Table of contents

Mode division multiplexing (MDM), where information is transmitted in the spatial modes of light, is mooted as a future technology with which to transmit large bits of information. However, one of the key issues in optical communication lies in connecting free-space to optical fiber networks, otherwise known as the 'last mile' problem. This is particularly problematic for MDM as the eigenmodes of free-space and fibers are in general not the same. Here we demonstrate a data transmission scheme across a free-space and fiber link using twisted light in the form of Laguerre–Gaussian (LG) azimuthal modes. As a proof-of-principle we design and implement a custom fiber where the supported LG modes can be grouped into five non-degenerate sets, and successfully transmit a gray-scale image across the composite link using one mode from each group, thereby ensuring minimal crosstalk.

Due to the increasing demand on high-efficiency organic photovoltaic (OPV) devices, light management technique has become an active research subject. Especially, plasmonic approach was proven to be suitable for application in OPV and has shown lots of successful results. In this review, we summarize recent studies on plasmonic nanostructures for OPV with their underlying enhancement mechanisms. Optical absorption enhancement by the resonant scattering and the strong plasmonic near field will be discussed for various implementation geometries including metal nanoparticles, patterned electrodes, and plasmonic metamaterials. In addition, we will also look into the electrical effects originating from plasmonic nanostructures, which inevitably affect the device's efficiency. Future research directions will be also discussed.

We investigate the dispersion relations of TE resonances in different graphene-dielectric structures. Previous work has shown that when a graphene layer is brought into contact with a dielectric material, a gap can appear in its electric band structure. This allows for the formation of TE-plasmons with unusual dispersion relations. In addition, if the dielectric has a finite thickness, graphene strongly modifies the behavior of the waveguiding modes by introducing dissipation above a well-defined cutoff frequency, thus providing the possibility of mode filtering. This cutoff and the properties of TE-plasmons are closely related to the pair-creation threshold of graphene, thus representing quantum mechanical effects that manifest themselves in the electromagnetic response.

We show that the ability to make direct measurements of momentum, in addition to the usual direct measurements of position, allows a simple configuration of two identical mechanical oscillators to be used for broadband back-action-free force metrology. This would eliminate the need for an optical reference oscillator in the scheme of Tsang and Caves (2010 Phys. Rev. Lett.  105 123601), along with its associated disadvantages. We also show that if one is restricted to position measurements alone then two copies of the same two-oscillator configuration can be used for narrow-band back-action-free force metrology.

The C-points in a space-variant optical field are polarization singularities possessing pure circular polarization and surrounded by an elliptically polarized environment. Depending on the rule of the alignment of the polarization ellipses around the C-point, three types of topological structures are considered in the forms Lemon, Star and Monstar. While the origin of Star and Lemon is related to the isotropic optical vortex in a circularly polarized component of a space-variant field, the Monstar-type C-point requires special conditions to appear. The conditions of the existence and the borders of a Monstar area are found analytically. Topological events of Monstar-form transformation to the Lemon form are inspected.

Transmission line formulation is used to analyze two-dimensional low-loss metallic gratings at optical frequencies when plasmonic waves propagate in the structure. This method, like the Fourier modal method, suffers from numerical instabilities when applied to such structures. A systematic approach to avoid these instabilities is presented. These numerical artifacts are attributed to the violation of Li's inverse rule and the appearance of higher-order spurious modes. In this paper, a new approach is proposed to identify and to greatly reduce the effect of these spurious modes based on the accuracy by which these modes are satisfying the conservation of momentum. Furthermore, the proposed scheme conserves power and improves the convergence, i.e., reduces the required truncation order.

