Gas sensing technology based on spectral absorption has been widely employed in various domains, such as industrial manufacturing and biomedical applications. Nevertheless, due to the constraints imposed by the Lambert-Beer law, the detection of weakly absorbing gases (such as ammonia) in the near-infrared (NIR) bands often necessitates the auxiliary utilization of multi-pass cells with ultra-long optical paths. This inevitably brings about a significant increase in equipment size and substantial manufacturing and operational costs. In recent years, it has been discovered that the photothermal spectroscopy (PTS) technique can compensate for the limitations of conventional spectral absorption-based gas sensing technologies, leading to extensive studies in this field. PTS adopts a pump-probe dual-light configuration, and the photothermal effect (PTE) induced by the non-radiative relaxation of gas molecules encodes the pump-light power variation onto the phase of the probe light. Subsequently, the phase fluctuation is extracted through a heterodyne or homodyne interferometer, and harmonic signals are demodulated through a lock-in amplifier. The PTE intensity (probe-light phase modulation) in the PTS system is directly related to the pump power density. As a result, the PTS system can achieve higher sensitivity on a much shorter sensing path, thus significantly reducing the equipment volume and costs. Hollow-core fiber (HCF), has been widely applied in PTS systems to realize ultra-sensitive sensing, benefiting from its capability to guide high power density. However, the sensing chamber formed by the elongated air core within the HCF needs to be uniformly filled with the target gas which hinders the real-time detection. The solution of drilling micro holes on the HCF surface using a femtosecond laser has emerged. With the assistance of a miniaturized high-pressure gas pump, this approach reduces the time required for filling the gas into the HCF and achieves a response time in dozens of seconds. Performing micron-level local drilling on the HCF places high demands on the fabrication process and leads to a sharp increase in costs undeniably. Therefore, the solution to high sensitivity, high integration, and low fabrication costs in sensing elements is crucial for the commercial application of PTS technology. Microfiber featuring high-power density, compact size, and cost-effective fabrication, can serve as an effective alternative.

We employ a tapered microfiber as the sensing element in the PTS system. Firstly, by building a cross-sectional model of microfiber-air in COMSOL and calculating the surface integration of the time-averaged power flux in different areas, the relationship between the evanescent field proportion and the microfiber diameter could be obtained. Based on the evanescent field proportion, the average power density of the in-fiber field and the evanescent field could be estimated. By utilizing the thermal-optic coefficients of air and SiO₂, the equivalent PTE intensity with varying diameters could be calculated and compared with the PTE intensity in the HCF. Subsequently, a tapered microfiber is fabricated through the fusion tapering approach and then encapsulated in a glass jar to form a gas chamber. Finally, the chamber is incorporated into the PTS system. The pump is tuned around 1512.24 nm (the NH₃ absorption line), while the center wavelength of the probe is 1550 nm. The average powers for the pump and probe are 3.6 mW and 1.4 mW respectively.

The tapered microfiber, with a waist diameter of 1 μm and a waist length of about 4 mm, exhibits an insertion loss of 0.75 dB/mm at the communication band (Fig. 1). The simulation results show that the evanescent field accounts for about 25% of the propagating pump light, providing a PTE intensity approximately 187 times higher than that of the HCF (Fig. 1). A heterodyne PTS gas sensing system is constructed by employing a microfiber as the key component (Fig. 2). A detailed analysis in both the frequency and time domains is conducted to examine the dynamic variations of the photothermal phase modulation during gas absorption (Fig. 3). The phase modulation intensities are found to be 8.1° and 11.6° when the pump scans away and is at the NH₃ absorption line respectively. Based on the demodulated second harmonics under the 10700×10^-6 NH₃ volume fraction and the noise, the 1σ equivalent detection limit of 39×10^-6 is obtained (Fig. 4). By calculating the 1σ standard deviation of the second harmonics over 30 pump scanning cycles, the system instability is 0.42% (Fig. 4). Additionally, by increasing the pump power from 1.2 mW to 3.6 mW and considering the corresponding noise and second harmonics, it is validated that the SNR can be enhanced by increasing the pump power (Fig. 5). Finally, through applying random vibration excitation to the system, the phase modulation amplitude increases by approximately 16 times during the occurrence of vibration, while the second harmonics remains stable. This demonstrates the system's ability to withstand ambient vibration noise.

