Meng-Ku Chen

Broadband IR supercontinuum generation using single crystal sapphire fibers.

In this paper, an investigation on broadband IR supercontinuum generation in single crystal sapphire fibers is presented. It is experimentally demonstrated that broadband IR supercontinuum spectrum (up to 3.2microm) can be achieved by launching ultra-short femtosecond laser pulses into single crystal sapphire fiber with a dimension 115microm in diameter and 5cm in length, which covers both the near IR spectral region and the lower end of the mid-IR spectral range. Furthermore, the mechanism of supercontinuum generation in single crystal sapphire fibers is briefly addressed. When the fiber length is shorter than the dispersion length, the self-phase modulation dominates the broadening effect. In this case, the broad supercontinuum spectrum with a smooth profile can be obtained. However, when the fiber length is longer than the dispersion length, the soliton-related dynamics accompanied by the self-phase modulation dominates the broadening effect. There are discrete spikes in the spectrum (corresponding to different order solitons). The above assumption of supercontinuum generation mechanism is quantitatively modeled by the computer simulation program and verified by the experimental results. Thus, one can adjust the spectral profile by properly choosing the length of the sapphire fibers. The broad IR spectral nature of this supercontinuum source can be very useful in a variety of applications such as broadband LADAR, remote sensing, and multi-spectrum free space communications.

Analysis of terahertz generation via nanostructure enhanced plasmonic excitations

In this paper, we conduct a quantitative study on the physical mechanism of electrons dynamics near the nanostructured metal film surfaces, as well as the efficiency of generated terahertz radiation associated with different types of nanostructures. The simulation results show that although the oscillating motion of emitted electrons outside the metal surface may affect the terahertz generation efficiency to some extent, this efficiency is predominantly determined by the electric field magnitude inside the metal film associated with nanostructure enhanced plasmonic excitations. Due to the field enhancement effect of the nanostructure, an appropriately designed nanostructured surface could greatly enhance the strength of generated terahertz signal via the increased nonlinear interactions between the light and the nanostructures.

Terahertz generation in multiple laser-induced air plasmas

An investigation of the terahertz wave generation in multiple laser-induced air plasmas is presented. First, it is demonstrated that the intensity of the terahertz wave increases as the number of air plasmas increases. Second, the physical mechanism of this enhancement effect of the terahertz generation is studied by quantitatively measuring the intensity of the generated terahertz wave as a function of phase difference between adjacent air plasmas. It is found out that the superposition is the main mechanism to cause this enhancement. Thus, the results obtained in this paper not only provide a technique to generate stronger terahertz wave but also enable a better understanding of the mechanism of the terahertz generation in air plasma.

Broadband supercontinuum generation covering UV to mid-IR region by using three pumping sources in single crystal sapphire fiber.

In this paper, we demonstrate that the the bandwidth of the supercontinuum spectrum generated in a large mode area sapphire fiber can be enhanced by employing triple pumping sources. Three pumping sources with wavelengths of 784 nm, 1290 nm, and 2000 nm are launched into a single crystal sapphire fiber that is 5 cm in length and has a core diameter of 115 microm. The nonlinear interactions due to self-phase modulation and four-wave mixing form a broadband supercontinuum that covers the UV, visible, near-IR and lower mid-IR regions. Furthermore, we explore the possibility of generating a broadband supercontinuum expanding from the UV to far-IR region by increasing the number of pumping sources with wavelengths in the mid- and far-IR.

Terahertz enhancement from terahertz-radiation-assisted large aperture photoconductive antenna

The observation of enhanced terahertz (THz) wave generation from the large aperture photoconductive (PC) antenna excited by both a femtosecond pump beam and a collinearly propagating ZnTe-pregenerated THz wave is reported within this paper. An analysis based on both the calculated and experimental results demonstrated that the superposition acts as the main physical mechanism of this THz enhancement effect due to the dominant contribution from the rapid change in photoexcited carrier density. A prerequisite for the THz enhancement requires that the polarization of the applied bias and the ZnTe-pregenerated THz should be identical in order to have a constructive superposition. Therefore, this observation introduces the possibility of recycling the unused portion of the pump beam to further improve the THz radiation. The enhancement effect could be optimized by changing the thickness of ZnTe, which could affect the photoexcited-free-carrier absorption of THz in the PC antenna and the bandwidth of final enha...

Middle-IR supercontinuum generations and applications

In this paper, the two different mechanisms of supercontinuum generation in single crystal sapphire fibers according to fiber lengths longer and shorter than dispersion length are theoretically and experimentally investigated. When the fiber length is shorter than the dispersion length, self-phase modulation is the dominant factor for supercontinuum broadening. A broad spectrum ranging from near-IR (1.2 μm) to the lower end of mid-IR (2.8 μm) is obtained. But, when the fiber length is longer than dispersion length, soliton-related dynamics with self-phase modulation is the dominant factor for supercontinuum. We further demonstrate that supercontinuum in a sapphire fiber can extend beyond the range of silica fibers by showing the spectrum from 2 μm to 3.2 μm. Also, we successfully apply the supercontinuum source generated from a sapphire fiber to IR spectroscopy. The spectra of pseudo-TNT chemical measured using our own supercontinuum source is in good agreement with those obtained by FTIR. Supercontinuum generation using a sapphire fiber, which has high damage threshold and broad transmission ranges can be used in many applications such as IR spectroscopy, broadband LADAR, remote sensing, and multi-spectrum free space communications.

THz Generation in Multiple Air Plasmas

Because the efficiency of THz generation in air plasma is quite low, the residual power of input beam after THz radiation is generated in air plasma remains almost the same. A new method, multiple air plasmas, is proposed. The residual power can be used to induce other air plasmas and generate THz radiation again. The multiple air plasmas method provides a potential way for the development of the intense THz source. The preliminary experimental results confirm the theoretical prediction. The multiple air plasmas generated THz can be very useful for remote THz generation and standoff detection.

Detection of concealed and buried chemicals by using multifrequency excitations

In this paper, we present a new type of concealed and buried chemical detection system by stimulating and enhancing spectroscopic signatures with multifrequency excitations, which includes a low frequency gradient dc electric field, a high frequency microwave field, and higher frequency infrared (IR) radiations. Each excitation frequency plays a unique role. The microwave, which can penetrate into the underground and/or pass through the dielectric covers with low attenuation, could effectively transform its energy into the concealed and buried chemicals and increases its evaporation rate from the sample source. Subsequently, a gradient dc electric field, generated by a Van De Graaff generator, not only serves as a vapor accelerator for efficiently expediting the transportation process of the vapor release from the concealed and buried chemicals but also acts as a vapor concentrator for increasing the chemical concentrations in the detection area, which enables the trace level chemical detection. Finally, ...

Analysis of Terahertz Generation and Control by Nanostructured Surfaces

In this paper, some of our recent works on the design of different types of nanostructured surfaces, the terahertz generation, terahertz lenses, and terahertz metamaterials are reviewed and discussed. The mechanism behind the terahertz radiation is the photoelectric emission effect, which leads to the oscillating motions of emitted electrons and are affected by the electric field inside the metal. Furthermore, by using those nanostructured surfaces, terahertz lenses, which are due to the excitation of surface plasmons, and terahertz metamaterials, which results from the effective inductor-capacitor resonator, are also presented.