Table of contents

Silicon waveguide optical nonreciprocal devices that use the magneto-optical phase shift are reviewed. The phase shift caused by the first-order magneto-optical effect is effective in realizing optical nonreciprocal devices on semiconductor waveguide platforms. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer contributes to the large penetration of the optical field into a magneto-optical material used as an over-cladding layer. This enhances the magneto-optical phase shift and, hence, contributes greatly to reducing the device footprint. A surface-activated direct bonding technique plays a key role in the fabrication of magneto-optical nonreciprocal devices. This technique makes it possible to use a high-quality single-crystalline magneto-optical garnet that exhibits a large first-order magneto-optical effect. An optical isolator based on the magneto-optical phase shift was demonstrated in a silicon waveguide with an optical isolation ratio as high as 30 dB and an insertion loss of 13 dB at a wavelength of λ = 1548 nm. Furthermore, a four-port optical circulator was demonstrated with maximum isolation ratios of 33.5 and 29.1 dB in the cross and bar ports, respectively, at λ = 1543 nm.

Gas-filled hollow-core photonic crystal fibers offer unprecedented opportunities to observe novel nonlinear phenomena. The various properties of gases that can be used to fill these fibers give additional degrees of freedom for investigating nonlinear pulse propagation in a wide range of different media. In this review, we will consider some of the the new nonlinear interactions that have been discovered in recent years, in particular those which are based on soliton dynamics.

We propose a technique to realize a tachyonic band structure in a coherent network, such as an array of coupled ring resonators. This is achieved by adding 'PT symmetric' spatially balanced gain and loss to each node of the network. In a square-lattice network, the quasi-energy bandstructure exhibits a tachyonic dispersion relation, centered at either the center or corner of the Brillouin zone. There is one tachyonic hyperboloid in each gap, unlike in PT-symmetric tight-binding honeycomb lattices where the hyperboloids occur in pairs. The dispersion relation can be probed by measuring the peaks in transmission across a finite network as the gain/loss parameter is varied.

In this review we will consider the retrieval of the wave at the exit surface of an object illuminated by a coherent probe from one or more measured diffraction patterns. These patterns may be taken in the near-field (often referred to as images) or in the far field (the Fraunhofer diffraction pattern, where the wave is the Fourier transform of that at the exit surface). The retrieval of the exit surface wave from such data is an inverse scattering problem. This inverse problem has historically been solved using nonlinear iterative methods, which suffer from convergence and uniqueness issues. Here we review deterministic approaches to obtaining the exit surface wave which ameliorate those problems.

We consider self-trapping of topological modes governed by the one- and two-dimensional (1D and 2D) nonlinear-Schrödinger/Gross–Pitaevskii equation with effective single- and double-well (DW) nonlinear potentials induced by spatial modulation of the local strength of the self-defocusing nonlinearity. This setting, which may be implemented in optics and Bose–Einstein condensates, aims to extend previous studies, which dealt with single-well nonlinear potentials. In the 1D setting, we find several types of symmetric, asymmetric and antisymmetric states, paying attention to scenarios of the spontaneous symmetry breaking. The single-well model is extended by including rocking motion of the well, which gives rise to Rabi oscillations between fundamental and dipole modes. Analysis of the 2D single-well setting gives rise to stable modes in the form of ordinary dipoles, vortex–antivortex dipoles (VADs), and vortex triangles (VTs), which may be considered as produced by spontaneous breaking of the axial symmetry. The consideration of the DW configuration in 2D reveals diverse types of modes built of components trapped in the two wells, which may be fundamental states and vortices with topological charges m = 1 and 2, as well as VADs (with m = 0) and VTs (with m = 2).

For the first time we have obtained the analytical expressions describing the spatial distribution of the polarization of a second harmonic beam's light field reflected from the surface of an isotropic gyrotropic medium in the case when a normally incident fundamental beam contains a polarization singularity of an arbitrary type. The contribution of bulk and surface responses of the nonlinear medium in the formation of the lines of circular polarization in the second harmonic beam were analyzed. The relation between topological characteristics and polarization states of the singularities in the incident and reflected beams was established.

We introduce a topological defect to a regular photonic crystal defect cavity with anisotropic unit cell. Spatially localized resonances are formed and have high quality factor. Unlike the regular photonic crystal defect states, the localized resonances in the topological defect structures support powerflow vortices. Experimentally we realize lasing in the topological defect cavities with optical pumping. This work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of lasing modes, enabling the manipulation of the field pattern and energy flow landscape.

