Table of contents

State-of-the-art x-ray light sources are nowadays based on large-scale electron accelerators, because the synchrotron radiation (SR) and x-ray free electron laser (XFEL) radiation generated by high-energy electron beams have many advantages over other alternatives in terms of the wavelength tunability, high brightness and flux, high coherence, flexible polarization states, and so on. This is the reason why SR and XFEL light sources have largely contributed to the evolution of x-ray science. This paper reviews the current status of such accelerator-based x-ray light source facilities and discusses their future perspectives.

Silicon photonics is a technology based on fabricating integrated optical circuits by using the same paradigms as the dominant electronics industry. After twenty years of fervid development, silicon photonics is entering the market with low cost, high performance and mass-manufacturable optical devices. Until now, most silicon photonic devices have been based on linear optical effects, despite the many phenomenologies associated with nonlinear optics in both bulk materials and integrated waveguides. Silicon and silicon-based materials have strong optical nonlinearities which are enhanced in integrated devices by the small cross-section of the high-index contrast silicon waveguides or photonic crystals. Here the photons are made to strongly interact with the medium where they propagate. This is the central argument of nonlinear silicon photonics. It is the aim of this review to describe the state-of-the-art in the field. Starting from the basic nonlinearities in a silicon waveguide or in optical resonator geometries, many phenomena and applications are described—including frequency generation, frequency conversion, frequency-comb generation, supercontinuum generation, soliton formation, temporal imaging and time lensing, Raman lasing, and comb spectroscopy. Emerging quantum photonics applications, such as entangled photon sources, heralded single-photon sources and integrated quantum photonic circuits are also addressed at the end of this review.

Optofluidics, nominally the research area where optics and fluidics merge, is a relatively new research field and it is only in the last decade that there has been a large increase in the number of optofluidic applications, as well as in the number of research groups, devoted to the topic. Nowadays optofluidics applications include, without being limited to, lab-on-a-chip devices, fluid-based and controlled lenses, optical sensors for fluids and for suspended particles, biosensors, imaging tools, etc. The long list of potential optofluidics applications, which have been recently demonstrated, suggests that optofluidic technologies will become more and more common in everyday life in the future, causing a significant impact on many aspects of our society. A characteristic of this research field, deriving from both its interdisciplinary origin and applications, is that in order to develop suitable solutions a combination of a deep knowledge in different fields, ranging from materials science to photonics, from microfluidics to molecular biology and biophysics, is often required. As a direct consequence, also being able to understand the long-term evolution of optofluidics research is not easy. In this article, we report several expert contributions on different topics so as to provide guidance for young scientists. At the same time, we hope that this document will also prove useful for funding institutions and stakeholders to better understand the perspectives and opportunities offered by this research field.

Controlling artificial Pearcey and swallowtail beams allows the realization of caustic lattices in nonlinear photosensitive media at very low light intensities. We examine their functionality as 2D and 3D waveguiding structures and show the potential of exploiting these lattices as linear beam splitters, which we name a 'Pearcey-Y-splitter'. For symmetrized Pearcey beams as auto-focusing beams, the formation of solitons in focusing nonlinearity is observed. Our original approach represents the first realization of caustic photonic lattices and can directly be applied in signal processing, microscopy and material lithography.

In this review we present the potentialities and the achievements of the use of non-classical photon-number correlations in twin-beam states for many applications, ranging from imaging to metrology. Photon-number correlations in the quantum regime are easily produced and are rather robust against unavoidable experimental losses, and noise in some cases, if compared to the entanglement, where losing one photon can completely compromise the state and its exploitable advantages. Here, we will focus on quantum enhanced protocols in which only phase-insensitive intensity measurements (photon-number counting) are performed, which allow probing the transmission/absorption properties of a system, leading, for example, to innovative target detection schemes in a strong background. In this framework, one of the advantages is that the sources experimentally available emit a wide number of pair-wise correlated modes, which can be intercepted and exploited separately, for example by many pixels of a camera, providing a parallelism, essential in several applications, such as wide-field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally, non-classical correlation enables new possibilities in quantum radiometry, e.g. the possibility of absolute calibration of a spatial resolving detector from the on-off single-photon regime to the linear regime in the same setup.