We designed all-silicon, multi-featured band-selective perfect absorbing surfaces based on CMOS compatible processes. The center wavelength of the band-selective absorber can be varied between 2 and 22 μm while a bandwidth as high as 2.5 μm is demonstrated. We used a silicon-on-insulator (SOI) wafer which consists of n-type silicon (Si) device layer, silicon dioxide (SiO₂) as buried oxide layer, and n-type Si handle layer. The center wavelength and bandwidth can be tuned by adjusting the conductivity of the Si device and handle layers as well as the thicknesses of the device and buried oxide layers. We demonstrate proof-of-concept absorber surfaces experimentally. Such absorber surfaces are easy to microfabricate because the absorbers do not require elaborate microfabrication steps such as patterning. Due to the structural simplicity, low-cost fabrication, wide spectrum range of operation, and band properties of the perfect absorber, the proposed multi-featured perfect absorber surfaces are promising for many applications. These include sensing devices, surface enhanced infrared absorption applications, solar cells, meta-materials, frequency selective sensors and modulators.

We propose an analytical model of quasi-normal mode (QNM) for resonant plasmonic nano cavities formed by sub wavelength grooves in metallic substrate. The QNM has shown great advantages in understanding and calculating the frequency response of resonant nano-structures. With our model we show that the QNM originates from a resonance of the fundamental mode in every individual groove and its interaction via surface waves. Analytical expression for the complex eigenfrequency as well as the field distribution of the QNM can be derived from the model. With the analytical model and a few assumptions on the scattered field, the legitimacy of the expansion of scattered field with QNMs under external illuminations is justified with the Mittag–Leffler's theorem of meromorphic function. The expansion coefficients of QNMs are analytically expressed with a finite set of elementary scattering coefficients, which avoids the calculation of the mode volume of QNMs that have a spatial divergence at infinity. The model clarifies the physical origin of QNMs and drastically reduces the computational load of QNMs, especially for a large ensemble of grooves for which brute-force numerical tools are not available. The validity of the proposed model is tested against fully vectorial numerical results.

A Fabry–Perot model is proposed to analyze the resonance behaviors of a metal–dielectric–metal waveguide with a rectangular side-coupled cavity that has a horizontal width below one wavelength. Two vertically propagating waveguide modes in the cavity are introduced in the model, and the vertical resonances in the cavity are quantitatively identified by two phase-matching conditions derived from the model. Thus other resonances from the prediction of the phase-matching conditions should be attributed to resonances of horizontally propagating modes in the cavity. These discussions can also give an explanation for the EIT-like transmission characteristics of such a structure. The present analysis provides helpful insight for the design of relevant devices that employ different types of resonances.

We investigate the optical response of square arrays of metallic nanoparticles where each lattice site is occupied by two particles, a dimer. In particular we examine the surface lattice resonances arising in these structures when the in-plane dipole moments associated with the plasmon modes of the nanoparticles couple together. The addition of a second particle to the basis leads to a more complex optical response, one that is anisotropic in the plane of the array. Extinction spectra are recorded at normal incidence for different orientations of the incident electric field. We compare our experimentally derived data with those from a coupled-dipole model. We show how the separation between the particles that comprise the dimer helps determine the overall response of the system.

We propose a highly sensitive curvature sensor based on cascaded single mode fiber (SMF) tapers with a microcavity. The microcavity is created by splicing a small piece of hollow core photonic crystal fiber (HCPCF) at the end of an SMF to obtain a sharp interference pattern. Experimental results show that two SMF tapers enhance the curvature sensitivity of the system and by changing the tapering parameters of the second taper, the curvature sensitivity of the system can be tailored, together with the fringe contrast of the interference pattern. A maximum curvature sensitivity of 10.4 dB/m⁻¹ is observed in the curvature range 0 to 1 m⁻¹ for a second taper diameter of 18 μm. The sensing setup is highly stable and shows very low temperature sensitivity. As the interrogation is intensity based, a low cost optical power meter can be utilized to determine the curvature.