We introduce a new family of nondiffracting full Poincaré beams based on a superposition of nondiffracting Mathieu beams, which we call the Mathieu–Poincaré beams (MPBs). We studied the polarization structure of the MPBs and how it is traced on the Poincaré sphere, and found that the first region mapping the Poincaré sphere is contained within an ellipse of circular polarization of constant size for all beam orders m for a given semi-focal distance and as expected a higher order $m\gt 1$ beam covers the Poincaré sphere m-fold in a nonuniform way given the noncircular symmetry of the Mathieu beams. Finally, we looked into the polarization singularities along the inter-focal line and observed that the all $m\;C$-points have a star (lemon) morphology for even (odd) beam order m when we used positive helical Mathieu beams to synthesize the MPBs, and that this relationship is reversed when we switched to a negative helical Mathieu beam.

We study theoretically the squeezing spectrum and second-order correlation function of the output light for an optomechanical system in which a mechanical oscillator modulates the cavity linewidth (dissipative coupling). We find strong squeezing coinciding with the normal-mode frequencies of the linearized system. In contrast to dispersive coupling, squeezing is possible in the resolved-sideband limit simultaneously with sideband cooling. The second-order correlation function shows damped oscillations, whose properties are given by the mechanical-like, the optical-like normal mode, or both, and can be below shot-noise level at finite times, ${g}^{(2)}(\tau )\lt 1.$

We model a dye-doped polymeric nanosphere as an ensemble of quantum emitters and use it to investigate the localized exciton–polaritons supported by such a nanosphere. By determining the time evolution of the density matrix of the collective system, we explore how an incident laser field may cause transient optical field enhancement close to the surface of such nanoparticles. Our results provide further evidence that excitonic materials can be used to good effect in nanophotonics.

In this paper, a novel structure of nano optical tweezers using triple-slit plasmonic grating structure is introduced and analyzed. The tweezers have deep potential wells that can trap sub-10 nm dielectric particle stably and efficiently. The resultant 50 KT potential well provides tight 2D trapping to the particle. The plasmonic device allows for steering the particle by simply changing the angle of the incident plane. This simple control allows efficient manipulation of the trapped particle over wide range of angles.

The phenomenon of extraordinary and multi-broadband optical transmission through sub-wavelength metallic grating with symmetry breaking has been theoretically investigated. Under normal incident light, the radiative and dark modes appear in adjacent slits of the grating with asymmetric heights. Through the destructive interference of alternative radiative and dark modes, multiple broadband transmission and enhanced light propagation is realized. The counter-propagating light circulation results in sharp dips in the transmission spectrum. These characteristics of the asymmetric grating could provide highly controllable ways to design novel devices.

We present an experimental study on a surface plasmon resonance (SPR) based fiber optic hydrogen gas sensor employing a palladium doped zinc oxide nanocomposite (ZnO_(1−x)Pd_x, 0 ≤ x ≤ 0.85) layer over the silver coated unclad core of the fiber. Palladium doped zinc oxide nanocomposites (ZnO_(1−x)Pd_x)  are prepared by a chemical route for different composition ratios and their structural, morphological and hydrogen sensing properties are investigated experimentally. The sensing principle involves the absorption of hydrogen gas by ZnO_(1−x)Pd_x, altering its dielectric function. The change in the dielectric constant is analyzed in terms of the red shift of the resonance wavelength in the visible region of the electromagnetic spectrum. To check the sensing capability of sensing probes fabricated with varying composition ratio (x) of nanocomposite, the SPR curves are recorded typically for 0% H₂ and 4% H₂ in N₂ atmosphere for each fabricated probe. On changing the concentration of hydrogen gas from 0% to 4%, the red shift in the SPR spectrum confirms the change in dielectric constant of ZnO_(1−x)Pd_x on exposure to hydrogen gas. It is noted that the shift in the SPR spectrum increases monotonically up to a certain fraction of Pd in zinc oxide, beyond which it starts decreasing. SEM images and the photoluminescence (PL) spectra reveal that Pd dopant atoms substitutionally incorporated into the ZnO lattice profoundly affect its defect levels; this is responsible for the optimal composition of ZnO_(1−x)Pd_x to sense the hydrogen gas. The sensor is highly selective to hydrogen gas and possesses high sensitivity. Since optical fiber sensing technology is employed along with the SPR technique, the present sensor is capable of remote sensing and online monitoring of hydrogen gas.

