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Nanoparticles enhance the second harmonic generation mechanism of lithium niobate


Recently, the team of Byron D. Gates from Simon Fraser University in Canada proposed a new mechanism: the second harmonic generation of individual lithium niobate particles can be significantly enhanced by non-radiative coupling to the local surface plasmon resonance of embedded gold nanoparticles. This new mechanism greatly improves the nonlinear efficiency, provides new ideas for the study of nonlinear optics, and plays an extremely important role in the design of new sublinear nonlinear optical platforms.

Research background

Ultrafast optical frequency conversion generated by the use of nonlinear optical harmonics in micro-nanomaterials has gradually become the key to the development of new sensing and imaging technologies in the fields of photonics foundation and application, materials science, chemistry and biology. Second harmonic generation (SHG) is one of the simplest nonlinear optical processes. In the SHG process, two input fields oscillating at fundamental frequencies are coherently combined to form an output field that oscillates at twice the fundamental frequency. SHG has attracted a lot of attention for its narrowband operation, relatively high efficiency in nonlinear optical processes, and natural generation in a variety of inorganic materials.

However, even in excellent nonlinear optical materials, the nonlinear fluctuations underpinning SHG are often still weaker or slower than the ~1 picosecond timescale required for ultrafast applications. In order to overcome this difficulty, scholars have carried out SHG photonics designs in large nonlinear optical crystals so that fundamental and SH waves can interfere with each other to overcome the inherently low SHG efficiency. However, due to the complex field distribution of nanostructured particles, this method is not effective for nanoscale SHG devices. This results in the absolute efficiency of nanoscale ultrafast nonlinear optical processes often depending on the microscopic volume of the nonlinear optical material used. Therefore, if the breakthrough of nonlinear optical efficiency limitation under nanometer size can be achieved, it will undoubtedly have extremely important research and application value.

Innovative research

In this study, the researchers solved the scientific and engineering challenges faced in nonlinear nanophotonics by creating a new energy transfer pathway for the local conversion of light in nanolocalization by using the surface localized near-field of the disordered array of gold nanoparticles. The disordered nanostructure in this scheme consists of mesoporous lithium niobate microspheres with a diameter of about 1 μm, and the microspheres are covered in a dispersed layer of gold nanoparticles with a diameter of 10 nm, which can directly realize the high-throughput synthesis process (Figure 1), while effectively avoiding the difficulties of precision nanofabrication, a nonlinear optical system with a large number of field strength enhancement regions is generated between the lithium niobate and gold surfaces.

The experimental results show that the geometry designed in this study enhances SHG by a factor of 32. Further analysis based on measurements and characterization showed that this enhancement derives from near-field localization and resonance behavior of metal nanoantennas that are generally considered unsuitable for far-field enhancement (Figures 2, 3). Therefore, the results not only achieve a nonlinear optical enhancement factor of a single disordered nanostructure greater than 10 for the first time, but also discover a new mechanism by which strong subwavelength nanoparticles can enhance SHG (Figure 4). This result is of great significance for the research and application of nonlinear optical devices under nanometer size, and also promotes the development of various fields using nonlinear optical technology.

Figure 1: Synthesis of hybrid gold-lithium niobate nanostructures. (a) Schematic representation of gold nanoparticles and lithium niobate hybrids prepared using in situ synthesis and characterized by scanning transmission electron microscopy (STEM) of the collection of gold-lithium niobate hybrids in the following modes. (b,c,d) High-angle annular dark-eld (HAADF) mode, energy dispersive X-ray spectroscopy (EDS) of gold-lithium niobate hybrids obtained by STEM technology was used to plot the following EDS plot. (e) Gold nanoparticles. (f) HAADF image of gold nanoparticles overlaying components. (g) Superimpose gold-niobium signals in these components.

Figure 2: Experimental process. (a) Schematic of the microscope configuration for characterizing the second harmonic generation (SHG) response of a single particle with a reflection mode setup with a femtosecond laser, half-wave plate, and objectives of different magnifications, including a 63x oil-immersion lens. (b) Schematic diagram of sample geometry of hybrid gold-lithium niobate particles.

Figure 3: Spectra obtained from experimental measurements and theoretical calculations. (a) Measured extinction spectra of pure lithium niobate particles (black) and hybrid gold-lithium niobate particles (red) compared to simulated dipolar plasma extinction (gold). These spectra show that hybrid gold-lithium niobate particles have plasma peaks at wavelength 530 nm, while pure lithium niobate particles are optically transparent. (b) Light scattering spectra of a single pure lithium niobate particle and a single hybrid gold-lithium niobate particle measured in a darkfield configuration illuminated by white light. The illustration in (b) depicts a gray image (red) of a single lithium niobate particle (black) and a single gold-lithium niobate mixed particle supported on a glass coverslip (scale 2 μm). The data is maximum-normalized. (c) The theoretical scattering cross-section (green) of model spheroids with a dielectric constant of 6.3+0.05i and a radius of 700 nm is normalized from 0 to 1 compared to the scattering data of a single pure lithium niobate particle (black) experimentally measured.

Figure 4: Second harmonic generation enhancement. (a) SHG spectra of hybrid gold-lithium niobate particles when FW is tuned from 800 nm (violet) to 960 nm (red). (b) Power dependence of SHG at 800 nm. The power scattered at SH (blue dot) versus the input laser power at FW b = 1.99 (95% confidence interval ± 0.02, see [黑线]Multiplied by the value of the normalization constant C, it is proportional to. (c) The scattering SHG power of a single hybrid gold-lithium niobate nanostructure was recorded as a function of the fundamental wavelength by scanning the excitation laser wavelength from 850 nm to 1070 nm in steps of 5 nm. The experimental results (blue, red) coincide nearly with the simulated near-field coupling enhancement (black). Illustration: Response function of a narrow Mie-resonance of a pure LiNbO3 sphere as a function of sphere radius and photon energy. The maximum value of the response function (gold/white) is located at the same resonance energy ωβ as the SHG enhancement peak, which means that as a2 increases (decreases), the narrow SHG enhancement feature undergoes a red (blue) shift. The white point indicates the offset peak position displayed in the main panel. (d) Optical energy transmission path diagram within the system. From the left, clockwise: incident light (deep red), SHG (blue arrow) transfer of energy from nonlinear polarization (blue vector field) to Mie resonance (halo), superradiated Prucell-enhanced SHG, and near-field enhanced SHG.

The article was published in Light: Science & Applications, a top international academic journal, entitled “Near-Field Enhancement of Optical Second Harmonic Generation in Hybrid Gold-Lithium Niobate Nanostructures”, by Rana Faryad Ali is the first author and Byron D. Gates is the corresponding author. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1092-8

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