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The radiation of electromagnetic and mechanical waves depends not only on the intrinsic properties of the emitter but also on the surrounding environment. This principle has laid the foundation for the development of lasers, quantum optics, sonar, musical instruments and other fields related to wave–matter interaction. In the conventional wisdom, the environment is defined exclusively by its eigenstates, and an emitter radiates into and interacts with these eigenstates. Here we show experimentally that this scenario breaks down at a non-Hermitian degeneracy known as an exceptional point. We find a chirality-reversal phenomenon in a ring cavity where the radiation field reveals the missing dimension of the Hilbert space, known as the Jordan vector. This phenomenon demonstrates that the radiation field of an emitter can become fully decoupled from the eigenstates of its environment. The generality of this striking phenomenon in wave–matter interaction is experimentally confirmed in both electromagnetic and acoustic systems. Our finding transforms the fundamental understanding of light–matter interaction and wave–matter interaction in general, and enriches the intriguing physics of exceptional points. The modes of the radiation field generated from an emitter are usually determined by the eigenstates of the surrounding environment. However, this scenario breaks down in a non-Hermitian system, at the spectral degeneracy known as an exceptional point.

Three-dimensional (3D) topological nodal points, such as Weyl and Dirac nodes have attracted wide-spread interest across multiple disciplines and diverse material systems. Unlike nodal points that contain little structural variations, nodal lines can have numerous topological configurations in the momentum space, forming nodal rings, nodal chains and potentially nodal links and nodal knots. However, nodal lines have much less development for the lack of ideal material platforms. In condensed matter for example, nodal lines are often fragile to spin-orbit-coupling, locating off the Fermi level, coexisting with energy-degenerate trivial bands and dispersing strongly in energy of the line degeneracy. Here, overcoming all above difficulties, we theoretically predict and experimentally observe nodal chains in a metallic-mesh photonic crystal having frequency-isolated linear bandtouching rings chained across the entire Brillouin zone (BZ). These nodal chains are protected by mirror symmetries and have a frequency variation less than 1%. We used angle-resolved transmission (ART) to probe the projected bulk dispersions and performed Fourier-transformed field scan (FTFS) to map out the surface dispersions, which is a quadratic touching between two drumhead surface bands. Our results established an ideal nodal-line material for further studies of topological line-degeneracies with nontrivial connectivities, as well as the consequent wave dynamics richer than 2D Dirac and 3D Weyl materials., Comment: 5 pages, 4 figures

We studied how the constitution parameters and applied bias magnetic field influence the bandwidth and frequency tunability of the one-way transmission (OWT) caused by magnetic surface plasmon resonance of the magnetic photonic crystals (MPC). We found that wide-band OWT could be obtained using ferrite rods with larger normalized radius and suppressing the bulk modes of MPC in the frequencies where the edge and bulk modes coexist. We verified that bias magnetic field tunes the frequencies of OWT but has little effects on its bandwidth by experiments, and a simple way is proposed to suppress the bulk modes by introducing air defect near the edge of the MPC and experimentally demonstrated its effectiveness. This work provides a way toward practical applications for wide-band tunable one-way devices.

China's greenhouse gas emission is very critical, how to develop a low-carbon economy, build a resource-saved and environment-friendly society become the key problems. This paper study on the key issues which influence the low carbon economy, explore new cooperation mechanisms among network enterprises, and construct a green economic develop model.

As silicon has a large refractive index and low loss in infrared wavelengths, it becomes an important optical material that has been widely used for integrated photonics applications. In this paper, we present some of our recent experimental works on infrared two-dimensional air-bridged silicon photonic crystal (PhC) slab devices that are based on both the band gap and band structure engineering. We have designed, fabricated, and characterized a series of PhC waveguides with novel geometries, resonant microcavities with fine tunability, and channel drop filters utilizing resonant coupling between waveguide and cavity. These devices are aimed to construct a more flexible network of transport channels for infrared light at micrometer/nanometer scale. We have also explored the remarkable dispersion properties of PhCs by engineering the band structures to achieve negative refraction, self-collimation, superprism, and other anomalous dispersion behaviors of infrared light beams. We have designed and fabricated a PhC structure with negative refraction effect and used scanning near-field optical microscopy to observe the negative refraction beam.