Liquid crystals allow for the real-time control of the polarization of light. We describe and provide some experimental examples of the types of general polarization transformations, including universal polarization transformations, that can be accomplished with liquid crystals in tandem with fixed waveplates. Implementing these transformations with an array of liquid crystals, e.g. a spatial light modulator, allows for the manipulation of the polarization across a beam's transverse plane. We outline applications of such general spatial polarization transformations in the generation of exotic types of vector polarized beams, a polarization magnifier, and the correction of polarization aberrations in light fields.

Using long-range surface plasmon polaritons light can propagate in metal nano-scale waveguides for ultracompact opto-electronic devices. Gold is an important material for plasmonic waveguides, but although its linear optical properties are fairly well understood, the nonlinear response is still under investigation. We consider the propagation of pulses in ultrathin gold strip waveguides, modeled by the nonlinear Schrödinger equation. The nonlinear response of gold is accounted for by the two-temperature model, revealing it as a delayed nonlinearity intrinsic in gold. The consequence is that the measured nonlinearities are strongly dependent on pulse duration. This issue has so far only been addressed phenomenologically, but we provide an accurate estimate of the quantitative connection as well as a phenomenological theory to understand the enhanced nonlinear response as the gold thickness is reduced. In comparison with previous works, the analytical model for the power-loss equation has been improved, and can be applied now to cases with a high laser peak power. We show new fits to experimental data from the literature and provide updated values for the real and imaginary parts of the nonlinear susceptibility of gold for various pulse durations and gold layer thicknesses. Our simulations show that the nonlinear loss is inhibiting efficient nonlinear interaction with low-power laser pulses. We therefore propose to design waveguides suitable for the mid-IR, where the ponderomotive instantaneous nonlinearity can dominate over the delayed hot-electron nonlinearity and provide a suitable plasmonics platform for efficient ultrafast nonlinear optics.

The nonlinear responses of different materials provide useful mechanisms for optical switching, low noise amplification, and harmonic frequency generation. However, the nonlinear processes usually have an extremely weak nature and require high input power to be excited. To alleviate this severe limitation, we propose new designs of ultrathin nonlinear metasurfaces composed of patterned graphene micro-ribbons to significantly enhance third harmonic generation (THG) at far-infrared and terahertz (THz) frequencies. The incident wave is tightly confined and significantly boosted along the surface of graphene in these configurations due to the excitation of highly localized plasmons. The bandwidth of the resonant response becomes narrower due to the introduction of a metallic substrate below the graphene micro-ribbons, which leads to zero transmission and standing waves inside the intermediate dielectric spacer layer. The enhancement of the incident field, combined with the large nonlinear conductivity of graphene, can dramatically increase the THG conversion efficiency by several orders of magnitude. In addition, the resonant frequency of the metasurface can be adjusted by dynamically tuning the Fermi energy of graphene via electrical or chemical doping. As a result, the THG wave can be optimized and tuned to be emitted at different frequencies without the need to change the nonlinear metasurface geometry. The proposed nonlinear metasurfaces provide a new way to realize compact and efficient nonlinear sources at the far-infrared and THz frequency ranges, as well as new frequency generation and wave mixing devices which are expected to be useful for nonlinear THz spectroscopy and noninvasive THz imaging applications.

In this paper, we investigate the recovery of range and spectral profiles associated with remote three-dimensional scenes sensed via single-photon multispectral lidar (MSL). We consider two different spatial/spectral sampling strategies and compare their performance for a similar overall number of detected photons. For a regular spatial grid of pixels, the first strategy consists of sampling all the spatial locations of the grid for each of the L wavelengths. The second strategy is consistent with the use of mosaic filter-based arrays and consists of acquiring only one wavelength (out of L) per spatial location. Despite the reduction of spectral content observed in each location, the second strategy has clear potential advantages for fast multispectral imaging using only a single frame read out. We propose a fully automated computational method, adapted for each of the two sampling strategies in order to recover the target range profile, as well as the reflectivity profiles associated with the different wavelengths. These strategies were also assessed with high ambient background. The performance of the two sampling strategies is illustrated using a single-photon MSL system with L = 4 wavelengths (473, 532, 589 and 640 nm). The results presented demonstrate that although the first strategy usually provides more accurate results, the second strategy does not exhibit a significant performance degradation, particularly for sparse photon data (down to 1 photon per pixel on average). These results suggest a way forward for the integration of single-photon detector arrays with mosaic filters for use in a range of emerging photon-starved two-dimensional and three-dimensional imaging applications.

