Second harmonic generation from an individual amorphous selenium nanosphere

Abstract

Among the numerous nonlinear optics effects, second harmonic generation (SHG) is always a hotspot and it is extensively used for optical frequency conversion, biomedical imaging, etc. However, SHG is forbidden in a medium with inversion symmetry under the electric-dipole approximation. Here, we demonstrated SHG from a single amorphous selenium (a-Se) nanosphere under near-infrared femtosecond pulse excitation. It was found that SH spectra are tunable with the size of a-Se nanospheres and the SHG efficiency of a single a-Se sphere with a diameter over 300 nm is estimated at 10(-8). We also established two physical mechanisms of SHG from the amorphous nanospheres. There is an electric-dipole contribution to the second-order nonlinearity in view of the inevitable structural discontinuity at the surface. The discontinuity of the normal component of the electric field strength leads to the quadrupole-type contributions arising from the large electric field gradient. The SHG process can be enhanced by resonance near the fundamental wavelength, giving rise to the detectable second harmonic (SH) spectra of a single a-Se nanosphere (d > 300 nm) or two small a-Se nanospheres (d = 200 nm) aggregated into a dimer, while the single nanosphere with smaller size (d > 300 nm) is undetectable. As an essential trace element for animals, a-Se features unique biological compatibility and has specific properties of optical nonlinearity within the optical window in biological tissue. This discovery makes a-Se nanospheres promising both in nonlinear optics and biomedicine.

Nonlinear Dielectric Nanoantennas and Metasurfaces: Frequency Conversion and Wavefront Control

“…Among them, surface SHG is expected to be the dominant component, following numerical studies on Si nanoantennas . In this scenario, a-Se nanoparticles and metasurfaces , have shown experimental SH conversion efficiencies up to 10 –6 , while a similar nonlinear response was measured for single poly-Si nanoparticles, where the inversion symmetry is broken at the many interfaces between the inner grains.…”

“…Although weaker, nanostructures made of centrosymmetric materials also exhibit SHG as the inversion symmetry breaks at interfaces. Polycrystalline Si (poly-Si), amorphous Si (a-Si), a-GaP, and a-Se , have been studied in this regard.…”

Nonlinear optics with resonant metasurfaces

“…48,50 Alternative realizations of metasurfaces and isolated nanoresonators were based on centrosymmetric materials, as e.g. amorphous selenium, 54 (001) GaP, 55 nano-crystalline Si, 56 and amorphous Si. 57 To induce the normally symmetryforbidden second-order nonlinear response from these nanostructures argument C from the discussion above was exploited using geometric symmetry breaking either at the surfaces or nanocrystals' boundaries in the nanoresonators.…”

Full-Color Laser Displays Based on Optical Second-Harmonic Generation from the Thin Film Arrays of Selenium Nanowires

“…A set of experiments clearly demonstrates the correlation between the strong polarization-dependent SHG signals and specific laser irradiation with polarization angles. The maximum SHG signals for the isolated single Se NW was collected by introducing different focused laser beams onto an individually isolated NW (i.e., λ = 875, 1075, and 1175 nm) as shown in Figure g, in which the SHG ability of Se NWs generated tunable and coherent light throughout the RGB spectrum via nonlinear optical properties . For the single SHG spectrum, the integrated intensity was displayed in the polar plot as a function of the polarization angle as presented in Figure h.…”