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Topological photonics has recently opened up a promising frontier for electromagnetic wave and light manipulation and has made great progress from unique physical concepts to novel practical photonic devices. Numerous works have discussed the realizations of chiral topological photonic states in magnetic photonic crystals with broken time-reversal symmetry; however, limited reports have been discussed to the achievements of antichiral topological photonic states. In this Perspective, we review recent progress in antichiral topological photonic states in magnetic photonic systems for the basic concepts, properties, and applications. Additionally, we provide an outlook for emerging frontier topics, promising opportunities, fundamental challenges, and potential applications for antichiral magnetic topological photonics. [ABSTRACT FROM AUTHOR]

We establish an effective and efficient theoretical approach called the spatial–temporal broadband nonlinear coupled wave theory (ST-BNCWT) to evaluate the spatial and temporal evolution behaviors of femtosecond laser pulse second harmonic generation (SHG) in a nonlinear medium. In this method, all the frequency components comprising the fundamental-wave laser pulse participate in a series of complicated three-wave mixing nonlinear coupling, including sum-frequency generation, difference-frequency generation, and the usual SHG, to create a second harmonic wave pulse. The contribution from each three-wave mixing process is strongly influenced by the corresponding phase matching or mismatching. We have used this method to analyze systematical transmission evolution characteristics of ultrashort laser pulses in a β-barium borate crystal and disclose the variation of a number of critical physical quantities. Our ST-BNCWT methodology can greatly facilitate the deep understanding of the SHG and various pulse transmission characteristics of ultrashort laser pulses and provide great support and guidance for experimental prediction. Moreover, our scheme opens up a promising path to explore and visualize novel nonlinear optical interactions in solid-state materials spanning the spatial and temporal domain, which are very helpful for building versatile ultrafast lasers in various spectral windows via powerful nonlinear frequency conversion technology against a basic high-performance ultrafast laser such as a Ti:sapphire femtosecond laser. [ABSTRACT FROM AUTHOR]

The reflectance-difference signal between s and p-polarized light when monitoring epitaxy growth of thin film was treated using a simplified model.

We demonstrate three-dimensional position and force detection of single gold nanosphere (GNP) and gold nanorod (GNR) particles in optical trap by combining backscattered light detection and dark field imaging. The trapping stiffness of the GNPs and GNRs for all three dimensions is measured. The results show that the spring constants in the propagation direction of the trapping laser are somewhat weaker than in other two directions for GNPs. While for GNRs, the spring constants in the polarization direction of the trapping laser are a little weaker than in other two directions. The effect of trapping laser polarization on the particles yields different spring constants in the transverse plane which is perpendicular to the propagation direction. And this effect is larger on GNRs than GNPs. [ABSTRACT FROM AUTHOR]

The coupling effect between two gold nanospheres (GNSs) is investigated for second-harmonic generation (SHG) in this paper. When the two GNSs approach each other, the coupling effect becomes stronger, and it consequently leads to stronger enhancement of SHG. A strong coupling area between two GNSs is also found. We propose a physical quantity, the coupling strength to describe the coupling effect on SHG enhancement and show that it can fully illustrate the relationship between the SHG enhancement and the gap of two GNSs. Moreover, further calculations reveal the existence of hot spots of both fundamental wave (FW) and excited second harmonic wave in the intermediate region of two GNSs. Our study can not only help to understand the coupling effect on SHG precisely but also offer some possibility to design a nonlinear plasmonic nano-ruler. [ABSTRACT FROM AUTHOR]

We present a general theoretical approach that combines the iterative technique with the nonlinear transfer matrix method to solve the general problems of three-wave mixing with pump depletion in nonlinear multilayer structures. The theoretical model is directly derived from nonlinear coupled wave equations, shows fast numerical convergence and maintains conservation of total energy perfectly for general structures. We have employed this approach to discuss second harmonic generation in photonic bandgap crystals with slow light effect and obtained a conversion efficiency of about 25% for both the forward and backward second harmonic waves. The detailed wave evolution process and energy conversion mechanism of sum frequency generation and difference frequency generation in perfect and imperfectly quasiphase-matching structures have been systematically investigated and their subtle and strong dependence on the initial conditions of incident waves has been revealed. The developed theoretical approach can be very useful for efficient and quantitative analysis of various nonlinear optical problems and help to design optimum nonlinear multilayer structures for specific high-efficiency nonlinear three-wave mixing processes. [ABSTRACT FROM AUTHOR]