A series of symmetrical nanoantennas with a symmetrical axis orientation angle of 45° or 135°, which are suitable for both X/Y linear and circular polarizations incidences simultaneously, have been designed and investigated in detail. We have deduced the transmitted matrix of the metasurface structure by rigorous mathematical theory, and found that the essential reason for the polarization-independence characteristics is that there are the same transmitted amplitudes and phases under the incidences of X/Y linear and circular polarization lights due to metasurface structure with the symmetrical axis's orientation angles of 45° or 135°. Based on the V-shaped, C-shaped, U-shaped and elliptical slit nanoantennas, we have verified the proposed theory fully by numerical simulations. The independence of the incident polarizations is very important for the practical applications and developments of the metasurfaces.

We studied the scattering and absorption of the H-polarized plane wave by a graphene-covered circular dielectric cylinder and a hollow tube with a graphene cover on the outer boundary in the terahertz range. The analytical solution of the wave-scattering problem was based on the Maxwell equations and the use of the separation of variables in the polar coordinates. We assumed the resistive boundary conditions on the zero-thickness graphene cover where the graphene electron conductivity was included as a parameter and was determined from the Kubo formalism. The computed spectra of the total scattering cross section and the absorption cross section displayed several types of resonances: the localized-surface-plasmon (LSP) resonances of the graphene cover and the resonances on the whispering-gallery modes of a dielectric cylinder or dielectric tube. The computed data can be useful for the design of graphene-based sensors of the changes in the refractive index of the host medium. For this purpose, the sensitivities and the figure-of-merit values of the LSP resonances were computed; their comparison demonstrated advantages of a thin-tube configuration.

The exciton–polariton propagation in resonant hybrid periodic stacks of isotropic/anisotropic layers, with misaligned in-plane anisotropy and Bragg photon frequency in resonance with Wannier exciton of 2D quantum wells is studied by self-consistent theory and in the effective mass approximation. The optical tailoring of this new class of resonant Bragg reflectors, where the structural periodicity of a multi-layer drives the in-plane optical $\hat{C}$-axis orientation, is computed for symmetric and asymmetric elementary cells by conserving strong radiation–matter coupling and photonic band-gaps. The optical response computation, on a finite cluster of N-asymmetric elementary cells, shows anomalous exciton–polariton propagation and absorbance properties strongly dependent on the incident wave polarizations. Finally, the behaviour of the so-called intermediate dispersion curves, close to the unperturbed exciton resonance, and located between upper and lower branches of the first band gap, is studied as a function of the in-plane $\hat{C}$-axis orientation. This latter optical property is promising for storing exciton–polariton impulses in this kind of Bragg reflector.

A theoretical investigation of the modulation instability (MI) in the three core triangular oppositely directed coupler with negative index material channel is presented. This class of couplers have an effective feedback mechanism due to the opposite directionality of the phase velocities in the negative and positive index channels. It is found that the MI in the nonlinear three core triangular oppositely directed coupler is significantly influenced by the ratio of the forward- to backward-propagating wave power and nonlinearity. Also, in the case of the normal dispersion regime a threshold-like behavior is observed, whereas this behavior is not identified in the anomalous dispersion regime. For the asymmetric case ($h\ne 1$), two pairs of instability bands are observed for both the nonlinear NIM and PIM channels, while a single pair of instability bands is noted for the symmetric case (h = 1). In the normal dispersion regime, the defocusing nonlinearity is found to suppress the MI by reducing both the gain and width of the instability band, whereas the MI is enhanced in the anomalous dispersion regime due to the defocusing nonlinearity. Thus we report new ways to generate and manipulate the MI and solitons in three-core triangular oppositely directed couplers with a particular emphasis on a negative-index material (NIM) channel.

