Photonic Crystal Unit Cell Research Articles

Symmetry breaking of bound states in the continuum in luminescence response of photonic crystal slabs with embedded Ge nanoislands

Bound states in the continuum (BIC) have attracted a great deal of attention in all-dielectric nanophotonics due to their ability to provide spectral features with a very high-quality factor. By definition, BIC cannot be observed in the far field because of the symmetry mismatch with the modes propagating in free space. Despite this, in systems with slightly reduced symmetry, the condition for BIC is lifted, which gives rise to the high-quality resonant features in their optical response. In particular, in photonic crystal slabs, which support the BIC states, the symmetry reduction allows modification of light propagation, reflection, or emission. In this work, using the photonic crystal slabs with embedded Ge nanoislands, we have shown the ability to control their light emission features by symmetry breaking. It was demonstrated that such symmetry breaking due to a change in the basis vectors of the photonic crystal unit cell or a change in the unit cell internal structure could provide independent control knobs to alter the spectral position of photonic crystal modes, their dispersion, and degeneracy. The obtained results reveal additional ways to manage the light emission of active media in photonic crystal slabs.

Enhanced self-collimation effect by low rotational symmetry in hexagonal lattice photonic crystals

In this study, we present the design of a photonic crystal (PC) structure with a hexagonal lattice, where adjustments to the PC unit cell symmetry reveal an all-angle self-collimation (SC) effect. By optimizing opto-geometric parameters, such as the rotational angle of auxiliary rods and adjacent distances, we analyze the SC property in detail, leveraging group velocity dispersion (GVD) and third-order dispersion (TOD) characteristics. We also investigate the relationship between symmetry properties and their influence on dispersion characteristics. Through symmetry manipulation, we gain a comprehensive understanding of the underlying mechanisms governing light collimation and confinement in the proposed configurations. The PC structure with a C1 symmetry group exhibits all-angle SC effect within the range of a/λ = 0.652 and a/λ = 0.668 normalized frequencies, with a bandwidth of Δω/ω c =2.4% Further breaking the symmetry, transforming from C1 to C2 group symmetry enhances the SC bandwidth to Δω/ω c =6.5% and reveals the perfect linear equi-frequency contours (EFC) at two different frequency bands: all angle SC between a/λ = 0.616 and a/λ = 0.344 normalized frequencies in the 4th transverse magnetic (TM) band and between a/λ = 0.712 and a/λ = 0.760 in the 5th TM band. Here, GVD and TOD values of the TM 4th band vary between 7.3 (a/2πc2)–254.3 (a/2πc2) and 449.2 (a2/4π 2c3)–1.3×105 (a2/4π 2c3), respectively. Also, GVD and TOD values of the TM 5th band vary between 182.5 (a/2πc2)–71.3 (a/2πc2) and −24380(a2/4π 2c3)–−9619 (a2/4π 2c3) values, respectively. Additionally, we propose a composite/hybrid PC structure resembling C2 group symmetry, where two auxiliary rods are replaced by rectangular photonic wires with the same refractive index and width equal to the diameter of auxiliary rods. This hybrid structure exhibits an all-angle SC effect with an operating bandwidth of Δω/ω c =11.7%, which displays near-zero GVD and TOD performance and offers enhanced robustness against potential fabrication precision issues.

Deep Learning-based Inverse Design of the Complete Photonic Band Gap in Two-Dimensional Photonic Crystals

Background: With the continuous development of computer science, data-driven computing methods have shown their advantages in various fields. In the field of photonics, deep learning (DL) can be used to inversely design the structure of optical devices. Objective: The two-dimensional (2D) photonic crystal (PCs) with adjustable structural parameters and a large complete photonic band gap (CPBG) are inversely designed in terms of DL neural network (NN) tagged to obtain a specified width of CPBG. Methods: The new PCs structure is designed by combining multiple factors that produce a CPBG. Tandem networks are used to speed up the training of the NN and tackle the problem of nonuniqueness that arises in inverse design. Results: After various attempts and improvements, the ideal PCs structure was obtained. It is found that the connecting channel between the primitives in the PCs unit cell has a dominate effect on the CPBG. The use of a tandem network enables better convergence of the network. Finally, suitable NN can be obtained, which can realize the forward prediction of the CPBG and the inverse design of the structure. Conclusion: DL can realize forward prediction and inverse design of 2D PCs targeting the width of the CPBG, which broadens the application scope of DL in the field of PCs.