Our study investigates PTS gas detection based on microfiber at the communication band with NH₃ as the target gas. Firstly, a tapered microfiber with a diameter of 1 μm is fabricated. The simulation indicates that the evanescent field accounts for about 25% of the pump power, bringing about a PTE intensity 187 times higher than that of HCF. Subsequently, a heterodyne PTS detection system is constructed. NH₃ detection under the 10^-6 level at 1512.24 nm is achieved with an ultra-short sensing length of 4 mm. With a pump power of 3.6 mW, the 1σ noise equivalent detection limit of 39×10^-6 is realized, and the instability of the detection signal within 30 pump tuning cycles is less than 0.5%. This system has a certain degree of immunity to ambient vibration, and the sensing sensitivity could be increased by boosting the pump power. The all-fiber, ultra-compact structure, high sensitivity, and low-cost characteristics make this system an affordable solution for gas detection in complex industrial processes.

Plasmon-induced transparency (PIT) refers to an atypical transmission phenomenon that results from the coupling of various resonance modes with the near-field electromagnetic wave. This usually results in a slowdown of the speed of light at the transmission peak due to significant dispersion, which is known as slow light performance. Numerous studies have demonstrated that slow light performance can be achieved in optical fibers, waveguides, and metasurfaces, which has led to wide-ranging applications in areas such as optical storage and optical modulation. Metasurfaces offer several advantages over optical fibers and waveguides, including small size, ease of fabrication, and excellent electromagnetic properties. Moreover, the addition of graphene to metasurfaces provides high-quality properties such as high transmittance, low loss, and dynamical adjustability. As a result, graphene-based metasurfaces hold significant potential in the study of slow light performance. However, the production and utilization of complex patterned graphene are limited by the current state of nanomanufacturing technology. Therefore, studying the PIT effect in simple structures with high quality is crucial for the practical production and application of PIT devices in experiments and real-life settings. Moreover, designing simple and manufacturable structures that produce high-quality multi-mode PIT is of great significance to optical device fabrication. This will significantly promote the rapid development and application of photonic devices based on the PIT effect.

In this paper, we investigate the PIT effect in monolayer patterned graphene metasurfaces using numerical simulations of the electromagnetic field. The simulations are performed using the full-wave electromagnetic software, namely CST Microwave Studio 2019, which utilizes the finite integration technique (FIT) to solve the discrete Maxwell equations. The metasurface structure consists of a split ring resonator (SRR) laterally coupled by metal-graphene-metal. Initially, we analyze the transmission spectrum and electric field distribution to gain insights into the PIT effect of the structure. Additionally, we derive a theoretical formula for graphene surface conductivity and investigate the effect of bias voltage on the dielectric constant of graphene by changing the Fermi level. Subsequently, we study the impact of different Fermi levels on amplitude modulation. Finally, we demonstrate the dynamic modulation of slow light performance by varying the bias voltage and graphene width.

In this paper, we present the design and analysis of a monolayer patterned metal-graphene metasurface that achieves a high-quality PIT effect (Fig. 1). We observe that as the Fermi level of graphene increases, the transmittance at resonance points also increases. Specifically, we achieve amplitude modulations of 96.05% and 65.40% at two different resonance points (Fig. 5). To quantify the relationship between the bias voltage and the Fermi level of graphene, we propose an equivalent capacitive coupling model, which shows that graphene primarily modulates the amplitude and has low sensitivity to frequency. Resonance frequency modulation can be achieved by changing the structural parameters such as the width of graphene (Fig. 7). We evaluate the performance of the slow light effect using parameters such as group delay, group refractive index, and delay bandwidth product (DBP). We find that the bias voltage is positively correlated with these parameters, while the graphene width is negatively correlated with them. For instance, when the bias voltage is set to 50 V, and the graphene width is stable, we obtain group delay, group refractive index, and DBP values of 44.11 ps, 276.19, and 3.66, respectively (Table 1). By reducing the graphene width, we further optimize these parameters, resulting in group delay, group refractive index, and DBP values of 93.12 ps, 756.67, and 9.31, respectively (Table 2). The Q value characterizes the loss of the device, with a higher Q value indicating a lower loss. When no bias voltage is applied, we obtain the largest Q value of 42.33. However, the Q value gradually decreases as the bias voltage increases, or the graphene width decreases. This suggests that high dispersion and low loss characteristics cannot be simultaneously achieved and need to be balanced in practical applications. Finally, we compare our device with relevant studies and demonstrate its significant advantages.

