Showing papers in "Journal of Optics in 2022"

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

Abstract: Abstract A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO 2 ) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO 2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO 2 , the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO 2 , the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO 2 , we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO 2 , the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

Strong coupling in two-dimensional materials-based nanostructures: a review

Abstract: Strong interaction between electromagnetic radiation and matter leads to the formation of hybrid light-matter states, making a system’s absorption and emission properties distinctively different from that at the uncoupled states. For instance, strong coupling between cavity photons and quantum emitters results in the emergence of Rabi splitting andnew polaritonic eigenmodes, exhibiting characteristic spectral anticrossing and ultrafast energy exchange. There has recnetly been a rapidly increasing number of studies focusing on strong coupling between photonic nanostructures and two-dimensional materials (2DMs), demonstrating exceptional nanoscale optical properties and applications. Here, we review the recent advances and important developments of strong light-matter interactions in hybrid photonic systems based on 2DMs, including graphene, black phosphorus, and transition-metal dichalcogenides. We adopt the coupled oscillator model to describe the strong coupling phenomena and give an overview of three classes of 2DMs-based nanostructures realizing this regime. Following this, we discuss potential applications that can benefit from strong coupling induced effects and conclude our review with a perspective on the future of this rapidly emerging field.

Geometric tilt-to-length coupling in precision interferometry: mechanisms and analytical descriptions

Abstract: Tilt-to-length (TTL) coupling is a technical term for the cross-coupling of angular or lateral jitter into an interferometric phase signal. It is an important noise source in precision interferometers and originates either from changes in the optical path lengths or from wavefront and clipping effects. Within this paper, we focus on geometric TTL coupling and categorise it into a number of different mechanisms for which we give analytic expressions. We then show that this geometric description is not always sufficient to predict the TTL coupling noise within an interferometer. We, therefore, discuss how understanding the geometric effects allows TTL noise reduction already by smart design choices. Additionally, they can be used to counteract the total measured TTL noise in a system. The presented content applies to a large variety of precision interferometers, including space gravitational wave detectors like LISA.

Mid-wave and long-wave infrared transmitters and detectors for optical satellite communications—a review

Abstract: There has been a recent surge in interest for optical satellite communication (SatCom) utilizing lasers. It is clear to see why, as optical SatCom is capable of higher speed, lighter weight, higher directionality, and higher efficiency versus their radio-based counterparts. Research into optical SatCom has focused on devices operating in the short-wave infrared (SWIR), which is due to the maturity and commercial availability of such component’s thanks to significant development in terrestrial telecommunications networks. However, SWIR performs poorly in fog and heavy weather, prompting investigations into longer mid-wave and long-wave infrared bands for optical communication instead due to reduced atmospheric losses. This paper provides a comprehensive review of laser transmitters, detectors, and the science behind selecting longer wavelengths for optical SatCom to boost optical SatCom between ground stations and low earth orbit satellite constellations being deployed.

Recent advances on strong light-matter coupling in atomically thin TMDC semiconductor materials

Abstract: The strong light–matter interaction between the exciton of atomically thin transition metal dichalcogenides (TMDCs) and photonic nanocavities leads to the formation of unique hybrid light-matter quasiparticles known as exciton-polaritons. The newly formed mixed state has the advantages of the photonic part such as rapid propagation and low effective mass and the highly desirable optical properties of TMDC’s exciton, including the interparticle strong interactions nonlinearity and spin-valley polarization. These joint properties make such systems an ideal platform for studying many compelling physics phenomena and open the possibility of designing novel optoelectronic devices. This work reviews recent progress of strong coupling between exciton in TMDC and different resonant photonic structures, such as optical microcavities, plasmonic and all-dielectric nanocavities. Furthermore, we discussed the unique valleytronic and nonlinear properties of TMDC monolayers in the strong coupling regime. Finally, we highlighted some of the challenges and potential future research opportunities in this field.

