Omni-polarized Faraday isolator based on non-Hermitian Faraday system.

Non-Hermitian systems have recently attracted significant attention in photonics due to the realization that the interplay between gain and loss can lead to entirely new and unexpected features. Here, we propose and demonstrate a non-Hermitian Faraday system capable of non-reciprocal omni-polarizer action at the exceptional point. Notably, both forward and backward propagating light with arbitrary polarization converge to the same polarization state. Leveraging the robustness and non-reciprocity of the non-Hermitian Faraday system, we realize an omni-polarized Faraday isolator that can effectively isolate any polarized light without the need for a polarizer at the incident port of backward propagation. Remarkably, under the given parameter configuration, the isolator achieves a maximum isolation ratio of approximately 100 dB and a minimum isolation ratio of around 45 dB for various polarized light, accompanied by near-zero insertion loss. Furthermore, our research reveals the remarkable tolerance of the non-Hermitian Faraday isolator to nonlinear effects. This unique characteristic allows us to harness nonlinear effects to achieve various optical functions, all while maintaining excellent isolation performance. The proposed non-Hermitian Faraday system paves the way for the realization of magnetically or optically switchable non-reciprocal devices.

Electro-Optical Comb Envelope Engineering Based on Mode Crossing.

Resonator-enhanced electro-optical (EO) combs could generate a series of comb lines with high coherence and stability. Recently, EO comb based on thin-film lithium niobate (TFLN) has begun to show great potential thanks to the high second-order nonlinearity coefficient of lithium niobate crystal. Here we demonstrate that EO comb envelope engineering based on mode crossing induced a quality factor reduction in the TFLN racetrack microcavity both in the numerical simulation and experiment. Our method paves the way for the generation of EO combs with an arbitrary envelope.

On-chip erbium-ytterbium-co-doped lithium niobate microdisk laser with an ultralow threshold.

Erbium-ion-doped lithium niobate (LN) microcavity lasers working in the communication band have attracted extensive attention recently. However, their conversion efficiencies and laser thresholds still have significant room to improve. Here, we prepared microdisk cavities based on erbium-ytterbium-co-doped LN thin film by using ultraviolet lithography, argon ion etching, and a chemical-mechanical polishing process. Benefiting from the erbium-ytterbium co-doping-induced gain coefficient improvement, laser emission with an ultralow threshold (∼1 µW) and high conversion efficiency (1.8 × 10-3%) was observed in the fabricated microdisks under a 980-nm-band optical pump. This study provides an effective reference for improving the performance of LN thin-film lasers.

Electrically tuned coupling of lithium niobate microresonators.

Microresonators coupled with integrated waveguides operate stably but usually lack tunability for an optimal coupling state. In this Letter, we demonstrate a racetrack resonator with an electrically modulated coupling on an X-cut lithium niobate (LN) platform by introducing a Mach-Zehnder interferometer (MZI) with two balanced directional couplers (DCs) to realize light exchange. This device provides a wide-range coupling regulation, from under-coupling and critical coupling to deep over-coupling. Importantly, it has a fixed resonance frequency when the DC splitting ratio is 3 dB. The measured optical responses of the resonator exhibit a high extinction ratio, exceeding 23 dB, and an effective half-wave voltage length Vπ·L of 0.77 V·cm, suitable for CMOS compatibility. Microresonators with tunable coupling and a stable resonance frequency are expected to find application in nonlinear optical devices on LN-integrated optical platforms.

Asymmetric reflection based on asymmetric coupling in single-layer extrinsic chiral metasurfaces.

We propose and experimentally demonstrate that giant asymmetric reflection of circularly polarized light based on asymmetric coupling can be achieved in single-layer extrinsic chiral metasurfaces at oblique incidence. The asymmetric coupling and asymmetric reflection in the extrinsic chiral metasurfaces are caused by extrinsic chirality, allowing them to have extremely high values. An asymmetric reflection of approximately 40% is measured. Furthermore, the asymmetric reflection of extrinsic chiral metasurfaces is demonstrated not only in intensity but also in phase retardation, which induces asymmetric polarization state conversion. An approximately 14° asymmetric reflected polarization offset from the symmetry axis is achieved. Our research provides an effective new method for constructing huge asymmetric coupled systems to manipulate electromagnetic waves.

Integrated ytterbium-doped lithium niobate microring lasers.