The study proposes a metal-graphene coupling structure to achieve a dynamically adjustable PIT effect, which results from the interference cancellation of the bright-bright mode. By adjusting the Fermi level of graphene, amplitude modulation can be achieved at 0.929 THz and 1.037 THz, with maximum modulation depths of 96.05% and 65.40%, respectively. By using the equivalent capacitive coupling circuit, the relationship between the bias voltage and Fermi level is calculated, and it is found that a bias voltage drop of Vg=30V and graphene width of w₂=2 μm can result in group delay, group refractive index, DBP, and Q values of 93.12 ps, 756.67, 9.31, and 10.19, respectively. These results hold significant value in slow optical device fabrication, THz communication, and detection research.

The advancement of micro-nanostructures has gained significant traction owing to their superior broadband antireflective attributes, which span a broad range of incident angles. This progress has expanded their application in photocells and photodetectors. However, these structures often possess subwavelength structural characteristics and high aspect ratio to manage the wavefront distortion of the target light field. The diminutive size and high aspect ratio of these periodic structural units make their surface susceptible to environmental damage, thereby affecting their optical performance. This paper proposes an antireflective micro-nanostructure surface with a composite grid structure. This innovative approach enhances the mechanical stability and longevity of the micro-nanostructure surface without altering its original design and optical properties.

We successfully proposed and fabricated an antireflection micro-nanostructure surface with a composite grid. This involved constructing a silicon oxide composite grid on a silicon substrate to protect the internal micro-nanostructural units. The optical and mechanical properties of the composite grid structure were optimized using appropriate material selection, morphological characterization, and size parameters. Moreover, the stress distributions of the three types of grid structure under a fixed load were analyzed using finite element analysis software. Based on the results of this theoretical analysis, the hexagonal composite grid antireflection micro-nanostructure was successfully fabricated by a combination of photolithography and etching technology. Furthermore, its morphology was evaluated using a scanning electron microscope (SEM), while a spectrometer measured its optical reflectivity. Lastly, an adhesive tape test was used to examine the sample surface and discuss the protective capacity of the composite grid for the antireflection micro-nanostructure.

The optical reflectivity test shows an average reflectivity difference of 0.068% between the antireflection micro-nanostructure surface attached to a composite grid and standalone micro-nanostructure [Fig.17(a)]. This result suggests that the grid structure has negligible impact on the micro-nanostructure's optical performance. The average reflectivity of the composite grid antireflection micro-nanostructure surface in the 3-5 μm frequency band is less than 4% for incident angles in the range of 8°-40°, demonstrating stable antireflection performance [Fig.17(b)]. The adhesive tape test on the composite grid antireflection micro-nanostructure confirms the effective maintenance of the micro-nanostructure (Fig.21) with no substantial change in its antireflection performance [Fig.23(a)]. In contrast, the surface of the micro-nanostructure without grid is damaged and its reflectivity is increased by 1.5% after the tape test [Fig.22 (b)]. These results validate the grid structure's protective role without altering the optical properties of the micro-nanostructure.

This study presents a successful fabrication of antireflection micro-nanostructure surface with composite grid by a combination of photolithography and etching. This design offers robust antireflection performance in the mid-infrared range across a wide incident angle. SEM is used to confirm the morphology of the antireflection micro-nanostructure surface with composite grid, showing structural parameters that closely resemble those of the simulation parameters. The Scotch 3M tape test is used to compare the antireflection micro-nanostructure surface with composite grid and single micro-nanostructure surface. The results indicate that the grid-structured antireflection micro-nanostructure surface maintains its original morphology and antireflection performance even after the tape test. Conversely, the micro-nanostructure surface without grid sustains damages, exhibiting a 1.5% increase in its reflectivity post-test. These findings reveal the grid structure's mechanical protective ability for the micro-nanostructure, improving its optical and mechanical properties. These advancements can propel future research and development of micro-nanostructures for optical and optoelectronic devices.