Dual-trap optical tweezers are widely used in biological sciences and many other areas. The two traps are generally considered to be independent of each other in most situations. But when they are close and even touch side by side, the influence may not be negligible. We calculate the optical force of dielectric spherical particles trapped in dual-trap optical tweezers by means of the discrete dipole approximation method and analyze the perturbation effect between the two traps. We find that the electric field does not change much when it is scattered by the trapped particles. As a result, the scattering light from one sphere has a little perturbation effect on the trapping force upon the other sphere. The perturbation mainly comes from the other trap and is thus affected by the distance between the two traps. In the experiment, the displacement of the sphere trapped in an immovable trap is observed when another trap is moving close. The experiment data of relative displacement match qualitatively with the calculation data. [ABSTRACT FROM AUTHOR]

We fabricate a series of square-lattice subwavelength circular, rectangular, and trapezoidal air-hole arrays drilled in opaque gold thin films and measured the transmission spectra of light passing through these metallic nanostructures in the near-infrared range. The measured results show strong dependence of extraordinary optical transmission on the lattice constant, hole size, and hole shape. The wavelengths of the transmission peaks and dips are mainly determined by the lattice constant. However, they are also influenced by the parameters of the hole size and hole shape. The experimental data agree well with numerical calculation results by means of a plane-wave transfer-matrix method when the detailed geometry of the air holes is fully considered. The results indicate that the extraordinary light transmission through subwavelength metallic nanostructures of air-hole arrays are governed by excitation of surface plasmon polaritons on the metal surface and their scattering by periodic air-hole arrays. [ABSTRACT FROM AUTHOR]

We theoretically investigate and discuss the response time, switching contrast, and pump power of all-optical switching in a nonlinear photonic crystal structure with high quality factor (high-Q) cavity. For the response time of all-optical switching, the drop and rise time are considered, respectively. Moreover, we find that when the duration of pump pulse is shorter than the lifetime, the response curve of all-optical switching is asymmetric, and the drop time is determined by both the lifetime of high-Q cavity and duration of pump pulse, and the rise time is mainly determined by lifetime of high-Q cavity. In contrast, when the duration of pump pulse is much longer than the lifetime, the dynamic response curve is symmetric, and both the drop and rise time are determined by the duration of pump pulse. On the other hand, the pump power can be significantly reduced by using a setup where the probe beam is located at the high-Q cavity mode with very narrow linewidth. Furthermore, if the central wavelength of pump pulse is also set to match with this or another high-Q cavity mode, the pump power will be reduced further due to the large field enhancement within the photonic crystal, and more importantly there is no extra prolonged response time of all-optical switching. By this method, the pump power as low as 210 KW/cm2 is observed in our model structure with the quality factor of only 32 096 for the ordinary polystyrene material. [ABSTRACT FROM AUTHOR]

Light transmitting through a subwavelength slit on an ordinary metal plate is diffracted to all directions but if the exit plane of the slit is patterned with periodical nanostructures, the diffracted light may be compressed into a collimated beam within a small angle. In this paper, we develop a rigorous theoretical method for solving the surface wave induced beam collimation in nanostructured subwavelength metallic slits. The method combines the analytical modal expansion method, the supercell technique, the transfer-matrix method, and the conventional Kirchhoff’s diffraction theory. It allows for quantitative investigation of coupling of the incident light into the guided wave of the slit and coupling of the guided wave out of the nanostructured exit plane. We have used the method to examine light transmission through the nanostructured metallic slit and the corresponding diffraction and beam collimation behaviors. We have extensively analyzed the angular transmission spectrum as a function of the nanostructure period and the incident light wavelength and revealed the condition at which good beam collimation can take place. The result shows that the beam collimation is caused by the excitation of the surface waves supported on the periodical nanostructured pattern and subsequent coupling into the radiation light. Several scattering channels can coexist for coupling the surface waves into the observed diffraction waves and they can act constructively to create one or more collimation beams with excellent directionality and high brightness. The diffraction field patterns in the large area confirm the angular spectrum analysis. [ABSTRACT FROM AUTHOR]

We fabricate periodic arrays of simple-square and composite-square air hole nanostructures on a 120 nm thick Au film deposited on glass substrate by focused ion beam technology and study their optical properties by examining the optical near-field distribution via scanning near-field optical microscopy in the near-infrared region. The simple-square nanostructure only contains one square air hole in each unit cell, while the composite-square one contains the same size square air hole in the center and eight smaller square air holes in the periphery. The measured optical near-field patterns for the two nanostructures show very different distribution features. High intensity light spots locate within the central square air hole in the simple-square structure, while they sit at the peripheral smaller square air holes in the composite-square structure. Numerical simulations based on the plane-wave transfer-matrix method have been carried out to analyze the optical near-field patterns for the two metallic nanostructures and agree well with the experimental data. The results indicate that light interaction with metallic nanostructures is very sensitive to even a small change in the subtle geometrical feature. Meanwhile, by comparing near-field patterns with dielectric nanostructures theoretically, we also find that optical confinement is better for gold nanostructures than for dielectric nanostructures. [ABSTRACT FROM AUTHOR]

