Azimuthal Vector Beam Research Articles

Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy

Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy

Multi-Function Reflective Vector Light Fields Generated by All-Dielectric Encoding Metasurface.

Traditional optics usually studies the uniform polarization state of light. Compared with uniform vector beams, non-uniform vector beams have more polarization information. Most of the research on generating cylindrical vector beams using metasurfaces focuses on generating transmitted beams using the geometric phase. However, the geometric phase requires the incident light to be circularly polarized, which limits the design freedom. Here, an all-dielectric reflective metasurface is designed to generate different output light according to the different polarization states of the incident light. By combining the two encoding arrangements of the dynamic phase and the geometric phase, the output light is a radial vector beam when the linearly polarized light is incident along the x-direction. Under the incidence of linearly polarized light along the y-direction, the generated output light is an azimuthal vector beam. Under the incidence of left-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of -1. Under the incidence of right-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of +1. The proposed reflective metasurface has potential applications in generating vector beams with high integration.

Excitation of surface plasmon polaritons by diffraction-free and vector beams.

Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse-magnetic (TM) Gaussian beam. Here we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel-Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization; then we couple the respective TM components of radial and azimuthal vector beam profiles to confirm polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at a greater extent, hereby outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits.

Nonlinear optical heating of all-dielectric super-cavity: efficient light-to-heat conversion through giant thermorefractive bistability

Abstract Optical heating of resonant nanostructures is one of the key issues in modern nanophotonics, being either harmful or desirable effect depending on the applications. Despite a linear regime of light-to-heat conversion being well-studied both for metal and semiconductor resonant systems is generalized as a critical coupling condition, the clear strategy to optimize optical heating upon high-intensity light irradiation is still missing. This work proposes a simple analytical model for such a problem, taking into account material properties changes caused by the heating. It allows us to derive a new general critical coupling condition for the nonlinear case, requiring a counterintuitive initial spectral mismatch between the pumping light frequency and the resonant one. Based on the suggested strategy, we develop an optimized design for efficient nonlinear optical heating, which employs a cylindrical nanoparticle supporting the quasi bound state in the continuum mode (quasi-BIC or so-called ‘super-cavity mode’) excited by the incident azimuthal vector beam. Our approach provides a background for various nonlinear experiments related to optical heating and bistability, where self-action of the intense laser beam can change resonant properties of the irradiated nanostructure.

High-power, femtosecond vortex beams generation in the visible and near-infrared region

Ultrafast vortex beams have been successfully utilized in a wide variety of applications. Here, we report on the generation of the visible and near-infrared cylindrical vector beams with femtosecond pulse duration and tens of Watts average output power. Based on an optimized fiber chirped-pulse amplification system and second harmonic generation technology, Gaussian beams are generated at the visible and near-infrared wavelength, respectively. After mode conversion operation, the high-quality cylindrical vector beams can be obtained with a central wavelength of 1036.13 nm and maximum output power and ultrafast pulse duration of 60.49 W/824 fs for radial vector beams and 59.67 W/783 fs for azimuthal vector beams. In the visible spectral region, the output cylindrical vector beams have a maximum output power of 30.78 W (radial vector beams) and 27.46 W (azimuthal vector beams) at 515.75 nm central wavelength. The pulse durations are measured to be 262 fs for radial vector beam and 273 fs for azimuthal vector beam at the maximum output power. The results demonstrate that the proposed laser systems may pay a promising way to develop high-power femtosecond cylindrical vector beams sources in the visible and near-infrared wavelength for optical communication, super-resolution imaging and laser processing.

Growth of Nanostructured Silver Flowers by Metal-Mediated Catalysis for Surface-Enhanced Raman Spectroscopy Application.

