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We present a single-shot detection method of terahertz-correlated second harmonic generation in plasma-based sources by directly mixing an optical probe into femtosecond laser-induced plasma filaments in air. The single-shot second harmonic trace is obtained by measuring second harmonic generation on a conventional CCD with a spatio-temporally distorted probe beam. The system shows a spectrometer resolution of 22 fs/pixel on the CCD and a true resolution on the order of the probe pulse duration. With considerable THz peak electric field strengths, this formalism can open the door to single-shot THz detection without bandwidth limitations.

To create self-controlled radiation photonics systems, it is necessary to have complete information about the nonlinear properties of the materials used. In this paper, the vibrational mechanism of the giant low-inertia cubic nonlinearity of water in the terahertz frequency range is experimentally proven. Its dominance which manifests itself when the temperature of the liquid changes is demonstrated. The measured nonlinear refractive index in the THz frequency range for water jet at temperatures from 14$^{\circ}$C to 21$^{\circ}$C demonstrates a correlation with the theoretical approach, varies from 4 to 10$\cdot$10$^{-10}$ cm$^2$/W and is characterized by a inertial time constant of less than 1 ps., Comment: 4 pages, 3 figures, 1 table

In contrast with imaging using position-resolving cameras, single-pixel imaging uses a bucket detector along with spatially structured illumination for image recovery. This emerging imaging technique is a promising candidate for a broad range of applications due to high signal-to-noise ratio (SNR) and sensitivity, and applicability in a wide range of frequency bands. Here, inspired by single-pixel imaging in the spatial domain, we demonstrate a temporal single-pixel imaging (TSPI) system that covers frequency bands including both terahertz (THz) and near-infrared (NIR) region. By implementing a programmable temporal fan-out (TFO) gate based on a digital micromirror device (DMD), we can deterministically prepare temporally structured pulses with a temporal sampling size down to 16.00$\pm$0.01 fs. By inheriting the advantages in detection efficiency and sensitivity from spatial single-pixel imaging, TSPI enables the compressive recovery of a 5 fJ THz pulse and two NIR pulses with over 97$\%$ fidelity. We demonstrate that the TSPI is robust against temporal distortions in the probe pulse train as well. As a direct application, we apply TSPI to machine-learning-aided THz spectroscopy and demonstrate a high sample identification accuracy (97.5$\%$) even under low SNRs (SNR $\sim$ 10).

Terahertz (THz) wave shows great potential in non-destructive testing, bio detection and cancer imaging. Recent progresses on THz wave near-field probes/apertures enable mechanically raster scanning of an object's surface in the near-field region, while an efficient, non-scanning, non-invasive, deeply sub-diffraction-limited imaging still remains challenging. Here, we demonstrate a THz near-field microscopy using a reconfigurable spintronic THz emitter array (STEA) with computational ghost imaging. By illuminating an object with the reconfigurable STEA in near field and computing the correlation measurements, we reconstruct its image with deeply sub-diffraction resolution. By circulating an external magnetic field, the in-line polarization rotation of THz waves is realized, making the fused image contrast polarization-free. The time-of-flight (TOF) measurements of coherent THz pulses further enables to resolve objects at different distances or depths. The demonstrated ghost spintronic THz-emitter-array microscope (GHOSTEAM) is a new imaging tool for THz near-field real-time imaging (with potential of video framerate), especially opening up paradigm-shift opportunities in non-intrusive label-free bioimaging in a broadband frequency range from 0.1 THz to 30 THz (namely 3.3-1000 cm-1).

Polar liquids are strong absorbers of electromagnetic waves in the terahertz range, therefore, historically such liquids have not been considered as good candidates for terahertz sources. However, flowing liquid medium has explicit advantages, such as a higher damage threshold compared to solid-state sources and more efficient ionization process compared to gases. Here we report systematic study of efficient generation of terahertz radiation in flat liquid jets under sub-picosecond single-color optical excitation. We demonstrate how medium parameters such as molecular density, ionization energy and linear absorption contribute to the terahertz emission from the flat liquid jets. Our simulation and experimental measurements reveal that the terahertz energy has quasi-quadratic dependence on the optical excitation pulse energy. Moreover, the optimal pump pulse duration, which depends on the thickness of the jet is theoretically predicted and experimentally confirmed. The obtained optical-to-terahertz energy conversion efficiency is more than 0.05%. It is comparable to the commonly used optical rectification in most of electro-optical crystals and two-color air filamentation. These results, significantly advancing prior research, can be successfully applied to create a new alternative source of terahertz radiation., Comment: 10 pages, 4 figures, 1 table, Optics Express

Recently, there has been growing interest in terahertz (THz) wave generation from liquids under optical excitation. Here, we propose and demonstrate the use of liquid lines in place of liquid films as THz emitters to boost THz signals. The geometry of the emitter eliminates the total internal reflection at the flat liquid-air interface. In addition, we observe that the polarity of the liquid has a significant influence on the THz wave generation. Alpha-pinene, a nonpolar liquid, offers much stronger THz radiation than water does. Besides paving the way to develop intense liquid THz sources, our work indicates that THz waves could be a tool for the further study of laser-liquid interaction.

Recently, computational sampling methods have been implemented to spatially characterize terahertz (THz) fields. Previous methods usually rely on either specialized THz devices such as THz spatial light modulators, or complicated systems requiring assistance from photon-excited free-carriers with high-speed synchronization among multiple optical beams. Here, by spatially encoding an 800 nm near-infrared (NIR) probe beam through the use of an optical SLM, we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera. This design does not require any dedicated THz devices, semiconductors or nanofilms to modulate THz fields. Through the use of computational algorithms, we successfully measure 128$\times$128 field distributions with a 62 $\mu m$ transverse spatial resolution, more than 15 times smaller than the central wavelength of the THz signal (940 $\mu m$). Benefitting from the non-invasive nature of THz radiation and sub-wavelength resolution of our system, this simple approach can be used in applications such as biomedical sensing, inspection of flaws. in industrial products, and so on., Comment: Main manuscript: 7 pages, 3 figures; Supplement: 8 pages, 3 figures

By using the Malliavin calculus and solving a control problem, Bismut type derivative formulae are established for a class of degenerate diffusion semigroups with non-linear drifts. As applications, explicit gradient estimates and Harnack inequalities are derived., Comment: 18 pages