Diffraction-free propagation of light has been demonstrated in free space for Bessel-like beams and for arbitrary beams in specially designed photonic crystals and metamaterials. The phenomenon is called self-collimation in photonic crystals and canalization in metamaterials, as the approaches to obtaining the effect are different. In both cases, however, diffraction-free propagation of light is achieved by making the dispersion surface of the material at a given frequency flat. In photonic crystals this is done by tuning the unit-cell dimensions close to the band-gap regime, and in metamaterials by tuning a hyperbolic-type metamaterial towards its transition to an ordinary elliptical metamaterial. In this work, we propose an alternative way to suppress optical diffraction in a metamaterial by adjusting the anisotropy of the finite-sized three-dimensional metamolecules and the material's spatial dispersion. The approach allows matching the wave impedance of the material to that of the surrounding medium in a wide range of incidence angles and thereby also suppressing optical reflection from the material's surface.

The gain competition is reported in an orthogonally linearly polarized (OLP) Nd:YVO₄ laser. Two a–cut Nd:YVO₄ crystals with their c–axes mutually orthogonal are employed as the gain medium. The wave equations for the gain-coupled polarized modes are derived and solved. A 32.7 W OLP laser was obtained and the output performances of the OLP laser are investigated experimentally and theoretically. The theoretical results agree well with the experimental ones, which indicates that the gain competition mechanism affects the output performance of this type of OLP laser significantly.

We had recently reported unique random laser action such as quasi-single-mode and low-threshold lasing from a submicrometre-sized spherical ZnO nanoparticle film with polymer particles as defects. The present study demonstrates a novel approach to realize single-mode random lasing by adjusting the sizes of the defect particles. From the dependence of random lasing properties on defect size, we find that the average number of lasing peaks can be modified by the defect size, while other lasing properties such as lasing wavelengths and thresholds remain unchanged. These results suggest that lasing wavelengths and thresholds are determined by the resonant properties of the surrounding scatterers, while the defect size stochastically determines the number of lasing peaks. Therefore, if we optimize the sizes of the defects and scatterers, we can intentionally induce single-mode lasing even in a random structure (Fujiwara et al 2013 Appl. Phys. Lett.102 061110).

Photometric stereo is an established three-dimensional (3D) imaging technique for estimating surface shape and reflectivity using multiple images of a scene taken from the same viewpoint but subject to different illumination directions. Importantly, this technique requires the scene to remain static during image acquisition otherwise pixel-matching errors can introduce significant errors in the reconstructed image. In this work, we demonstrate a modified photometric stereo system with perfect pixel-registration, capable of reconstructing 3D images of scenes exhibiting dynamic behavior in real-time. Performing high-speed structured illumination of a scene and sensing the reflected light with four spatially-separated, single-pixel detectors, our system reconstructs continuous real-time 3D video at ∼8 frames per second for image resolutions of 64 × 64 pixels. Moreover, since this approach does not use a pixelated camera sensor, it can be readily extended to other wavelengths, such as the infrared, where camera technology is expensive.

We combine single- and two-photon interference procedures for characterizing any multi-port linear optical interferometer accurately and precisely. Accuracy is achieved by estimating and correcting systematic errors that arise due to spatiotemporal and polarization mode mismatch. Enhanced accuracy and precision are attained by fitting experimental coincidence data to curve simulated using measured source spectra. We employ bootstrapping statistics to quantify the resultant degree of precision. A scattershot approach is devised to effect a reduction in the experimental time required to characterize the interferometer. The efficacy of our characterization procedure is verified by numerical simulations.

For the capture and sorting of multiple cells, a sensitive and highly efficient polarization-controlled three-beam interference set-up has been developed. With the theory of superposition of three beams, simulations on the influence of polarization angle upon the intensity distribution and the laser gradient force change with different polarization angles have been carried out. By controlling the polarization angle of the beams, various intensity distributions and different sizes of dots are obtained. We have experimentally observed multiple optical tweezers and the sorting of cells with different polarization angles, which are in accordance with the theoretical analysis. The experimental results have shown that the polarization angle affects the shapes and feature sizes of the interference patterns and the trapping force.

