Shi-Tong Xu

Intensity clamping during laser filamentation by TW level femtosecond laser in air and argon

A systematic research on intensity clamping phenomenon was conducted both in air and argon by using a TW level femtosecond laser. Though the laser peak power was increased from 0.1 up to 1.5 TW in the experiment, highly stabilized peak intensity inside the filament was observed in both gases. The peak intensities inside filaments were experimentally determined to be about 6.4 × 1013 W/cm2 (f = 20 cm) in air and 1.2, 1.3, and 1.7 × 1014 W/cm2 when different focal lenses (f = 100, 60, and 20 cm) were used in argon, respectively.

Artificial high birefringence in all-dielectric gradient grating for broadband terahertz waves

Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6–1.4 THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material.

Terahertz polarization converter and one-way transmission based on double-layer magneto-plasmonics of magnetized InSb

In this work, we investigate the nonreciprocal circular dichroism for terahertz (THz) waves in magnetized InSb by the theoretical calculation and numerical simulation, which indicates that longitudinally magnetized InSb can be applied to the circular polarizer and nonreciprocal one-way transmission for the circular polarization THz waves. Furthermore, we propose a double-layer magnetoplasmonics based on the longitudinally magnetized InSb, and find two MO enhancement mechanisms in this device: the magneto surface plasmon resonance on the InSb-metal surface and Fabry-Pérot resonances between two orthogonal metallic gratings. These two resonance mechanisms enlarge the MO polarization rotation and greatly reduce the external magnetic field below 0.1T. The one-way transmission and perfect linear polarization conversion can be realized over 70dB, of which the transmittance can be modulated from 0 to 80% when the weak magnetic field changes from 0 to 0.1T under the low temperature around 200K. This magnetoplasmonic device has broad potential as a THz isolator, modulator, polarization convertor, and filter in the THz application systems.

Intensity clamping during dual-beam interference

One has prospected to overcome the intensity clamping phenomenon during femtosecond laser filamentation by superimposing two beams. This experimental scheme is thus studied in our work. We have realized the interference of two ultrashort laser beams and one of them creates a filament in air. The formation of a dynamic plasma grating induced by the interference pattern is observed. However, qualitative analysis of the experimental results indicates that intensity clamping still governs the intensity evolution during the dual-beam interaction. The expectation of greatly increasing laser intensity by dual beams superposition might not be realistic.

Tunable terahertz wave-plate based on dual-frequency liquid crystal controlled by alternating electric field

In this work, the optically anisotropic property of dual-frequency liquid crystals (DFLC) in terahertz (THz) regime has been experimentally investigated, which indicates that the refractive index and birefringence of DFLC can be continuously modulated by both the alternating frequency and intensity of the alternating electric field. This tunability originates from the rotation of DFLC molecules induced by alternating electric fields. The results show that by modulating the alternating frequency from 1 kHz to 100 kHz under 30 kV/m electric field, the 600 μm thickness DFLC cell can play as a tunable quarter-wave plate above 0.68 THz, or a half-wave plate above 1.33 THz. Besides, it can be viewed as a tunable THz phase shifter from 0 to π. Therefore, due to its novel tuning mechanism, DFLC will be of great significance in dynamic manipulating on THz phase and polarization.

Terahertz wave modulation enhanced by laser processed PVA film on Si substrate

An optically pumped ultrasensitive broadband terahertz (THz) wave modulator based on polyvinyl alcohol (PVA) film on Si wafer was demonstrated in this work. The THz time domain spectroscopy experiments confirm that the PVA/Si can drastically enhance the photo-induced THz wave modulation on the Si surface, especially when the PVA film is heated by a high-power laser. A modulation depth of 72% can be achieved only under 0.55 W/cm2 modulated laser power, which is superior significantly to the bare Si. The numerical simulations indicate that the laser processed PVA (LP-PVA) film increases the photo-generated carrier concentration on the Si surface in two orders of magnitude higher than that of bare Si. Moreover, the modulation mechanism and the dynamic process of laser heating on the PVA/Si have been discussed. This highly efficient THz modulation mechanism and its simple fabrication method have great application potentials in THz modulators.

Mechanical modulation of terahertz wave via buckled carbon nanotube sheets

Manipulation of terahertz (THz) wave plays an important role in THz imaging, communication, and detection. The difficulty in manipulating the THz wave includes single function, untunable, and inconvenient integration. Here, we present a mechanically tunable THz polarizer by using stretchable buckled carbon nanotube sheets on natural rubber substrate (BCNTS/rubber). The transmittance and degree of polarization of THz wave can be modulated by stretching the BCNTS/rubber. The experiments showed that the degree of polarization increased from 17% to 97%, and the modulation depth reached 365% in the range of 0.2-1.2 THz, as the BCNTS/rubber was stretched from 0% to 150% strain. These changes can be also used for high strain sensing up to 150% strain, with a maximum sensitivity of 2.5 M/S. A spatial modulation of THz imaging was also realized by stretching and rotating BCNTS/rubber. The theoretical analysis and numerical modeling further confirm the BCNTS/rubber changes from weak anisotropic to highly anisotropic structure, which play key roles in THz wave modulation. This approach for active THz wave manipulation can be widely used in polarization imaging, wearable material for security, and highly sensitive strain sensing.

Coupled dielectric-metal subwavelength gratings for terahertz polarization converter and one-way transmission

A dielectric-metal double layer grating structure is proposed for high performance of polarization converter and one-way transmission in the terahertz (THz) region. The results show that this composite grating structure achieves a high polarization conversion of 90% and a broadband one-way transmission with an extinction nearly 30dB from 0.2 to 1.2THz. It provides a simple way towards practical applications for THz polarization control and one-way transmission.

Terahertz polarization mode conversion in compound metasurface

A compound metasurface for terahertz (THz) wave polarization mode conversion has been experimentally investigated, which is integrated with an H-shaped metallic metamaterial and 45° arranged subwavelength dielectric grating on the two surfaces of a Si substrate. The polarization mode conversion from a TM to a TE resonance mode is achieved at 1.3 THz for forward transmission and 0.63 THz for backward transmission. Based on this property, a unidirectional transmission is obtained with the highest extinction of 23 dB at 0.63 THz. Moreover, due to the multiple reflections and subwavelength integration, a localized resonance mechanism in this metasurface greatly enhances the polarization conversion rate, reduces the insertion loss, and expands the operating bandwidth from 0.3 to 1.6 THz, not merely at the π phase matching point, which are quite different with the properties of the discrete metallic metamaterial and dielectric grating. This work provides an efficient way towards practical applications in THz br...