We present a comprehensive investigation to ascertain the impact of gold and silver films on modifying the quality-factor (Q-factor) of optical Tamm-plasmon (OTP) resonance in a metal-distributed Bragg reflector (M-DBR) geometry. Here, OTP mode is excited using direct incidence of white-light-source at normal incidence as well as oblique incidence on M-DBR geometry. The lifetime of OTP in gold and silver deposited films on DBR mirror was determined from OTP resonance linewidth. The lifetime and the Q-factor of OTP modes are found to depend on DBR bilayers, metal film thickness as well as on different plasmon active metals. This finding would facilitate tuning the Q-factor and consequently, the lifetime of OTP modes for various applications in all-optical switches and modulators. In addition, we discuss the spectral characteristics of OTP modes excited using normal and oblique incident of source.

Measuring the three-dimensional point spread function (3D PSF) of imaging spectrometers is a challenging task since it requires a small, monochromatic and bright source. Here we introduce a powerful and practical new approach for 3D PSF measurement on the basis of a bright virtual monochromatic point-like source, which is formed by a collimated light beam and a convex spherical mirror. The effectiveness of the proposed methodology is demonstrated and discussed through 3D PSF measurements of an acousto-optic tunable filter based imaging spectrometer.

Grating coupled near-field interference lithography has the ability to produce deep-subwavelength interference patterns. Simulations of these systems is very computationally intensive. An inverse design procedure employing a genetic algorithm is utilized here to massively reduce the computational load and allow for the design of systems capable of interfering extremely high numerical apertures. This method is used to optimize systems with an interference patterns with a half pitch of $\lambda /40$ corresponding to a numerical aperture of 20. It is also used to demonstrate interference of higher $| m|$ diffraction orders.

Leakage radiation microscopy (LRM) is used to investigate the optical properties of surfaces. The front-focal plane (FFP) image with LRM reveals the structural features on the surfaces. A back-focal plane (BFP) image with LRM reveals the angular distribution of the radiation. Herein, we experimentally demonstrate that the out-of-focal plane (OFP) images present a link between the FFP and BFP images and provide optical information that cannot be resolved by either FFP or BFP images. The OFP image provides a link between the spatial location of the emission and the angular distribution from the same location, and thus information about the film's discontinuity, nonuniformity or variable thickness can be uncovered. The use of OFP imaging will extend the scope and applications of the LRM and coupled emission imaging, which are powerful tools in nanophotonics and high throughput fluorescence screening.

We present an intrinsic polarization splitting characteristic of low-symmetric photonic crystals (PCs) formed by unit-cells with C₂ rotational symmetry. This behavior emerges from the polarization sensitive self-collimation effect for both transverse-magnetic (TM) and transverse-electric (TE) modes depending on the rotational orientations of the unit-cell elements. Numerical analyzes are performed in both frequency and time domains for different types of square lattice two-fold rotational symmetric PC structures. At incident wavelength of λ = 1550 nm, high polarization extinction ratios with ∼26 dB (for TE polarization) and ∼22 dB (for TM polarization) are obtained with an operating bandwidth of 59 nm. Moreover, fabrication feasibilities of the designed structure are analyzed to evaluate their robustness in terms of the unit-cell orientation: for the selected PC unit-cell composition, corresponding extinction ratios for both polarizations still remain to be over 18 dB for the unit-cell rotation interval of θ = [40°–55°]. Taking all these advantages, two-fold rotationally symmetric PCs could be considered as an essential component in photonic integrated circuits for polarization control of light.