We present a semiclassical theory of the linear and nonlinear optical response of graphene. The emphasis is placed on the nonlinear optical response of graphene from the standpoint of the underlying chiral symmetry. The Bloch quasiparticles in the low-energy limit around the degeneracy points are dominantly chiral. It is shown that this chiral behavior in conjunction with scale invariance in graphene around the Dirac points results in the strong nonlinear optical response. Explicit expressions for the linear and nonlinear conductivity tensors are derived based on semiconductor Bloch equations (SBEs). The linear terms agree with the result of Kubo formulation. The three main additive mechanisms contribute in the nonlinear optical response of graphene: pure intraband, pure interband and the interplay between them. For each contribution, an explicit response function is derived. The Kerr-type nonlinearity of graphene is then numerically studied and it is demonstrated that the nonlinear refractive index of graphene can be tuned and enhanced by applying a gate voltage. It is also discussed that a strong Kerr nonlinearity can be achieved in a gated graphene monolayer. However, this nonlinearity is accompanied with a significant amount of absorption loss.

The second harmonic generation (SHG) response was measured for arbitrarily oriented linear input polarization on Si(111) surfaces in rotational anisotropy experiments. We show for the first time, using the simplified bond hyperpolarizability model (SBHM), that the observed angular shifts of the nonlinear peaks and symmetry features—related to changes in the input polarization—help to identify the corresponding interface dipolar and bulk quadrupolar SHG sources, yielding excellent agreement with the experiment. Additionally, we evaluate for the s-in/p-out (sp) and p-in/p-out (pp)-polarization SHG intensities the contributions from the individual Si bonds. Furthermore, a relation between the four parameters arising from SBHM and six coefficients of the phenomenological SHG theory needed to reproduce experimental data is established.

We investigated the feasibility of using a WS₂-deposited side-polished fiber as a harmonic mode-locker to produce a femtosecond fiber laser with a frequency of 1.51 GHz. Our work focuses on using a side-polished fiber platform with non-uniform WS₂ particles prepared through liquid phase exfoliation method without centrifugation. Femtosecond optical pulses were generated from an all-fiberized erbium-doped fiber-based ring cavity by increasing the pump power to achieve a tunable pulse repetition rate from 14.57 MHz to 1.51 GHz (104th harmonic). The characteristics of the output pulse were systematically investigated to analyze the pulse repetition rate, harmonic order, average output power, pulse energy, and pulse width as a function of the pump power. The output performance of the laser was compared to that of a laser based on a microfiber-based WS₂ film SA described in (Yan et al 2015 Opt. Mater. Express5 479–89). This experimental demonstration reaffirms that a side-polished fiber is an effective platform to implement an ultrafast harmonic mode-locker, and non-uniform WS₂ particles prepared via simple liquid phase exfoliation method without centrifugation provide a suitable saturable absorption response at 1.55 μm.

We propose and numerically verify a novel scheme of frequency-shift free optical phase conjugation by counter-propagating dual pump four-wave mixing in nonlinear fiber. The two counter-propagating pumps create a Bragg grating inside the fiber, which diffracts the forward propagating signal and generates a backward propagating idler wave whose phase is conjugate of signal phase. The two pump frequencies are placed symmetrically about signal frequency to ensure that idler wave will have same frequency as that of signal wave. Since the signal and idler waves appear at opposite ends, the idler is easily filtered out from the rest of the spectrum. Using nonlinear Schrödinger equation, we derive equations of signal and idler evolution. We obtain expressions for idler phase and show that perfect phase conjugation is achieved at an optimum length of fiber for a given pump power. We study the effect of fiber length and pump power on phase conjugation. Simulation results show the perfect phase conjugation at optimum fiber length under lossless conditions and small phase-offset when fiber loss and self and cross phase modulations are included. The small phase-offset is avoided by choosing fiber length smaller than optimum fiber length. Simulation results exhibit close agreement to theoretical values, which validates our simulations.