We study the optical properties associated with both the polariton gap and the Bragg gap in periodic resonator–waveguide coupled systems, based on the temporal coupled mode theory and the transfer matrix method. Using the complex band and the transmission spectrum, it is feasible to tune the interaction between multiple Bragg scattering and local resonance, which may give rise to analogous phenomena of electromagnetically induced transparency (EIT). We further design a plasmonic slot waveguide side-coupled with local plasmonic resonators to demonstrate the EIT-like effects in the near-infrared band. Numerical calculations show that realistic amounts of metal Joule loss may destroy the interference and the total absorption is enhanced in the transparency window due to the near zero group velocity of the guiding wave.

Optical loss induced by the arbitrary structure discontinuity in the Au stripe that supports long-range surface plasmon-polaritons (LRSPPs) propagation is investigated in this paper. A broad range of arbitrary gap sizes, 4 to 20 μm, is reported for 25 nm thick Au stripe with different widths embedded in 5 μm thick optically symmetric polymer SU-8. The simulations and experimental data find a high tunneling efficiency of the long-range mode power transmission of larger than 50% over a 10 μm gap at a wavelength of 1550 nm. Accordingly, a thermally activated in-line Mach–Zehnder interferometer switch based on the guiding of LRSPPs along Au stripe with gaps is designed and fabricated. Switching characteristics are analyzed upon heating one arm of the interferometer through the passage of current therein. The fabricated switch exhibits an extinction ratio of 17 dB with a driving power of 13 mW. The results prove that LRSPPs are insensitive to technological imperfections and waveguide interruptions, which is of benefit to active plasmonic device applications.

We show that the optical force exerted on a finite size chiral sphere by a surface plasmon mode has a component along a direction perpendicular to the plasmon linear momentum. We reveal how this chiral lateral force, pointing in opposite directions for opposite enantiomers, stems from an angular-to-linear crossed momentum transfer involving the plasmon transverse spin angular momentum density and mediated by the chirality of the sphere. Our multipolar approach allows us discussing the inclusion of the recoil term in the force on a small sphere taken in the dipolar limit and observing sign inversions of the lateral chiral force when the size of the sphere increases.

In this article, it has been theoretically shown that broad angle negative refraction is possible with asymmetric anisotropic metamaterials (AAMs) constructed by only dielectrics or lossless semiconductors at the telecommunication and relative wavelength range. Though natural uniaxial materials can exhibit negative refraction, the maximum angle of negative refraction and critical incident angle lie in a very narrow range. This problem can be overcome by our proposed structure. In our structures, negative refraction originates from the highly asymmetric elliptical iso-frequency. This is artificially created by the rotated multilayer sub-wavelength dielectric or semiconductor stack, which acts as an effective AAM. This negative refraction is achieved without using any negative permittivity materials such as metals. As we are using simple dielectrics, fabrication of such structures would be less complex than that of the metal based metamaterials. By considering the time harmonic field incidence, negative refraction has been demonstrated for two dimensional bi-dielectric structures for TM polarization with realistic parameters. Our proposed ideas have been validated by the full wave simulations considering both the effective medium approach and realistic structure model. This device might find some important applications in photonics and optoelectronics.

The tunnelling of a long-time (about 100 μs) frequency-modulated electromagnetic pulse through a one-dimensional active dielectric photonic crystal and the influence of the frequency modulation of the wavepacket on its time delay and peak velocity is theoretically investigated. The possibility of tuning the characteristics of the transmitted pulse using frequency modulation is demonstrated.

We investigate the localized surface modes in a structure consisting of the cholesteric liquid crystal layer, a phase plate, and a metal layer. These modes are analogous to the optical Tamm states. The nonreciprocal transmission of polarized light propagating in the forward and backward directions is established. It is demonstrated that the transmission spectrum can be controlled by external fields acting on the cholesteric liquid crystal and by varying the plane of polarization of the incident light.

An efficient sensitivity analysis approach for quantum nanostructures is proposed. The imaginary time propagation method (ITP) is utilized to solve the time dependent Schrödinger equation (TDSE). Using this method, an extraction of all the modes and their sensitivity with respect to all the design parameters have been performed with minimal computational effort. The sensitivity analysis is done using the adjoint variable method (AVM) and results are comparable to those obtained using central finite difference method (CFD) applied directly on the response level.

We demonstrate experimentally the feasibility of a two-state quantum bit commitment protocol, which is both concealing and partially binding, assuming technological limitations. The security of this protocol is based on the lack of long-term stable quantum memories. We use a polarization-encoding scheme and optical fiber as a quantum channel. The measurement probability for the commitment is obtained and the optimal cheating strategy demonstrated. The average success rates for an honest player in the case where the measurements are performed using equal bases are 93.4%, when the rectilinear basis is measured, and 96.7%, when the diagonal basis is measured. The rates for the case when the measurements are performed in different bases are 52.9%, when the rectilinear basis is measured, and 55.4% when the diagonal basis is measured. The average success rates for the optimal cheating strategy are 80% and 73.8%, which are way below the success rates of an honest player. Using a strict numerical validity criterion, we show that, for these experimental values, the protocol is secure.

