Jiayu Zhao

Terahertz imaging with sub-wavelength resolution by femtosecond laser filament in air

Terahertz (THz) imaging provides cutting edge technique in biology, medical sciences and non-destructive evaluation. However, due to the long wavelength of the THz wave, the obtained resolution of THz imaging is normally a few hundred microns and is much lower than that of the traditional optical imaging. We introduce a sub-wavelength resolution THz imaging technique which uses the THz radiation generated by a femtosecond laser filament in air as the probe. This method is based on the fact that the femtosecond laser filament forms a waveguide for the THz wave in air. The diameter of the THz beam, which propagates inside the filament, varies from 20 μm to 50 μm, which is significantly smaller than the wavelength of the THz wave. Using this highly spatially confined THz beam as the probe, THz imaging with resolution as high as 20 μm (~λ/38 at 0.4 THz) can be realized.

Simple method of measuring laser peak intensity inside femtosecond laser filament in air

Measurement of laser intensity inside a femtosecond laser filament is a challenging task. In this work, we suggest a simple way to characterize laser peak intensity inside the filament in air. It is based on the signal ratio measurement of two nitrogen fluorescence lines, namely, 391 nm and 337 nm. Because of distinct excitation mechanisms, the signals of the two fluorescence lines increase with the laser intensity at different orders of nonlinearity. An empirical formula has been deduced according to which laser peak intensity could be simply determined by the fluorescence ratio.

Femtosecond laser filament array generated with step phase plate in air.

Femtosecond laser filament arrays are generated in air by using three kinds of step phase plates with π phase lag, namely, the semicircular phase plate (SCPP), the quarter-circle phase plate (QCPP) and eight-octant phase plate (EOPP). Experimental results and simulations show that filament arrays consisting of two, four and eight filaments, respectively, are produced by three phase plates. The transverse patterns of the filament arrays are determined by the geometrical shapes of the phase plates. At the same time, the separation distances are found to vary with the focal lengths of the used lenses. We further propose that by using an axicon, filament array in the form of ring shape could be realized while the lengths of the filaments could be significantly elongated at the same time. Our study has suggested a realistic method to generate filament array by the step phase plate with π phase lag.

3D printed low-loss THz waveguide based on Kagome photonic crystal structure

A low-loss hollow core terahertz waveguide based on Kagome photonic crystal structure has been designed and fabricated by 3D printing. The 3D printed waveguide has been characterized by using THz time-domain spectroscopy. The results demonstrate that the obtained waveguide features average power propagation loss of 0.02 cm-1 for 0.2-1.0 THz (the minimum is about 0.002 cm-1 at 0.75 THz). More interesting, it could be simply mechanically spliced without any additional alignment, while maintaining the excellent performance. The 3D printing technique will be a promising solution to fabricate Kagome THz waveguide with well controllable characteristics and low cost.

Impressive laser intensity increase at the trailing stage of femtosecond laser filamentation in air

The longitudinal distribution of the laser peak intensity inside a half meter long femtosecond laser filament in air is studied by measuring the signal ratio of two nitrogen fluorescence lines, 391 nm and 337 nm. The experimental results reveal that laser peak intensity initially remains almost constant (~4.3 × 10(13) W/cm2) inside the filament. However, before the end of the filament, surprisingly the laser intensity undergoes dramatic increase. A maximum intensity as high as 2.8×10(14) W/cm2 could be reached. The experimental result is unexpected by the conventional intensity clamping scenario, according to which the laser peak intensity would feature low variation along a filament. The experimental result is then interpreted as being due to the generation of a short pulse at trailing stage of the filamentation with reduced diameter. This phenomenon might be of great interest owing to its potential application in high-order-harmonic generation and producing isolated single attosecond laser pulse through simple experimental approach.

Propagation of terahertz wave inside femtosecond laser filament in air

Terahertz (THz) generation in femtosecond laser filament has recently been found to greatly expand THz applications in atmospheric remote sensing. In this article, by investigating the THz waveform emitted from different length of filaments, the refractive index of the THz wave has been found to be smaller than the unity within the filament region. It indicates that the THz pulse may propagate inside the filament though the diameter of the filament is much smaller than the wavelength of the THz wave. The hypothesis is supported by further numerical simulation, which considers the radially non-uniform plasma density distribution of the filament.

Simple method to enhance terahertz radiation from femtosecond laser filament array with a step phase plate

In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate a filament array. This simple method makes full use of the laser energy and could potentially open a new approach to remotely enhance the THz emission in air.

Backward angular distribution of air lasing induced by femtosecond laser filamentation

Angular distribution of the backscattered nitrogen fluorescence induced by femtosecond laser filamentation in air has been studied. The experimental results demonstrate that the fluorescence intensity forms multiple rings and is amplified in the backward direction through amplified spontaneous emission (ASE). More importantly, the ASE signal features an angular dependent gain coefficient. The results are valuable for the optimization of the remote-sensing setup by the filamentation in air.