Photonic Band Gap Region Research Articles

Enhancement of photoluminescence from defect states in ZnS random photonic crystal: An effect of electronic and photonic mode coupling

This paper reports on the enhanced defect state emission from ZnS in the form of a random photonic crystal (RPC) medium. ZnS photonic crystals with varied randomness are fabricated by colloidal self assembly of ZnS nanospheres (215 ± 10 nm). Reflection and transmission studies reveal mid band gap wavelength at ∼435 nm. The band structure calculated for BCC lattice with reduced packing fraction (53%) is in good agreement with experimental results. The reflection due to the photonic band gap diminishes with increased randomness in the nanosphere arrangement. The features of fluorescence from ZnS are modified in the RPC medium, resulting in suppression at wavelengths in the photonic band gap region and an enhancement at band edge wavelengths of 415 and 468 nm. This enhancement becomes less prominent with increasing randomness in the structure. Interestingly these two modes correspond to the electronic defect states of ZnS. Emission enhancement is shown to be due to the strong coupling of electronic defect states and photonic band edge states which is facilitated by randomly scattering slow Bloch modes in the ZnS RPC. Fabrication of RPCs by colloidal self-assembly with specifically designed degrees of randomness (leading to controllable features of emission) provides scope for the design of low threshold random lasing systems.

Fresnel lenses fabricated by femtosecond laser micromachining on polymer one-dimensional photonic crystal

We report the fabrication of micro-Fresnel lenses by femtosecond laser surface ablation on one-dimensional (1-D) polymer photonic crystals. This device is designed to focus the transmitted wavelength (520 nm) of the photonic crystal and filter the wavelengths corresponding to the photonic band-gap region (centered at 630 nm, ranging from 530 to 700 nm). Integration of such devices in a wavelength selective light harvesting and filtering microchip is envisaged.

One-dimensional photonic structure with multilayer random defect

A photonic structure composed of two dielectric mirrors with a random multilayer defect embedded is presented. Simulating the transmission spectra of 30,028 permutations of the random defect with the transfer matrix method, we have observed defect lines in the photonic bad gap region, with intensity and spectral position depending on the different permutations. We have compared their optical properties with the ones of two types of structures: (i) conventional defect and (ii) periodic multilayer defect photonic structures. We have noticed that about 15% of the random defect structures, showing very intense defect peaks in the photonic band gap region, exhibit total light transmission in such range of wavelengths that is higher with respect to the other structures. Moreover, random defect structures can show two defect lines with relatively high transmission maxima, in spite of the only one defect line or the two weak defect lines of the other multilayer sequences. These results can be interesting for the fabrication of one- or two-colour photonic crystal defect lasers, with an appropriate choice of random defect sequences and emitting media.

Fresnel Lenses fabricated by femtosecond laser micromachining on Polymer 1D Photonic Crystal

We report the fabrication of micro Fresnel lenses by femtosecond laser surface ablation on polymer 1D photonic crystals. This device is designed to focus the transmitted wavelength of the photonic crystal and filter the wavelengths corresponding to the photonic band gap region. Integration of such devices in a wavelength selective light harvesting and filtering microchip can be achieved.

Bandgap Calculation of 2D Hexagonal Photonic Crystal Structures Based on Regression Analysis

We introduce a novel method based on mathematical equations to evaluate Photonic Band Gap (PBG) of the 2D hexagonal structures. By using regression analysis, extraction of PBG in all of 2D hexagonal structures is possible. We show that the mathematical equations that reached in this work have sufficient validity with comparing other numerical methods such as Plane Wave Expansion (PWE) and so etc. We demonstrate this validity for both 2D hexagonal air holes in dielectric medium and dielectric rods in air cases. The results show that the PBG region of both above mentioned kinds tend toward lower normalized frequencies by increasing the refractive index of dielectric material. The developed formulas in this work consist all of the physical parameters of structures such as lattice constant, refractive indices and dielectric radiuses. The resulting equations are suitable for applicable optical devices designing to tune the desired PBGs. These equations have minimum errors compared with other methods and it has been shown that results are matched very well with numerical simulations.

Modified annular photonic crystals with enhanced dispersion relations: polarization insensitive self-collimation and nanophotonic wire waveguide designs

A novel (to our best knowledge) type of photonic crystal (PC) structure called modified annular PC (MAPC) that is composed of dielectric rods with off-centered air holes is thoroughly studied. The plane wave expansion method is applied for spectral analysis. A complete photonic bandgap region with a considerable value of gap width Δω/ω=7.06% is achieved by optimizing the structural parameters of the proposed periodic medium. By introducing geometrical asymmetry to the primitive cell of PC, we engineer the dispersion properties of the proposed photonic structure such that conventional equifrequency contours for the second band can be transformed into tilted rectangular shapes. This feature enables us to demonstrate the polarization insensitive tilted self-collimation effect. A hybrid structure composed of dielectric nanowire and MAPCs is offered to obtain a high degree of polarization independent guiding of light. The two-dimensional finite-difference time-domain method is carried out to verify the light guiding efficiencies. Polarization insensitive optical functionalities achieved by MAPC structure can be deployed in integrated optical circuits.

