OVERVIEW
Terahertz time-domain spectroscopy (THz-TDS) is a coherent technique covering spectral range of 0.1 – 100 THz and providing access to the absorption as well as dispersion characteristics of the sample. The advent of efficient THz sources and suitable detection techniques [Dragoman 2004, Reiman 2007] boosted areas such as biomedical imaging, security, industrial quality control and others. For instance, extension of the THz-TDS into 2D allows to perform tomographic imaging without the need for biologically active energetic photons (X-Ray) with corresponding health risks.
On the other hand, the recent advent of high-intensity THz sources based on wave-mixing in plasma [Thomson 2007, Thomson 2010] provided access to sources with extremely high THz bandwidths of the order of 100 THz and field intensities of the order of MV cm-1. This allows construction of broadband THz spectrometers [Ho, 2010] that significantly extend performance of classical FTIR spectrometers. Moreover the technique is homodyne and coherent meaning that besides transmission properties of the sample the dispersion (phase-change) can be extracted as well. The accessible intensities of the current state-of-the-art THz sources already allow a design of novel nonlinear THz elements with a notable impact for optical communications and THz detection. A significant issue that remains however is the lack of fundamental nonlinear optical parameters in the THz range. Although there is a certain recent progress in the area [Wen, 2008], the field still remains largely unexplored.
The initial phase of the project aims on the design of a broadband FWM based THz source in plasma using the process χ3(ω, ω, -2ω, Δω):
EΩ(t)∝χ‹3›Eω1(t)E*ω2(t)Eω3(t)cos(φω1-φω2+φω3)
Ω=ω1-ω2+ω3.
being for ω1=ω3=ω; ω2=2ω reduced to the degenerate case i.e. mixing of fundamental and second harmonic.
The THz generation by FWM in plasma is schematically depicted in Fig. 1. Two photons at the fundamental frequency (at 800 nm) are mixed with a single photon at the second harmonic (400 nm) inside of a χ3 medium. Since the process involves nonmonochromatic radiation, that is pulses with a finite spectral bandwidth, a small frequency ofset in the THz range results from difference frequency generation ω+ω-2ω. Consequently, one would assume a tunable broadband THz source when using the nondegenerate scheme with the approach being demonstrated by our coworkers for the first time [Balciunas, 2010].
Literature
D.Dragoman, M.Dragoman; Prog. Qant. Electron. 28, 1 (2004).
K.Reimann; Rep. Prog. Phys. 70, 1597 (2007).
M. Thomson, M. Kress, T. Loffler, H.G. Roskos, Laser & Photon. Rev. 1, No. 4, 349–368 (2007).
M.D. Thomson, V. Blank, and H.G. Roskos, Opt. Express 18, 23173-23182 (2010).
I-Chen Ho, Xiaoyu Guo, and X.-C. Zhang, Opt. Express 18, 2872-2883 (2010).
H. Wen, M. Wiczer, and A. M. Lindenberg, Phys. Rev. B 78, 125203 (2008).
Balčiūnas T., Lorenc D., Ivanov M., Smirnova O., Pugžlys A., Zheltikov A.M., Dietze D., Darmo J., Unterrainer K., Rathje T., Paulus G.G., and Baltuška A., International Conference on Ultrafast Phenomena 18.7.-23.7.,Snowmass Village, CO (2010).