We present a novel one-step calibration methodology for geometrical distortion correction for optical coherence tomography (OCT). A calibration standard especially designed for OCT is introduced, which consists of an array of inverse pyramidal structures. The use of multiple landmarks situated on four different height levels on the pyramids allow performing a 3D geometrical calibration. The calibration procedure itself is based on a parametric model of the OCT beam propagation. It is validated by experimental results and enables the reduction of systematic errors by more than one order of magnitude. In future, our results can improve OCT image reconstruction and interpretation for medical applications such as real time monitoring of surgery.

A rough non-uniform ZnSe thin film on a GaAs substrate is optically characterised using imaging spectroscopic reflectometry (ISR) in the visible, UV and near IR region, applied as a standalone technique. A global-local data processing algorithm is used to fit spectra from all pixels together and simultaneously determine maps of the local film thickness, roughness and overlayer thickness as well as spectral dependencies of film optical constants determined for the sample as a whole. The roughness of the film upper boundary is modelled using scalar diffraction theory (SDT), for which an improved calculation method is developed to process the large quantities of experimental data produced by ISR efficiently. This method avoids expensive operations by expressing the series obtained from SDT using a double recurrence relation and it is shown that it essentially eliminates the necessity for any speed–precision trade-offs in the SDT calculations. Comparison of characterisation results with the literature and other techniques shows the ability of multi-pixel processing to improve the stability and reliability of least-squares data fitting and demonstrates that standalone ISR, coupled with suitable data processing methods, is viable as a characterisation technique, even for thin films that are relatively far from ideal and require complex modelling.

We investigate the role of surface nanoscale topographies in the inhomogeneous coupling of laser radiation to Ni-based superalloy CMSX-4 surfaces and in the formation of laser-induced periodic surface structures. The initial surface arbitrary roughness is already able, upon laser exposure, to induce low-spatial-frequency and high-spatial-frequency structures, actively determining an interference of incoming and scattered fields resulting in spatial energy modulation. A topology variation with the incoming dose via the number of pulses determines a correlated evolution in the regular ripple arrangements. The scattering pattern is severely influenced by the scattering source geometries. Therefore, we equally study experimentally the role of one-dimensional nanoscale grooves in determining polarization-dependent structuring patterns at increasing irradiation dose. Finally, the role of surface nanostructures in generating the surface modulation of deposited energy is analyzed by finite-difference-time-domain simulation. Ripple formation in multi-pulses is a result of the feedback process between light and nanostructures.

We report the observation of intensity feedback random lasing at 645 nm in disperse orange 11 dye-doped PMMA (DO11/PMMA) with dispersed ZrO₂ nanoparticles (NPs). The lasing threshold is found to increase with concentration, with the lasing threshold for 0.1 wt% being 75.8 ± 9.4 MW cm⁻² and the lasing threshold for 0.5 wt% being 121.1 ± 2.1 MW cm⁻², with the linewidth for both concentrations found to be ≈10 nm. We also consider the material's photostability and find that it displays fully reversible photodegradation with the photostability and recovery rate being greater than previously observed for DO11/PMMA without NPs. This enhancement in photostability and recovery rate is found to be explicable by the modified correlated chromophore domain model, with the NPs resulting in the domain free energy advantage increasing from 0.29 eV to 0.41 eV. Additionally, the molecular decay and recovery rates are found to be in agreement with previous measurements of DO11/PMMA (Ramini et al 2013 Polym. Chem. 4 4938). These results present new avenues for the development of robust photodegradation-resistant organic dye-based optical devices.

We consider the possibility that the chirality parameters and the non-reciprocity parameters appearing in the constitutive relations for the displacement and magnetic induction fields in a bi-isotropic medium might not be equal and thereby shed light on the physical significance of the fact that they are the same. We find, in particular, that they must be equal in order to retain the local conservation of energy.

The regime of multiple filamentation of gigawatt-power femtosecond laser pulses in fused silica bars is theoretically investigated. Numerical simulations are used to analyze the fine spatial structure of the plasma region formed due to photoionization of silica and accompanying pulse filamentation. The dependence of the number, spatial position, and length of different generations of plasma channels on the energy and focusing conditions of the optical pulse is studied. The role of pulse sequential refocusing in the formation of the plasma region is discussed.

