A spectrally wide-band terahertz modulator based on monolayer graphene on germanium (GOG) is proposed. Utilizing a homemade THz-TDS (Terahertz-time domain spectroscopy) system, we experimentally demonstrated that the THz modulator can be tuned by a 1550 nm pump beam in a frequency range from 0.2 to 1.5 THz. The average transmittance of THz decreases from 40% to 22% when the pump power is increased to 250 mW, while the absorption coefficient averaged increases from 19 cm<sup>-1</sup> to 44 cm<sup>-1</sup>. The maximum modulation depth of the GOG modulator can reach as high as 62% at 0.38 THz and in a frequency range from 0.2 to 0.7 THz, the modulation depth is over 50%. Compared with bare Ge, it was proved that the modulation performance can be moderately enhanced by introducing monolayer graphene. This novel optically controlled graphene based THz modulator provides a feasible method for terahertz applications in communication and imaging.
Single or double femtosecond Bessel laser beams are employed to generate multiple annular beams with different central wavelengths. A set of annular beams with bandwidths of several tens of nanometers and different peak wavelengths discretely distributed in the visible and infrared wavelength range are generated. No supercontinuum emission is observed. The propagation directions of these annular beams are wavelength-dependent and different from the propagation direction of the pump femtosecond Bessel beams, so four-wave mixing and secondary parametric process are considered to be the generation mechanism of these annular beams, which is further confirmed by numerical simulations. The energy conversion efficiency of single generated annular beam is in the order of 10-6. It is expected that specific resonant structures of the third-order nonlinear optical susceptibility of the sample used in experiments can enhance the energy conversion efficiency at certain wavelengths. These simultaneously generated colorful annular beams are actually composed of light pulses with nearly the same durations as the pump femtosecond pulses, which can find important applications where multi-wavelength ultrashort light pulses are needed, such as optical telecommunications, sensing, and pump-probe measurements. It is considered that stimulated four-wave mixing and corresponding cascaded parametric process are the main generation mechanism.