We report experimental observation of enhanced near infrared transmission through periodic H-shaped arrays in metallic thin films. A broad transmission band centered at about 1.6 μm is found and the transmission coefficient can be as high as about 16.3% for an arm width of 120 nm for the H-pattern. The position of the transmission peak shifts to a short wavelength by about 140 nm when the arm width is increased to 200 nm. This variation feature is inverse to that found in periodic hole arrays. It indicates that the localized surface plasmon is an important parameter contributing to the enhanced transmission. The H-shaped arrays exhibit a strong polarization effect consistent with the geometry anisotropy. Numerical simulations by using a transfer matrix method reproduce quite well the obtained transmission spectrum.

Nonlinear frequency conversion offers an effective way to expand the laser wavelength range based on birefringence phase matching (BPM) or quasi-phase-matching (QPM) techniques in nonlinear crystals. So far, efficient high-harmonic generation is enabled only via multiple cascaded crystals because of the extreme difficulty to simultaneously satisfy BPM or QPM for multiple nonlinear up-conversion processes within a single crystal. Here we report the design and fabrication of a chirped periodic poled lithium niobate (CPPLN) nonlinear crystal that offers controllable multiple QPM bands to support 2nd-8th harmonic generation (HG) simultaneously. Upon illumination of a mid-IR femtosecond pulse laser, we observe the generation of an ultrabroadband visible white light beam corresponding to 5th-8th HG with a record high conversion efficiency of 18%, which is high compared to conventional supercontinuum generation, especially in the HG parts. Our CPPLN scheme opens up a new avenue to explore and engineer novel nonlinear optical interactions in solid state materials for application in ultrafast lasers and broadband laser sources.

Second-harmonic generation (SHG) from periodic arrays of subwavelength rectangular air hole with various aspect ratios perforated in gold thin films can get resonantly enhanced for some specific geometric shapes. Here we clarify the physical origin of this shape resonance effect. A nonlinear coupled-mode theory is set up to solve energy conversion from fundamental wave (FW) mode to second-harmonic wave (SHW) mode within the nanoscale air hole. It reveals that several physical mechanisms, including the FW mode excitation amplitude, FW-SHW modal spatial overlap, FW-SHW mode phase mismatch, and SHW mode attenuation, are all geometric shape sensitive and altogether act to induce the SHG shape resonance effect. The theory agrees well with experimental observations and provides an accurate and complete explanation for the long-emphasized but elusive shape effect. The study may stimulate deeper insights to visualize general nonlinear nanophotonic processes and pave the way to engineering high-efficiency nonlinear nanophotonic devices.

One-dimensional PhC mirrors are constructed in a single-mode silica slab waveguide with a row of elliptical holes. The photonic band gap (PBG) of the PhC structure is attained by fast eigen-mode calculations. Being aware that component radiated waves of the PhC mirror are generated at interfaces between different waveguide sections, when propagating guided waves impinge on these interfaces, we point out that the total radiation loss of the PhC mirror is consequence of interferometric interplays of component radiated waves. We visualize this radiation generation process with intuitive pictures. We also estimate total radiation losses of PhC mirrors by using an analytical model. For uniform PhC mirrors, our model explains the oscillations of the total radiation loss with the increase of the period number. The calculated results agree well with the numerical simulations in terms of the oscillation period, the damping speed, the initial phase, and the relative intensity. For non-uniform PhC mirrors, our model finds that the progressively tapered transition from the feeding waveguide to the PhC mirror does not yield the lowest radiation loss. This finding is against to the well known “impedance-matching” picture. The matching of our model with the simulated results certifies the interferometric nature of the radiation generation process in a PhC mirror especially when a low-index waveguide is considered.

By utilizing a vector network analyzer, the field distributions on the surface of a three-dimensional woodpile photonic crystal with a straight waveguide or a bend waveguide buried under the surface were measured in the microwave regime. The information of field profile and propagation characteristics of the guided modes can be successfully extracted from the surface near-field measurement. This work indicates that the near-field detection can become a promising means for experimental characterization of three-dimensional photonic crystal devices in supplement to the usual transmission spectrum measurement.