Coupled Optical Tamm States in a Planar Dielectric Mirror Structure Containing a Thin Metal Film

The coupling between two optical Tamm states (OTSs) with the same eigenenergy is numerically investigated in a planar dielectric mirror structure containing a thin metal film. The reflectivity map in this structure at normal incidence is obtained by applying the transfer matrix method. Two splitting branches appear in the photonic bandgap region when both adjacent dielectric layers of metal film are properly set. The splitting energy of two branches strongly depends on the thickness of the metal film. According to the electric field distribution in this structure, it is found that the high-energy branch corresponds to the antisymmetric coupling between two OTSs, while the low-energy branch is associated with the symmetric coupling between two OTSs. Moreover, the optical difference frequency of two branches is located in a broad terahertz region.

Bottom-up Photonic Crystal Lasers

The directed growth of III-V nanopillars is used to demonstrate bottom-up photonic crystal lasers. Simultaneous formation of both the photonic band gap and active gain region is achieved via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. The nanopillars implement a GaAs/InGaAs/GaAs axial double heterostructure for accurate, arbitrary placement of gain within the cavity and lateral InGaP shells to reduce surface recombination. The lasers operate single-mode at room temperature with low threshold peak power density of ∼625 W/cm2. Cavity resonance and lasing wavelength is lithographically defined by controlling pillar pitch and diameter to vary from 960 to 989 nm. We envision this bottom-up approach to pillar-based devices as a new platform for photonic systems integration.

Bottom-up Photonic Crystal Cavities Formed by Patterned III–V Nanopillars

We report on the formation and optical properties of bottom-up photonic crystal (PC) cavities formed by III-V nanopillars (NPs) via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. This method of NP synthesis allows for precise lithographic control of NP position and diameter enabling simultaneous formation of both the photonic band gap (PBG) region and active gain region. The PBG and cavity resonance are determined by independently tuning the NP radius r, pitch a, and height h in the respective masked areas. Near-infrared emission at 970 nm is achieved from axial GaAs/InGaAs heterostructures with in situ passivation by laterally grown InGaP shells. To achieve out-of-plane optical confinement, the PC cavities are embedded in polydimethylsiloxane (PDMS) and removed from the growth substrate. Spatially and spectrally resolved 77 K photoluminescence demonstrates a strong influence of the PBG resonance on device emission. Resonant peaks are observed in the emission spectra of PC cavities embedded in PDMS.

Influence of filling fraction on the defect mode and gap closing of a one-dimensional photonic crystal: An analytical approach

Study of the optical properties of the one-dimensional defective photonic crystals using the gap map is improving through the emergence of new analytical methods, which are easy and without any physical restrictions. Gap map is able to monitor the changes in the defect mode frequencies and photonic band gap regions as a function of filling fractions, and all visible spectra in a single graphic presentation. In this paper, by utilizing a novel technique based on Green’s function method for analyzing the defect modes, the gap map and gap closing point of a one-dimensional defective photonic crystal have been demonstrated. This method enables study of the defect modes inside the omnidirectional band gap, which is an important object in the designing of the optical filters. Moreover, as a designing criterion, obtaining the gap closing points inside the gap map enables finding of some filling fraction intervals that each one contains several distinct omnidirectional band gaps simultaneously, using a single photonic crystal. This method has been employed for the design of an optical filter at 1.3 and 1.55 μm, which is applicable for telecommunication.

Photonic band-gap maps for different two dimensionally periodic photonic crystal structures

In this paper, the photonic band-gap (PBG) maps of fundamental photonic crystal (PhC) lattices are presented, and discussed. Two fundamental types of lattice: square and hexagonal (triangular, graphite or honeycomb and kagome) with circular, square, and hexagonal hole shapes are considered. Because they show the largest area gap map and because of the relative ease of fabrication of circular holes, it can be asserted that square and triangular lattices of circular holes offer the best choice of two dimensionally periodic photonic crystal (PhC) structure. Graphite and kagome lattices of circular holes in GaAs also show large area gap maps (and the largest gap map is for the second or third higher PBG region). So graphite and kagome lattices are also relevant for exploitation in PhC devices. At the scale required, fabrication process limitations are a significant problem for the realization of hole shapes other than circular.

Spontaneous Emission of Terbium Complex In-Filled in Single-Crystal Colloidal Multilayer

Highly monodisperse submicron-sized silica spheres were synthesized by a chemical method, and single-crystal colloidal multilayers were prepared by a vertical deposition technique. Rare earth complex Tb(ACAC)3phen was in-filled in this structure to investigate the photonic bandgap effect on the spontaneous emission. The photon density of states in the photonic crystal was calculated numerically to investigate the origin of the abnormally enhanced photoluminescence in the photonic bandgap region. Our results are suggestive of a new way to probe the position of fluorescent molecules in such photonic bandgap materials.

