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.

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.

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.

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.

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.

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 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 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.

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.

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.

Nanoscale beam splitters based on gradient metasurfaces.

Beam splitters are essential components in various optical and photonic applications, for example, interferometers, multiplexers, and so on. Present beam splitters based on cubes or plates are normally bulky. Realizing beam splitters in nanoscales is useful to reduce the total size of photonic devices. We demonstrate here a beam splitter with nanoscale thickness based on a gradient metasurface comprising lithium niobate cylinder arrays. Since one unit cell of metasurface comprising two cylinder rows shows two opposite phase gradients, the incident light is split into different directions according to the generalized Snell's law. The split ratio is proven to be effectively tunable.

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.

Tunable dual-wavelength fiber laser with unique gain system based on in-fiber acousto-optic Mach-Zehnder interferometer.

A fast tunable dual-wavelength laser based on in-fiber acousto-optic Mach-Zehnder interferometer (AO-MZI) with new fabrication process is proposed. Not only could the center wavelength of the output laser be optimized with enhanced tuning range about 30 nm by tuning the polarization and the driving frequency of the radio frequency (RF) signal accordingly, but also the spectral spacing between the two output wavelengths could be tuned from ~0 nm to 2.65 nm by controlling the power of the RF signal. The tuning mechanism was also discussed.

9.80-W and 0.54-mJ actively Q-switched Nd:YAG/Nd:YVO4 hybrid gain intracavity Raman laser at 1176 nm.

Here we propose an efficient diode-end-pumped actively Q-switched 1176-nm Nd:YAG/Nd:YVO4 hybrid gain intracavity Raman laser. By virtue of the construction of a coaxial double crystal, the laser not only can operate efficiently at low pulse repetition frequencies (PRFs), thereby realizing relatively high-energy and high-peak-power pulsed output, but also is capable of generating a high average output power at high PRFs. A maximum pulse energy of 0.54 mJ for the 1176-nm Stokes light is achieved at the PRF of 10 kHz, and the maximum average output power up to 9.80 W is obtained at the PRF of 100 kHz, while the incident pump power is 42.0 W.

Ultra-strong enhancement of electromagnetic fields in an L-shaped plasmonic nanocavity.

Enhancements up to four orders of magnitude for electric intensity and three orders of magnitude for magnetic intensity are realized in a novel 2D L-shaped nanocavity. This structure makes full use of the dimension confinement, cavity resonance and tip enhancement to increase the electromagnetic intensity. An impedance matching model is developed to design this cavity by regarding the cavity as a load impedance where electromagnetic fields are maximally enhanced when maximum electromagnetic energy is delivered to the load impedance. Our L-shaped nanocavity promises a variety of useful functionalities in sensing, nonlinear spectroscopy and signal processing.

Mode conversion in a tapered fiber via a whispering gallery mode resonator and its application as add/drop filter.

Based on the conversion between the fundamental mode (LP01) and the higher-order mode (LP11) in a tapered fiber via a whispering gallery mode resonator, an add/drop filter was proposed and demonstrated experimentally, in which the resonator only interacted with one tapered fiber, rather than two tapered fibers as in conventional configurations. The filter gains advantages of easy alignment and low scattering loss over the other filters based on tapered fiber and resonator, and will be useful in application.

Efficient 914-nm Nd:YVO4 laser under double-end polarized pumping.

We report herein the enhancement of output power and optical efficiency of a quasi-three-level Nd:YVO4 laser through a double-end polarized pumping scheme, which improves the usually insufficient pump absorption of the short laser gain medium with low doping concentration, and meanwhile alleviates the influence of thermal effect. 17.7 W laser output at 914 nm is obtained under the launched 808-nm pump of 53.0 W, corresponding to an optical efficiency of 33.4%.

5.2-W high-repetition-rate eye-safe laser at 1525 nm generated by Nd:YVO4₋YVO4stimulated Raman conversion.

We report herein an efficient eye-safe Raman laser, which is based upon Nd:YVO4₋YVO4 and in-band pumped by a wavelength-locked laser diode array at 878.6 nm. By virtue of mitigated thermal load and improved pump absorption, a maximum average output power of 5.2 W at 1525 nm is obtained under the incident pump power of 30.6 W with the pulse repetition frequency of 140 kHz, corresponding to an optical efficiency of 17.0%.