Jianming Dai, Xu Xie, Nicholas Karpowics, Hua Zhong, and X.-C. Zhang
W.M. Keck Laboratory for Terahertz Science, Center for Terahertz Research
Rensselaer Polytechnic Institute, Troy, NY 12180, USA
Email address: firstname.lastname@example.org
We demonstrate generation, manipulation, amplification,and detection of highly directional broadband THz waves in laser-induced air plasma.
After the first demonstration of THz wave time-domain spectroscopy in the late 80¡¯s, the time-domain THz community commonly uses coherent emitters and detectors for pulsed THz waves, which are mainly limited to photoconductive dipole antennas and electro-optic crystals [1, 2]. Ambient air, a third-order nonlinear material, exhibits a remarkable performance for the generation, manipulation, amplification, and detection of pulsed THz waves with excitation by femtosecond laser beams.
II. EXPERIMENTAL RESULTS AND ANALYSIS
Experimentally, a desktop Ti:sapphire regenerative amplifier is used as the femtosecond laser source for the generation, detection,and amplification of THz waves. Air plasma (ionized gas molecules), created by a mixed optical beam (fundamental w(800 nm) and its second harmonic wave 2w(400 nm)), produces very intense, highly-directional, and ultra-broadband THz waves through third-order optical nonlinearity [3, 4].
By changing the relative phase between the ¨´ and 2¨´ pulses (i.e.time delay), the amplitude and polarity of the THz wave can be precisely controlled . As the reciprocal process of THz wave generation, we study the use of the air sensor to coherently detect pulsed THz waves . The dynamic range obtained with the airsensor
technique is about 1000.
To investigate the amplification effect of THz waves in air plasma, a gain beam, as well as its second-harmonic, is focused by an optical lens, forming an air plasma spot. An input THz beam is focused onto the same plasma spot in a collinear geometry by a parabolic mirror with a hole to allow the gain beam to pass through,and then the output THz beam (i.e., the amplified THz waveform) is detected by an 1-mm thick <110> oriented ZnTe crystal.
Fig. 1 shows the waveforms and their corresponding spectra without and with air plasma by blocking and unblocking the gain beam, respectively. With a plasma length of about 2 mm, the input peak THz signal is amplified by over 50%.
Fig. 2 plots the gain spectrum, indicating a gain obtained in laserinduced air plasma over the frequency range from 0.5 THz to 3 THz.
The amplification effect demonstrated here is basically attributed to high-order optical parametric processes. Further theoretical analysis and calculation is currently under investigation.
We demonstrated the generation, manipulation, amplification and detection of THz waves in laser-induced air plasma. Our demonstration, rich with science and challenging technology, provides a new direction for THz remote sensing.
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