As one of the most important semiconductors, silicon has been used to fabricate electronic devices, waveguides, detectors, solar cells, etc. However, the indirect bandgap and low quantum efficiency (10⁻⁷) hinder the use of silicon for making good emitters. For integrated photonic circuits, silicon-based emitters with sizes in the range of 100−300 nm are highly desirable. Here, we show the use of the electric and magnetic resonances in silicon nanoparticles to enhance the quantum efficiency and demonstrate the white-light emission from silicon nanoparticles with feature sizes of ~200 nm. The magnetic and electric dipole resonances are employed to dramatically increase the relaxation time of hot carriers, while the magnetic and electric quadrupole resonances are utilized to reduce the radiative recombination lifetime of hot carriers. This strategy leads to an enhancement in the quantum efficiency of silicon nanoparticles by nearly five orders of magnitude as compared with bulk silicon, taking the three-photon-induced absorption into account.

Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety of modern photonic applications, but it remains challenging in nanophotonics. Surface second-order nonlinearity of noble metals can be significantly boosted up by plasmon-induced field enhancement, however the related far-field second-harmonic generation (SHG) may also be quenched in highly symmetric plasmonic nanostructures despite huge near-field amplification. Here, we demonstrate that the SHG from a single gold nanosphere is significantly enhanced when tightly coupled to a metal film, even in the absence of a plasmon resonance at the SH frequency. The light-induced electromagnetic asymmetry in the nanogap junction efficiently suppresses the cancelling of locally generated SHG fields and the SH emission is further amplified through preferential coupling to the bright, bonding dipolar resonance mode of the nanocavity. The far-field SHG conversion efficiency of up to $$3.56\times 10^{-7}$$3.56 × 1 0 − 7 W⁻¹ is demonstrated from a single gold nanosphere of 100 nm diameter, two orders of magnitude higher than for complex double-resonant plasmonic nanostructures. Such highly efficient SHG from a metal nanocavity also constitutes an ultrasensitive nonlinear nanoprobe to map the distribution of longitudinal vectorial light fields in nanophotonic systems.

The high spatial frequency periodic structures induced on metal surface by femtosecond laser pulses was investigated experimentally and numerically. It is suggested that the redistribution of the electric field on metal surface caused by the initially formed low spatial frequency periodic structures plays a crucial role in the creation of high spatial frequency periodic structures. The field intensity which is initially localized in the grooves becomes concentrated on the ridges in between the grooves when the depth of the grooves exceeds a critical value, leading to the ablation of the ridges in between the grooves and the formation of high spatial frequency periodic structures. The proposed formation process is supported by both the numerical simulations based on the finite-difference time-domain technique and the experimental results obtained on some metals such as stainless steel and nickel.

Periodic surface structures with periods as small as about one-tenth of the irradiating femtosecond (fs) laser light wavelength were created on the surface of a titanium (Ti) foil by exploiting laser-induced oxidation and third harmonic generation (THG). They were achieved by using 100-fs laser pulses with a repetition rate of 1 kHz and a wavelength ranging from 1.4 to 2.2 μm. It was revealed that an extremely thin Ti_xO_y layer was formed on the surface of the Ti foil after irradiating fs laser light with a fluence smaller than the ablation threshold of Ti, leading to a significant enhancement in THG which may exceed the ablation threshold of Ti_xO_y. As compared with Ti, the maximum efficacy factor for Ti_xO_y appears at a larger normalized wavevector in the direction perpendicular to the polarization of the fs laser light. As a result, the THG-dominated laser ablation of Ti_xO_y induces 100-nm periodic structures parallel to the polarization of the fs laser light. The depth of the periodic structures was found to be ~10 nm by atomic force microscopy and the formation of the thin Ti_xO_y layer was verified by energy dispersive X-ray spectroscopy.

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.