In this paper, we report on the preparation of graphene oxide and graphene oxide-Au nanodispersions in various solvents, such as water, DMF (N,N-dimethylformamide) and NMP (N-methyl-2-pyrrolidone). Optical, structural and nonlinear optical properties of all the samples have been studied. The nonlinear optical properties have been measured using the z-scan technique. It is shown that the incorporation of Au nanoparticles can greatly improve the nonlinear optical properties of graphene oxide. More importantly, the fact is recognized that the media that surround the nonlinear sample can influence its nonlinear optical properties by their nonlocal action. The nonlocal z-scan theory has been used to estimate the role of the surrounding medium in changing the samples' nonlinear responses.

We report the modulated Cerenkov up-conversion in a LiTaO₃ waveguide with an annular domain structure. As a result of the continuous rotational symmetry of such a structure, the phase velocity of the nonlinear polarization wave has a wide modulation tolerance. The reciprocal vectors which could be involved in the interaction had a threshold range associated with the waveguide parameters. The experimental results and simulations demonstrated that the radiation intensity had a close relationship with the overlap between the pump and the radiated wave, which could be decreased by non-collinear reciprocal vectors. Our detailed discussions on the Cerenkov frequency-doubling process using such an annular-poling crystal may lay the groundwork for the further study of the modulation of other nonlinear Cerenkov processes.

The nonlinear optical response of a material system contains detailed information about its electronic structure. Standard approaches to nonlinear spectroscopy often use multiple beams crossed in a sample, and detect the wave vector matched polarization in transmission. Here, we apply a phase-synchronous digital detection scheme using an excitation geometry with two phase-modulated collinear ultrafast pulses. This scheme can be used to efficiently detect nonlinear coherent signals and incoherent signals, such as higher harmonics and multiphoton fluorescence and photocurrent, from various systems including a photocell device. We present theory and experiment to demonstrate that when the phase of each laser pulse is modulated at the frequency ${\phi }_{1}$ and ${\phi }_{2},$ respectively, nonlinear signals can be isolated at the frequencies $n({\phi }_{2}-{\phi }_{1}),$ where $n=0,1,2,\ldots .$ This approach holds promise for performing nonlinear spectroscopic measurements under low-signal conditions.

We present a numerical study of the propagation dynamics of accelerating and decelerating truncated Airy pulses (TAPs) in the anomalous dispersion region of optical fibres, with inclusion of the Raman scattering effects, by analysing their cross-correlation frequency resolved optical gating traces. We identify the differences between the evolution dynamics of a decelerating and accelerating TAP. It is shown that the main lobe of the pulse is capable of shedding solitons, which are delayed due to the Raman effects. For the decelerating pulse, however, the soliton is dragged from the original pulse and never meets the input oscillatory Airy tail due to the deceleration of both the pulse and the soliton. The rest of the decelerating pulse rebuilds a new Airy waveform with a stronger degree of truncation compared with that of the incident pulse. For the accelerating TAP, the soliton collides continuously with the tail of the pulse and thus gains further energy by means of their nonlinear interaction. As a consequence, the remaining pulse cannot develop a new Airy waveform. In addition, under the same conditions, the Raman-induced frequency shift of the accelerating TAP is much larger compared with that of the decelerating one.

We present a monolithic high power narrow-linewidth linearly polarized nanosecond all-fiber laser in a master oscillator power amplifier configuration. The pulsed seed is generated by modulating a single-frequency continuous wave laser at 1064 nm using an electro-optic intensity modulator. In order to suppress stimulated Brillouin scattering (SBS), the pulse width is set to be ∼4 ns. When the repetition rate is set to be 20 MHz or 10 MHz, SBS has been observed in the main amplifier. The corresponding SBS thresholds are ∼100 W and 120 W respectively. At the repetition rate of 5 MHz, the pulse width is compressed to ∼3 ns and an average power of 136 W is obtained, corresponding to peak power of 8.5 kW. Further power scaling is limited by stimulated Raman scattering. At maximum output power, the beam quality (M² factor) is ∼1.1 and the polarization extinction ratio is >15.8 dB.

We propose and experimentally observe a novel family of Airy-like beams. First, we theoretically investigate the physical generation of our proposed controllable Airy-like beams by introducing a rotation angle factor into the phase function, which can regulate and flexibly control the beam wavefront. Meanwhile we can also readily control the main lobes of these beams to follow appointed parabolic trajectories using the rotation angle factor. We also demonstrate that the controllable Airy-like beams lack the properties of being diffraction-free and self-healing. The experiments are performed and the results are in accord with the theoretical simulations. We believe that the intriguing characteristics of our proposed Airy-like beams could provide more degrees of freedom, and are likely to give rise to new applications and lend versatility to the emerging field.

