Small Refractive Index Research Articles

A Review of Integrated Photonic Devices Using Sb2Se3

AbstractThe silicon photonic technology is a highly promising option for photonic integrated circuits and has attracted intensive interests, particularly since it can utilize complementary metal‐oxide‐semiconductor processing techniques and facilities, thereby realizing high‐density photonic integrations with low‐cost. Unfortunately, the thermo‐optic and the carrier dispersion effects, which are the typical means of tuning silicon photonics devices, bring the drawbacks of high power consumption and large device size due to the relatively weak effect with a small refractive index change and being volatile. For overcoming these drawbacks, phase‐change materials are introduced into silicon photonic devices, where VO2 and Ge2Sb2Te5 are the most commonly used ones. However, the key disadvantage of large loss resulting from them limits further improving the performances of integrated photonic devices. Therefore, Sb2Se3 has seen increasing interests recently in the design of silicon photonic integrated devices, benefiting from the advantages of having extremely low loss over the C‐band and being non‐volatile. In this paper, the trending recent studies about integrated optical devices are systematically reviewed using Sb2Se3, which are classified according to the device function.

Effect of biaxial strain on the electronic structure and optical properties of two-dimensional Bi2Te2S

In this work, the effects of biaxial strain on the electronic structure and optical properties of monolayer Bi2Te2S are studied by the first-principles methods. The calculated results show that the monolayer Bi2Te2S is an indirect band gap semiconductor with a band gap of 1.0 eV. The absence of imaginary frequency in the phonon spectrum indicates that the structure can exist stably. With the increase of tensile strain, the band gap value decreases approximately quasi-linearly. When 10 % tensile strain is applied, the band gap value is reduced to 0 eV, achieving the transition from an indirect bandgap semiconductor to a direct bandgap semiconductor. With the increase of compressive strain, the band gap value increases first and then decreases, and the band gap value reaches a maximum of 1.28 eV at −4 % strain. Combined with the density of states analysis, the reason for this change in the band structure is that the contribution of Bi 6p, Te 5p and S 3p state electrons to the conduction band and valence band changes under different strains. The effect of strain on the optical properties shows that when different strains are applied, the monolayer Bi2Te2S has a high absorption coefficient in the entire visible region. The single-layer Bi2Te2S material has a smaller refractive index under tensile strain. The static dielectric function value increases with the increase of tensile strain, and the peak value of the dielectric function decreases and moves to the low energy direction. This indicates that the tensile strain will enhance the migration of photogenerated electron-hole pairs, which is beneficial to improving the utilization of light. This work will provide a theoretical reference for the subsequent study of the electronic and optical properties of monolayer Bi2Te2S.

Influence of Hot Carrier Cooling and Band‐Filling Effect on Linewidth of Perovskite Microlasers

AbstractIn semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers.

Periodic stub implementation with plasmonic waveguide as a slow-wave coupled cavity for optical refractive index sensing

Optical biosensors based on plasmonic nanostructures have attracted great interest due to their ability to detect small refractive index changes with high sensitivity. In this work, a novel plasmonic coupled cavity waveguide is proposed for refractive index sensing applications. The structure consists of a metal–insulator–metal waveguide side coupled to an array of asymmetric H-shape element, designed to provide dual-band resonances. The sharp transmission dips and large field enhancements associated with dual-band resonances can enable sensitive detection of material under test. The resonator array creates a slow light effect to improve light-matter interactions. The structure was simulated using the finite integration technique as the full-wave technique, and the sensitivity and figure of merit were extracted for different ambient refractive indices. The maximum sensitivity of 1774 nm/RIU and high figure of merit of 2 × 104 RIU−1 for the basic model and 1.15 × 105 RIU−1 for the modified model were achieved, demonstrating the potential for high-performance sensing. The unique transmission characteristics also allow for combined spectral shaping and detection over a broad bandwidth. The simple, compact geometry makes the design suitable for on-chip integration. This work demonstrates a promising refractive index sensor based on coupled dual-band resonators in a plasmonic waveguide.