Roadmap on multimode photonics

Abstract: Multimode devices and components have attracted considerable attention in the last years, and different research topics and themes have emerged very recently. The multimodality can be seen as an additional degree of freedom in designing devices, thus allowing for the development of more complex and sophisticated components. The propagation of different modes can be used to increase the fiber optic capacity, but also to introduce novel intermodal interactions, as well as allowing for complex manipulation of optical modes for a variety of applications. In this roadmap we would like to give to the readers a comprehensive overview of the most recent developments in the field, presenting contributions coming from different research topics, including optical fiber technologies, integrated optics, basic physics and telecommunications.

Light localization in a linearly graded-index substrate covered by intensity dependent nonlinear self-focusing cladding

Abstract: The new features of the light localization along the graded-index substrate covered by the cladding characterized by a self-focusing nonlinearity are described theoretically. In the model of nonlinearity the dielectric function in substrate depends linearly on the distance from the cover-substrate interface. The optical characteristics of Kerr type positive nonlinear response in the cover is changed abruptly with an increasing light intensity. The light intensity distribution in across interface direction described by exact solution to nonlinear wave equation is calculated and analyzed. It is found that increasing the width of the graded-index layer leads to a shift in the position of the maximum intensity. The intensity maximum appears in the graded-index substrate after the effective refractive index reaches the critical value. The thickness of nonlinear near-surface layer, which is formed after the light intensity reaches the critical value, monotonically decreases with increasing width of the graded-index layer. The possibility of realizing a multimode regime of the light propagation, which can be realized with an increasing width of the graded-index layer and it is characterized by a deep penetration of the light field into the substrate, is derived. The possibility of control the distribution of the light power between the cover and substrate by change in the effective refractive index and the width of the graded-index layer is described.

Self-healing of structured light: a review

Abstract: Self-healing of light refers to the ability of a light field to recover its structure after being damaged by a partial obstruction placed in its propagation path. Here, we will give a comprehensive review of the history and development of self-healing effects, especially highlighting its importance in vector vortex beams carrying spin and orbital angular momenta. Moreover, an unified zoology of self-healing, structured light is proposed to unveil a deeper understanding of its physical mechanism and provide a bird’s eye view on diverse forms of self-healing effects of different kinds of complex structured light. Finally, we outline the open challenges we are facing, potential opportunities and future trends for both fundamental physics and applications.

Microdroplet-tin plasma sources of EUV radiation driven by solid-state-lasers (Topical Review)

Abstract: Plasma produced from molten-tin microdroplets generates extreme ultraviolet light for state-of-the-art nanolithography. Currently, CO2 lasers are used to drive the plasma. In the future, solid-state mid-infrared lasers may instead be used to efficiently pump the plasma. Such laser systems have promise to be more compact, better scalable, and have higher wall-plug efficiency. In this Topical Review, we present recent findings made at the Advanced Research Center for Nanolithography (ARCNL) on using 1 and 2 µm wavelength solid-state lasers for tin target preparation and for driving hot and dense plasma. The ARCNL research ranges from advanced laser development, studies of fluid dynamic response of droplets to impact, radiation-hydrodynamics calculations of, e.g. ion ‘debris’, (EUV) spectroscopic studies of tin laser-produced-plasma as well as high-conversion efficiency operation of 2 µm wavelength driven plasma.

A polarization-insensitive triple-band perfect metamaterial absorber incorporating ZnSe for terahertz sensing

Abstract: In this work, a triple-band polarization-insensitive metamaterial structure with perfect absorption is proposed by incorporating a zinc selenide (ZnSe) spacer. The structure was optimally designed by varying the type of the spacer and the unit cell dimensions. The structure was simulated using the finite integration technique, and the results showed that the proposed design achieved a near-perfect absorption of about 99%, 99%, and 100% at 22.50, 28.98, and 35.14 THz, respectively. Its absorption characteristics were insensitive to the polarization angle and a wide range of incidence angles, making it an ideal absorber. Further investigations of the electric field, magnetic field, and surface current distributions were carried out to elaborate on the absorption characteristics at various resonance frequencies. The proposed device can also be used as a sensor that can detect the depth of the surrounding analyte and its refractive index. The device could detect the depth of the analyte with a peak sensitivity of 2.76 THz μm−1 and its refractive index with a peak sensitivity of 1.55 THz RIU−1. Thus, the design could have interesting terahertz applications.