Terahertz (THz) waves are electromagnetic ones with frequencies ranging from 0.1 THz to 10 THz. Due to their high penetration, low photon energy, and high communication capacity, terahertz waves are widely employed and have important applications in broadband communication, medical imaging, nondestructive testing, security, and other fields. However, as how to generate high-quality terahertz waves becomes a major technical problem, the THz frequency band is once called the THz gap. With the rapid development of ultrafast optoelectronics, photoconductive antenna (PCA), a THz source involved in electronics and photonics, can be applied at room temperature, with high frequency of THz wave generation and low requirements for laser pump power, which makes it stand out among other THz sources. However, due to the high refractive index of photoconductive materials, the photoconversion efficiency of THz PCA is low. Meanwhile, due to the shielding effect of electric fields, the THz radiation power is easily saturated and difficult to improve.

The radiation power of THz PCA is affected by many parameters, such as current density, bias voltage, selected laser power, and repetition frequency. However, the micro-nano structures can effectively improve carrier mobility to form more obvious local enhancement of electric fields at the interface with LT-GaAs substrate. Therefore, based on the surface plasmas theory, our paper adopts the finite difference time domain (FDTD) method. With the purpose to study the efficiency enhancement of THz PCA, the period and structural parameters of micro-nano structures are calculated and simulated by FDTD software. In simulating THz wave radiation of PCA, the laser irradiates at the gap between the PCA electrodes, which stimulates the transient photocurrent in the substrate. The transient photocurrent is incorporated into the FDTD calculation as the current source, which makes the FDTD algorithm can be employed in semiconductor calculation.

Adding a dielectric anti-reflection layer on the substrate surface can increase the absorption rate of incident light of micro-nano structures. Therefore, the Si₃N₄ anti-reflection layer can be added during simulation to improve efficiency. After the THz PCA model is built, micro-nano columnar structures are added on the substrate surface to study the enhancement of electric fields and the reduction of reflection after-wave. The changes in transmittance and reflectance monitors before and after adding micro and nano structures are observed and recorded. It should be noted that if the micro-nano structures between the electrodes are too small, it is easy to melt under a strong photocurrent, which results in a short PCA circuit. If the micro-nano structures among the electrodes are too thick, the absorption rate will also be greatly reduced despite significantly reduced reflectivity, causing decreased overall efficiency of the THz PCA. Therefore, a balanced structure should be selected during the simulation to reduce the reflectivity with a high absorption rate. Additionally, the distribution period of the columnar micro-nano structures also affects the generation of THz waves. The micro-nano structures with different arrangement distribution forms and densities are simulated respectively, and the results of single-layer structures are selected (Fig. 4).

To explore the relationship between the period and the transmittance of double-layer micro-nano structures, we expand the simulation range to try a variety of different period combinations, which can achieve maximum efficiency and ensure feasibility. During the simulation, other parameters of the upper micro-nano structures are not changed to ensure that the selected structure has the same upper layer transmittance, except for the increasing number after expanding the simulation range. In addition, the lower micro-nano structures should be interspersed below the gap of the upper micro-nano structures at an appropriate spacing. On the contrary, the generation efficiency of THz waves may be reduced if the lower micro-nano structures are too dense.

Under the 1550 nm 100 fs laser pulse light source, the cylindrical micro-nano structure with a diameter of 0.1225 μm and a height of 0.75 μm arranged by 3×4 triangles with a period of 1 μm has a small transmittance to generate THz waves, which means a high absorption rate. Meanwhile, the electric field intensity (Fig. 5) and the transmittance (Fig. 6) are shown in the figures. The maximum intensity of the central electric field is 0.55 and the transmittance is about 0.16. The double-layer heterogeneous micro-nano structure (Fig. 9), electric field intensity (Fig. 10), and transmittance obtained through calculation and scanning (Fig. 11) are shown. The transmittance is about 0.09, and the electric field intensity of the central part reaches the highest value of 1.02, which is 185.45% of the single-layer micro-nano structure. It can be concluded that under the 1550 nm 100 fs laser pulse light source, the addition period is 1 μm and the diameter of the 4×5 equilateral triangle arrangement is 0.1225 μm, with the height of 0.75 μm and the period of 0.5 μm. The cylindrical micro-nano structure with a diameter of 0.17 μm and a height of 0.5 μm in the 8×8 square arrangement has a higher absorption rate.

