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.
The novel time-resolved terahertz (THz) Kerr effect (TKE) spectroscopy is a powerful tool for the study of the intermolecular structure and low-frequency molecular dynamics of liquid water. Here, we further observe the Kerr effect response in free-flowing aqueous membranes induced by strong ultra-broadband THz pulses. The intermolecular hydrogen bonding dynamics of a series of ionic aqueous solutions were investigated and a significantly enhanced Kerr effect response associated with polarization anisotropy was obtained. We analyze the spectra of the obtained Kerr effect responses of water and aqueous solutions and focus on the vibrational features of multiple molecular modes on the spectra. We decompose the molecular responses of ionic aqueous solutions into Debye relaxation related to molecular reorientation motions, intermolecular hydrogen bond bending and stretching modes associated with restricted translational motions, and specific ion-water hydrogen bond vibrational modes. The effects of different ions and vibrational modes on the hydrogen bonding network excited by the THz electric field and their corresponding manifestations in the spectrum are analyzed and compared. Our measurements positively affect the deeper understanding of intermolecular dynamics in liquid media.
In this work, we use terahertz (THz) pulses and near-infrared laser pulses to excite molecular motion modes of liquid water in resonant and non-resonant states, and record the corresponding THz Kerr effect (TKE) and optical Kerr effect (OKE) curves at the sub-picosecond time scale. We analyze the frequency dependence and energy dependence of TKE and OKE responses in water. The characteristics and advantages of the two schemes in low-frequency molecular dynamics observation of liquid water are summarized by calculating the signal-to-noise ratio (SNR). Our work provides valuable insights into the microscopic origin of dielectric properties in water, and provides a new method for observing low-frequency molecular motion in various aqueous solutions and even in biological tissues.
The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.
Terahertz wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. For further understanding of the mechanism of the terahertz wave generation in laser-induced plasma as well as the terahertz modulator base on pre-formed air plasma, the generation of terahertz radiation using an effective wavelength scaling mechanism is examined when two-color laser fields are mixed in pre-formed plasma created by synchronized 800nm laser pulse. In our experiment, the effect of preformed plasma is investigated using an orthogonal pumping geometry. With a preformed plasma, both the modulation depth of terahertz radiation energy and the change of terahertz radiation polarization increases with increasing excitation laser wavelength. We found that the terahertz modulation depth and terahertz polarization changes increase as a function of the energy of the 800nm-prepulse. Some possible reasons are discussed. We attribute the terahertz polarization rotation to additional relative phase of the two-color fields introduced by the preformed air plasma. This provides a practical way to control the polarization and energy of terahertz pulses for potential applications.
Terahertz (THz) wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. The most popular and efficient laser-plasma scheme used for THz generation is the two-color scheme, in which a laser pulse at the fundamental frequency is supplemented by its second harmonic that is obtained with the use of a nonlinear crystal. The type-I β-barium borate (BBO) crystal plays a very important role in second harmonic generation. In this research, we investigate the THz generation efficiency with changing the thicknesses of the BBO crystals. Moreover, the wavelength of the excitation laser is tunable from 1200 nm to 1600 nm. The THz generation efficiency is characterized by rotating the BBO crystal with the same tilt angle, changing laser wavelength with invariant pump power. And we also record the two orthogonal components of THz electric field by rotating the ZnTe crystal. We think that the thickness of BBO crystal affects the phase difference between the two components along the ordinary axis and extraordinary axis, resulting in the change of the polarization state of the fundamental wave. Meanwhile, the frequency doubling efficiency of BBO has an impact on the power ratio of the two-color laser. This provides a practical way to control the polarization of THz pulses for potential applications.
For further understanding of the mechanism regarding terahertz (THz) wave generation in laser-induced plasma, the pump wavelength dependence of THz emission is examined when two-color laser fields are mixed in pre-formed plasma created by another 800nm laser pulse. In our experiment, the effect of pre-formed plasma is investigated using an orthogonal pumping geometry. With a pre-formed plasma, the power of THz wave generated by the pump pulse reduces significantly, and the THz modulation increases with the growth of pump wavelength. Possible reason for the result is discussed in terms of tunneling ionization in the THz generation mechanism.
In our experiments, terahertz radiation via two-color generated laser plasma gas targets is studied using nitrogen and the noble gases (helium, neon, argon, krypton, and xenon) as the generation media. Carried out at the infrared beam of the advanced laser light source, we studied the effects of different pump wavelengths (between 1200 nm and 1600 nm) on THz generation. Terahertz pulse energy is measured as functions of input pulse energy, gas species, gas pressure. The experimental results show that the terahertz pulse energy approach a maximum value of 0.0578 μJ per pulse in xenon gas when the input 1600 nm pulse energy is 0.4 mJ per pulse.
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