Plasmonic vortices: a review

Abstract: Surface plasmon polaritons (SPPs), surface electromagnetic waves propagating along metal-dielectric interfaces, have found numerous applications in integrated photonic devices, optical storage, and optical sensing, etc. In recent years, there has been a surge of interest in the fundamental and applications of SPPs carrying orbital angular momentum, namely SPP vortices or plasmonic vortices. In this review, we summarize the fundamental concepts of plasmonic vortices, and highlight recent advances in the generation and applications of plasmonic vortices, from SPPs at lightwave frequencies to spoof SPPs at microwave and Terahertz frequencies.

High tunability and sensitivity of 1D topological photonic crystal heterostructure

Abstract: Abstract A modality to high tunability and sensing performance of a one-dimensional (1D) topological photonic crystal (PC) heterostructure is realized, based on a new mechanism through a 1D topological PC. By inserting an aqueous defect layer as a sandwich between two 1D PCs, transmittance gradually decreases with the increasing thickness of the defect layer. However, when the two layers of the topological heterostructure interface are replaced by the defect layer, the tunability and all sensing capabilities are improved, and the principle of topology is preserved. A topologically protected edge state is formed at the heterostructure interface with a highly localized electric field. For glucose sensing, high sensitivity S = 603.753 nm RIU −1 is obtained at the low detection limit of about DL = 1.22 × 10 − 4 RIU with high-quality factor Q = 2.33 × 10 4 and a high figure of merit FOM = 8147.814 RI U − 1 . Besides, the transmittance can be maintained at over 99% at low and/or high glucose concentrations, due to the coupling topological edge mode between defect mode and topological edge state. An excellent platform is examined to design a topological photonic sensor. This flexible platform can be used for any type of sensor solely by replacing the interface layers with different sensor materials. Thus, our results will promote the development of 1D topological photonic devices.

Surface waves propagating along the interface between a parabolic graded-index medium and a self-focusing nonlinear medium: exact analytical solution

Abstract: The waveguide properties of interface between a parabolic graded-index medium and a self-focusing nonlinear medium are described theoretically. The exact analytical solution to the wave equation with dielectric permittivity dependent on the distance from interface and on the electric field intensity is found. The obtained solution describes the new type of nonlinear surface wave. It is shown that the electric field is localized completely inside the parabolic graded-index layer. The dispersion equation determining explicit dependence of the effective refractive index on the thickness of the parabolic graded-index layer and the change in dielectric constant in it is found in a particular case corresponding to the exotic surface wave propagation. The influence of the optical parameters on the distribution profile of the electric field across is analyzed. The decrease in the field in surface waves with distance from the interface is non-exponential.

The surface waves propagating along the contact between the layer with the constant gradient of refractive index and photorefractive crystal

Abstract: Abstract We consider the photorefractive crystal with diffusion nonlinearity mechanism contacting with the linearly graded-index medium. The new types of the surface waves propagating along the contact are found and their properties are described. The transition between two types of the surface waves can be controlled by the waveguide temperature. The light field penetrates into the photorefractive crystal much deeper than into the graded-index medium. The maxima intensity monotonically increase with an increase in the propagation constant, and monotonically decrease with an increase in the temperature. The localization length of the surface waves decreases with an increase in the characteristic distance the graded-index medium and with a decrease in the propagation constant and the temperature. The discrete spectrum of the propagation constant for the special waveguide mode corresponding to the case of exponential damping of intensity maxima of the surface waves with an increasing temperature is obtained. It is shown that the intensity maxima in the photorefractive crystal depend parabolic on the temperature at low temperature waveguide mode.