Single-layer equilateral triangular cylindrical nanocrystals are proven to be better than other single-layer structures in other conditions. The double-layer heterogeneous micro-nano structures are superior to the single-layer micro-nano structures and other double-layer structures. This double-layer heterogeneous micro-nano structure has structural innovation, which can make the THz PCA generate high-quality THz waves. Additionally, the depth-to-width ratio and frequency of the proposed micro-nano structure can be processed by the existing plasma etching technology.

Due to the toxicity of lead, commercial application of lead halide perovskites will cause environmental pollution. Therefore, replacing lead with nontoxic elements has been a focus in this field. Tin-based halide perovskite has a near-infrared optical response, which can effectively solve the toxicity of lead-based perovskite and exhibit properties comparable to lead-based perovskite. High-quality CH₃NH₃SnI₃ nanoplatelet with a smooth surface, regular shape, and controllable size is of great significance for the development of micro- or nano-optoelectronic devices. Currently, CH₃NH₃SnI₃ is mainly synthesized through the solution method. The development of novel synthetic routes for high-quality CH₃NH₃SnI₃ is critical for high-performance lead-free optoelectronic devices. The chemical vapor deposition method without the use of solvents, which can prevent the evolution of grain boundaries and surface defects, has been proven in the preparation of high-quality micro- or nano-structured perovskite. In this paper, a two-step vapor deposition method is developed to prepare high-quality CH₃NH₃SnI₃ nanoplatelets. The dependence of sizes and compositions of the nanoplatelets on deposition time, H₂ flow rate, conversion time, and Ar flow rate is systematically studied through experiments combined with crystal nucleation theory. High-quality CH₃NH₃SnI₃ nanoplatelets with controllable sizes and uniform surfaces are achieved, where the sizes can be controlled between 8-41 μm. They show good near-infrared (920 nm) photoluminescence performance. In addition, to slow down the oxidation rate of tin-based perovskite, which is another big challenge, researchers have proposed various solutions such as solution doping. Unfortunately, the effect of doping will also change the overall structure. Therefore, we achieve high-stability (more than 48 h in N₂ atmosphere) CH₃NH₃SnI₃ nanoplatelets by passivating the surface of the prepared nanoplatelets through polymethyl methacrylate (PMMA) coating, which does not destroy the molecular structure of the perovskite. This lead-free perovskite nanomaterial with controllable size and composition can be applied to develop near-infrared optoelectronic devices in the future.

We employ a two-step chemical vapor deposition method. Firstly, SnI₂ nanoplatelet precursor with a smooth surface, regular shape, and controllable size (8-41 μm) is successfully prepared on a mica substrate by adjusting the H₂ flow rate and reaction time. Then, the SnI₂ nanoplatelets are converted into CH₃NH₃SnI₃ by using an Ar-driven reaction between CH₃NH₃I and the precursor. The surface morphology and chemical compositions of the nanoplatelets are analyzed through optical microscopy and X-ray diffraction. Based on crystal nucleation theory, the effects of H₂ flow rate and reaction time on the surface morphology and size of SnI₂ nanoplatelets are systematically studied. The effects of Ar flow rate and conversion time on the composition and photoluminescence properties of the prepared CH₃NH₃SnI₃ nanoplatelets are also studied. Then absorption and photoluminescence of CH₃NH₃SnI₃ nanoplatelets prepared under appropriately optimized conditions are measured to characterize their quality. Finally, stability tests are conducted on the prepared nanoplatelets to characterize their stability through the passivation effect of PMMA film in the N₂ atmosphere.

The prepared SnI₂ nanoplatelets have uniform colors and regular shapes. By adjusting the H₂ flow rate and reaction time, nanoplatelets with controllable sizes ranging from 8 to 41 μm are prepared (Fig. 2). We find that the driving force of flow rate will affect the uniformity of deposition. As the reaction time increases, the desorption rate of the substrate increases, and appropriate conditions are critical for fabricating nanoplatelets with controllable sizes. The X-ray diffraction images and absorption spectra of the prepared CH₃NH₃SnI₃ nanoplatelets show that the nanoplatelet is mainly composed of CH₃NH₃SnI₃ and have narrower bandgaps. Subsequently, the nanoplatelet also exhibits good near-infrared (920 nm) photoluminescence characteristics (Fig. 4). A PMMA thin film that is spin-coated on the surface of the prepared nanoplatelets is also demonstrated to prevent the oxidation characteristics of tin. X-ray diffraction images of PMMA-passivated CH₃NH₃SnI₃ at different time shows that the perovskite exhibits great stability under a N₂ atmosphere for more than 48 h [Fig. 5(b)].