We propose a line defect waveguide structure along the Gamma-Mu direction in two-dimensional triangular lattice silicon photonic crystal slabs. The modal dispersion relation and the transmission spectra of this waveguide are studied. The results show that by perturbing the width of the line defect and the diameter of the air holes adjacent to the waveguide core, one can control the width of the single mode transmission window and make it far broader than the original one. The proposed Gamma-Mu waveguide will help to build a more flexible network of interconnection channel of light in two-dimensional photonic crystal slabs.

A waveguide coupler is designed and realized in a three-dimensional woodpile photonic crystal at microwave regime. This waveguide coupler shows good energy transfer property, which is confirmed through measurement of transmission spectrum, internal field distribution and surface field distribution using Agilent microwave network analyzer.

Artificial optical birefringence can be realized in three-dimensional photonic crystals with a uniaxial structural symmetry: e.g., woodpile photonic crystals with a tetragonal lattice structure in the long-wavelength limit. The ordinary and extraordinary indices of refraction are determined from calculation of the reflection coefficient for a plane wave incident on the surface of a semi-infinite photonic crystal at different angles. We find that the anisotropy can be widely tuned by simply changing the width and thickness of the dielectric rod. A large relative negative anisotropy over 33% is found. A transition from positive anisotropy to negative anisotropy can be readily achieved. At certain parameters, a structurally anisotropic nanostructure can behave like an optically isotropic medium. Our study opens a window to use artificial nanostructures to create an arbitrary optical anisotropy that is not possible in natural crystals.

A large relative anisotropy can be realized in three-dimensional woodpile photonic crystals in the long-wavelength limit. We find that the anisotropy can be tuned. Transition from positive anisotropy to negative anisotropy can be widely achieved.

We designed waveguide channels in two types of square spiral photonic crystals. Wide polarization gaps, in which only one circular polarization wave is allowed while the other counter-direction circular polarization wave is forbidden, can be opened up on the waveguide modes within the fundamental photonic band gap according to the calculation of band structures and transmission spectra. This phenomenon is ascribed to the chirality of the waveguide and is independent of the chirality of the background photonic crystal. Moreover, the transmission spectra show a good one-way property of the waveguide channels. The chiral quality factor demonstrates the handedness of the allowed and impeded chiral waveguide modes, and further proved the property of the waveguide-mode polarization gap. Such waveguides with waveguide-mode polarization gap are a good candidate for one-way waveguides with robust backscattering-immune transport.

LiNbO3 nonlinear photonic crystals with two kinds of Sierpinski fractal superlattice structures, in which their domain walls are circle and square cylindrical surfaces for the second order, respectively, are successfully fabricated. Quasi-phase matching and nonlinear Cerenkov harmonics under pulse laser beams are measured. The spot- and line-shaped light sources are realized by quasi-phase matching harmonics. Ten kinds of spot-shaped outputs can be accomplished, in which the quasi-phase matching harmonic conversion efficiency for the fundamental wavelength 1.352 μm reaches 27%. A green line-shaped light source with expansion angle of about 3° is observed, which originates from the coexistence of imperfect collinear and non-collinear quasi-phase matching processes. In addition, the ring-shaped light sources of continuous wavebands from 425 nm to 675 nm are obtained by nonlinear Cerenkov harmonics. The optical properties of quasi-phase matching and Cerenkov harmonics in two kinds of crystals are highly similar to each other.

Metal films grown on Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the mid-infrared regime has been observed. High order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. Si(1,1) peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface plasmon polaritons (SPPs) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPPs diffraction by the lattice., 4 pages, 5 figures, 26 references

We use near-field scanning optical microscopy (NSOM) operating in collection mode to map the optical field distribution of continuous wave infrared light propagating along an air-bridged W3 silicon photonic crystal (PC) slab multimode waveguide in a subwavelength resolution. The detected near-field optical intensity distribution patterns show very different longitudinal propagation features and transverse field profiles at different wavelength ranges. We have analyzed the dispersion of the guided modes in the PC waveguide and the corresponding eigenfield profile for each guided mode by means of the three-dimensional plane-wave expansion simulations. It is found that the experimentally observed complex while interesting near-field pattern features can be well explained by the field profiles of the multiple waveguide modes and their superposition at different excitation wavelengths. The detection and analysis of this multimode PC slab waveguide via NSOM could help us to understand complex wave propagation behavior and to further design novel PC devices.