Tunable plasmon-induced transparency in graphene-based plasmonic waveguide for terahertz band-stop filters

Abstract: Terahertz (THz) tunable filters have great advantages in miniaturizing integrated and multiband communication systems. Here, THz band-stop filter with switchable single/double plasmon-induced transparency (PIT) based on metal-dielectric-metal waveguide consisting of a ring-stub cavity and two graphene ribbons (GRs) on stub sidewalls is proposed. The switchable characteristics of single and dual-PIT are investigated numerically and theoretically by the finite-difference time-domain and the radiating multi-oscillator theory, displaying good correspondence. The single-PIT is excited by the destructive interference between bright mode and dark mode, which possesses significant tunability of resonant frequency and transmission amplitude due to the existence of GRs. When independently regulating chemical potentials of GRs on the left and right sidewalls, the dual-PIT emerges. And the filter based on dual-PIT switches from single stopband to dual-stopband or even multi-stopband filtering. Besides, the band-stop filtering performance of the tunable PIT can be further optimized by increasing the number of ring-stub cavities. The tunable PIT in graphene-based plasmonic waveguide holds potential for THz multiband communications and subwavelength plasmonic devices, such as filters, switches, modulators.

Electrically tunable toroidal Fano resonances of symmetry-breaking dielectric metasurfaces using graphene in the infrared region

Abstract: The magnitudes of coupling strength play an important role in various resonant phenomena such as Fano resonances (FRs). However, the coupling strength within the FRs using dielectric metasurfaces cannot be easily manipulated once they have been made. In this paper, toroidal FR is excited using the silicon metasurface with symmetry-breaking nanocylinders. Inserting a graphene layer with an ion-gel top gate onto the silicon metasurface, actively tunable response of a toroidal FR resulting from the manipulated coupling strength and the phase shift between two states. The hybrid graphene-silicon metasurface realize tunable Fano parameter (q) from −1.38 to −1.85 with applied voltage ranging from 0 to 2 v. Theoretical results predicted that higher q values are reachable relying on the hybrid graphene-silicon metasurface. The high-quality(Q)-factor (∼444) tunable FR of metasurface in the near-infrared region is observed. By applying a bias voltage to graphene obtain a blueshift of resonant wavelength (∼4 nm) with a maximum change of transmission spectrum peak up to 30%. These results have potential in high-efficient tunable electro-optic modulators, near-infrared optical switches, etc.

Generation and evolution of Vortex array with variable-ratio lateral-shearing interferometry

Abstract: Different from the method by plane-wave interference, here an efficient approach is proposed to generate optical vortex array (VA) based on lateral-shearing interferometer (LSI), in which the evolution from light spot array to VA can be observed by continuously variable shear ratio in a certain range. VAs with topological charge 2 and 1 are simulated by software GLAD and proved to be effectiveness through optical experiment. Theoretical analysis and experimental results show that when the shear ratio approaches to zero, we can stably obtain a vortex array with high density and variable topological charge.

Optimal trap velocity in a dynamic holographic optical trap using a nematic liquid crystal spatial light modulator

Abstract: A dynamic holographic optical trap uses a dynamic diffractive optical element such as a liquid crystal spatial light modulator to realize one or more optical traps with independent controls. Such holographic optical traps provide a number of flexibilities and conveniences useful in various applications. One key requirement for such a trap is the ability to move the trapped microscopic object from one point to the other with the optimal velocity. In this paper we develop a nematic liquid crystal spatial light modulator based holographic optical trap and experimentally investigate the optimal velocity feasible for trapped beads of different sizes, in such a trap. Our results show that the achievable velocity of the trapped bead is a function of size of the bead, step size, interval between two steps and power carried by the laser beam. We observe that the refresh rate of a nematic liquid crystal spatial light modulator is sufficient to achieve an optimal velocity approaching the theoretical limit in the respective holographic trap for beads with radius larger than the wavelength of light.

The tight-focusing properties of radially polarized symmetrical power-exponent-phase vortex beam

Abstract: In this paper, the radially polarized (RP) new kind of power-exponent-phase vortex (NPEPV) beam, with rotationally symmetrical phase structure, was introduced and the tightly focused properties of the RP NPEPV beam passing through a high numerical aperture objective lens were studied numerically. The results show that with the increase of topological charge l, there are multiple intensity points in the focal region, and the number is consistent with the topological charge. In addition, as the power order n increases, the light intensity gradually concentrates on the central optical axis and the surrounding intensity points gradually disappear, which finally presents a Gaussian intensity distribution with the dark cores gradually move away from the optical axis and disappear. These unique properties will have potential applications in particle trapping and laser fabrication, especially for simultaneous trapping of multiple particles and fabrication of chiral microstructures.