A passively mode-locked thulium-doped fiber laser (TDFL) based on a nonlinear amplifying loop mirror (NALM) is presented. By adjusting the polarization controllers, stable noise-like (NL) mode-locked femtosecond pulse operation is obtained at the 2 μm band. In the experimental period of 200 min, the output power fluctuation is less than 0.06 dB and the 3 dB spectral bandwidth variation is less than 0.02 nm, indicating that the pulsed TDFL possesses good long-term stability. To the best of our knowledge, this is the first 2 μm band NALM-based TDFL with small net anomalous dispersion for a NL femtosecond pulse. At the maximum pump power of 3.52 W, the emitting laser has a NL pulse width of 460 fs, the repetition rate of 9.1 MHz, and the NL pulse energy of 32.72 nJ.

Since the random edges of practically manufactured grating can be described by the self-affine fractal model, this paper investigates theoretically Fresnel diffraction of grating with rough edges on the basis of the self-affine fractal theory and discusses the variation of the Talbot image of grating with the rough parameters of edges. The amplitude gratings with different rough edges are produced with the help of the correlation function of the random distribution. Then, simulations of the diffraction intensity distributions of rough gratings are performed, and the modulation effect of speckles on Talbot image are shown. In order to explain the variation of the Talbot image of grating with rough edges, the theoretical analysis of the Talbot effect of grating with rough edges is given according to the statistic optics theory. The presented approximate analytic expression of the average diffraction intensity indicates the relationship between the diffraction and rough parameters of grating edges. The conclusions of this paper are useful for evaluating the Talbot image of practical grating.

Holographic optical tweezers provide a contactless way to trap and manipulate several microobjects independently in space using focused laser beams. Although the methods of fast and efficient generation of optical traps are well developed, their user friendly control still lags behind. Even though several attempts have appeared recently to exploit touch tablets, 2D cameras, or Kinect game consoles, they have not yet reached the level of natural human interface. Here we demonstrate a multi-modal 'natural user interface' approach that combines finger and gaze tracking with gesture and speech recognition. This allows us to select objects with an operator's gaze and voice, to trap the objects and control their positions via tracking of finger movement in space and to run semi-automatic procedures such as acquisition of Raman spectra from preselected objects. This approach takes advantage of the power of human processing of images together with smooth control of human fingertips and downscales these skills to control remotely the motion of microobjects at microscale in a natural way for the human operator.

Time-dependent Bragg diffraction by multilayer gratings working by reflection or by transmission is investigated. The study is performed by generalizing the time-dependent coupled-wave theory previously developed for one-dimensional photonic crystals (André J-M and Jonnard P 2015 J. Opt.17 085609) and also by extending the Takagi–Taupin approach of the dynamical theory of diffraction. The indicial response is calculated. It presents a time delay with a transient time that is a function of the extinction length for reflection geometry and of the extinction length combined with the thickness of the grating for transmission geometry.

Based on the cross-correlation function, a method for determining the azimuthal mode index (topological charge) ${\ell }$ of a partially coherent vortex beam, assuming the beam possesses a radial index p = 0, has recently been proposed. Here, we show a new kind of spatial correlation function, i.e., the mutual coherence function of two points (x, y) and (2x, 2y), which we call the double-correlation function. Unlike the cross-correlation singularity, which depends on both radial and azimuthal mode indices, the double-correlation singularity is dependent on the radial mode index only. Based on this property, we propose a method for determining the azimuthal and radial mode indices of a partially coherent beam with a nonzero radial mode index, to our knowledge, for the first time.

Recently, we have indirectly demonstrated that nanostructure reconstruction accuracy is degraded by the outliers in optical scatterometry, and we have applied the robust estimation method to suppress these outliers. However, the existence of a possible heavy masking effect could result in the risk of low measurement accuracy, since the detection of outliers is simply based on the judgment of residual value. In this work, a novel method is introduced to directly detect outliers, which can provide the intuitional display of outliers in a two-dimensional coordinate system. Moreover, a robust correction step based on the principle of least trimmed squared estimator regression is proposed to replace the conventional Gauss–Newton iteration step, by which the more reliable and accurate nanostructure reconstruction is achieved. The improved reconstruction of a one-dimensional etched Si grating has demonstrated the feasibility of the proposed methods.