Conventional band demultiplexer with high quality factor and transmission power based on four optimized shaped photonic crystal resonators

Abstract In this work, we propose a new filter design based on a ring resonator. This structure has 99% transmission ratio and high sensitivity to detect small refractive index variations of the order of 0.002. More specifically, the small size of this resonator gives the advantage to model a demultiplexer of size 463 µm2 based on four ports which operates in the conventional transmission band. Also, we show that our demultiplexer structure can reach more than 90% transmission ratio with an optical quality factor of about 3800, the spectral width is 0.72 nm and a crosstalk between −13.28 and −32.13. In our study, we emphasized the plane wave method to study the photonic band gap and FDTD to determine the transmission spectrum.

Enhanced Temperature Sensing by Multi‐Mode Coupling in an On‐Chip Microcavity System

AbstractThe microcavity is a promising sensor platform, but any perturbation will disturb its linewidth, cause resonance shift or splitting. However, such sensing resolution is limited by the cavity's optical quality factor and mode volume. Here, an on‐chip integrated microcavity system is proposed and demonstrated that the resolution of a self‐referenced sensor can be enhanced with multi‐mode coupling. In experiments, inter‐mode coupling strength is carefully optimized with a pulley waveguide and observed a resolution improvement of nearly 3 times in the frequency domain. While experiencing small refractive index change tuned by temperature, mode‐coupled system shows a 7.2 times sensitivity enhancement that is than the uncoupled system on the same chip and a very significant lineshape contrast ratio change as great reference for minor frequency shifts. This approach will help design microcavity sensors to improve detection sensitivity and resolution under limited manufacture precision.

A Ferroelectric Nonlinear Optical Crystal for Deep‐UV Quasi‐Phase‐Matching

AbstractPhase matching is indispensable for high efficiency of second‐order nonlinear optical (NLO) crystals. Thousands of these crystals rely on birefringence phase matching, whereas only a few of them are quasi‐phase matching without the restriction of birefringence. Herein, using oriented seeds inch‐sized LiNH4SO4 (LAS) single crystals are successfully grown. LAS is NLO‐active with a very short deep‐UV absorption edge of 171 nm and high laser damage threshold up to 1.47 GW cm−2 at 1064 nm, 10 ns, but its birefringence is merely 0.0078@532 nm, which is too small to realize birefringence phase matching in deep‐UV region. Fortunately, P−E hysteresis loop, variable‐temperature NLO tests, and ferroelectric domain observations confirm that LAS is ferroelectric. Furthermore, LAS exhibits small refractive index dispersion resulting in a relatively long periodic length of 1.4 µm and its single domain structure can be easily obtained by electrical poling. These attributes make LAS a scarce NLO candidate for deep‐UV quasi‐phase matching. This work highlights the longly overlooked potential of sulfates and provides new opportunities for deep‐UV NLO materials.

Unraveling the effect of pressure on structural phase transition, electronic, and optical properties of Hf1-xSixO2 (x = 0, 0.03, 0.06, 0.09): A first-principles investigation

The non-centrosymmetric phase of HfO2 with excellent Si compatibility has great importance in integrated photonic circuits, non-volatile memory, and ferroelectric field effect transistors. We report a theoretical study of pressure-induced phase transition from a centrosymmetric m-HfO2 to non-centrosymmetric o-HfO2 with Si doping. The reported phase transition pressures, i.e. 15, 14, 8, and 8 GPa for x = 0, 0.03, 0.06, and 0.09, respectively, for Hf1-xSixO2 are in excellent agreement with available experimental results. With increasing Si concentrations, the transition pressures reduce significantly, which is explained in terms of bond length and charge transfer. The thermal stability of the obtained o-HfO2 phase is examined via ab-initio molecular dynamics up to its synthesis temperature. The negative formation energy also confirms its thermodynamical stability. Density of states indicates the noticeable appearance of Si states in the lower conduction band and an increase in the extent of hybridization between Hf-5d, O-2p, and Si-2p atomic states. The doping changes the nature of band gap from indirect to direct, concurrently reducing the magnitude. The o-HfO2 shows a low value of static dielectric constant, which results in a smaller refractive index (1.82) and lower reflectivity in the infrared and visible regions.

