experimental absorption spectra match well with the calculated results, which is identified by electronic calculations to be caused by d-d transitions of TM²⁺ ions.

Peculiar physical properties of graphene offer remarkable potential for advanced photonics, particularly in the area of nonlinear optics at deep-subwavelength scale. In this article, we use a theoretical and computational analysis to demonstrate an efficient mechanism for enhancing the third-harmonic generation in graphene diffraction gratings. By taking advantage of the relation between the resonance wavelength of localized surface-plasmon polaritons of graphene ribbons and disks their specific geometry, we can engineer the spectral response of graphene gratings so as strong plasmonic resonances exist at both the fundamental frequency and third-harmonic (TH). As a result of this dual resonance mechanism for optical near-field enhancement, the intensity of the TH can be increased greatly.

We describe a powerful quasinormal mode (QNM) approach to characterizing the decay properties of quantum emitters in metal-dielectric resonator systems, including hybrid plasmonic-photonic coupled cavities as well as hyperbolic metamaterial resonators. We quantify both the radiative and non-radiative decay rates in these complex structures using these QNMs and a Green function expansion, which yields an excellent agreement with full-dipole calculations of Maxwell’s equations. Using this analytical QNM theory, we can map the Purcell factor for the system over a wide range of frequencies and dipole positions. We further show that how individual QNMs of these systems contribute to the underlying physics, whether it is strong interference effects between the sub-systems, in the case of a hybrid structure, or it is a physically meaningful explanation of very low beta factor for single photon emission in the case of hyperbolic metamaterials.

We study a device that consist of quantum resonators coupled to a mesoscopic photonic structure, such as a metasurface or a 2D metamaterial. For metasurfaces, we use surface Bloch modes in order to reach various coupling regimes between the metasurface and a quantum emitter, modelized semi-classically by an oscillator. Using multiple scattering theory and complex plane techniques, we show that the coupling can be characterized by means of a pole-and-zero structure. The regime of strong coupling is shown to be reached when the pole-and- zero pair is broken. For 2D metamaterial, we show the possibility of controlling optically the opening or closing of a gap.

Integration of solid-state quantum emitters such as NV center in diamond with tapered optical fiber is demanded by number of applications ranging from sensing and imaging to quantum communications and computations. Nevertheless, utilization of a single NV center coupled with an optical fiber meets significant challenge of fiber fluorescence that can considerably mask emission of the quantum object. In this paper, we analyze main sources of such fluorescence for the case of NV center coupled to tapered single mode optical fiber and discuss possible ways of improving signal to noise ratio in this case.

The negatively charged nitrogen vacancy (NV) center in diamond is a promising solid-state quantum memory. However, developing networks comprising such quantum memories is limited by the fabrication yield of the quantum nodes and the collection efficiency of indistinguishable photons. In this letter, we report on advances on a hybrid quantum system that allows for scalable production of networks, even with low-yield node fabrication. Moreover, an NV center in a simple single mode diamond waveguide is shown in simulation and experiment to couple well to a single mode SiN waveguide with a simple adiabatic taper for optimal mode transfer. In addition, cavity enhancement of the zero phonon line of the NV center with a resonance coupled to the waveguide mode allows a simulated <1800 fold increase in the collection of photon states coherent with the state of the NV center into a single frequency and spatial mode.

We experimentally demonstrate the realization of a parity-time (PT) symmetry breaking in optically coupled semiconductor lasers (SCLs). The two SCLs are identical except for a detuning between their optical emission frequencies. This detuning is analogous to the gain-loss parameter found in optical PT systems. To model the coupled SCLs, we employ the standard rate equations describing the electric field and carrier inversion of each SCL, and show that, under certain conditions, the rate equations reduce to the canonical, two-site PT- symmetric model. This model captures the global behavior of the laser intensity as the system parameters are varied. Overall, we find that this bulk system (coupled SCLs) provides an excellent test-bed to probe the characteristics of PT-breaking transitions, including the effects of time delay.