We present all the periodic Green function dyadics that enter a description of a 2D array of emitters at the level that includes the electric dipole, magnetic dipole and electric quadrupole moment of each emitter. We find a concise analytic form for the radiative contributions to the periodic Green function dyadics that give rise to radiation reaction fields, and so our description of the scattered light explicitly satisfies the optical theorem; we give the non-radiative contributions that do not affect energy balance in the form of rapidly converging series. Finally, we present an approximation scheme for evaluating periodic Green function dyadics at long wavelengths that rigorously respects energy conservation. The scheme extends the range of validity of the usual static approximation by the inclusion of a simple dynamic correction.

We observe the crossing and anti-crossing behaviors of nearly degenerate mode pairs in a rolled-up tubular microcavity, which can be explained by weak and strong couplings between the modes. Exceptional points (EPs) are thus obtained in the tubular microcavity since they are the critical point where a transition from strong to weak coupling occurs. Rolled-up tubular microcavities with a given resonant mode approaching an EP in parameter space expanded by two continuous variables are also realized without using near-field probes. Microcavities with EPs prepared in a rolled-up way could be mechanically stable and would be used for optofluidic detection.

In this paper, a new kind of liquid-crystal microlens array with graphene electrodes controlled electrically are designed and fabricated successfully. The graphene-based liquid-crystal microlens arrays (GLCMAs) exhibit excellent beam focusing performances in both the visible and near infrared (NIR) wavelength regions and also synthetic aperture imaging function. The graphene films used to fabricate the electrodes of the GLCMAs are grown by chemical vapor deposition over copper foils, demonstrating several characters of low sheet resistance and high transmittance in both wavelength ranges above. The key processes for shaping the GLCMAs include: transferring graphene films from copper foils to wafers selected, conventional UV-photolithography, ICP etching, and liquid-crystal encapsulation. Through performing common optical measurements, the point spread functions of incident lasers with different wavelength, such as red lasers of ∼600 nm, green lasers of ∼532 nm, and NIR lasers of ∼980 nm, have been obtained. Several key parameters including focal spots size, average normalized light intensity, focal length, average deviation rate and contrast ratio have been acquired and analyzed. A particular synthetic-aperture imaging based on the GLCMA is realized so as to certify a fact that a single target pattern can be constructed effectively based on some sub-aperture patterns with several tens or hundreds of micrometer scale, and thus highlight a way to fast process partial or small-zoned patterns for enhancing the detection efficiency of special targets.

In this work, we comprehensively investigate a Dammann grating (DG) that can generate a 5 × 5 diffraction spot array with an extending angle of $18^\circ \times 18^\circ$ around the fiber communication wavelength of $1550\,\mathrm{nm}$. The DG is a simple metasurface structure composed of a silicon cuboid nanorod array on a silica substrate, and only two different sizes of nanorods with square cross-sections and uniform spatial orientations are used. These simple units and this configuration are favorable in practice, and the C4 symmetry cross section of the nanorods ensures the polarization-independent operation of the DG. The phase modulation of the nanorods is achieved by the guiding mode propagating in them rather than electric or magnetic Mie-type resonance, which makes the design of the cuboid nanorods easy and robust. More importantly, the two-dimensional nanorod array is generated from a one-dimensional array, which further decreases the design and fabrication complexity.

Excitation of the second harmonic of THz radiation is investigated theoretically in the planar multilayered structure dielectric-graphene-dielectric-graphene–.... It is studied the case of the oblique incidence of the s-polarized fundamental wave, where the electric field is parallel to the interfaces, and generation of the p-type second harmonic wave occurs. The original concept is proposed to employ the double resonance arrangement for the effective generation of the second harmonic. The double resonant case can be realized when a high-permittivity dielectric is at the input of the structure and the vacuum is at the output. The high efficiency is demonstrated; the second harmonic reflectance coefficient is ≥0.01 under realistic values of the collision frequency in graphene >10¹² s⁻¹. Such a great efficiency, which is four–five orders of magnitude higher than reported for the graphene-dielectric structures previously, is proposed for the first time. To compute the nonlinear surface currents, two approaches were used, the kinetic and the hydrodynamic. A qualitative agreement between two approaches, proven in the present modeling, ensures an applicability of the results.