Dynamic measurement of an angular Goos–Hänchen shift at a surface plasmon resonance in liquid

We developed a surface plasmon resonance (SPR)-enhanced angular Goos–Hänchen (GH) shift measurement system capable of tracking small refractive index changes with high sensitivity in a liquid environment. Our method can be performed in angular interrogation schemes, where we demonstrate a simple zero-finding algorithm to locate the SPR angle instead of the complicated data processing algorithms used in conventional sensors. We also propose a displacement interrogation scheme for dynamic measurement of small refractive index changes in the sample. The main advantage of our method is the controllability of the measured displacement by standard geometrical optics, allowing measurement sensitivity enhancement without the need to modify the sensor material.

Resonant-Cavity-Enhanced Electrochromic Materials and Devices.

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Extracellular Vesicle Refractive Index Derivation Utilizing Orthogonal Characterization.

The analysis of small particles, including extracellular vesicles and viruses, is contingent on their ability to scatter sufficient light to be detected. These detection methods include flow cytometry, nanoparticle tracking analysis, and single particle reflective image sensing. To standardize measurements and enable orthogonal comparisons between platforms, a quantifiable limit of detection is required. The main parameters that dictate the amount of light scattered by particles include size, morphology, and refractive index. To date, there has been a lack of accessible techniques for measuring the refractive index of nanoparticles at a single-particle level. Here, we demonstrate two methods of deriving a small particle refractive index using orthogonal measurements with commercially available platforms. These methods can be applied at either a single-particle or population level, enabling the integration of diameter and scattering cross section values to derive the refractive index using Mie theory.

DeepGT: Deep learning-based quantification of nanosized bioparticles in bright-field micrographs of Gires-Tournois biosensor

Rapid and decentralized quantification of viral load profiles in infected patients is vital for assessing clinical severity and tailoring appropriate therapeutic strategies. Although microscopic imaging offers potential for label-free and amplification-free quantitative diagnostics, the small size (∼100 nm in diameter) and low refractive index (n ∼1.5) of bioparticles present challenges in achieving accurate estimations, consequently increasing the limit of detection (LoD). In this study, we present a novel synergistic biosensing approach, DeepGT, combining Gires-Tournois (GT) sensing platforms with deep learning algorithms to enhance nanoscale bioparticle counting accuracy. The GT sensing platform serves as a photonic resonator, increasing bioparticle visibility in bright-field microscopy and maximizing chromatic contrast. By employing a back-end with a dilated convolutional neural network architecture, DeepGT effectively refines artifacts and color deviations, significantly improving particle estimation accuracy (MAE ∼2.37 across 1596 images) compared to rule-based algorithms (MAE ∼ 13.47). Notably, the enhanced accuracy in detecting invisible particles (e.g., two- or three-particles) enables an LoD of 138 pg ml−1, facilitating a dynamic linear correlation at low viral concentration ranges within the clinical spectrum of infection, from asymptomatic to severe cases. Leveraging transfer learning, DeepGT, which relies on a chromatometry-based strategy instead of a spatial resolution approach, exhibits exceptional precision when analyzing particles of diverse dimensions smaller than the microscopy system’s minimum diffraction limit in visible light (< 258 nm). The DeepGT approach holds promise for early screening and triage of emerging viruses, reducing costs and time requirements in diagnostics.

Optical Sensor Methodology for Measuring Shift, Thickness, Refractive Index and Tilt Angle of Thin Films

We propose a simple optical method and device design for the non-contact determination of small shift, thickness, refractive index, and tilt angle of thin films. The proposed sensor consists of a laser light source, a third- or two-order spiral amplitude zone plate with a high numerical aperture, and a CCD camera connected to a computer. It is shown that the third-order zone plate transforms the incident Gaussian beam into a three-petal rotating beam. By measuring the rotation angle of the three-petal intensity distribution, one can measure the following: a minimum shift along the optical axis of about 7 nm (the wavelength is 532 nm), a change in the plate thickness by 3 nm, a change in the tilt angle of the plate by 0.1 degrees, and a change in the refractive index by 0.01.