Slow-light effect in symmetry-reduced non-defect photonic crystals

In this study, a two-dimensional low-symmetric photonic crystal (PC) configuration with elliptical geometry is presented and its slow-light (SL) effect is investigated. Reducing the symmetry in the PC unit cell provides slow modes at the higher transverse electric bands. The calculated group index and the corresponding normalized bandwidth equal to {ng, BW}={63.56, 0.0065}. That corresponds to a value of figure of merit (FOM)=0.4344 defined by the product of the average group index and the normalized bandwidth, FOM=〈ng〉BW, which is comparable to the values available in literature. Tracing the whole edges of the Brillouin zone, strongly excited SL modes are observed only along Γ-X but not along Γ-X’ or Γ-M. That condition allows for the design of low-symmetric PC waveguides with finite thicknesses at the expense of lowering group index value. The SL effect is still obtained for the proposed low-symmetric PCs having finite thicknesses, which is numerically proved via finite-difference time-domain methods. It is important to note that non-dispersive SL Bloch modes exist through the non-zero k-vector components of Brillouin zone. Hence, such a defect-free (without either point- or line- defect) SL PC design may have a great potential for the use of compact photonic devices such as in optical switching and biochemical sensing applications.

Photonic crystal based interferometric design for label-free all-optical sensing applications.

Optical sensing devices has a great potential in both industrial and biomedical applications for the detection of biochemicals, toxic substances or hazardous gases thanks to their sustainability and high-selectivity characteristics. Among different kinds of optical sensors based on such as fibers, surface plasmons and resonators; photonic crystal (PC) based optical sensors enable the realization of more compact and highly efficient on-chip sensing platforms due to their intriguing dispersive relations. Interferometric devices based on PCs render possible the creation of biochemical sensors with high sensitivity since a slight change of sensor path length caused by the captured biochemicals could be detected at the output of the interferometer via the interferences of separated beams. In this study, a new type of Mach-Zehnder Interferometer (MZI) using low-symmetric Si PCs is proposed, which is compatible with available CMOS technology. Intended optical path difference between the two MZI channels is provided by the periodic alignments of symmetry-reduced PC unit cells in the MZI arms. Unlike the conventional symmetrical PC based MZIs, Fano resonances exist for the proposed MZI design, i.e. transmission dips and peaks appear in the output spectrum, and the location of dip and peak frequencies in transmission spectra can be efficiently controlled by utilizing interference phenomenon. Exploiting this effect, any refractive index change at the surrounding medium could be distinctly observed at the transmission spectra. In the view of such results, it is convenient to say that the proposed MZI configuration is suitable for efficient optical sensing of toxic gases as well as liquids. The designed all-dielectric MZI system is numerically investigated in both spectral and spatial domains to analyze its interferometric tunability: an optical sensitivity of about 300 nm/RIU is calculated for gaseous analytes whereas that sensitivity value is around 263.2 nm/RIU in the case of liquid analytes. Furthermore, high quality factor of Q > 45000 is obtained at Fano resonances with Figure-of-Merit (FoM) value of FoM ∼ 8950 RIU-1(7690 RIU-1) in the case of gas analytes (liquid analytes), which is the indication of enhanced optical sensing performance of the proposed MZI design. Considering all the above-mentioned advantages, the proposed interferometric configurations based on low-symmetric PCs could be utilized for efficient photonic sensor applications that require controllable output power or sensing of gaseous and liquid substances.

Photonic metacrystal: design methodology and experimental characterization

We report a design methodology for creating high-performance photonic crystals with arbitrary geometric shapes. This design approach enables the inclusion of subwavelength shapes into the photonic crystal unit cell, synergistically combining metamaterials concepts with on-chip guided-wave photonics. Accordingly, we use the term "photonic metacrystal" to describe this class of photonic structures. Photonic metacrystals exploiting three different design freedoms are demonstrated experimentally. With these additional degrees of freedom in the design space, photonic metacrystals enable added control of light-matter interactions and hold the promise of significantly increasing temporal confinement in all-dielectric metamaterials.