Integrated and stable microlasers are indispensable building blocks of micro-photonics. Here, we report the realization of an ytterbium-doped lithium niobate microring laser operating in the 1060-nm band under the pump of a 980-nm-band laser. The monolithic laser has a low threshold of 59.32 µW and relatively high output power of 6.44 µW, a state-of-the-art value for rare-earth ions-doped lithium niobate thin-film lasers. The monolithic laser with desirable performance and attractive scalability may find many applications in lithium niobite photonics.

On-chip ytterbium-doped lithium niobate microdisk lasers with high conversion efficiency.

Integrated optical systems based on lithium niobate on insulator (LNOI) have attracted the interest of researchers. Recently, erbium-doped LNOI lasers have been realized. However, the reported lasers have a relatively lower conversion efficiency and only operate in the 1550 nm band. In this paper, we demonstrate an LNOI laser operating in the 1060 nm band based on a high Q factor ytterbium-doped LNOI microdisk cavity. The threshold and the conversion efficiency of the laser are 21.19 µW and 1.36%, respectively. To our knowledge, the conversion efficiency is the highest among the reported rare-earth-doped LNOI lasers. This research extends the operating band of LNOI lasers and shows the potential in realizing high-power LNOI lasers.

Coin Paradox Spin-Orbit Interaction Enhances Magneto-Optical Effect and Its Application in On-Chip Integrated Optical Isolator.

We designed a simple on-chip integrated optical isolator made up of a metal-insulator-metal waveguide and a disc cavity filled with magneto-optical material to enhance the transverse magneto-optical effect through the coin paradox spin-orbit interaction (SOI). The simulation results of the non-reciprocal transmission properties of this optical structure show that a high-performance on-chip integrated optical isolator is obtained. The maximum isolation ratio is greater than 60 dB with a corresponding insertion loss of about 2 dB. The great performance of the optical isolator is attributed to the strong transverse magneto-optical effect, which is enhanced by the coin paradox SOI. Moreover, the enhancement of the transverse magneto-optical effect through the coin paradox SOI is more substantial for smaller azimuthal mode number n. Benefiting from this, the transverse magneto-optical effect remains strong in a wide wavelength range. Additionally, a smaller cavity has a stronger transverse magneto-optical effect in the same wavelength range. Our research provides a new perspective for creating highly integrated magneto-optical devices.

A Plasmonic Infrared Multiple-Channel Filter Based on Gold Composite Nanocavities Metasurface.

A plasmonic near-infrared multiple-channel filter is numerically and experimentally investigated based on a gold periodic composite nanocavities metasurface. By the interference among different excited plasmonic modes on the metasurface, the multipeak extraordinary optical transmission (EOT) phenomenon is induced and utilized to realize multiple-channel filtering. Investigated from the simulated transmission spectrum of the metasurface, the positions and intensity of transmission peaks are tuned by the geometrical parameters of the metasurface and environmental refractive index. The fabricated metasurface approached transmission peaks at 1128 nm, 1245 nm, and 1362 nm, functioning as a three-passbands filter. With advantages of brief single-layer fabrication and multi-frequency selectivity, the proposed plasmonic filter has potential possibilities of integration in nano-photonic switching, detecting and biological sensing systems.

Bidirectional Electromagnetically Induced Transparency Based on Coupling of Magnetic Dipole Modes in Amorphous Silicon Metasurface.

A bidirectional electromagnetically induced transparency (EIT) arising from coupling of magnetic dipole modes is demonstrated numerically and experimentally based on nanoscale a-Si cuboid-bar metasurface. Analyzed by the finite-difference time-domain (FDTD) Solutions, both the bright and dark magnetic dipole mode is excited in the cuboid, while only the dark magnetic dipole mode is excited in the bar. By breaking the symmetry of the cuboid-bar structure, the destructive interference between bright and dark magnetic dipole modes is induced, resulting in the bidirectional EIT phenomenon. The position and amplitude of simulated EIT peak is adjusted by the vertical spacing and horizontal spacing. The EIT metasurface was fabricated by Electron-Beam Lithography and deep silicon etching technique on the a-Si film deposited by Plasma-Enhanced Chemical Vapor Deposition. Measured by a convergent spectrometer, the fabricated sample achieved a bidirectional EIT peak with transmission up to 65% and 63% under forward and backward incidence, respectively. Due to the enhanced magnetic field induced by the magnetic dipole resonance, the fabricated bidirectional EIT metasurface provides a potential way for magnetic sensing and magnetic nonlinearity.

On-chip erbium-doped lithium niobate microring lasers.