In the paper, we prepare high-quality single crystal CH₃NH₃SnI₃ nanoplatelets with controllable sizes by using a two-step chemical vapor deposition method and systematically study the effects of flow rate, reaction time, and other factors on the crystal size and morphology of CH₃NH₃SnI₃ nanoplatelets. We find that when the H₂ flow rate ranges from 14 mL/min to 18 mL/min, and the reaction time is 35 min, the prepared nanoplatelets has a regular shape and smooth surface. The size of the nanoplatelets increases with the increase in flow rate, and the average side length increases from 8 μm to 41 μm. At the same time, the density of nanoplatelets also increases. In the second step, the Ar flow rate is tuned from 38 mL/min to 42 mL/min, and the content of CH₃NH₃SnI₃ in the nanoplatelets increases first and then decreases. As the reaction time increases, the CH₃NH₃SnI₃ content in the nanoplatelets show similar behavior. Full conversion into CH₃NH₃SnI₃ is achieved at an Ar flow rate of 40 mL/min and reaction time of 160 min. The study of absorption spectra and steady-state PL spectra shows that CH₃NH₃SnI₃ nanoplatelets have good near-infrared (920 nm) photoluminescence characteristics. In addition, we also present a passivation method for adding PMMA thin films on the surface of nanoplatelets by spin coating, which can effectively isolate the water oxygen problem of Sn²⁺ and has good stability in the N₂ atmosphere for over 48 h. With the further improvement of the stability of tin-based perovskite nanoplatelets, the photoluminescence properties of tin-based perovskite nanoplatelets will be improved. This work lays the foundation for the research on lead-free perovskite nanoplatelets and is expected to significantly improve the performance of optoelectronic devices.

The whispering-gallery mode (WGM) optical microresonator facilitates the continuous propagation of light waves with minimal loss, owing to total internal reflection. This characteristic can considerably enhance the interaction between light and matter, increase the efficiency of nonlinear effects, and remarkably reduce the threshold for nonlinear effects. Various microresonator-based nonlinear optical effects, such as stimulated Raman scattering, stimulated Brillouin scattering, and four-wave mixing, have been extensively researched. Studies of microresonator-based nonlinear optics have been applied in several research avenues, including optical switches, nonlinear optical devices, and precision measurement. Indeed, the study of nonlinear optics in WGM optical microresonators is of considerable importance. Crystalline optical microresonators offer unique benefits over silica-based WGM optical microresonators. One major example is the calcium fluoride (CaF₂) crystalline microresonator, which has emerged as an ideal platform for studying nonlinear optics due to its high nonlinear coefficient, low absorption coefficient, and suitability for long-term storage after processing. Researching the nonlinear effects based on CaF₂ crystalline microresonators entails excellent prerequisites. However, in the current scientific landscape, research into CaF₂ crystalline microresonators has not been extensively pursued. Additionally, the study of CaF₂ crystalline microresonator-based nonlinear optics is not widespread. In light of the above discussion, this study aims to further explore the potential of CaF₂ crystalline optical microresonators, particularly in the research of stimulated Brillouin scattering, stimulated Raman scattering, and other nonlinear effects in microresonators. Additionally, it aims to provide a preliminary foundation for subsequent nonlinear applications in CaF₂ crystalline microresonators.