Dual-band all-dielectric chiral photonic crystal

We present an all-dielectric chiral photonic crystal that guides the propagation of electromagnetic waves without backscattering for dual bands. The chiral photonic crystal unit cell is composed of four dielectric cylinders with increasing inner diameter clockwise or anticlockwise, which leads to chirality. It is demonstrated that the proposed chiral photonic crystal can generate dual band gaps in the gigahertz frequency range and has two types of edge states, which is similar to topologically protected edge states. Hence, the interface formed by the proposed 2D chiral photonic crystal can guide the propagation of electromagnetic waves without backscattering, and this complete propagation is immune to defects (position disorder or frequency disorder). To illustrate the applicability of the findings in communication systems, we report a duplexer and a power divider based on the presented all-dielectric chiral photonic crystal.

Remote Temperature Sensor Based on Tamm Resonance

A highly-sensitive remote temperature sensor based on Tamm resonance is proposed using a one-dimensional photonic crystal. The proposed structure is prism/Ag/Toluene/SiO2 /(PSi1/PSi2)N/Si. The transfer matrix method is used to discuss the interaction between the structure and the S-polarization of the incident radiation waves. We optimized the structure by studying the effect of the incident angle, the thickness of the first and second layers of the photonic crystal unit cell, the porosity of them, and the thickness of the toluene layer. High sensitivity, high signal-to-noise ratio, and very low resolution are achieved due to the coupling between the porous silicon photonic crystal properties and Tamm resonance that makes it very distinguished compared to previous works.

How Light Passes Through Ordered Macroporous Photonic Crystals Immersed in Solvents

Understanding light transmission through 3D macroporous photonic crystals is essential for linking their optical characteristics to any application, particularly when immersed in solvents. We examined TiO2 and SnO2 IOs in a series of common solvents to fix existing issues in Bragg-Snell theory and the definition of the photonic crystal unit cell and pore infilling. Using a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure, we demonstrate experimentally and theoretically that low fill factors are caused by porous material infilling all interstitial vacancies of the colloidal opal template, instead of complete filling of most interstitial voids. By also including an optical interference condition for inverse opals with a larger effective refractive index than their substrate, the optical data for macroporous photonic crystals are now consistent with microscopy measurements. This analysis provides an accurate correlation between the true response of a macroporous photonic crystal to tits primary properties including the index contrast with a solvent, infilled structure and the unit cell definition, and the directionality of the photonic band gap. As control in functional porous and ordered photonic materials has broadening to include biosensing and energy storage for example, a comprehensive and consistent description of how light passes through ordered macroporous photonic crystal structures is an important requirement.

How Light Passes Through Ordered Macroporous Photonic Crystals Immersed in Solvents

A better understanding of light transmission and relation to photonic bandgap in inverse opal photonic crystals is important when these structures are examined in liquids or solvents. We present a study of light tranmission through TiO2 and SnO2 inverse opals in several common solvents. The data and analysis identifies a correction to the Bragg-Snell theory and the photonic crystal unit cell definition that consolidates some inconsistencies between Bragg-Snell theory in various index solvents and physical measurements of inverse opals. We show experimental studies that suggest low fill factors in inverse opals are defined by porous material infilling all interstitial vacancies in the opal template to form an IO.

Predictive and generative machine learning models for photonic crystals

AbstractThe prediction and design of photonic features have traditionally been guided by theory-driven computational methods, spanning a wide range of direct solvers and optimization techniques. Motivated by enormous advances in the field of machine learning, there has recently been a growing interest in developing complementary data-driven methods for photonics. Here, we demonstrate several predictive and generative data-driven approaches for the characterization and inverse design of photonic crystals. Concretely, we built a data set of 20,000 two-dimensional photonic crystal unit cells and their associated band structures, enabling the training of supervised learning models. Using these data set, we demonstrate a high-accuracy convolutional neural network for band structure prediction, with orders-of-magnitude speedup compared to conventional theory-driven solvers. Separately, we demonstrate an approach to high-throughput inverse design of photonic crystals via generative adversarial networks, with the design goal of substantial transverse-magnetic band gaps. Our work highlights photonic crystals as a natural application domain and test bed for the development of data-driven tools in photonics and the natural sciences.