The periodic arrays of two-stage cross-dipole fractal slits etched on a $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick gold film exhibit two resonance pass bands in the near- and mid-infrared region. The sample was fabricated by electron beam lithography and reactive ion etching system, and the width of the fractal slits is about $120\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The transmission coefficient exceeds 1 at the peak wavelength if normalized to the area of the fractal slits. The measured transmission spectrum in the frequency domain around the short wavelength pass band agrees well with the simulation results obtained via the transfer-matrix method. The existence of two resonance bands and their relative positions can be well explained by the geometric self-similarity of the fractal structure. Both transmission spectrum and electric field distribution analysis show that the two resonance bands result from the interaction of the incident light with the individual primary and secondary elements of the two-stage fractal structure.

in this paper, we experimentally investigate a channel drop filter in a three-dimensional layer-by-layer photonic crystal (PC) based on the resonant coupling between waveguides and cavities. The working frequency range of the PC device is in the microwave regime. We gradually increase the cavity size and measure the transmission spectra, and found that the channel drop filter is tunable in a wide range of frequency bands. Furthermore, it has excellent frequency selectivity.

We have designed and fabricated a lithium niobate (LN) nonlinear photonic crystal (NPC) with a two-dimensional (2D) ellipse structure of inverse poling domains. The structure can offer continuously varying reciprocal lattice vectors in different directions to compensate the phase-mismatching during the second harmonic generation (SHG) for diverse pump wavelengths. We consider three propagation directions with large effective nonlinear susceptibility and measure the nonlinear conversion efficiency of SHG. The experimental data are in good agreement with the quantitative calculation results using the effective susceptibility model with pump depletion. With high-efficiency SHG in multiple propagation direction, the 2D ellipse structure of LN NPC has the potential to realize various broadband nonlinear frequency conversion processes in different propagation direction with a single crystal.

Ultrabroadband laser sources are highly desirable in a wide variety of modern science disciplines ranging from physics, chemistry and materials science to information communications and processing. Here we present the design and fabrication of a chirped periodically poled lithium niobate (CPPLN) nonlinear photonic crystal that supports multiple orders of quasiphase matching with finite bandwidth and allows for the simultaneous broadband generation of second and third harmonics with high conversion efficiency. Moreover, the chirp rate has a significant influence on the conversion efficiency and bandwidth. The CPPLN scheme offers a promising approach for the construction of short-wavelength laser sources and enables the generation of the three primary colors-red, green and blue-from a single crystal, which may have potential applications in large-screen laser displays.

Gyromagnetic photonic crystal (GPC) offers a promising way to realize robust transport of electromagnetic waves against backscattering from various disorders, perturbations and obstacles due to existence of unique topological electromagnetic states. The dc magnetic field exerting upon the GPC brings about the time-reversal symmetry breaking, splits the band degeneracy and opens band gaps where the topological chiral edge states (CESs) arise. The band gap can originate either from long-range Bragg-scattering effect or from short-range localized magnetic surface plasmon resonance (MSP). These topological edge states can be explored to construct backscattering-immune one-way waveguide and other nonreciprocal electromagnetic devices. In this paper we review our recent theoretical and experimental studies of the unique electromagnetic properties of nonreciprocal devices built in GPCs. We will discuss various basic issues like experimental instrumental setup, sample preparations, numerical simulation methods, tunable properties against magnetic field, band degeneracy breaking and band gap opening and creation of topological CESs. We will investigate the unidirectional transport properties of one-way waveguide under the influence of waveguide geometries, interface morphologies, intruding obstacles, impedance mismatch, lattice disorders, and material dissipation loss. We will discuss the unique coupling properties between one-wave waveguide and resonant cavities and their application as novel one-way bandstop filter and one-way channel-drop filter. We will also compare the CESs created in the Bragg-scattering band gap and the MSP band gap under the influence of lattice disorders. These results can be helpful for designing and exploring novel nonreciprocal electromagnetic devices for optical integration and information processing.

Interferometric behaviours in radiation generation process of a one-dimensional photonic-crystal (PhC) reflector are investigated on a silica waveguide. An analytical model, which can calculate total radiation losses, is presented after analysing eigen-mode properties and radiation pattern characteristics. Our model takes into account the interference of component radiated waves generated at interfaces between different waveguide sections and is verified by comparing with numerical simulations of periodic and non-periodic one-dimensional PhC reflectors. Its capability to quickly find minimum-radiation tapering geometries helps circumvent time-consuming numerical simulations. Our model negates the conventional knowledge that a gradually tapered transition from uniform waveguide to PhC waveguide yields lower radiation loss. In one example, the gradually tapered transition produces 2.3dB more radiation than the optimum geometry.

