Asymmetric Cherenkov radiation for improved terahertz generation in the Si-prism-coupled LiNbO_3 layer

Abstract

We show that a Cherenkov emission of terahertz waves from a femtosecond optical pulse propagating in a LiNbO(3) crystal can be strongly spatially asymmetric with respect to the direction of the optical pulse propagation. We propose using this phenomenon to improve the spectral characteristics of one of the most efficient optical-to-terahertz converters: a thin LiNbO(3) layer attached to a Si-prism outcoupler.

Unidirectional Cherenkov Radiation for improved terahertz generation in the Si-prism-coupled LiNbO<inf>3</inf> Layer

“…We show that in LN the orientation of the crystallographic axes can be arranged in such a way that the line-like (stretched along the y axis) nonlinear polarization P NL (x, ξ), produced by a focused-to-aline pump laser pulse (ξ = tz/V, V is the group velocity), will be almost perfectly orthogonal to the electric field E (tilted with respect to the displacement D) on a half of the Cherenkov wedge ( Fig. 1) [4]. As a result, this half-wedge will be practically not generated and almost all terahertz radiation will be emitted in the direction normal to the other half of the Cherenkov wedge.…”

Selective imaging of the terahertz electric field of the phonon-polariton in LiNbO3

“…A pump pulse traveling in the z direction with a speed of c/n g generates THz electric polarization P THz through optical rectification, where c is the speed of light in vacuum and n g = 2.27 is the group refractive index at 1300 nm [39]. By using the optical rectification tensor [36], the THz electric polarization in a cylindrical coordinate (r, φ, z), as indicated in Fig. 1(b) is expressed as follows:…”

“…We numerically calculated the THz electric field E THz of the phonon-polariton to explain the experimental results. In the calculation, we referred to a theoretical framework [33,35,36,41] that describes the one-dimensional propagation of the THz electric field in LiNbO 3 (x cut) by using the Maxwell equations. We extended this theory to two-dimensional propagation in LiNbO 3 (z cut) with the cylindrical coordinate system:…”

“…LiNbO 3 offers a wide range of optical applications, such as THz-wave emitters [20][21][22] and quasi-phase-matched devices [23][24][25][26], owing to its significant nonlinear optical effects and ferroelectricity. LiNbO 3 exhibits high optical-to-THz conversion efficiency, particularly when using the tiltedpump-pulse-front scheme [13,22,[27][28][29][30][31] or Si-prism coupling method [32][33][34][35][36]. Thus, it is necessary to survey the response to the ultrashort pulse laser and to analyze the THz electric field.…”

Selective imaging of terahertz electric field of phonon-polariton in LiNbO$_3$

“…LiNbO 3 offers a wide range of optical applications, such as THz-wave emitters [20][21][22] and quasi-phase matched devices [23][24][25][26] owing to its significant nonlinear optical effects and ferroelectricity. LiNbO 3 exhibits high optical-to-THz conversion efficiency, particularly when using the tilted-pump-pulse-front scheme [13,22,[27][28][29][30][31] or Si-prism coupling method [32][33][34][35][36]. Thus, it is necessary to survey the response to the ultrashort pulse laser and to analyze the THz electric field.…”