Filling in the gaps: The nature of light transmission through solvent-filled inverse opal photonic crystals

Understanding the nature of light transmission and the photonic bandgap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. We examined TiO2 and SnO2 IOs in a range of common solvents to solve the conflict between Bragg-Snell theory, optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic bandgap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theroetically that low fill factors are caused by less desne material infilling all interstitial vancancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than its substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with physical measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic bandgap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystals structures and their primary optical signatures is a fundamental requirement for application.

Air-Mode Photonic Crystal Micro-Ring Resonator With Enhanced Quality Factor for Refractive Index Sensing

Intense light-matter interaction is critical for enabling high-sensitivity refractive index sensors, high-efficiency light sources, and high-performance nonlinear devices. A bowtie-hole photonic crystal micro-ring resonator is proposed and numerically demonstrated to enhance the light-matter interaction effectively through achieving strong spatial and temporal light confinement, simultaneously. Through engineering the photonic crystal unit cell, the bowtie-hole photonic crystal is proved to possess the ability to localize light with a highly tight spatial confinement. By optimizing the bowtie-hole radius and the bowtie angle, the Q factor of the air mode in the resonator is improved to 1.42 × 105, which denotes an excellent temporal confinement. With the advantages addressed, the refractive index sensing based on the high-Q air mode in the bowtie-hole photonic crystal micro-ring resonator is investigated. For the air mode in the bowtie-hole photonic crystal micro-ring resonator, the Q factor is comparable with that in the reported research, meanwhile the sensing sensitivity has a 2-fold enhancement.

Band gap engineering and applications in compound periodic structure containing hyperbolic metamaterials

Behaviours of light in materials strongly depend on the topological structure of the iso-frequency surface (IFS). The usual materials, of which the unit cell of photonic crystal is made up, are dielectrics, whose IFSs have the same closed topological structure. As a simplest photonic crystal, one-dimensional photonic crystal (1DPC) has attracted intensive attention due to its simple fabrication technique as well as numerous applications. However, in a conventional all-dielectric 1DPC, photonic band gaps (PBGs) for both transverse magnetic (TM) and transverse electric (TE) polarizations will shift toward short wavelengths (i.e. blueshift) as incident angle increases. The underlying physical reason is that the propagating phase in isotropic dielectric will decrease as incident angle increases. The blueshift property of band gap for TM and TE polarization will limit the band width of omnidirectional band gap and the range of operating incident angles in some PBG-based applications, including near-perfect absorption, polarization selection and sensitive refractive index sensing. However, for TM polarization, the propagating phase in a hyperbolic metamaterial (HMM) will increase with incident angle increasing. This special phase property of HMM provides us with a way to flexibly tune the angle-dependent property of band gap in periodic compound structure composed of alternative HMM with open IFS and dielectric with close IFS. In this review, we realize zeroshift (i.e. angle-independent) band gaps as well as redshift band gaps in 1DPCs containing HMMs, which can be utilized to realize near-perfect absorption, sensitive refractive index sensing and polarization selection working in a wide range of incident angles.

Two-Dimensional Hybrid Photonic Crystal With Graded Low-Index Using a Nonuniform Voltage

Abstract We proposed a new method for designing graded index lens using liquid crystal infiltration into annular photonic crystals. Applying an external nonuniform voltage in the transverse direction perpendicular to the direction of light propagation yields different orientation of liquid crystal molecules inside the photonic crystal unit cells. As a result, a gradient refractive index was modulated. We numerically investigate focusing properties of the designed graded index structure using plane-wave expansion and finite-difference time-domain methods. The gradient refractive index profile was adjusted by varying the nonuniform voltage excitations, which consequently altered the focal distance of the graded index structure. A wide tuning range of 1856 nm was achieved for focal distance by the proposed graded index structure. This feature can be implemented for planning a flat lens with tunable focal distance based on electro-optic effect. These achievements may have future applications in some optical devices such as near-field imaging and scanning.