The fabrication of CaF₂ crystalline microresonators with an ultrahigh quality factor up to 3.6 × 10⁸ was achieved using an ultraprecision polishing technique. This provided the prerequisite foundation for the study of nonlinear optics. We designed and constructed an experimental platform for studying nonlinear optics, where the pump laser's wavelength was manipulated using a tunable laser. The pump laser was amplified until its laser power approached the threshold; this amplification was achieved by employing an erbium-doped fiber amplifier. Thereby, CaF₂ microresonator-based nonlinear effects were excited. In the study of stimulated Brillouin scattering, Brillouin lasers and low-noise Brillouin cascade lasers were efficiently generated by modifying the pump wavelength and increasing the pump laser power. In order to acquire a Brillouin optical frequency comb, the pump wavelength was adjusted to scan from short to long wavelength. Consequently, we achieved a first-order Brillouin optical frequency comb with a perfect comb tooth state. Given that stimulated Raman scattering exhibited an ultrawide gain range, in our study, four-wave mixing assisted by stimulated Raman scattering can generate optical frequency combs at longer wavelengths. Different pump-wavelength detuning and pump power can be adjusted to optimize the signal-to-noise ratio of Raman lasers and the output of Raman combs. Furthermore, in the experimental demonstration of ultrawide Raman spectra, the coupling and interaction among numerous modes in the resonator may cause asymmetric comb tooth spacing distribution and power distribution.

We fabricate CaF₂ optical microresonators using ultraprecision machining techniques with a custom-built machining system. The quality factor of the microresonator attains a value of 3.6×10⁸, providing an appropriate platform for nonlinear optics (Fig. 3). We acquire substantial nonlinear experimental results, including a signal-to-noise ratio of 56.23 dB for the first-order stimulated Brillouin laser (Fig. 5) and 60 dB for the stimulated Raman laser (Fig. 7). Even for the fourth-order stimulated Brillouin laser in cascaded Brillouin systems, a signal-to-noise ratio of 26 dB (Fig. 5) is preserved. Furthermore, we manage to satisfy both the phase and energy requirements of stimulated Brillouin scattering and four-wave mixing by employing precise control mechanisms. The coupled-Brillouin optical frequency comb achieves a perfect state, resulting in an optical frequency comb with a single multiple of the free spectral range (Fig. 6). The four-wave mixing assisted by Raman laser generates an optical frequency comb with a bandwidth of 900 nm (Fig. 8), extending the comb tooth range into the visible light spectrum. Noteworthily, our results of CaF₂ microresonator-based nonlinear optics are more comprehensive compared with the results of early studies on CaF₂ microresonators. Additionally, certain experimental results were not demonstrated in early studies on CaF₂ microresonators. Our results provide strong evidence for CaF₂ microresonators being an ideal platform in nonlinear optics research.

Targeted investigations are conducted to explore the remarkable performance of CaF₂ crystalline optical microresonators in the realm of nonlinear optics. The CaF₂ microresonator, obtained using ultraprecision machining techniques, exhibits an ultrahigh quality factor of 3.6×10⁸. We have succeeded in exciting nonlinear effects. In the experiments, we simultaneously excite stimulated Brillouin scattering, stimulated Raman scattering, and the four-wave mixing effect in the CaF₂ microresonators at submilliwatt power levels. The results demonstrate the high-efficiency generation of stimulated Raman laser and Brillouin lasers. In addition, a fourth-order cascaded Brillouin laser is achieved by satisfying the frequency shift condition of Brillouin scattering. Particularly, Brillouin-coupled four-wave mixing, Raman-assisted Kerr effect, and ultrabroadband Raman optical frequency comb have been achieved by satisfying the corresponding phase-matching and energy-conservation conditions. The four-wave mixing process, with assistance from the Raman laser, yields optical frequency combs with a bandwidth of 900 nm. The spectral range of the optical frequency comb has been extended to the visible light wavelength regime. This achievement provides a foundation for subsequent research and development of applications such as lasers in the visible light wavelength range.

With the development of informatization, there is a growing demand for information transmission and processing. In this situation, it becomes urgent to increase the channel capacity. Currently, there are multiple multiplexing techniques available to enhance channel capacity, such as wavelength division multiplexing, polarization multiplexing, and mode division multiplexing (MDM). Among them, MDM utilizes different modes and polarizations of light to carry data and parallelly transmits multiple data channels in optical waveguides or fibers using only a single wavelength laser source, which can be seen as a new dimension to expand the capacity of optical fiber communication. The lithium niobate-on-insulator (LNOI) platform, with its strong electro-optic effect, low material loss, and wide transparent window, can achieve high-speed electro-optic modulators and optical nonlinear devices while providing high refractive index contrast waveguides, which thus makes it capable of manufacturing high-speed and high-density on-chip optical devices. However, there are few reports on MDM devices on LNOI platforms, and they mainly focus on the principle of phase matching of directional couplers loaded with LNOI platforms with silicon nitride. Although the mode converter based on the principle of phase matching of directional couplers has low processing difficulty and good scalability, it may have the problem of a relatively large footprint, which is not conducive to large-scale on-chip integration.