We introduce a new kind of partially coherent beam, non-uniformly Hermite–Gaussian correlated beam, by employing a non-uniformly Hermite function to modulate the spectral degree of coherence. The evolution of such scalar beam on propagation in free space and turbulent atmosphere are investigated. It is demonstrated that the spectral intensity distributions exhibit extraordinary propagation characteristics, such as self-focusing and laterally shifted intensity maxima. The position of the maximum intensity and the intensity profile can be controlled by the order of the Hermite function. The results can be useful in free-space optical communications and beam shaping.

When a collimated light beam is passed consequently along the optic axes of two identical biaxial crystals, the conical refraction produces in the focal image plane a specific light pattern consisting of a ring and a central spot. The ring is formed due to the additive action of two crystals, while the spot results from the reversed conical refraction in such a degenerated cascade arrangement. The relative intensity of these two components depends on the azimuth angle between the orientations of the crystals about the beam axis. It is shown that this dependence arises due to the interference of pairs of waves produced by conical refraction in two crystals. If a part of these waves is blocked by polarization selection of beam components, the dependence of the light pattern on the azimuth angle vanishes. In this case, the outgoing light profile consists of a ring and a central spot with fixed intensities so that the total beam power is divided equally between these two components. Depending on the applied polarization, the central spot appears either as a restored input beam or a charge-two optical vortex. The results of numerical simulations of the effect are in a very good agreement with the experimental observations.

A single-beam phase retrieval method with partially coherent illumination is proposed. By using an obverse and reverse iterative (ORI) algorithm, objects can be reconstructed within less time by recording a sequence of diffraction patterns at different axial planes under partially coherent light illumination. Partially coherent light illumination reduces coherent noise and the number of diffraction patterns needed for reconstruction. Thus, the whole process is fast and has high immunity to external perturbation due to the reference-less configuration. Both simulation and experimental results are presented to demonstrate the feasibility of the proposed approach.

We show the inline selection of transmission or reflection spectrum of one of two fiber Bragg gratings (FBGs) with different Bragg wavelengths by incorporating a polarization-diversity loop without reconfiguring the filter structure. The proposed filtering apparatus consists of a fiber-pigtailed polarization beam splitter, two FBGs, and three quarter-wave plates (QWPs). Without optical switches and couplers, the proposed filter can flexibly choose the transmission or reflection spectrum of each FBG through proper control of the QWPs contained in the filter. The fabricated filter shows an average insertion loss of ∼4.43 dB, average band rejection ratio of ∼17.92 dB, and average side-mode suppression ratio of ∼19.73 dB.

Computational ghost diffraction (CGD) with a higher-order cosh-Gaussian modulated incoherent source is investigated theoretically. The corresponding numerical simulations are given to see clearly the effects of the parameters of the higher-order cosh-Gaussian source on the imaging quality. Our results show that the resolution of the CGD patterns can be significantly improved by properly varying the source parameters. In addition, we numerically study the effect of the propagation distances in the CGD system and explore the CGD applicability in coherent diffraction imaging. These results may be helpful for implementation of high-resolution x-ray diffraction.

An approach to implement photonic-assisted time-interleaved analog-to-digital conversion and its calibration method are presented. The analog modulated optical signal is divided into M channels, suffering different time delay induced by optical delay lines which provide great flexibility in producing time intervals and is then sampled by electronic analog-to-digital converters (ADCs). The channel mismatches resulting in performance degradation are estimated by a modified sine wave fitting method. The time mismatch and other mismatches are corrected by fine optical delay adjustment and digital processing, respectively. A four-channel photonic-assisted time-interleaved analog-to-digital converter (TIADC) system operating at 40 GSa s⁻¹ was demonstrated experimentally. The photonic-assisted TIADC system was tested with a 6.31 GHz sine wave signal, exhibiting 40.3 dB signal-to-noise and distortion ratio (SINAD) and 57.6 dBc spurious-free dynamic range (SFDR). It is shown that the SINAD is dominated by the signal-to-noise ratio (SNR) of the analog optical link and the SFDR of the proposed system is limited by the linearity of the link.

We present and analyze a one-port sensor based on a single diffraction grating delineated over a planar optical waveguide. Distinct to previously reported devices, the grating we use here is used not only as I/O coupler, but also provides a built-in reference beam that is basically unaffected by the sensing process. The sensing process causes two effects simultaneously: a change in the angle of the out-coupled beam and a change in the phase accumulated by that beam. Both changes can be determined by their conjunction with the reference beam back-diffracted directly by the grating. These two effects are expected to have despair sensitivities, the angle changing effect being coarse and the interferometric phase-change effect being highly sensitive. Sensing simultaneously at two different scales enlarges to a great extent the sensing dynamic range. Theoretical analysis and simulations of a specific implementation example of the device are presented.