Over the past five years, open systems with balanced gain and loss have been investigated for extraordinary properties that are not shared by their closed counterparts. Non-Hermitian, Parity-Time (PT ) symmetric Hamiltonians faithfully model such systems. Such a Hamiltonian typically consists of a reflection-symmetric, Hermitian, nearest-neighbor hopping profile and a PT-symmetric, non-Hermitian, gain and loss potential, and has a robust PT -symmetric phase. Here we investigate the robustness of this phase in the presence of long-range hopping disorder that is not PT-symmetric, but is periodic. We find that the PT-symmetric phase remains robust in the presence of such disorder, and characterize the configurations where that happens. Our results are found using a tight-binding model, and we validate our predictions through the beam-propagation method.

Parity time (PT) symmetric systems are known to exhibit two distinct phases: those associated with an unbroken and broken symmetry. In the domain of optics, PT-symmetry can be established by incorporating a balanced distribution of gain and loss in a system. Under linear conditions, in a coupled dimer, composed of two cavities or waveguides, if the gain-loss contrast increases beyond a critical value with respect to the coupling constant, a transition is expected from the unbroken symmetry to the broken symmetry regime. However, in the presence of nonlinearity, this transition behavior can be drastically modified. We here study a system of two coupled semiconductor-based resonators that are lasing around an exceptional point. The quantum wells in such structures not only provide gain but also lead to strong levels of saturable loss in the absence of any optical pumping. Interestingly, in sharp contrast with linear PT-symmetric configurations, such nonlinear processes are capable of reversing the order in which the symmetry breaking occurs. If the ratio of the net loss to coupling is less than unity in one of the cavities, as the pumping level in the other resonator is increased, the nonlinear eigenmodes move from an unbroken symmetric state to a broken one. Moreover, in this nonlinear domain, the structural form of the resulting solutions are isomorphic to the corresponding linear eigenvectors expected above and below the phase transition point. Experimental results are in good agreement with these predictions.

We discuss asymmetric reflectance in surface plasmon Bragg gratings incorporating optical gain, referred to as active asymmetric surface plasmon Bragg gratings. It is shown that balanced modulation of index and gain/loss with quarter pitch spatial shift causes unidirectional coupling between contra-propagating modes in long-range surface plasmon polariton Bragg gratings. Such gratings operate at the breaking threshold of parity-time symmetry (exceptional point). Two active asymmetric surface plasmon Bragg gratings designs are proposed and their performance is examined through modal and transfer matrix method computations.

PT-symmetric structures in photonic crystals, combining refractive index and gain-loss modulations is becoming a research field with increasing interest due to the light directionality induced by these particular potentials. Here, we consider PT-symmetric potentials with axial symmetry to direct light to the crystal central point obtaining a localization effect. The axial and PT-symmetric potential intrinsically generates an exceptional central point in the photonic crystal by the merge of both symmetries. This particular point in the crystal lattice causes field amplitude gradients with exponential slopes around the crystal center. The field localization strongly depends on the phase of the central point and on the complex amplitude of the PT-potential. The presented work analyzes in a first stage 1D linear PT-axisymmetric crystals and the role of the central point phase that determines the defect character, i.e. refractive index defect, gain-loss defect or a combination of both. The interplay of the directional light effect induced by the PT-symmetry and the light localization around the central point through the axial symmetry enhances localization and allows higher field concentration for certain phases. The linearity of the studied crystals introduces an exponential growth of the field that mainly depends on the complex amplitude of the potential. The work is completed by the analysis of 2D PT-axisymmetric potentials showing different spatial slopes and growth rates caused by symmetry reasons.

We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. We also show that slow-light-enhanced ultra-compact plasmonic Mach-Zehnder interferometer sensors, in which the sensing arm consists of a waveguide system based on a plasmonic analogue of electromagnetically induced transparency, lead to an order of magnitude enhancement in the refractive index sensitivity compared to a conventional metal-dielectric-metal plasmonic waveguide sensor. Finally, we show that plasmonic coaxial waveguides offer a platform for practical implementation of plasmonic waveguide-cavity systems.