We present a three-level model based on a density matrix to examine the influence of coherence and dephasing on the gain spectrum of mid-infrared quantum cascade lasers. The model is used to examine a quantum cascade active region with multiple optical transitions. We show how coherence can explain the origin of additional peaks in the gain spectrum. We also analyze the spectra calculated using the three-level model with a rate equation formalism to demonstrate the importance of considering interface roughness and limitations of the rate equation formalism. Specifically, we present how interface roughness influences the broadening and oscillator strength that are recovered using a rate equation analysis. The results of this work are important when considering the design of active regions with multiple optical transitions and could lead to devices with improved performance.

We present an algorithm of extracting the wavelet coefficients of object based on ghost imaging (GI) system. Through modification of the projected random patterns by using a series of templates, wavelet transform GI (WTGI) can directly measure the high frequency components of wavelet coefficients without needing the original image. In this study, we theoretically and experimentally perform the high frequency components of wavelet coefficients detection with an arrow and a letter A based on GI and WTGI. Comparing with the traditional method, the use of the algorithm proposed in this paper can significantly improve the quality of the image of wavelet coefficients in both cases. The special advantages of GI will make the wavelet coefficient detection based on WTGI very valuable in real applications.

A spatio-temporal model for investigating the characteristics of laser-induced plasmas in aqueous media is developed by modifying the general form of the well-known rate equation and simultaneously accounting for the influences of multiphoton and cascade ionization on the propagation of short laser pulses. In this model, the nonlinear absorption of laser pulse energy is considered to be time and space dependent inside the computational volume. The model is verified by comparing the results of three-dimensional axisymmetric numerical simulations with existing experimental data for laser pulses of 30 ps, 1064 nm at focusing angles between 4° and 28° with energies in the wide range between 0.1 to 6000 μJ. This model could reasonably predict the various characteristics of a laser-induced plasma, such as breakdown threshold, size, shape and energy transmitted through the plasma. Also the transmitted energy data obtained from the model is within 10% of the experimental data at the largest focusing angle and 20% at the smallest angle. To compare the calculations with plasma photographs, electron density values are transformed into a gray scale. The simulated plasma shapes correlate well with the existing experimental observations. The outcomes of the model, such as spatial distribution of plasma energy density, could be used as input for a hydrodynamic model to estimate the strength of the mechanical effects associated with plasma formation.

We propose a technique to control the spectral and temporal coherence properties of pulsed beams of light via time-dependent manipulation of the spectral phase. Modulation schemes for the generation of partially coherent pulse trains from a train of fully coherent pulses are presented. The feasibility of experimental realization of the method is confirmed by numerical estimates.

We demonstrate supercontinuum (SC) generation in a 2.8-cm-long chalcogenide double-clad fiber (Ch-DCF). The calculated chromatic dispersion of the fundamental mode shows that the Ch-DCF has flattened chromatic dispersion, which is within −10 ± 10 ps/nm/km from 3.8 to 12.6 $\mu {\rm{m}}$ in the normal dispersion regime. The variation of SC spectra is investigated by changing the pump wavelength and pump peak power. The broadband SC spectra extending from 2 to 14 μm at the −40 dB level is observed when the pump wavelength of 10 μm and pump peak power of 1.3 MW. The SC generation is simulated by the scalar generalized nonlinear Schrödinger equation. The simulation results are mostly the same as the experimental results and show that the supercontinua generated in the Ch-DCF are highly coherent.

We study the possibility of efficient self-compression of femtosecond laser pulses in nonlinear media with anomalous dispersion of group velocity during the self-focusing of wave packets with a power several times greater than the critical self-focusing power. The results of qualitative analysis of the evolution of three-dimensional wave packets with the quasi-soliton field distribution are confirmed by the computer simulation. The simulation proves that the considered regime of compression of high-power laser pulses with initial durations of about ten optical cycles is stable relative to filamentation instability, due to the influence of the nonlinear dispersion. We demonstrate the possibility of self-compression of laser pulses at a multi-millijoule energy level and up to one optical cycle with an energy efficiency of more than 50%.

