Tong Wu

Intense thermal terahertz-to-infrared emission from random metallic nanostructures under femtosecond laser irradiation

We report intense (~10 mW), ultra-broadband (~150 THz wide), terahertz-to-infrared, Gaussian-wavefront emission from nanopore-structured metallic thin films under femtosecond laser pulse irradiation. The proposed underlying mechanism is thermal radiation. The nanostructures of the metal film are produced by random holes in the substrate. Under pulse-train femtosecond laser irradiation, we found dramatically enhanced optical absorption, with an absorptivity that was equal to as much as 95% of the metallic surface nanostructure, due to both an antireflection mechanism and dissipation of excited surface plasmon polaritons into the metal surface.

Contrast-enhanced continuous-terahertz-wave imaging based on superparamagnetic iron oxide nanoparticles for biomedical applications

We present a novel contrast-enhanced continuous-terahertz-wave imaging modality based on magnetic induction heating of superparamagnetic iron oxide nanoparticles (SPIOs), which yields a highly sensitive increment in the reflection terahertz (THz) signal in SPIO solution upon exposure to an alternating magnetic field. In the differential and relative refection change focal-plane images before and after alternating magnetic field exposure, a dramatic contrast is demonstrated between water with and without SPIOs. This low-cost, simple, and stable contrast-enhanced continuous-THz-wave imaging system is suitable for miniaturization and real-time imaging application.

Terahertz-to-infrared emission through laser excitation of surface plasmons in metal films with porous nanostructures.

We report on the investigation of terahertz-to-infrared (THz-to-IR) thermal emission that relies on the excitation of surface plasmons in metal films deposited on a substrate with randomly ordered nanoscale pore arrays. The THz-to-IR radiation was observed both in the direction of laser beam propagation and the reverse direction. The intensity ratio between backward and forward radiation is exponentially dependent on the nominal thickness of the porous metal films. The findings are discussed in view of the proposed generation mechanism based on propagating surface plasmon polaritons on both air/metal and metal/substrate interfaces.

Excitation-wavelength dependent terahertz wave polarization control in laser-induced filament

We examine the terahertz (THz) emission from air filament driven by two-color lasers with relatively longer wavelengths than 800 nm. The THz energy dependence on the input laser energy increases more rapidly with a longer laser wavelength, and the scaling laws of THz energy as a function of fundamental wavelength vary for different optical powers, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, the THz polarization undergoes a continuous rotation as a function of the laser wavelength, since the relative phase and polarization of the two pulses are adjusted through changing the excitation wavelength in the frequency doubling crystal. Our results contribute to the understanding of THz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.

Continuous-terahertz-wave molecular imaging system for biomedical applications

Abstract. Molecular imaging techniques are becoming increasingly important in biomedical research and potentially in clinical practice. We present a continuous-terahertz (THz)-wave molecular imaging system for biomedical applications, in which an infrared (IR) laser is integrated into a 0.2-THz reflection-mode continuous-THz-wave imaging system to induce surface plasmon polaritons on the nanoparticles and further improve the intensity of the reflected signal from the water around the nanoparticles. A strong and rapid increment of the reflected THz signal in the nanoparticle solution upon the IR laser irradiation is demonstrated, using either gold or silver nanoparticles. This low-cost, simple, and stable continuous-THz-wave molecular imaging system is suitable for miniaturization and practical imaging applications; in particular, it shows great promise for cancer diagnosis and nanoparticle drug-delivery monitoring.

Experimental 210GHz terahertz nondestructive testing for aerospace composite materials

In this paper we proposed a terahertz nondestructive testing method to achieve the image of the aerospace composite materials. We commence in the following manner, using the continuous back wave oscillator (BWO) as the source of terahertz radiation, which the frequency is 210 GHz. By the reflection type imaging testing system, the terahertz probe focus moves from the surface to inside of the concealed composite materials. And the materials are bonded on the two-dimensional scanning translation units towards the focal of the terahertz mirror, which can move in real-time to get the data by the phase-locked amplifier. All the system is controlled by the computer. The interior structure can be tested by the images range from 0.5mm to 1mm resolution. The images are calculated by principal component analysis (PCA) and the least square method(LSM). With the proper math to calculate in Matlab, the structure, the size, position and shape defects such as cracks, inclusions, empty and bubbles in the aerospace insulation composite materials can be tested by the terahertz image system. It is a more convenient and intuitive method of imaging inner aerospace composite material.

Wavelength Scaling of Terahertz Wave Absorption via Preformed Air Plasma

Two-color laser mixing in gases can result in the emission of broadband, single-cycle terahertz (THz) pulses. In this study, we find that THz wave absorption is observed when the THz wave and preplasma beam are focused on the same point, and that the wavelength of the preplasma beam can influence the absorption. The experimental results reveal that the THz wave absorption becomes less pronounced with increasing preplasma wavelength. We found that the absorption decreased sharply at a wavelength of 1400 nm, which may be related to the preplasma density, and the preplasma fluorescence profiles were measured using a charge-coupled device arrangement to prove our conjecture. In addition, the full-width at half-maximum of the frequency of the THz wave becomes broadened during absorption. The experimental results agree well with our presented model.

The study of gas species on THz generation from laser-induced air plasma

Intense Terahertz waves generated from air-induced plasma and serving as broadband THz source provide a promising broadband source for innovative technology. Terahertz generation in selected gases has attracted more and more researchers’ interests in recent years. In this research, the THz emission from different atoms is described, such as nitrogen, argon and helium in Michelson. The THz radiation is detected by a Golay Cell equipped with a 6-mm-diameter diamond-inputting window. It can be seen in the first time that when the pump power lies at a stable level, the THz generation created by the femtosecond laser focusing on the nitrogen is higher than which focusing on the helium, and lower than that produced in the argon gas environment. We believe that the THz intensity is Ar > N > Ne because of its atomic mass, which is Ar > N > Ne as well. It is clear that the Gas molecular decides the release of free electrons ionized from ultra short femtosecond laser through the electronic dynamic analysis. The higher the gas mass is, the stronger the terahertz emission will be. We further explore the THz emission at the different laser power levels, and the experimental results can be commendably quadratic fitted. It can be inferred that THz emission under different gas medium environment still complies with the law of four-wave mixing (FWM) process and has nothing to do with the gas environment: the radiation energy is proportional to the quadratic of incident laser power.

Bi-directional terahertz-to-infrared emission from metal-coated nanostructures upon femtosecond laser irradiation.

We report on the investigation of bi-directional terahertz-to-infrared (THz-to-IR) radiation from a metal film coated on a substrate with randomly ordered pore arrays by irradiation of femtosecond laser pulses. THz-to-IR radiation was observed both for front-side excitation (laser incident on the metal surface) and for rear-side excitation (laser incident on the substrate). In both cases, the radiation was observed both in the propagation direction of the laser beam and in the reverse direction. Considering these findings, we propose a thermal emission mechanism based on the production of surface plasmons, either delocalized (through phase-matched excitation) or localized (through surface roughness) at the air/metal and metal/substrate interfaces.

Terahertz wave absorption via femtosecond laser-filament concatenation

Terahertz (THz) emission from laser-induced air plasma is a well known and widely used phenomenon. We report that when two laser beams from the laser creating two plasma filaments interact with each other, THz absorption is observed. We believe that a change in the refractive index of the plasma causes the THz-wave absorption. The following experimental results reveal that the THz absorption becomes more pronounced with increasing pump power and that the gas species surrounding the femtosecond laser filament can also influence the THz absorption rate.