Thursday, June 25, 2015

Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO3

Spotlight Summary by József A. Fülöp

Visualizing a phenomenon is often an important driving force for scientific results and new measurement techniques. It is probably the best way also for learning and understanding. The work of Frank Hegmann’s group on ultrafast imaging of terahertz (THz) waves in lithium niobate (LiNbO3) benefits both technology and understanding.

The researchers at University of Alberta in Edmonton, Canada, applied a technique developed about one and half decades ago to map the generation and propagation of THz Cherenkov waves and what they call transition-like THz radiation in LiNbO3, a nonlinear material commonly used for THz pulse generation. The technological novelty in the work is the use of phase contrast imaging to visualize the full profile of the THz Cherenkov cone in bulk LiNbO3 in a transverse imaging geometry, where an expanded optical probe (imaging) beam travels in a direction perpendicular to the optical pump pulse generating the THz radiation. The THz electric field modulates the phase of the optical probe pulse and the phase modulation is converted to amplitude modulation through Talbot imaging, where the camera is moved out of the image plane of the sample. Talbot imaging is easy to set up and yields qualitative field images.

Ultrafast phase-contrast imaging can find applications in the development and optimization of pulsed optical or THz sources. The technique enables the direct observation and visualization of various nonlinear optical interaction processes. For example, one of the most widely used methods for the generation of intense THz pulses is optical rectification of femtosecond pulses with tilted pulse front. Such sources use a combination of a diffraction grating and imaging optics to generate the pump pulse-front tilt. Imaging can introduce distortions which limit the useful pumped area and therefore the achievable THz pulse energy. Ultrafast phase-contrast imaging can help to measure and minimize such distortions by providing a direct and easy-to-interpret method, superior, for example, to measuring the output beam characteristics of the THz radiation.

Last, but not least, I would like to emphasize the educational value of the work. The scheme is very well suited for an advanced student laboratory course where a suitable femtosecond laser is available. It can help students to get acquainted with important concepts like pulse front tilt or (non-collinear) phase matching.

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