Abstract-Multicycle terahertz pulse generation by optical rectification in LiNbO3, LiTaO3, and BBO crystals

Dogeun Jang and Ki-Yong Kim

 (a) Experimental setup for multicycle THz generation and detection. (b) Input laser spectrum measured (black line) before entering the experimental setup with a Gaussian fit (red line). (c) Measured (black dotted line) and estimated (red solid line) laser pulse duration as a function of input GDD (top 𝑥$x$-axis), practically controlled by varying the distance (bottom 𝑥$x$-axis) between the grating pair in the laser compressor. (d) Focused THz beam profile captured by a microbolometer focal plane array.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-14-21220

We report multicycle, narrowband, terahertz radiation at 14.8 THz produced by phase-matched optical rectification of femtosecond laser pulses in bulk lithium niobate (LiNbO3) crystals. Our experiment and simulation show that the output terahertz energy greatly enhances when the input laser pulse is highly chirped, contrary to a common optical rectification process. We find this abnormal behavior is attributed to a linear electro-optic (EO) effect, in which the laser pulse propagating in LiNbO3 is modulated by the terahertz field it produces, and this in turn drives optical rectification more effectively to produce the terahertz field. This resonant cascading effect can greatly increase terahertz conversion efficiencies when the input laser pulse is properly pre-chirped with additional third order dispersion. We also observe similar multicycle terahertz emission from lithium tantalate (LiTaO3) at 14 THz and barium borate (BBO) at 7 THz, 10.6 THz, and 14.6 THz, all produced by narrowband phase-matched optical rectification.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Anharmonic phonon-polariton dynamics in ferroelectric LiNbO 3 studied with single-shot pump-probe imaging spectroscopy

T. Kuribayashi, T. Motoyama, Y. Arashida, J. Takeda,

https://www.researchgate.net/publication/325007933_Anharmonic_phonon-polariton_dynamics_in_ferroelectric_LiNbO_3_studied_with_single-shot_pump-probe_imaging_spectroscopy

We demonstrate that single-shot pump-probe imaging spectroscopy with an echelon mirror enables us to disclose the ferroelectric phonon-polariton dynamics across a wide temperature range from 10 K to 375 K while avoiding the photorefractive effects that appear prominently at low temperatures. The E-mode phonon-polaritons corresponding to the two transverse optical modes, TO1 and TO3, up to ∼7 THz were induced in LiNbO3 through an impulsive stimulated Raman scattering process. Subsequently, using single-shot pump-probe imaging spectroscopy over a minimal cumulative time, we successfully visualized the phonon-polariton dynamics in time-wavelength space even at low temperatures. We found that the phase-matching condition significantly affected the observed temperature-dependent phonon-polariton frequency shift. The anharmonicity of the TO1 and TO3 modes was then evaluated based on an anharmonic model involving higher-order interactions with acoustic phonons while eliminating the influence of the frequency shift due to the phase-matching condition. The observed wavenumber-dependent damping rate was analyzed by considering the bilinear coupling of the TO1 or TO3 modes with the thermally activated relaxation mode. We found that the phonon-polariton with a higher frequency and wavenumber had a higher damping rate at high temperatures because of its frequent interaction with the thermally activated relaxation mode and acoustic phonons. The TO3 mode displayed greater bilinear coupling than the TO1 mode, which may also have contributed to the observed high damping rate. Thus, using our unique single-shot spectroscopy technique, we could reveal the overall anharmonic characteristics of the E-mode phonon-polaritons arising from both the acoustic phonons and the relaxation mode.

Abstract-Terahertz generation based on difference frequency generation by rotating fan-out poled LiNbO3

Zhongyang Li, Degang Xu, and Jianquan Yao
https://www.osapublishing.org/ao/abstract.cfm?uri=ao-55-35-10073

Theoretical analysis of terahertz (THz) generation based on difference frequency generation (DFG) by rotating fan-out poled LiNbO3 is presented. The uncertainty of poling vectors during clockwise and counterclockwise rotation of the fan-out poled LiNbO3 at the center of the crystal input face is investigated. The poling period along the propagation direction of the input pump and signal waves during clockwise and counterclockwise rotation is calculated. The dependence of THz vectors on rotation angles is numerically simulated. The relative intensities of a THz wave during clockwise rotation of the fan-out poled LiNbO3LiNbO 3 are calculated.

© 2016 Optical Society of AmericaFull Article  |  PDF Article

Abstract-Hybrid tilted-pulse-front excitation scheme for efficient generation of high-energy terahertz pulses

L. Pálfalvi, Z. Ollmann, L. Tokodi, and J. Hebling
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-8-8156

Conception of a hybrid type tilted-pulse-front pumping scheme for the generation of high-energy terahertz pulses is presented. The proposed setup is the combination of the conventional setup containing imaging optics and the contact grating. The solution was developed for nonlinear materials requiring large pulse-front-tilt angle, like LiNbO3. Due to the creation of the pulse-front-tilt in two steps the limitations of imaging errors can be significantly reduced. Furthermore the necessary grating constant of the contact grating can be larger compared to the simple contact grating scheme making possible the fabrication of the grating profile with significantly higher precision. A detailed optimization procedure with respect to the diffraction efficiency on the contact grating is given for LiNbO3. Instructions are also given how to construct the geometry of the setup in order to minimize imaging errors. Examples are given for LiNbO3 based practically realizable, optimized schemes with reduced imaging errors and high diffraction efficiency on the contact grating.

© 2016 Optical Society of America

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Abstract-Suppressed Terahertz Generation in LiNbO3 via Optical Rectification by Three-Photon Absorption (3PA) under Intense Femtosecond Laser Excitation

Li-Guo Zhu
https://www.osapublishing.org/abstract.cfm?URI=nlo-2015-NM3A.5

We’ll demonstrate that 3PA induced by intense laser is the main limit of THz generation in LiNbO3. A 3D model’ll be proposed. Optimized setups (pump fluence, duration, geometry, etc) for tilted-pulse-front experiments will be given.

© 2015 OSA

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

Spotlight Summary by József A. Fülöp 

https://www.osapublishing.org/spotlight/summary.cfm?URI=oe-23-6-8073

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.