Optical waveguides are characterized by high-efficiency transmission of electromagnetic (EM) waves and optical cavities have frequency selective property. The combination of optical waveguides and cavities can result in a large amount of devices in optical integrated circuit, among which a channel-drop filter is an important member. A three-dimensional (3D) woodpile photonic crystal (PC) can possess a complete band gap that allows perfect confinement of EM waves. We theoretically and experimentally study the property of the x-type waveguide and acceptor-type defect cavity that are created in the 3D woodpile PC working in the microwave regime. On the basis of resonant coupling between the x-type wave guide and acceptor-type cavity, a series of three-port in-plane-type channel-drop filters are built: single-cavity, double-cavity, three-cavity, and four-cavity channel-drop filters. All of them exhibit good frequency responding property by changing the size of cavity. The multicavity channel-drop filter demonstrates the possibility to build multichannel wavelength division multiplexers in the 3D PC platform. [ABSTRACT FROM AUTHOR]

The transmission efficiency of electromagnetic waves through waveguide bends in a three-dimensional woodpile photonic crystal is investigated experimentally in the microwave regime. A stable technique of field extrapolation is employed to evaluate the coefficient of transmitted waves both for the reference straight waveguide and waveguide bend. We compare and analyze the transmission spectra of a wide variety of bend structures, and find that the multiface mirror scattering effect and cavity resonant effect in the bend zone are two major factors that influence the transmission property of the waveguide bends. A simple picture of the physical mechanism governing the electromagnetic wave transport at the bend zone can be drawn; this allows easier design of high-efficiency waveguide bend structures. [ABSTRACT FROM AUTHOR]

We investigated the influence of the irregular shape of air holes on the optical characteristics of channel-drop filters built in a two-dimensional photonic crystal slab. Three differently shaped structures are tested by experiments and simulations to demonstrate the shape-tuning capability of the optical devices. A four-channel filter based on this shape design is presented. The photonic crystal consists of a triangular array of air holes fabricated by focused ion beam lithographic technology on a silicon slab with thickness of 260 nm. The filter contains a heterostructure with different lattice constants of 420 and 430 nm. In each channel, the filtering functionality is guaranteed by the indirect resonant coupling between waveguide and cavity. The elliptical air holes that surround the cavity have different parameters, and this leads to a fine tuning of the cavity’s resonant frequencies. The performance of the multichannel filter is found to be very encouraging. [ABSTRACT FROM AUTHOR]

By the numerical simulation based on the finite-difference time-domain method, we investigate the adjustability of image distance for the same object distance in two-dimensional photonic crystals (PCs). When we add a fraction of a metallic component to the center of each dielectric bar, the PC slab lens can form a non-near-field image and the image distance changes for different surface terminations formed by introducing cylinders at the surface layers whose geometric and physical parameters are different from those of the PC bulk. Furthermore, the image distance can be further tuned by combining the two kinds of cylinders at the surface layers with different ratios of slab length. These simulation results clearly show that the imaging properties can be controlled effectively by changing the surface termination of PC slab lenses. [ABSTRACT FROM AUTHOR]

We examine the optical properties of three-dimensional metallic photonic crystals made from a periodic stacking of thin metallic mesh layers separated by homogeneous dielectric films by means of a combination of the plane-wave-based transfer-matrix method and analytical modal solution approach. Although each metallic mesh layer can serve as a frequency-selective surface and involves an intrinsic long-wavelength waveguide cutoff to electromagnetic waves, pass bands and new band gaps can exist far below the cutoff frequency due to the global coupling effect among different mesh layers. The results for the transmission spectra and photonic band structures are in good agreement with existing experimental measurements. It is found that the position of the pass bands and band gaps strongly depends on the thickness and composite of the separation layer between the adjacent metallic mesh layers. [ABSTRACT FROM AUTHOR]

Transfer-matrix methods have been routinely used to calculate the transmission and reflection spectra of electromagnetic waves propagating through a photonic crystal slab. In this article we show that the intrinsic lattice symmetries within a photonic crystal can be used to reduce numerical computations of the electromagnetic problem. We have employed this lattice-symmetry enhanced transfer-matrix method to handle two-dimensional photonic crystal slabs of square and triangular lattices and three-dimensional layer-by-layer photonic crystal slabs. The calculated transmission spectra agree excellently with the photonic band structures in predicting the photonic band gap positions, indicating the accuracy and efficiency of this proposed approach. [ABSTRACT FROM AUTHOR]

Theoretical models have been developed to investigate the reflection and transmission spectra, and photonic band structure for a midinfrared layer-by-layer metallic photonic crystal. It is found that photonic band gap effects due to global coupling between different unit cells in different layers play a key role in the formation of the lowest stop band gap extending to zero frequency and a high transmission passband adjacent to the stop band gap. Excellent agreement of theoretical results with experimental measurements is achieved. [ABSTRACT FROM AUTHOR]