We report the theory and experiment of how an ultrathin (<80 nm) layer of vanadium dioxide (VO₂) can be used to control and adjust the polarization state of light. The refractive index of vanadium dioxide undergoes large changes when the material makes a phase transition from semiconductor to metal at a temperature of 68 °C. In a thin film, this results in optical phase shifts that are different for s- and p-polarizations in reflection or transmission. We investigate the conditions under which the polarization state would changes between linear or circular or between linear polarizations oriented differently during the material’s phase transition. The effect is demonstrated from 600 nm to 1600 nm and optical devices are proposed based on experimental data on refractive indices with temperature.

Achieving active control of the flow of light in nanoscale photonic devices is of fundamental interest in nanophotonics. For practical implementations of active nanophotonic devices, it is important to determine the sensitivity of the device properties to the refractive index of the active material. Here, we introduce a method for the sensitivity analysis of active nanophotonic waveguide devices to variations in the dielectric permittivity of the active material. More specifically, we present an analytical adjoint sensitivity method for the power transmission coefficient of nanophotonic devices, which is directly derived from Maxwell’s equations, and is not based on any specific numerical discretization method. We show that in the case of symmetric devices the method does not require any additional simulations. We apply the derived theory to calculate the sensitivity of the power transmission coefficient with respect to the real and imaginary parts of the dielectric permittivity of the active material for both two-dimensional and three-dimensional plasmonic devices. We consider Fabry-Perot cavity switches consisting of a plasmonic waveguide coupled to a cavity resonator which is filled with an active material with tunable refractive index. To validate our method, we compare it with the direct approach, in which the sensitivity is calculated numerically by varying the dielectric permittivity of the active material, and approximating the derivative using a finite difference. We find that the results obtained with our method are in excellent agreement with the ones obtained by the direct approach. In addition, our method is accurate for both lossless and lossy devices.

In this paper we investigate the potentials of liquid crystalline elastomer microstructures for the realization of optically tunable photonic microstructures. While certain limitations regarding the compromise between feature size and structure warping have been observed, it turns out that the simultaneous presence of a refractive index tuning effect and of a shape tuning effect intrinsic to the LCE material can be harnessed to design tunable photonic devices with unique behavior.

A series of dipolar and quadrupolar two-photon absorption (2PA) photoinitiators (PIs) based around the well-known triphenylamine (TPA) core and tricyanofuran (TCF) acceptors have been prepared for use in two-photon polymerisation (TPP). The synthesised dipolar species are designated as 5 and 7, and the remaining quadrupolar species are 6, 8, 9 and 10. Large two-photon absorption cross-sections (δ_2PA) ranging between 333 - 507 GM were measured at 780 nm using the z-scan technique. Fluorescence quantum yields (ΦF) were below 3% across the series when compared to Rhodamine 6G as a reference standard. Finally, TPP tests were conducted on PIs 7 and 8 to assess their ability to initiate the polymerisation of acrylate monomers using an 800 nm femtosecond Ti:Sapphire laser system.

We analyze amplification of terahertz plasmons in a grating-gate semiconductor hetero-structure. The device consists of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT), i.e. a HEMT structure with a double-barrier gate stack enabling resonant tunneling from gate to channel. In these devices, the key element enabling substantial power gain is the coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e. the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as in previous works, enabling amplification with associated power gain >> 30 dB at room temperature.

Using computer simulation, we show a possibility of ultrafast switching between stable states of an optical bistable device based on nano-film of semiconductor. Optical bistability occurs because of nonlinear dependence of semiconductor absorption coefficient on electric field potential. Electric field is induced by a laser pulse due to charged particles generation. The main feature of this bistable element is low absorption energy, which is necessary for switching, in comparison with bistable element based on other physical mechanism of laser energy absorption. For computer simulation of a problem under consideration a new finite-difference scheme is proposed using the original iterative process.