Dynamically Tuning the Density of Mode in a Photonic Crystal With Double Defects

The density of mode in acoustically modulated one-dimensional photonic crystal with two defects embedded has been calculated using the Wigner time phase approach. The acoustic wave redistributes the periodicity and the refractive index of each layer in the unit cell of photonic crystal. It is observed that there are two peaks in the density of mode plots whose intensity as well as frequency domain are dynamically tuned as these peaks are affected by the fractional change in the time period of acoustic waves. In case of synchronous modulation of density of mode, coupled and uncoupled defect modes are observed, which might be useful in realizing future optoelectronic devices.

Experimental realization of deep-subwavelength confinement in dielectric optical resonators.

The ability to highly localize light with strong electric field enhancement is critical for enabling higher-efficiency solar cells, light sources, and modulators. While deep-subwavelength modes can be realized with plasmonic resonators, large losses in these metal structures preclude most practical applications. We developed an alternative approach to achieving subwavelength localization of the electric and displacement fields that is not accompanied by inhibitive losses. We experimentally demonstrate a dielectric bowtie photonic crystal structure that supports mode volumes commensurate with plasmonic elements and quality factors that reveal ultralow losses. Our approach opens the door to the extremely strong light-matter interaction regime with, simultaneously incorporating both an ultralow mode volume and an ultrahigh quality factor, that had remained elusive in optical resonators.

Enhanced light trapping in dye-sensitized solar cell by coupling to 1D photonic crystal and accounting for finite coherence length

Theoretical analysis of dye-sensitized solar cell integrated with photonic crystals based on coupled coherent and geometrical optics formalism has presented. The effect of structural parameters, such as thickness of layer, number of unit cells and light incident angle on the optical properties and photo-current magnification for the proposed photonic crystal based dye-sensitized solar cell have studied. The angular response of the cell in terms of light harvesting efficiency and cumulative photo-generation rate has also analysed. A strategy has presented to enhance the performance of the cell under oblique incidence. The effect of number of photonic crystal unit cells has also analysed in view of percentage enhancement in cumulative photo-generation rate. This work provides new insight into the design and tailoring of the photonic crystals to enhance the light harvesting efficiency in the solar cells.

Design of Photonic Crystal Cavities for Extreme Light Concentration

The fields of photonic crystals and plasmonics have taken two different approaches to increasing light–matter interaction. Photonic crystal cavities increase temporal confinement of light in a material, as represented by their high quality factor, while plasmonic structures increase spatial confinement, as represented by their low mode volume. However, the inability to simultaneously attain extreme temporal and spatial confinement of light remains a barrier to realizing ultimate control of light in a material and maximum performance in photonic devices. Here, by engineering the photonic crystal unit cell to incorporate deep subwavelength dielectric inclusions, we show that it is possible in a single structure to achieve a mode volume commensurate with plasmonic elements while maintaining a quality factor that is characteristic of traditional photonic crystal cavities. Manipulating the geometric design of the unit cell leads to precise control of the band structure and mode distribution in the photonic cry...

Sandwich-typed resonator cavity based on a regular photonic crystal nanobeam

The authors propose and numerically examine a two-component design of an optical nanocavity. The design utilizes the effective medium theory. Such a nanocavity consists, first, of a photonic crystal nanobeam, in which the photonic crystal unit cell is not changed. Second, the cavity contains a fragment of some supplementary material of the size of several or several tens of photonic crystal unit cells. This paper describes the cavity model in which the both components have an equal thickness and can be fabricated from the same silicon wafer. While combining the two components, the defect has formed, in which the resonant mode can be excited. The advantages of the proposed cavity model are reported, particularly the easiness with which the cavities array can be fabricated and the possibility of implementing electrically pumped light sources and amplifiers. The fabrication tolerances of the proposed nanocavity were investigated. It has been found that existing structural layers alignment technologies can be used for fabricating the suggested cavity.