Lithium niobate on insulator (LNOI), regarded as an important candidate platform for optical integration due to its excellent nonlinear, electro-optic, and other physical properties, has become a research hotspot. A light source, as an essential component for an integrated optical system, is urgently needed. In this Letter, we reported the realization of 1550 nm band on-chip LNOI microlasers based on erbium-doped LNOI ring cavities with loaded quality factors higher than 1 million at ∼970nm, which were fabricated by using electron beam lithography and inductively coupled plasma reactive ion etching processes. These microlasers demonstrated a low pump threshold of ∼20µW and stable performance under the pump of a 980 nm band continuous laser. Comb-like laser spectra spanning from 1510 to 1580 nm were observed in a high pump power regime, which lays the foundation of the realization of pulsed laser and frequency combs on a rare-earth ion-doped LNOI platform. This Letter effectively promotes the development of on-chip integrated active LNOI devices.

Broadened Angle-Insensitive Near-Perfect Absorber Based on Mie Resonances in Amorphous Silicon Metasurface.

A broadband near-perfect absorber is analyzed by an amorphous silicon (a-Si) hook shaped nanostructure metasurface. The transmission and reflection coefficients of the metasurface are investigated in the point electric and magnetic dipole approximation. By combining square and semicircle nanostructures, the effective polarizabilities of the a-Si metasurface calculated based on discrete dipole approximation (DDA) exhibit broadened peaks of electric dipole (ED) and magnetic dipole (MD) Mie resonances. The optical spectra of the metasurface are simulated with different periods, in which suppressed transmission are shifted spectrally to overlap with each other, leading to broadened enhanced absorption induced by interference of ED and MD Mie resonances. The angle insensitive absorption of the metasurface arrives 95% in simulation and 85% in experiment in spectral range from 564 nm to 584 nm, which provides potential applicability in nano-photonic fields of energy harvesting and energy collection.

Broadened band near-perfect absorber based on amorphous silicon metasurface.

A dielectric broadened band near-perfect absorber based on an amorphous silicon(a-Si) T-shaped nanostructure metasurface is investigated numerically and experimentally. The simultaneous suppressed transmission and reflection of the a-Si nanostructure metasurface are achieved by investigating the interference of the periodically adjustable electric dipole(ED) and magnetic dipole(MD) Mie resonances. The absorption of the a-Si nanostructure metasurface approaches the maximum of 95% in simulation and 80% in experiment with a top-hat shape in the spectral range from 580 nm to 620 nm by employing the T-shaped nanostructure. The proposed near-perfect absorber provides a new approach for expanding absorption bandwidth by integrating different nanostructures in metasurface, which is potentially applicable in nanophotonic fields of optical isolation, optical trapping and energy harvesting.

Plasmonic transmitted optical differentiator based on the subwavelength gold gratings.

A nanoscale plasmonic optical differentiator based on subwavelength gold gratings is investigated theoretically and experimentally without Fourier transform lenses and prisms. In the vicinity of surface plasmon resonance (SPR), the transfer function of subwavelength gold gratings is derived by optical scattering matrix theory. Simulated by the finite difference time domain (FDTD) method, the wavelengths of optical spatial differentiation performed by subwavelength gold gratings are tuned by the grating period and duty cycle, while the throughput of edge extraction is mainly adjusted by the grating thickness. Without Fourier transformation, the fabricated plasmonic optical differentiator experimentally achieves real-time optical spatial differentiation in transmission and implements SPR enhanced high-throughput edge extraction of a microscale image with a resolution of 10 µm at 650 nm, which has potential applications in areas of optical analog computing, optical imaging, and optical information processing.

Evolution of the Raman beam quality in a folded-coupled-cavity Nd:YVO4/YVO4 Raman laser.

An end-pumped actively $Q$Q-switched ${\rm Nd}\!:\!{{\rm YVO}_4}/{{\rm YVO}_4}$Nd:YVO4/YVO4 Raman laser with a folded coupled cavity is demonstrated to study the evolution of Raman beam quality. The theoretical mechanism of the beam cleanup effect of stimulated Raman scattering is analyzed. The beam quality ($M^2$M2) of the Raman beam and the fundamental beams before and after the Raman conversion are measured experimentally. The results show that with the incident pump power increasing, the ${M^2}$M2 of the fundamental beam increases from 1.85 to 3.08, while the ${M^2}$M2 of the Raman beam increases from 1.21 to 1.69. The beam quality of the Raman laser and its degradation are better than that of the fundamental laser.