A tunable broadband absorber induced by resonant tunneling is investigated in a coupled quantum-dot (CQD) system. The CQDs show a narrow absorption in a linear case. When the inter-dot tunneling arises, constructive interference occurs in a nonlinear absorption, which exhibits a large absorption in a wide frequency range. Therefore, the total absorption spectrum is much more broadened, and its bandwidth under given conditions can be one order of magnitude larger than that of the liner absorption. Bright-state coherence is employed to qualitatively explain the numerical results. Furthermore, the dependence of the broad absorption on the tunneling strength and analytical expression of the nonlinear absorption spectrum are also provided.

Fabrication capabilities of high optical quality hexagonal superstructures by chemical etching of inverted ferroelectric domains in lithium niobate platform suggests a route for efficient implementation of compact hexagonal microcavities. Such nonlinear optical hexagonal micro-resonators are proposed as a platform for second harmonic generation (SHG) by the combined mechanisms of total internal reflection (TIR) and quasi-phase-matching (QPM). The proposed scheme for SHG via TIR-QPM in a hexagonal microcavity can improve the efficiency and also the compactness of SHG devices compared to traditional linear-type based devices. A simple theoretical model based on six-bounce trajectory and phase matching conditions was capable for obtaining the optimal cavity size. Furthermore numerical simulation results based on finite difference time domain beam propagation method analysis confirmed the solutions obtained by demonstrating resonant operation of the microcavity for the second harmonic wave produced by TIR-QPM. Design aspects, optimization issues and characteristics of the proposed nonlinear device are presented.

In this paper, we report two different saturable absorbers based on WS₂ film and Bi₂Te₃ film with similar preparation processes. A high-energy stable Q-switching pulse is achieved in identical cavity configurations for each absorber. A modulation depth of 10.17% and a maximum single pulse energy of 56.50 nJ are obtained when employing the WS₂ film. However, using the Bi₂Te₃ film we obtain a higher modulation depth of 23.04% and a larger single pulse energy of 61.80 nJ, and stable dual-wavelength Q-switching operation was shown at a pump power of 92 mW.

In this paper, first a detailed investigation of the various behaviors of the near- and far-field diffractions from multiplicatively separable (MS) structures in the x and y directions is presented. It is shown that in near-field propagation, the diffraction pattern of a 2D MS structure is same as the product of the corresponding near-field diffractions of the 1D individual components of the structure. For the far-field diffraction, although the resulting diffraction pattern is not equal to the product of the individual 1D structures' diffraction patterns, we show that it is still a 2D MS pattern. Second, a detailed theoretical investigation of the contrast enhancement effect for the multiplication of two 1D orthogonal intensity patterns (not necessarily periodic) is presented. By merging the above-mentioned facts, we deduced that in near-field diffraction, the contrast of the diffraction pattern of a 2D MS structure is always larger than the contrast of each of the diffraction patterns of the corresponding individual 1D structures. In the second part of this paper, we implement the intensity contrast as a quantity to describe self-images of the 2D MS periodic structures. For the first time, two very important results are obtained based on the contrast enhancement effect. We show that the depth of focus of the self-images increases for the 2D periodic structures in comparing to their corresponding 1D structures. We also predict the existence of additional self-images in addition to the Talbot images located at the least common multiple of each of the individual 1D Talbot distances for the 2D MS periodic structures. In addition, in this work we present a very straightforward manner for the prediction of the 1D or 2D forms of the diffraction pattern and the direction of the 1D pattern strips at given propagation distances from the 2D structure by introducing another intensity-contrast-based parameter. Furthermore, we show that the diffraction pattern of a 2D MS periodic structure depends strongly on the square of the ratio of the 1D structure periods in the x and y directions. These theoretical achievements are verified by some experimental works.

In this study, a polarization-independent microlens array composed of two layered nematic liquid crystal (LC) materials with orthogonal alignment and separated by a double-sided indium-tin oxide silica with a thickness of ∼500 μm, is proposed. The functioned stacking of two polarization LC microstructures lead to an electrically tuning polarization and also polarization-independent light control architecture. Experimental investigations reveal key characters such as a high beam utilization efficiency of more than 90% and a compound point spread function tuned electrically. Dual-mode imaging including the polarization mode and the polarization-independent mode can be realized based on the novel configuration with both independent voltage signals applied over two functioned LC layers. The normalized focusing intensity demonstrates a special property of no polarization dependence on beams processed. An image criteria for evaluating the quality of polarization-insensitive images based on the difference image between the horizontal and vertical polarization images, is introduced. Considering the advantages of electrically tunable composed focal length and then switching the polarization and polarization-independent mode, it can be expected that the developed devices can not only be used to measure and further tune the polarization state of beams propagating in LC layers but also acquire high quality target images.

