Impact of geometry on the TM photonic band gaps of photonic crystals and quasicrystals

Freeze-derived heterogeneous structural color films

Structural colors have a demonstrated value in constructing various functional materials. Efforts in this area are devoted to developing stratagem for generating heterogeneous structurally colored materials with new architectures and functions. Here, inspired by icing process in nature and ice-templating technologies, we present freeze-derived heterogeneous structural color hydrogels with multiscale structural and functional features. We find that the space-occupying effect of ice crystals is helpful for tuning the distance of non-close-packed colloidal crystal nanoparticles, resulting in corresponding reflection wavelength shifts in the icing area. Thus, by effectively controlling the growth of ice crystals and photo-polymerizing them, structural color hydrogels with the desired structures and morphologies can be customized. Other than traditional monochromatic structure color hydrogels, the resultant hydrogels can be imparted with heterogeneous structured multi-compartment body and multi-color with designed patterns through varying the freezing area design. Based on these features, we have also explored the potential value of these heterotypic structural color hydrogels for information encryptions and decryptions by creating spatiotemporally controlled icing areas. We believe that these inverse ice-template structural color hydrogels will offer new routes for the construction and modulation of next generation smart materials with desired complex architectures.

Polarization Singularities of Photonic Quasicrystals in Momentum Space

We report the observation of polarization singularities in momentum space of 2D photonic quasicrystal slabs. Supercell approximation and band-unfolding approach are applied to obtain approximate photonic dispersions and the far-field polarization states defined on them. We discuss the relations between the topological charges of the polarization vortex singularities at Γ points and the symmetries of photonic quasicrystal slabs. With a perspective of multipolar expansions for the supercell, we confirm that the singularities are protected by the point-group symmetry of the photonic quasicrystal slab. We further uncover that the polarization singularities of photonic quasicrystal slab correspond to quasibound states in the continuum with exceptionally high-quality factors. Polarization singularities of different topological charges are also experimentally verified. Our Letter introduces core concepts of optical singularities into quasiperiodic systems, providing new platforms for explorations merging topological and singular optics.

On-Demand Design of Tunable Complete Photonic Band Gaps based on Bloch Mode Analysis

The fundamental property of photonic crystals is the band gap effect, which arises from the periodic dielectric modulation of electromagnetic waves and plays an indispensable role in manipulating light. Ever since the first photonic-bandgap structure was discovered, the ability to tune its bandgap across a wide wavelength range has been highly desirable. Therefore, obtaining photonic crystals possessing large on-demand bandgaps has been an ever-attractive study but has remained a challenge. Here we present an analytical design method for achieving high-order two-dimensional photonic crystals with tunable photonic band gaps on-demand. Based on the Bloch mode analysis for periodic structures, we are able to determine the geometric structure of the unit cell that will realize a nearly optimal photonic band gap for one polarization between the appointed adjacent bands. More importantly, this method generates a complete bandgap for all polarizations, with frequencies tuned by the number of photonic bands below the gap. The lowest dielectric contrast needed to generate a photonic band gap, as well as conditions for generating complete bandgaps, are investigated. Our work first highlights the systematic approach to complete photonic band gaps design based on Bloch mode analysis. The physical principles behind our work are then generalized to other photonic lattices.

Optical bistability and four-wave mixing with a single nitrogen-vacancy center coupled to a photonic crystal nanocavity in the weak-coupling regime

We explore optical bistability and degenerate four-wave mixing of a hybrid optical system composed of a photonic crystal nanocavity, a single nitrogen-vacancy center embedded in the cavity, and a nearby photonic waveguide serving for in- and outcoupling of light into the cavity in the weak-coupling regime. Here the hybrid system is coherently driven by a continuous-wave bichromatic laser field consisting of a strong control field and a weak probe field. We take account of the nonlinear nature of the nitrogen-vacancy center in the Heisenberg-Langevin equations and give an effective perturbation method to deal with such problems in the continuous-wave-operation regime. The results clearly show that the bistability region of the population inversion and the intensity of the generated four-wave mixing field can be well controlled by properly adjusting the system practical parameters. The nanophotonic platform can be used to implement our proposal. This investigation may be useful for gaining further insight into the properties of solid-state cavity quantum electrodynamics system and find applications in all-optical wavelength converter and switch in a photonic crystal platform.

Magnetic fluid-filled microhole in the collapsed region of a photonic crystal fiber for the measurement of a magnetic field

A method for measurement of a magnetic field by filling a microhole with magnetic fluid (MF) in a photonic crystal fiber (PCF) is presented and experimentally demonstrated. A microhole is created in the collapsed region between the PCF and the single-mode fiber by using femtosecond laser micromachining, and a PCF-based Mach-Zehnder interferometer is formed. The MF is filled into the microhole. Due to the tunable refractive index property of the MF, the mode field diameter of the propagation light is changed with the external magnetic field, and the magnetic field can be detected by measuring the visibility contrast of the white light interferogram. The experimental results show that sensitivity of up to 0.042 dB/Oe is achieved.

An optimal substrate design for SERS: dual-scale diamond-shaped gold nano-structures fabricated via interference lithography

Dual-scale diamond-shaped gold nanostructures (d-DGNs) with larger scale diamond-shaped gold nanoposts (DGNs) coupled to smaller scale gold nanoparticles have been fabricated via interference lithography as a highly reliable and efficient substrate for surface enhanced Raman scattering (SERS). The inter- and intra-particle plasmonic fields of d-DGNs are varied by changing the periodicity of the DGNs and the density of gold nanoparticles. Because of the two different length scales in the nanostructures, d-DGNs show multipole plasmonic peaks as well as dipolar plasmonic peaks, leading to a SERS enhancement factor of greater than 10(9). Simulations are carried out by finite-difference time-domain (FDTD) methods to evaluate the dependence of the inter- and intra-particle plasmonic field and the results are in good agreement with the experimentally obtained data. Our studies reveal that the combination of two different length scales is a straightforward approach for achieving reproducible and great SERS enhancement by light trapping in the diamond-shaped larger scale structures as well as efficient collective plasmon oscillation in the small size particles.