Metallic flowers with nanoscale surface roughness can provide a platform for highly sensitive and reproductive surface-enhanced Raman spectroscopy (SERS). Here, we present a method to grow a nanostructured silver flower (NSF) at the apex of a plasmonic tip based on metal-mediated catalysis, where the NSF was rapidly generated in no more than 1 min. The NSF was used as the SERS substrate under linear polarization beam (LPB) excitation to achieve a 10–9 M detection sensitivity for the malachite green analyte. The reproducibility for SERS is examined to have been guaranteed by comparing Raman intensity enhanced by different NSFs. Compared with the LPB, the azimuthal vector beam (AVB) excitation can further improve the SERS activity of the NSF, which is consistent with the simulation result that the gap mode can be effectively generated between two adjacent Ag nanoparticles (NPs) and between the NPs and the Ag pyramids on the surface of the NSF under AVB illumination. This work makes it promising for plasmonic tip-mediated catalysis to be applied in nanofabrication, the products of which can be further exploited in nanostructure-based ultrasensitive detection.

Second-order surface optical nonlinear response of plasmonic tip axially excited via ultrafast vector beams

We present the second-order surface optical nonlinear response of a plasmonic tip under axial excitation of ultrafast vector beams. Theoretical calculations show that the tip has an optimized nanofocusing characteristic under axial excitation of the radial vector beam (RVB), and the electric-field intensity enhancement factor is higher than that of linear polarization beam (LPB) and azimuthal vector beam (AVB) excitations. In the experiment, the second harmonic spectra are clearly measured under three excitation beams. The intensity of the second harmonic obtained via RVB excitation is one order of magnitude higher than that of LPB and AVB excitations.

Plasmon-enhanced nonlinear nanofocusing of gold nanoprisms driven via an ultrafast azimuthal vector beam.

We present the plasmon-enhanced nonlinear nanofocusing of a gold (Au) nanoprism array substrate (ANAS) driven via an ultrafast azimuthal vector beam (AVB). Theoretical calculations show that the electric-field intensity of the ANAS vertically excited via the femtosecond AVB is higher than that of LPB excitation. In this experiment, the second-order surface nonlinear optical response of the ANAS is adopted to examine the nonlinear plasmonic nanofocusing of the ANAS, and it was observed that the second harmonic (SH) intensity of the ANAS excited via the femtosecond AVB is ∼3.8 times higher than that of LPB excitation, revealing that the ANAS under AVB excitation has a better nonlinear plasmonic nanofocusing characteristic than that under LPB excitation. Furthermore, the GaSe nanosheets are transferred on the ANAS to examine the nonlinear plasmonic nanofocusing of the ANAS. The SH intensity of the GaSe nanosheets deposited on the ANAS via the femtosecond AVB excitation has been enhanced ∼4.7 times than that of LPB excitation, indicating that the ANAS via AVB excitation has better nonlinear plasmonic nanofocusing than that of LPB excitation. This method may be used as a nonlinear nanofocusing light source to increase the light-matter nonlinear interaction.

Azimuthal vector beam exciting silver triangular nanoprisms for increasing the performance of surface-enhanced Raman spectroscopy

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 10−13 M (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 (10−11 M). 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.

Plasmonic tip internally excited via an azimuthal vector beam for surface enhanced Raman spectroscopy

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.

Study on the properties of vector beams generated by a curved wave plate in the strong-focusing regime

We propose a new scheme to generate the axisymmetric vector beam. A curved wave plate, designed by using the birefringence properties of a crystal, can generate two different phase distributions with respect to the o-light and e-light, and it then can transform the linearly polarized light into the radial or azimuthal vector beam with the property of rotational symmetry. The above scheme has the advantage of having a simple light path and can be adjusted conveniently. According to Richards-Wolf's model of the classical vector diffraction, we calculate the distributions of the diffracted electromagnetic field that is illuminated by a hollow Gaussian beam and focused by a lens with high numerical aperture. Results show that the hollow vector beam has a very high intensity and intensity gradient and longitudinal distribution of the electric or magnetic field even if illuminated by the laser with a power of 0.5 W. In addition, real-time adjusted distributions of the photon angular momentum can be generated. This scheme has good application prospects in the manipulations of the microscopic particles.