Ultra-high sensitivity terahertz sensor based on a five-band absorber

Abstract: Terahertz sensing is one of the most promising methods for label-free and noninvasive detection of refractive index changes. However, the figure of merit (FOM) of terahertz sensors in practical applications has been low. In this paper, a metamaterial sensor based on simple stacking of gold and silicon dioxide is proposed, through whose structure not only narrow-band absorption with five absorption peaks is realized, but FOM is also improved to 1792. The excellent sensing performance and the mature manufacturing technology of this kind of structure provide a platform for the design of multi-band photodetectors and high-sensitivity sensors.

Generation and characterization of complex vector modes with digital micromirror devices: a tutorial

Abstract: Complex vector light modes with a spatial variant polarization distribution have become topical of late, enabling the development of novel applications in numerous research fields. Key to this is the remarkable similarities they hold with quantum entangled states, which arises from the non-separability between the spatial and polarisation degrees of freedom (DoF). As such, the demand for diversification of generation methods and characterization techniques have increased dramatically. Here we put forward a comprehensive tutorial about the use of DMDs in the generation and characterization of vector modes, providing details on the implementation of techniques that fully exploits the unsurpassed advantage of Digital Micromirrors Devices (DMDs), such as their high refresh rates and polarisation independence. We start by briefly describing the operating principles of DMD and follow with a thorough explanation of some of the methods to shape arbitrary vector modes. Finally, we describe some techniques aiming at the real-time characterization of vector beams. This tutorial highlights the value of DMDs as an alternative tool for the generation and characterization of complex vector light fields, of great relevance in a wide variety of applications.

RamanLIGHT—a graphical user-friendly tool for pre-processing and unmixing hyperspectral Raman spectroscopy images

Abstract: Raman spectroscopy is a valuable tool for non-destructive vibrational analysis of chemical compounds in various samples. Through 2D scanning, it one can map the chemical surface distribution in a heterogeneous sample. These hyperspectral Raman images typically contain spectra of pure compounds that are hidden within thousands of sum spectra. Inspecting each spectrum to find the pure compounds in the dataset is impractical, and several algorithms have been described in the literature to help analyze such complex datasets. However, choosing the best approach(es) and optimizing the parameters is often difficult, and the necessary software was not yet combined in a single program. Therefore, we introduce RamanLIGHT, a fast and simple app to pre-process Raman mapping datasets and apply up to eight unsupervised unmixing algorithms to find endmember spectra of pure compounds. The user can select from six smoothing methods, four fluorescence baseline-removal methods, four normalization methods, and cosmic-ray and outlier removal to generate a uniform dataset prior to the unmixing. We included the most promising pre-processing methods, since there is no routine that perfectly fits all types of samples. Unmixed endmember spectra can be further used to visualize the distribution of compounds in a sample by creating abundance maps for each endmember separately, or a single labeled image containing all endmembers. It is also possible to create a mean spectrum for each endmember, which better describes the true compound spectrum. We tested RamanLIGHT on three samples: an aspirin-paracetamol-caffeine tablet, Alzheimer’s disease brain tissue and a phase-separated polymer coating. The datasets were pre-processed and unmixed within seconds to gain endmembers of known and unknown chemical compounds. The unmixing algorithms are sensitive to noisy spectra and strong fluorescence backgrounds, so it is important to apply pre-processing methods to a suitable degree. RamanLIGHT is freely available as an MATLAB and soon as standalone app.

