Optical cavities play crucial roles in enhanced light–matter interaction, light control, and optical communications, but their dimensions are limited by the material property and operating wavelength. Ultrathin planar cavities are urgently in demand for large-area and integrated optical devices. However, extremely reducing the planar cavity dimension is a critical challenge, especially at telecommunication wavelengths. Herein, we demonstrate a type of ultrathin cavities based on large-area grown Bi₂Te₃ topological insulator (TI) nanofilms, which present distinct optical resonance in the near-infrared region. The result shows that the Bi₂Te₃ TI material presents ultrahigh refractive indices of >6 at telecommunication wavelengths. The cavity thickness can approach 1/20 of the resonance wavelength, superior to those of planar cavities based on conventional Si and Ge high refractive index materials. Moreover, we observed an analog of the electromagnetically induced transparency (EIT) effect at telecommunication wavelengths by depositing the cavity on a photonic crystal. The EIT-like behavior is derived from the destructive interference coupling between the nanocavity resonance and Tamm plasmons. The spectral response depends on the nanocavity thickness, whose adjustment enables the generation of obvious Fano resonance. The experiments agree well with the simulations. This work will open a new door for ultrathin cavities and applications of TI materials in light control and devices.

We demonstrate spectral-furcated vector solitons in normal-dispersion fiber lasers comprising a section of polarization-maintaining fiber. The spectrum of each orthogonal-polarized component is confined by the birefringence-related phase-matching principle, and the bicorn spectral structure corresponds to the zero-order sidebands of two vector modes. Due to the Hopf bifurcation effect, the vector soliton evolves into a breathing state at the higher pump level, accompanied by an extra set of sub-sidebands that continuously exchange energy with the zero-order sidebands. Simulation results fully reproduce experimental observations of the spectral furcation and soliton breathing, offering comprehensive insights into the pulse-shaping mechanism of the birefringence-managed soliton.

As a universal phenomenon in nonlinear optical systems, intermittency is usually accompanied by the coherence loss such as soliton explosions in fiber lasers. Based on real-time spectroscopy, we revealed the coherent dissipative soliton intermittency in normal-dispersion fiber lasers. By increasing the pump strength, the intermittency transforms from the transient pulsation to the bi-stable soliton. It is demonstrated that the slow-gain effect dominates such coherent intermittency. Our results provide novel insights into laser physics, offering a promising approach for studying the bi-stable dissipative soliton.

Cylindrical vector beams and vortex beams, two types of typical singular optical beams characterized by axially symmetric polarization and helical phase front, possess the unique focusing property and the ability of carrying orbital angular momentum. We discuss the formation mechanisms of such singular beams in few-mode fibers under the vortex basis and show recent advances in generating techniques that are mainly based on long-period fiber gratings, mode-selective couplers, offset-spliced fibers, and tapered fibers. The performances of cylindrical vector beams and vortex beams generated in fibers and fiber lasers are summarized and compared to give a comprehensive understanding of singular beams and to promote their practical applications.

Surface-enhanced Raman spectroscopy (SERS) with high-sensitivity performance is a very necessary detection technology. Here, we present a method for increasing the performance of SERS based on silver triangular nanoprism arrays (ATNAs) vertically excited via a focused azimuthal vector beam (AVB). The ATNA substrates with different structural parameters are prepared based on the traditional self-assembled and modified film lift-off technique. Based on a theoretical model established adopting the structural parameters of the ATNA substrates, theoretical simulation results show that AVB excitation can achieve greater electric-field enhancement than linearly polarized beam (LPB) excitation. Experimental result indicates that SERS sensitivity obtained via AVB excitation is 10 13M (1 M = 1 mol/L) using rhodamine 6G (R6G) as the target analyte, which is 2 orders of magnitude lower than that of LPB excitation (10 11M). Meanwhile, the uniformity and reproducibility of the ATNA substrates are examined using Raman mapping and batch-to-batch measurement, respectively, and the Raman enhancement factor is calculated to be ～3.3×107. This method of vector light field excitation may be used to improve the SERS performance of the substrates in fields of ultra-sensitive Raman detection.

The synergy of a plasmonic tip and fiber-based structure light field excitation can provide a powerful tool for Raman examination. Here, we present a method of Raman spectrum enhancement with an Ag-nanoparticles (Ag-NPs)-coated fiber probe internally excited via an azimuthal vector beam (AVB), which is directly generated in a few-mode fiber by using an acoustically induced fiber grating. Theoretical analysis shows that gap mode can be effectively generated on the surface of the Ag-NPs-coated fiber probe excited via an AVB. The experimental result shows that the intensity of Raman signal obtained with analyte molecules of malachite green by exciting the Ag-NPs-coated fiber probe via an AVB is approximately eight times as strong as that via the linear polarization beam (LPB), and the activity of the AVB-excited fiber probe can reach 10 11 mol/L, which cannot be achieved by LPB excitation. Moreover, the time stability and reliability are also examined, respectively.

We demonstrate an all-fiber Q-switched cylindrical vector laser based on a black phosphorus saturable absorber and a transverse mode converter. The saturable absorber is prepared by incorporating the polyvinyl alcohol with anti-oxidized black phosphorus nanosheets exfoliated in aqueous poly(dimethyldiallyl ammonium chloride) solution. The mode converter is composed of a tapered two-mode fiber and a single-mode fiber, and it can excite switchable azimuthally and radially polarized beams by modulating the input polarization. By enhancing the pump power from 64.68 to 174.82 mW, the repetition rate of the Q-switched azimuthally/radially polarized laser enlarges from 16.72/19.25 to 30.71/37.82 kHz.

We present a detailed analysis on mode evolution of grating-coupled surface plasmonic polaritons (SPPs) on a conical metal tip based on the guided-wave theory. The eigenvalue equations for SPPs modes are discussed, revealing that cylindrical metal waveguides only support TM₀₁ and HE_m1 surface modes. During propagation on the metal tip, the grating-coupled SPPs are converted to HE₃₁, HE₂₁, HE₁₁ and TM₀₁ successively, and these modes are sequentially cut off except TM₀₁. The TM₀₁ mode further propagates with drastically increasing effective mode index and is converted to localized surface plasmons (LSPs) at the tip apex, which is responsible for plasmonic nanofocusing. The gap-mode plasmons can be excited with the focusing TM₀₁ mode by approaching a metal substrate to the tip apex, resulting in further enhanced electric field and reduced size of the plasmonic focus.