A dual-parameter sensor based on a fiber-optic interferometer consisting of a non-adiabatic fiber taper and a long-period fiber grating (LPFG) integrated with magnetic nanoparticle fluids has been proposed and experimentally demonstrated. Due to the Mach–Zehnder interference induced by the concatenation of the fiber taper and long-period grating, an interferometric spectrum could be acquired within the transmission resonance spectral envelope of the LPFG. Thanks to different magnetic field and temperature sensitivities of difference interference dips, simultaneous measurement of the magnetic field intensity and environmental temperature could be achieved. Moreover, due to the variation in coupling coefficients of the fiber taper and the LPFG in response to the change of the applied magnetic field intensity, some of the interference dips would exhibit opposite magnetic-field-intensity-dependent transmission loss variation behavior. Magnetic field intensity and temperature sensitivities of 0.017 31 dB Oe⁻¹ and 0.0315 dB K⁻¹, and −0.024 55 dB Oe⁻¹ and −0.056 28 dB K⁻¹ were experimentally acquired for the experimentally monitored interference dips.

Efficiency enhancement of a hydrogenated amorphous-silicon carbide (a-SiC:H) solar cell using downshifting and upconversion of photovoltaic (PV) glasses doped with transition metal (TM) ions and rare earth (RE) ions are investigated. P₂O₅-Li₂O-Al₂O₃-Sb₂O₃-MnO-Yb₂O₃-Er₂O₃ glass doped with Sb³⁺-Mn²⁺-Yb³⁺-Er³⁺ ions is prepared and the PV glass is placed on an a-SiC:H solar cell. The performance of the cell in combination with the PV glass is simulated and measured, and the results show that the theoretical and experimental efficiencies are both enhanced compared to the bare one. The potential of TM-RE quad-doped glasses for improving the efficiency of a-SiC:H PV modules are explored.

Metallic nanoparticles (NPs) have not been effective in improving the overall performance of the cells with micrometer-thick absorbing layers mainly due to the parasitic optical dissipation in the metal. Here, using both experiment and theory, we demonstrate that aggregates of metallic NPs enhance the light absorption of dye-sensitized solar cells of a few micrometer-thick light absorbing layers. The composite electrode containing the optimal concentration of 5 wt% Au@SiO₂ aggregates shows the enhancement of 80% and 52% in external quantum efficiency and photocurrent density, respectively. The superior performance of the aggregates relative to NP is attributed to their larger scattering efficiency using full-wave optical simulations. This is further confirmed by optical spectroscopic measurements showing that a large fraction of the incident light couples into the diffused components because of the presence of these metallic aggregates. The optical absorption enhancement is broadband and it is particularly strong at wavelengths larger than 680 nm where the optical absorption of dye molecules is weak.

Conventional c-Si solar cells employ micron-sized pyramids for achieving reduced reflection (∼10%) and enhanced light trapping by multiple bounces (maximum 3) of the incident light. Alternatively, bio-mimetic, moth-eye sub-wavelength nanostructures offer broadband antireflection properties (∼3%) suitable for solar cell applications in the optical regime. However, such structures do not provide any advantage in the charge carrier extraction process as radial junctions cannot be formed in such sub-wavelength dimensions and they have high surface area causing increased charged carrier recombination. The choice of the geometry for achieving optimum photon–electron harvesting for solar applications is therefore very critical. Cross-fertilization of the conventional solar cell light-trapping techniques and the sub-wavelength nanostructures results in unique micro-nanostructures (structures having sub-wavelength dimensions as well as dimensions of the order of few microns) which provide advanced light management capabilities along with the ability of realizing radial junctions. It is seen that an ultralow reflection along with wide angle light collection is obtained which enables such structures to overcome the morning, evening and winter light losses in solar cells. Further, super-scattering in the structures offer enhanced light trapping not only in the structure itself but also in the substrate housing the structure. Ray and wave optics have been used to understand the optical benefits of the structures. It is seen that the aspect ratio of the structures plays the most significant role for achieving such light management capabilities, and efficiencies as high as 12% can be attained. Experiments have been carried out to fabricate a unique micro-nanomaze-like structure instead of a periodic array of micro-nanostructures with the help of nanosphere lithography and the MacEtch technique. It is seen that randomized micro-nanomaze geometry offers very good antireflection properties (∼1%) which are close to the expected optical behaviour of the periodic array at a much lower surface area.

We derive planar permittivity profiles that do not reflect perpendicularly exiting radiation of any frequency. The materials obey the Kramers–Kronig relations and have no regions of gain. Reduction of the Casimir force by means of such materials is also discussed.