A complete single-point statistical description of a narrow-band partially polarized optical field is developed in terms of the 2D period-averaged probability density function (PA-PDF) of the electrical field vector. This statistic can be measured using the coherent (heterodyne) detection. PA-PDF carries more information about the partially polarized light than the traditional Stokes vector. For a simple Gaussian partially polarized field the PA-PDF depends on 13 real parameters in contrast to the four parameters of the Stokes vector or coherence tensor. We show on several examples that the polarization state of the wave, as described by PA-PDF can vary significantly even while Stokes vector remains fixed.

Based on the extended Huygens-Fresnel integral, the analytical expressions for the Wigner distribution function (WDF) and kurtosis parameter of partially coherent flat-topped vortex (PCFTV) beams propagating through atmospheric turbulence and free space are derived. The WDF and kurtosis parameter of PCFTV beams through turbulent atmosphere are discussed with numerical examples. The numerical results show that the beam quality depends on the structure constants, the inner scale turbulence, the outer scale turbulence, the spatial correlation length, the wave length and the beam order. PCFTV beams are less affected by turbulence than partially flat-topped coherent (PCFT) beams under the same conditions, and will be useful in free-space optical communications.

Phase only spatial light modulators (SLMs) have become the tool of choice for shaped light generation, allowing the creation of arbitrary amplitude and phase patterns. These patterns are generated using digital holograms and are useful for a wide range of applications as well as for fundamental research. There have been many proposed methods for optimal generation of the digital holograms, all of which perform well under ideal conditions. Here we test a range of these methods under specific experimental constraints, by varying grating period, filter size, hologram resolution, number of phase levels, phase throw and phase nonlinearity. We model beam generation accuracy and efficiency and show that our results are not limited to the specific beam shapes, but should hold for general beam shaping. Our aim is to demonstrate how to optimise and improve the performance of phase-only SLMs for experimentally relevant implementations.

Image haze removal has attracted much attention in optics and computer vision fields in recent years due to its wide applications. In particular, the fast and real-time dehazing methods are of significance. In this paper, we propose a fast dehazing method in hue, saturation and intensity colour space based on the polarimetric imaging technique. We implement the polarimetric dehazing method in the intensity channel, and the colour distortion of the image is corrected using the white patch retinex method. This method not only reserves the detailed information restoration capacity, but also improves the efficiency of the polarimetric dehazing method. Comparison studies with state of the art methods demonstrate that the proposed method obtains equal or better quality results and moreover the implementation is much faster. The proposed method is promising in real-time image haze removal and video haze removal applications.

Several applications, such as optical tweezers and atom guiding, benefit from techniques that allow the engineering of spatial field profiles, in particular their longitudinal intensity patterns. In cylindrical coordinates, methods such as frozen waves allow an advanced control of beam characteristics, but in Cartesian coordinates there is no analogous technique. Since Cartesian beams may also be useful in applications, we develop here a method to modulate on demand the longitudinal intensity pattern of any (initially) unidimensional Cartesian beam with concentrated angular spectrum (thus encompassing all unidimensional paraxial beams) in lossless and lossy media. To this end, we write the total beam as a product of two unidimensional beams and explore the degree of freedom provided by the additional Cartesian coordinate. While in the plane where this coordinate is zero the chosen unidimensional beam keeps its structure with the additional desired intensity modulation, a sinusoidal-like oscillation appears in the direction of this variable and creates a spot whose size is tunable. Examples with Gaussian and Airy beams are presented and their corresponding experimental demonstrations in free-space are performed to show the validity of the method.