We numerically and experimentally generate a type of autofocusing beam that is combined from multiple Bessel-like beams. The beams are combined from two- and four-component Bessel-like beams. We demonstrate that the intensity of these beams can suddenly increase by orders of magnitude right before the target. With infinite components, the combined beams have a smaller focal spot, a longer focal length, and can morph into nondiffracting Bessel beams in their far field. Through numerical simulation comparisons of the propagation dynamics of these beams with those of other combined beams, it can be seen that these beams exhibit better focusing features.

The optical Talbot effect has been used to explore the topological charges of optical vortices. We recorded the self-imaging of a diffraction grating in the near-field regime with the optical vortex of several topological charges. Our twisted light was generated by a spatial light modulator (SLM). Previous studies showed that interferometric methods can determine the particular orbital angular momentum (OAM) states, but a large number of OAM eigenvalues are difficult to distinguish from the interference patterns. Here, we show that the Talbot patterns can distinguish the charges as well as the OAM of the vortices with high orders. Owing to the high sensitivity and self-imaging of the Talbot effect, several OAM eigenvalues can be distinguished by direct measurement. We check the experimental results with our theory. The present results are useful for classical and quantum metrology as well as future implementations of quantum communications.

In this paper we propose a photonic architecture as an alternative tool to distribute point to multipoint analog and digital information over a hybrid wireless visible optical communication system. The experimental set-up is composed of a red laser pointer, an acousto-optic modulator, a sinusoidal grating and a photo-detector array. By using a simple and variable interferometric system, diffraction gratings with different spatial frequencies are generated and recorded on a photoemulsion which is composed of vanilla with dichromate gelatin. Analog video and digital information are first transmitted and recovered over a wireless communication system using a microwave carrier at 4.52 GHz which is generated by distributed feedback lasers operating in the low laser threshold current region. Separately, the recovered video information and digital data are combined with a radio frequency signal of 80 MHz, obtaining a subcarrier of information that is imposed on the optical carrier of the pointer laser using an acousto-optic modulator which is operated with an angle of incident light that satisfies the Bragg condition. The modulated optical carrier is sent to a sinusoidal grating, the diffraction pattern is photo-detected using an array of PIN photo-detectors. The use of sinusoidal gratings with acousto-optic modulators allows that number of channels to be increased when both components are placed in cascade.

An attack-free four random phase mask cryptosystem is breached in the paper. The decryption key of the system can be easily accessed by the opponent by using a new type of powerful three-dimensional phase retrieval method. Our result, to the best of our knowledge, is the first work to show this security flaw in the attack-free four random phase mask cryptosystem. Meanwhile, a set of numerical simulations is provided to demonstrate the robustness of the presented method.

We propose to use a super-structured waveplate (called an S-waveplate) for vectorial optical vortex filtering, and experimentally demonstrate the radial Hilbert transform and selective edge enhancement. Based on the Jones calculus of polarization states and Fourier analysis, we calculate and analyze the point spread function of an optical 4-f system including an S-waveplate filter having the vectorial vortex of topological charge 1 (TC = 1). Numerical simulations and optical experiments demonstrate that a vectorial optical vortex filter can be used to implement selective edge enhancement with an analyzer before the output plane. The edge enhancement can be obtained even when the center of the filter is off-axis or the illuminating light is non-monochromatic.

The properties of the guided modes, including the single-mode conditions and the coupling of different polarized modes in the single-crystal lithium niobate photonic wires, were analyzed in detail. One-dimensional photonic crystal micro-cavities with several different patterns, which could be used as an ultra-compact optical filter, were designed and simulated in order to get high transmission at the resonant wavelength and the best preferment. The designed structure, with the whole size of 6.5 × 0.7 μm², was fabricated on a single-mode photonic wire. A measured peak transmission of 0.34 at 1400 nm, an extinction ratio of 12.5 dB and a Q factor of 156 were obtained. The measured transmission spectrum was basically consistent with the simulation, although a slight shift of resonant wavelength occurred due to the fabrication errors.