Giant Tunable Circular Dichroism of Large-Area Extrinsic Chiral Metal Nanocrescent Arrays.

Circular dichroism (CD) is an interesting phenomenon originating from the interaction of light with chiral molecules or other nanostructures lacking mirror symmetries in three-dimensional (3D) or two-dimensional (2D) space. While the observable effects of optical chirality are very weak in most of the natural materials, they can be designed and significantly enhanced in synthetic chiral structures, where the spatial symmetry of their component are broken on a nanoscale. Therefore, fabrication of composites capable of cheap, time-saving, and giant CD is desirable for the advanced optical technologies. Here, the giant CD of large-area metal nanocrescent array structures was investigated theoretically and experimentally. The largest value of the CD spectrum measured was larger than 0.5, and the CD spectrum was tuned effectively and extensively while maintaining a large peak intensity, which can be attributed to the selective excitation of the lattice surface modes (LSMs) by circularly polarized light. The analysis of the extrinsic chiral structure shows potential applications in chiral molecule sensing and polarizing imaging.

Tunable polarization-independent dual-band coherent perfect absorber based on metal-graphene nanoring structure.

A dual-band polarization-independent coherent perfect absorber(CPA) based on metal-graphene nanostructure is proposed, which is composed of golden nanorings with different sizes on graphene monolayer. Based on the finite-difference time-domain (FDTD) solutions, coherent perfect absorptions of the metal-graphene CPA are achieved at frequencies of 50.54 THz and 43.60 THz, which are resulted from the excited surface plasmon resonance induced by different size nanorings. Through varying the relative phase of two incident countering-propagating beams, the absorption peaks are all-optically tuned from 98.3 % and 98.4 % to nearly 0, respectively. By changing the gate-controlled Fermi energy of the graphene layer, the resonance frequencies of the CPA are tuned simultaneously without changing the geometrical parameters. And polarization independence of the metal-graphene CPA is revealed due to the center symmetry of nanoring structure. The electrical tunability of resonance frequency and polarization independence enable the proposed CPA to be widely applied in optoelectronic and engineering technology areas for tunable active multiple-band regulation and control.

10.3-W actively Q-switched Nd:YVO4/YVO4 folded coupled-cavity Raman laser at 1176 nm.

We report herein an efficient actively Q-switched Nd:YVO4/YVO4 intracavity Raman laser operating at 1176 nm. Factors such as resonator geometry and pumping scheme are optimized to strengthen the power scalability and the conversion efficiency of the intracavity Raman laser. With a folded coupled cavity adopted to make full use of the high pump intensity on the Raman crystal, the first-order Stokes output of 10.32 W at 1176 nm is achieved under the incident pump power of 39 W and pulse repetition frequency of 160 kHz. The corresponding optical efficiency reaches 26.4%, and even higher efficiency of 27.8% is obtained at lower incident pump of 34.4 W.

High-performance second-Stokes generation of a Nd:YVO4/YVO4 Raman laser based on a folded coupled cavity.

We report an actively Q-switched Nd:YVO4/YVO4 intracavity Raman laser at second-Stokes wavelength of 1313.6 nm, which is capable of operating efficiently under pulse repetition frequency higher than 80 kHz. A folded coupled cavity is adopted to optimize the fundamental and the Stokes resonators individually and make full use of the high pump intensity on the Raman crystal. With relatively high output coupling of 82% at 1313 nm, the average output power of 5.16 W at 1313 nm is achieved under the incident pump power of 36.7 W. The cascaded Raman emission at both the first- and second-Stokes wavelength of 1176 and 1313 nm is investigated to discuss the optimization of the second-Stokes generation.

Giant circular dichroism of large-area extrinsic chiral metal nanocrecents.

In this work, we demonstrate the strong extrinsic chirality of the larger-area metal nanocrescents by experiments and simulations. Our results show that the metal nanocrescent exhibits giant and tunable circular dichroism (CD) effect, which is intensively dependent on the incident angle of light. We attribute the giant extrinsic chirality of the metal nanocrescent to the excitation efficiencies difference of localized surface plasmon resonance (LSPR) modes for two kinds of circularly polarized light at a non-zero incident angle. In experiment, the largest CD of 0.37 is obtained at the wavelength of 826 nm with the incident angle of 60°. Furthermore, the CD spectra can be tuned flexibly by changing the metal nanocrescent diameter. Benefitting from the simple, low-cost and mature fabrication process, the proposed large-area metal nanocrescents are propitious to application.