In this study, we propose a new nonlinear optical image encryption technique using spiral phase transform (SPT). First, the primary image is phase encoded and multiplied with a random amplitude mask (RAM), and using power function, the product is then powered to m. This powered output is Fresnel propagated with distance z₁ and then modulated with a random phase mask (RPM). The modulated image is further Fresnel propagated with distance z₂. Similarly, a security image is also modulated with another RAM and then Fresnel propagated with distance z₃. Next, the two modulated images after Fresnel propagations, are interfered and further Fresnel propagated with distance z₄ to get a complex image. Finally, this complex image is SPT with particular spiral phase function (SPF), to get the final encrypted image for transmission. In the proposed technique, the security keys are Fresnel propagation distances, the security image, RPM, RAMs, power order, m, and order of SPF, q. Numerical simulation results confirm the validity and effectiveness of the proposed technique. The proposed technique is robust against noise and brutal force attacks.

We propose a new approach to realize switchable mode operation in a few-mode erbium-doped fiber laser. The ring fiber laser structure is constructed with a core-offset splicing between single-mode fiber and dual-mode fiber. Stable operating on the fundamental mode laser and second-order mode laser individually or simultaneously is realized by appropriately adjusting the state of the polarization controller and bending status of the few-mode fiber Bragg grating. The narrow 3 dB linewidth less than 0.02 nm and high optical signal to noise ratio more than 42 dB are obtained for both modes in either separate laser or simultaneous laser operating conditions.

An iterative method based on coupled mode theory and the steepest descent principle in a microwave waveguide is proposed in the optical waveguide for the design of a LP_0m mode converter. After iterative design, a LP₀₂-LP₀₁ mode converter and a LP₀₃-LP₀₂ mode converter can achieve 99% conversion efficiency in our model. The 1dB bandwidth of a LP₀₂-LP₀₁ mode converter is measured as more than 130 nm. These optimum waveguides are simulated by RSoft software and indicate high conversion efficiency as well. Furthermore, we investigate the compatibility of the iterative method for mode converter design with initial waveguides of multiple taper shapes. The proposed method is promising for applications in mode division multiplexing systems.

In this work, the generation of optical vortices in an optical integrated circuit is numerically demonstrated. The optical vortices with topological charge m = ±1 are obtained by the coherent superposition of the first order modes present in a waveguide with a rectangular cross section, where the phase delay between these two propagating modes is Δφ = ±π/2. The optical integrated circuit consists of an input waveguide continued with a y-splitter. The left and the right arms of the splitter form two coupling regions K₁ and K₂ with a multimode output waveguide. In each coupling region, the fundamental modes present in the arms of the splitter are selectively coupled into the output waveguide horizontal and vertical first order modes, respectively. We showed by employing the beam propagation method simulations that the fine tuning of the geometrical parameters of the optical circuit makes possible the generation of optical vortices in both transverse electric (TE) and transverse magnetic (TM) modes. Also, we demonstrated that by placing a thermo-optical element on one of the y-splitter arms, it is possible to switch the topological charge of the generated vortex from m = 1 to m = −1.

We present a theoretical investigation of the angular Goos–Hänchen shift (GHS) of a Gaussian light beam upon reflection from a multilayered structure consisting of a nematic liquid crystal (LC) cell sandwiched between electrodes and deposited on a magneto-electric/non-magnetic bilayer. We show that the angular GHS can be enhanced and controlled both via a voltage applied to the LC cell and a magnetization reversal in the magneto-electric film. We describe the principle of an optical sensor of chemical vapors in the vicinity of the structure based on the voltage-induced tunability of the angular GHS.

Spectral management is one of the promising ways to increase the efficiency of modern photovoltaic devices. We study the performance of phosphor-filled luminescent down-shifting (LDS) layers. We focus on four powder phosphors with refractive indices in the range of 1.66–1.84 and similar particle size distributions. Using experimental characterization as well as 3D optical simulations, we identify key parameters of the phosphor particles and LDS layers that primarily affect the optical transmittance, absorptance, and photoluminescence quantum yield of the layers. We investigate the influence of the medium located beneath the LDS layer and reveal a strong increase in the performance when the layer is applied directly onto the solar cell. Finally, the optimal combination of the particle, binder and layer parameters that render the highest performance of the LDS layers are also indicated and discussed.