Numerical investigations on a photonic nanojet coupled plasmonic system for photonic applications

Abstract: A photonic nanojet (PNJ) from a microcavity is a narrow and intense beam of light used to enhance the emerging electric field. Metal nanoparticles (NPs), on the other hand, confine a strong field in their vicinity due to the resonance of the free electrons with the incident field. A hybrid combination of a microcavity with a NP can drastically enhance the output field. In this work, a systematic numerical study of the microcavity-NP system has been carried out to investigate the effect of the shape of the metal NPs on the output field strength. The single and their dimer NPs with different dimer nanogaps with PNJ producing microcavity have been investigated. Splitting of the broad dipole mode of the NP has also been observed. As an application of this study, the surface enhanced Raman spectroscopy factor of the order of 107 has been estimated for nano-cube dimer NP-microcavity hybrid system.

Ultranarrow dual-band metamaterial perfect absorber and its sensing application

Abstract: In this paper, a dual-band metamaterial absorber (MMA) with wide-angle and high absorptivity is proposed. The MMA consists of two silver layers separated by a dielectric layer. Its top resonant element is constituted by two concentric ring resonators connected with four strips. Based on electromagnetic field simulation, the proposed MMA has two narrow absorption peaks with an absorption rate of 99.9% at 711 nm and 99.8% at 830 nm, and the corresponding line width of the two absorption peaks are only 9.7 and 9.8 nm. The dual-band MMA shows high absorptivity under wide incident angles. The simulated field pattern shows that dual-band perfect absorption is the combined result of the interaction of two concentric ring resonators and unit cell coupling. In addition, the hexapole plasmon mode can be observed at the outer ring at one absorption peak. The narrow plasmon resonance has a potential application in optical sensing, and can be used to measure the concentration of aqueous glucose with two frequency channels. The proposed MMA with high absorptivity is simple to manufacture, and has other potential applications, such as narrow-band filters, energy storage device, and so on.

Label-free Raman and fluorescence imaging of amyloid plaques in human Alzheimer’s disease brain tissue reveal carotenoid accumulations

Abstract: Alzheimer’s disease (AD) is a neurodegenerative disease, characterized by the presence of extracellular deposits (plaques) of amyloid-beta peptide and intracellular aggregates of phosphorylated tau. In general, these hallmarks are studied by techniques requiring chemical pre-treatment and indirect labeling. Imaging techniques that require no labeling and could be performed on tissue in its native form could contribute to a better understanding of the disease. In this article a combination of label-free and non-invasive techniques is presented to study the biomolecular composition of AD human brain tissue. We build on previous research that already revealed the autofluorescence property of plaque, and the presence of carotenoids in cored plaques. Here, we present further results on cored plaques: showing blue and green autofluorescence emission coming from the same plaque location. Raman microscopy was used to confirm the presence of carotenoids in the plaque areas, with clear peaks around 1150 and 1514 cm−1. Carotenoid reference spectra were recorded in hexane solution, but also adsorbed on aggregated Aβ42 peptides; the latter agreed better with the Raman spectra observed in plaques. From the six single carotenoids measured, lycopene matched closest with the peak positions observed in the cored plaques. Lastly, stimulated Raman scattering (SRS) microscopy measurements were performed, targeting the shift of the beta-sheet Amide I peak observed in plaques. Employing SRS in the C–H stretch region we also looked for the presence of a lipid halo around plaque, as reported in the literature for transgenic AD mice, but such a halo was not observed in these human AD brain samples.

Generation of stable temporal doublet by a single-mode silicon core optical fiber

Abstract: A highly nonlinear single mode anomalous dispersion silicon-core fiber is designed and optimized at the operating wavelength of 2.2 µm for the purpose of generating stable temporal pulse doublets. To designate the output pulse pair as a perfect Gaussian doublet, two new parameters, dissimilarity factor ( ρg ) and co-similarity index (μ cs) are introduced. Different input pulse parameters such as power, pulse width and chirp are optimized to obtain Gaussian temporal doublets at the shortest optimum length (∼few cm) which is sufficiently smaller in comparison to silica fibers reported earlier. The output pulse pairs remain as a doublet for quite a good stability length. In view of serving practical purposes, the possibilities of fluctuations of input power and pulse width are included to investigate the changes in stability length and effective repetition rate (ERR). The change in ERR along the fiber length produces a remarkable change in free carrier concentration in core, which has also been taken into account for the first time as per our knowledge to obtain the temporal pulse doublet in the so designed Si-core fiber.