We theoretically and experimentally study the side coupling between guided modes and cavity modes in a one-way waveguide that is composed of a regular photonic crystal and a gyromagnetic photonic crystal. At the cavity resonant wavelength, the backward mode can be completely blocked while the forward mode is only slightly influenced in the transmissivity for a specially designed waveguide. This unique light transport property can be exploited to construct a unidirectional band stop filter and a unidirectional channel-drop filter that can selectively process a light signal propagating only along a particular direction.

One-way edge states realized in magneto-optical photonic crystals (MOPCs) is generally believed to be robust to scattering from disorders and even strong perturbations. In this work, we experimentally and theoretically study the influence of obstacles on the transport of forward and backward edge states in two-dimensional MOPCs under a dc magnetic field. We find that the passage of electromagnetic wave around the obstacles mainly takes advantage of the formation of new edge states at the new interfaces formed between the obstacle and bulk MOPC. The forward edge states can be blocked when the new edge state channel is cut off or the coupling with the original edge states to the new edge states is strongly reduced.

We experimentally realize a one-way waveguide made from a gyromagnetic photonic crystal (GPC) and a normal dielectric photonic crystal in their common photonic band gap. Due to the time reversal symmetry breaking of the GPC under a dc magnetic field, the edge modes exhibit a one-way property. We investigate the unidirectional propagation character originating from the chiral edge states at different widths of the waveguide. Both the forward modes, which are robust to various obstacles, and the backward modes show an obvious frequency shift for different waveguide widths. Numerical simulations by means of the transfer-matrix method and multiple scattering methods agree well with the experimental data.

We theoretically investigated the transmission properties of an H-shaped periodic array built on a metallic thin film by combining the transmission spectra with the near-field amplitude and phase distribution analysis. Because of the geometric anisotropy of the H-shaped structure, the structure exhibits very different transmission behaviors for x-polarization and y-polarization incident light, in particular, the number, position, lineshape and broadness of the resonant peaks. In order to understand the root of these different transmission behaviors for the two polarizations, the characteristics of the transmission near-field, both the amplitude and phase distribution behind the metal structure, are analyzed in detail. At the transmission peaks, the high field amplitude domains always have a homogeneous phase distribution. Both the global and local surface plasmon resonance (SPR) effect can play an active role in shaping the transmission behaviors, and their existence can be uncovered by the optical near-field patterns at the transmission peaks. Whether the global or local SPR effect plays a dominant role strongly depends on the incident light polarization. Even for the same polarization state, the situation can still be very different for different resonant peaks. In some special cases these two resonance mechanisms can act collectively and result in a very sharp SPR peak in the transmission spectrum, which can find potential applications in sensitive optical sensors. Our studies clearly indicate the importance of optical near-field analyses in revealing the major physical mechanisms underlying light interaction with complex metallic nanostructures.

We build a two-dimensional square-lattice gyromagnetic photonic crystal from magneto-optical material of yttrium-iron-garnet (YIG) and demonstrate its tunable electromagnetic properties by placing the photonic crystal in a tunable dc magnetic field and measuring the transmission spectra for microwaves. The magnetic field breaks the intrinsic degeneracy at the M symmetry point between the 2-3 photonic bands and opens a band gap. Near the resonance frequency of YIG, a band gap is also opened due to the metallic properties of YIG. Our work opens a window to exploit magneto-optical photonic crystals as tunable integrated optical devices.

We demonstrate the design, fabrication, and characterization of a multichannel filter in a two-dimensional triangular-lattice photonic crystal slab. The output signal channel, which directs in the Γ-M crystalline direction, is orthogonal to the input signal channel that directs in the Γ-K crystalline direction. In each channel, the filtering function is guaranteed by the indirect resonant coupling between the waveguide and the cavity. Four resonant cavities with different sizes provide the mechanism of finely tuning the output wavelength. Our experimental results are in good agreement with simulations.

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. (C) 2008 American Institute of Physics.

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.