Spectrally selective infrared absorption in defect-mode photonic-crystal-slab cavity

Significantly enhanced absorption at the defect mode can be obtained at surface-normal direction in a dielectric single-defect photonic-crystal-slab, with an absorption enhancement factor greater than 4,000. Complete absorption suppression within the photonic bandgap region can also be observed in defect-free photonic crystal cavities. High spectral selectivity and tunability is feasible with defect mode engineering, making photonic crystal defect cavities a promising nanophotonic platform for the spectrally selective infrared sensing and hyper-spectral imaging, with the incorporation of quantum well or quantum dot infrared photodetector heterostructures.

Geometric phases for a three-level Λ -type system in one-dimensional photonic band gaps

The geometric phase of a three-level atom interacting with single-mode in the presence of the one-dimensional photonic band gap material is studied. We consider dipole emitters situated in the thin slab region between two semi-infinite one-dimensionally periodic photonic crystals, a situation reminiscent of planar cavity laser structures. It shows that the Pancharatnam phase depends crucially on the presence of the photonic band gap. This feature is attributed to the fact that in the photonic band gap region electromagnetic modes are not allowed to propagate into the dielectric slab and hence no interaction can take place in this region. The outcomes are illustrated with numerical simulations applied to GaAs.

Optical properties of porous silicon Thue‐Morse structures

AbstractDielectric aperiodic Thue‐Morse structures up to 128 layers have been realized by the porous silicon technology. Normal incidence reflectivity measurements have been performed to investigate the photonic properties of the devices. A partial photonic band gap region, centered at 1100 nm and 70 nm wide has been observed for the S6 and S7 Thue‐Morse structures. The S6 multilayer has been studied as sensor device on exposure to several chemical substances. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cavity modes of exciton–polaritons in an organic crystal with distributed Bragg reflectors

We study optical spectra of cavity modes of exciton–polaritons in an organic crystal. The cavity is composed of distributed Bragg reflectors and anthracene crystals occupy the gap of the cavity. Many transmittance peaks corresponding to the cavity modes appear in the photonic band gap region of the transmittance spectra. The spectral positions of the cavity modes converge to the exciton resonance in anthracene. Dispersion of the cavity modes is obtained from the spectral peaks. It looks like a dispersion of the exciton–polariton in anthracene; however it differs slightly from the dispersion curve of the polariton. This discrepancy between the dispersion of the modes and the polariton curve is ascribed to the effective cavity length varying with the refractive index of anthracene.

Photonic band gaps analysis of Thue-Morse multilayers made of porous silicon

Dielectric aperiodic Thue-Morse structures up to 128 layers have been fabricated by using porous silicon technology. The photonic band gap properties of Thue-Morse multilayers have been theoretically investigated by means of the transfer matrix method and the integrated density of states. The theoretical approach has been compared and discussed with the reflectivity measurements at variable angles for both the transverse electric and transverse magnetic polarizations of light. The photonic band gap regions, wide 70 nm and 90 nm, included between 0 and 30 degrees , have been observed for the sixth and seventh orders, respectively.

Color separating with integrated photonic bandgap optical diodes: a numerical study

An optical diode (OD) and its application based on the photonic band-gap (PBG) property of chiral photonic crystals are presented. The OD is constructed from wave-plates and cholesteric liquid crystals with two different pitches. The component is polarization sensitive and exhibits passive anisotropic transmission analogous to the electronic diode. The non-reciprocal transmission of linearly polarized light at the different PBG regions is discussed. A new scheme of a color-separating system with the OD is proposed. The OD behavior constitutes the operational mechanism for a color splitter.

Electro-tunable optical diode based on photonic bandgap liquid-crystal heterojunctions

Manipulation of light is in strong demand in information technologies. Among the wide range of linear and nonlinear optical devices that have been used, growing attention has been paid to photonic crystals that possess a periodic modulation of dielectric function. Among many photonic bandgap (PBG) structures, liquid crystals with periodic structures are very attractive as self-assembled photonic crystals, leading to optical devices such as dye lasers. Here we report a new hetero-PBG structure consisting of an anisotropic nematic layer sandwiched between two cholesteric liquid-crystal layers with different helical pitches. We optically visualized the dispersion relation of this structure, displaying the optical diode performance: that is, the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions. Transmittance spectra with circularly polarized light also reveal the diode performance, which is well simulated in calculations that include an electro-tunable diode effect. Lasing action was also confirmed to show the diode effect with a particular directionality.

Design of air-guiding honeycomb photonic bandgap fiber

We introduce a design procedure for hollow-core photonic bandgap (PBG) fiber with a cladding made of air-silica honeycomb photonic crystal (PC). It is found that air-guiding can be realized in both fundamental and secondary PBG regions of the cladding PC. Dispersion and radiation loss of the fundamental mode for two types of fiber structure are theoretically calculated. The fibers show promising waveguiding ability.