Achieving efficient and intense second-harmonic generation (SHG) in the terahertz (THz) spectrum holds great potential for a wide range of technical applications, including THz nonlinear functional devices, wireless communications, and data processing and storage. However, the current research on THz harmonic emission primarily focuses on inorganic materials, which often offers challenges in achieving both efficient and broadband SHG. Herein, the remarkable efficiency of organic materials in producing THz harmonics is studied and demonstrated, thereby opening up a new avenue for searching candidates for frequency-doubling devices in the THz band. By utilizing DAST, DSTMS, and OH1 crystals, we showcase their superior frequency conversion capabilities when pumped by the narrowband THz pulses centered at 2.4, 1.6, and 0.8 THz. The SHG spans a high-frequency THz domain of 4.8 THz, achieving an unprecedented conversion efficiency of ∼1.21% while maintaining a perturbative nonlinear response. The highly efficient SHG of these materials is theoretically analyzed by considering the combined effects of dispersion, phonon absorption, polarization, and the nonlinear susceptibility of organic crystals. This work presents a promising platform for efficient THz frequency conversion and generation across a wide range of frequencies, offering new opportunities for novel nonlinear THz applications in next-generation electronics and optics.
In this work, we experimentally demonstrate the Dyakonov surface wave mode at visible frequency in a hyperbolic
metasurface. The extremely strong anisotropy of the hyperbolic metasurface enables two Dyakonov surface waves on the
two surfaces of the hyperbolic metasurface. Strong coupling between the two surface waves forms a Dyakonov type
surface wave mode, which is highly directional and lossless, and has significant applications in two-dimensional photonic
circuits and devices.
It is generally believed that scaling the lattice constant is a basic method to achieve a frequency response due to the
sub-wavelength feature of metamaterials (MMs). In this paper, an alternative MMs design method by changing the line
width to configure resonance is proposed. Three planar arrays of electric-field-coupled (ELC) MMs with the line width
of 3μm, 4.5μm and 6μm for the THz frequency responses have been designed and fabricated, and their characteristics are
investigated both by simulations and experiments. Measurement results show that the ELC resonators show strong
couplings at the target frequency and an evident blue shift is obtained as the line width increased. An ELC resonator with
the resonance of 1.3THz is also simulated by increasing the line width to 12μm, which has altered 160% compared with
that of the 1μm line width ELC. The blue shift feature can also be applied in other MM structures for the microwave,
infrared and optical frequencies.
The surface plasmon resonance (SPR) between metal and dielectric material has a good enhancement on
electromagnetic wave transmission. In this paper, a series of two-dimension (2D) metal gratings and spiral
structures with different geometrical size were experimentally tested by Terahertz time-domain spectroscopy
(THz-TDS). The experiment results show that the 2D metal gratings have almost 70% increment on terahertz
transmission than the pure silicon substrate in the range of 0.2-2.5THz, which indicates a strong coupling in the
terahertz range, and the resonance mode shows a blue shift. On the other hand, the influence of different radiation
directions was analyzed. It presents that the slightly higher transmission can be achieved when terahertz wave
radiate from the front side than the back side. It reveals that surface plasmon resonance can enhance the terahertz
transmission efficiently and has potential applications in security imaging, biological analysis and spectroscopy.
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