Compact planar meta‐lenses for nondiffracting surface wave deflection

AbstractA novel planar lens structure is proposed by using metasurfaces to convert cylindrical‐like surface wave to inclined nondiffracting wave with a deflection angle. Based on the geometric optics, the refractive index distribution of the surface wave lens is derived. A square metal ring‐shaped metasurface cell is used for realizing small refractive index whilst two opposite E‐shaped metal patches are used to form the meta‐cell with large refractive index. By adjusting the size of meta‐cells, the desired refractive index distribution is obtained after optimization. Measured results show that the inclined nondiffractive surface wave can be achieved with the maximum propagation distance of 99.72 mm at the tilted angle of 26.8°, which are consistent with the full‐wave simulation and numerical calculation results. The proposed meta‐lens achieves compact size, large titled angle, and long nondiffraction propagation distance, therefore could be well utilized in surface wave communication and nondestructive detection of surface defects.

Phase wavefront perturbation calculation model for spectroscopic refractive index matching of hybrid materials.

A low-cost flexible spectroscopic refractive index matching (SRIM) material with bandpass filtering properties without incidence angle and polarization dependence by randomly dispersing inorganic C a F 2 particles in organic polydimethylsiloxane (PDMS) materials was proposed in our previous study. Since the micron size of the dispersed particles is much larger than the visible wavelength, the calculation based on the commonly used finite-difference time-domain (FDTD) method to simulate light propagation through the SRIM material is too bulky; however, on the other hand, the light tracing method based on Monte Carlo theory in our previous study cannot adequately explain the process. Therefore, a novel approximate calculation model, to the best of our knowledge, based on phase wavefront perturbation is proposed that can well explain the propagation of light through this SRIM sample material and can also be used to approximate the soft scattering of light through composite materials with small refractive index differences, such as translucent ceramics. The model simplifies the complex superposition of wavefront phase disturbances and the calculation of scattered light propagation in space. The scattered and nonscattered light ratios; the light intensity distribution after transmission through the spectroscopic material; and the influence of absorption attenuation of the PDMS organic material on the spectroscopic performance are also considered. The simulation results based on the model are in great agreement with the experimental results. This work is important to further improve the performance of SRIM materials.

10 dB emission suppression in a structured low index medium

Significant suppression of radiation in 3D structured media with small refractive indices of 1.4–1.6, such as glass or polymers, is a desirable feature yet to be obtained. We present here quasiperiodic 3D structures with a homogeneous distribution of Bragg peaks on a sphere in reciprocal space obtained by an overlap of multiple gratings. Simulations on 3D structures revealed a limitation of emission suppression contrasting from theoretical predictions. The effect of number of gratings and refractive index contrast on the obtained suppression is investigated. We demonstrate an almost refractive index independent −10dB suppression of emission for a refractive index as low as 1.30.

Sensitivity enhanced tunable plasmonic biosensor using two-dimensional twisted bilayer graphene superlattice

Abstract This study theoretically demonstrated an insight for designing a novel tunable plasmonic biosensor, which was created by simply stacking a twisted bilayer graphene (TBG) superlattice onto a plasmonic gold thin film. To achieve ultrasensitive biosensing, the plasmonic biosensor was modulated by Goos–Hänchen (GH) shift. Interestingly, our proposed biosensor exhibited tunable biosensing ability, largely depending on the twisted angle. When the relative twisted angle was optimized to be 55.3°, such a configuration: 44 nm Au film/1-TBG superlattice could produce an ultralow reflectivity of 2.2038 × 10−9 and ultra-large GH shift of 4.4785 × 104 µm. For a small refractive index (RI) increment of 0.0012 RIU (refractive index unit) in sensing interface, the optimal configuration could offer an ultra-high GH shift detection sensitivity of 3.9570 × 107 µm/RIU. More importantly, the optimal plasmonic configuration demonstrated a theoretical possibility of quantitatively monitoring severe acute respiratory syndrome coronavirus (SARS-CoV-2) and human hemoglobin. Considering an extremely small RI change as little as 3 × 10−7 RIU, a good linear response between detection concentration of SARS-CoV-2 and changes in differential GH shift was studied. For SARS-CoV-2, a linear detection interval was obtained from 0 to 2 nM. For human hemoglobin, a linear detection range was achieved from 0 to 0.002 g/L. Our work will be important to develop novel TBG-enhanced biosensors for quantitatively detecting microorganisms and biomolecules in biomedical application.