According to the coupled mode theory, when light propagates in a medium, the energy of the light field can be coupled from one mode to another mode by designing the appropriate medium structure. In this process, the perturbation of the medium structure not only satisfies the phase matching requirement of the two converting modes along the propagation direction of the z-axis but also has an appropriate refractive index distribution in the lateral direction to obtain an appropriate coupling coefficient and achieve a shorter coupling length. Metasurfaces are two-dimensional artificial materials with subwavelength features that can manipulate the phase, amplitude, and polarization of light waves at subwavelength scales through a special refractive index distribution. Integrating metasurfaces into optical waveguides can help deal with the relatively large footprint of converters based on the principle of phase matching of directional couplers, which is not conducive to large-scale on-chip integration. By leveraging the subwavelength-scale manipulation of light waves offered by metasurface structures, this study proposes a compact lithium niobate (LN) waveguide mode converter that can achieve TE₀-TE₁ or TE₀-TE₂ conversion in LN waveguides. In order to achieve coupling between modes, a reverse design method is adopted, and three-dimensional (3D) electromagnetic field simulation is utilized to optimize the subwavelength periodic stripe etching structure parameters of the device, so as to meet the phase matching requirements along the propagation direction and the lateral direction refractive index distribution with a high coupling coefficient.

Figure 3 shows the top view of the designed TE₀-TE₁ LN waveguide mode converter, the simulated modal field distribution, insertion loss, and crosstalk between modes, as well as the modal field distribution of the cross-sectional planes that are perpendicular to the propagation directions at the input and output. It can be seen that the energy of the TE₀ mode gradually decreases during propagation and is gradually converted into that of the TE₁ mode. Within the bandwidth of 1400-1700 nm, the insertion loss is less than 0.8 dB, with a minimum of 0.3 dB at 1520 nm; the crosstalk between modes is less than -10 dB, with a minimum of -38 dB at 1473 nm, and the extinction ratio is 37.3 dB. The low crosstalk between modes means that most of the energy of the TE₀ mode is converted into the energy of the TE₁ mode, and the energy loss is not significant, thus making the mode converter suitable for the field of optical communication. To demonstrate the scalability of this design, the TE₀-TE₂ mode conversion design is also presented in Fig. 4. It shows that the energy of the TE₀ mode gradually decreases during propagation and is gradually converted into that of the TE₂ mode. Within the bandwidth of 1400-1700 nm, the insertion loss is less than 2.4 dB, and the crosstalk between modes is less than -10 dB. The low crosstalk between modes means that most of the energy of the TE₀ mode is converted into the energy of the TE₂ mode, and the energy loss is not significant. In order to evaluate the effect of process errors on the performance of the designed structure and ensure the reproducibility of the device, finite-difference time-domain (FDTD) simulations of the insertion loss and crosstalk between modes in TE₀-TE₁ and TE₀-TE₂ mode conversions are carried out for processing errors of the etching groove width d and etching groove sidewall angle α (Figs. 5-8). It can be seen that the designed device has good tolerance to process errors in d and α.

This study proposes a compact LN waveguide mode converter based on metasurface structures, which can achieve TE₀-TE₁ and TE₀-TE₂ conversions. In order to achieve efficient coupling between modes, a reverse design method is adopted, and 3D electromagnetic field simulation is utilized to optimize the parameters of the tilted periodic sub-wavelength stripe etching structure of the device, which complies with the phase matching requirements of mode conversion along the propagation direction and the transverse refractive index distribution requirements of short coupling length. Simulation results show that the device has an insertion loss of less than 0.8 dB and crosstalk between modes of less than -10 dB in the wavelength range of 1400-1700 nm for TE₀-TE₁ conversion, with a conversion length of about 20 μm. In addition, the device has good scalability and process tolerance for higher-order mode conversion, making it a good candidate for mode converters in high-density integrated MDM systems in future LNOI.