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Papers by Srinivasa Rao Ragam

Terahertz Sources Based on Difference-Frequency Generation for Spectroscopy and Biomaterial Analysis

Terahertz Biomedical and Healthcare Technologies

Journal of Lightwave Technology, Aug 1, 2009

We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by l... more We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by laser diode (LD) pumping in noncollinear phase-matched difference frequency generation (DFG). The effects of interaction length and beam spot size of input lasers (near-IR) in GaP crystals were studied. The THz wave power dependence on various sizes of GaP crystals was investigated and it was observed that an output power of 4 nW was obtained with a 20 mm long GaP crystal at 1.5 THz. Also, the THz wave absorption coefficient was dominant for longer GaP crystals at high frequencies (above 2.5 THz). The THz wave power dependence on beam spot size (1.2 mm-300 m) of near-IR lasers at 1.62 THz was studied, and an improvement of THz wave power being seen with a 500 m beam spot size, while the beam divergence effect was dominant for beam spot sizes below 500 m.

GaP crystal length and beam spot size dependences on the output power of CW THz waves (電子デバイス)

電子情報通信学会技術研究報告 Ed 電子デバイス, Dec 12, 2008

Structural, optical and electronic properties of homoepitaxial GaN nanowalls grown on GaN template by laser molecular beam epitaxy

RSC Adv., 2015

We have grown homoepitaxial GaN nanowall networks on GaN template using an ultra-high vacuum lase... more We have grown homoepitaxial GaN nanowall networks on GaN template using an ultra-high vacuum laser assisted molecular beam epitaxy system by ablating solid GaN target under a constant r.f. nitrogen plasma ambient.

CW THz-wave generation from GaP pumped with semiconductor lasers and its applications for high-resolution spectroscopy

2009 34th International Conference on Infrared, Millimeter, and Terahertz Waves, 2009

Abstract We have developed a continuous wave (CW) terahertz (THz) wave generation by near infrare... more Abstract We have developed a continuous wave (CW) terahertz (THz) wave generation by near infrared laser diode (NIR-LD) pumping based on non-collinear phase-matched difference frequency generation (DFG) in a GaP crystal. The THz output power is ...

Compact and portable terahertz source based on frequency mixing of solid-state laser beams

2011 ICO International Conference on Information Photonics, 2011

312. Accession number:20113414247719 Title:Compact and portable terahertz source based on frequen... more 312. Accession number:20113414247719 Title:Compact and portable terahertz source based on frequency mixing of solid-state laser beams Authors:Zhao, Pu (1); Ragam, Srinivasa (1); Ding, Yujie J. (1); Zotova, Ioulia B. (2) Author affiliation:(1) Department of Electrical and ...

Semiconductor Laser Diode Technology and Applications, 2012

Review of Scientific Instruments, 2009

We constructed a high resolution terahertz ͑THz͒ spectroscopic system with an automatic scanning ... more We constructed a high resolution terahertz ͑THz͒ spectroscopic system with an automatic scanning control using a continuous wave ͑cw͒ THz wave generator based on difference frequency generation method by excitation of phonon-polariton mode in GaP. The pump and signals lasers were compact, tunable external cavity laser, and distributed feedback ͑DFB͒ lasers, respectively. The generated THz waves were tuned automatically by changing the temperature of the DFB laser using a system control. We present the water vapor transmission characteristics of the THz wave and also absorption spectrum of a white polyethylene in the frequency range of 1.97-2.45 THz. The spectroscopic measurements performed at an output power level of 2 nW, which was obtained with a 15-mm-long GaP crystal at 2 THz. The advantage of this cw THz spectrometer is wide frequency tuning range ͑0.7-4.42 THz͒ with an estimated linewidth of full width at quarter maximum Ͻ8 MHz and this system has a potential application in high resolution spectroscopy.

Journal of the Japan Institute of Metals, 2010

We have developed a high resolution and continuous wave (CW) terahertz spectroscopic system bas... more We have developed a high resolution and continuous wave (CW) terahertz spectroscopic system based on difference frequency generation (DFG) with an excitation of phonon polariton in Gallium phosphide (GaP). The pump and signal near infrared (NIR) lasers used in this system are an external cavity laser diode (ECLD) and a distributed feedback laser diode (DFB), respectively. Each line width of the NIR laser is less than 4 MHz, thus the quality factor Q of generated THz waves is more than 10 6. The terahertz wave frequency can be tuned automatically by changing wavelength of the DFB laser. As an example for the THz spectroscopic application for non destructive inspection of ultra high molecular weight polyethylene (UHMWPE), we have shown terahertz spectra of UHMWPE with some degrees of deformation.

Journal of Infrared, Millimeter, and Terahertz Waves, 2010

We constructed two types of terahertz (THz) spectrometers with automatic scanning control based o... more We constructed two types of terahertz (THz) spectrometers with automatic scanning control based on the difference frequency generation method by the excitation of the phonon-polariton mode in GaP. The pulsed THz wave spectroscopic systems were based on an optical parametric oscillator and Nd: YAG laser sources having a frequency resolution of 1.5 GHz, and on Cr:fosterite sources with a resolution of 20 GHz. Following these, we recently constructed a continuous wave (CW) THz wave spectroscopic system with laser diode excitation. One of the advantages of the CW THz wave spectrometer is its wide frequency tuning range with fine frequency resolution of<8 MHz. In this study, we compare both types of spectrometers (pulsed versus CW) to show the characteristics of each system in terms of frequency resolution. The absorption spectra of a non-deformed white polyethylene crystal and ultra-high molecular weight polyethylene with/without deformation are measured by using the CW THz wave spectrometer and pulsed THz wave spectrometer. The effect of the high-resolution CW THz wave spectrometer is shown based on the THz spectroscopic results.

IEEE Photonics Technology Letters, 2000

We constructed a two-directional continuous-wave (CW) terahertz (THz) wave generation system with... more We constructed a two-directional continuous-wave (CW) terahertz (THz) wave generation system with laser diode pumping. The generation method is based on difference frequency generation (DFG) of two near-infrared (NIR) lasers by excitation of phonon polaritons in a GaP crystal. The two NIR lasers for DFG are amplified simultaneously using one ytterbium-doped fiber amplifier (FA). Efficient beam overlapping of the NIR lasers was achieved on the GaP crystal surface. This system consists of a simple optical design with a single FA. Narrow linewidth CW THz waves, which are generated in two directions, can be applied to high-resolution spectroscopy.

The impact of RF-plasma power in carrier relaxation dynamics of unintentional doped GaN epitaxial layers grown by MBE

Optical Materials, 2016