In-fiber interferometry sensors for refractive index

Compact interferometers based on waveguiding structures have found countless applications in refractive index measurements, chemical sensing, as well as temperature and pressure measurements. The most common fiber devices are based on Mach–Zehnder interferometry and Michelson interferometry—two design concepts that can readily be implemented using simple fiber optic components, such as mode splitters and combiners, fiber optic gratings, and fiber tapers, among others. Fiber interferometry can also be conducted based on the Sagnac effect and the Young (double-slit) interferometer. In this review, we examine and compare over 400 fiber optic interferometers as well as more than 60 fiber optic refractive sensors based on fiber optic cavities. Even though many of the devices show temperature-, strain-, and pressure-sensitivity, we focus our review on refractive index measurements, as these are the most common applications. Many devices were characterized by their inventors using their sensitivity to refractive index changes. While the sensitivity is an important characteristic of the device, it does not easily relate to the smallest resolvable refractive index change or the limit of detection when applied to chemical measurements. Instead, we propose here that one should use the figure of merit, which is defined through the refractive index sensitivity and the width of an interferometer fringe. Using simple assumptions, we were able to mathematically relate the sensitivity and the figure of merit to common design parameters, such as the length of the interferometer arms, the operating wavelength, refractive indices of the fiber and the sample, as well as an overlap parameter, which describes the fraction of the guided wave in the sensing arm that interacts with the sample. We determined this overlap parameter for each reviewed device from the reported interferograms. Our meta-analysis provides for the first time simple and easily applicable guidance to increase the figure of merit of fiber optic interferometers and fiber optic cavities with regard to their ability to detect small refractive index changes. A high figure of merit allows measuring very small refractive index changes such as those of gases at different pressures or of very dilute solutions.

Er-Yb fibers based on aluminophosphorosilicate glass

The paper presents results of studying properties of Er-Yb fibers based on aluminophosphorosilicate glass matrix. The dependence of the pump-to-signal conversion efficiency on the glass composition is revealed. Optimal core compositions, which makes it possible to provide a short amplifier length (no more than 1 meter) and a small core-clad refractive index difference (below 0.0035), has been determined. It opens up the possibility of creating single-mode Er-Yb fibers with a large mode area and a high threshold of nonlinear effects.

A Review of Silicon‐Based Integrated Optical Switches

AbstractRecently, silicon‐integrated optical circuits have attracted intensive interests, thanks to the compatibility with the complementary metal‐oxide‐semiconductor (CMOS) technology that enables mass production at low cost. The optical switch is an essential part of optical integrated circuits, with broad applications in optical communications and networks, optical computing, and sensing such as LiDAR. In general, the silicon‐integrated optical switch adopts thermo‐optic or carrier dispersion effect to realize reconfigurable signal routing. However, the use of thermo‐optic effect leads to high power consumption, and the carrier dispersion effect has the disadvantage of small refractive index change. In addition, both effects are non‐latching, and hence, continuous power consumption is required even when switching is not needed. For overcoming these drawbacks, phase‐change materials (PCMs) have been introduced into silicon‐integrated optical switches. In this paper, silicon‐integrated optical switches are classified according to the underlying structure and recent research is reviewed. Recent studies on silicon‐integrated optical switches incorporating PCMs are also reviewed. Furthermore, the pros and cons of different types of integrated optical switches with and without PCMs are compared and discussed.