Nonlinear THz Generation through Optical Rectification Enhanced by Phonon–Polaritons in Lithium Niobate Thin Films

Abstract

We investigate nonlinear THz generation from lithium niobate films and crystals of different thicknesses by optical rectification of near-infrared femtosecond pulses. A comparison between numerical studies and polarization-resolved measurements of the generated THz signal reveals a 2 orders of magnitude enhancement in the nonlinear response compared to optical frequencies. We show that this enhancement is due to optical phonon modes at 4.5 and 7.45 THz and is most pronounced for films thinner than 2 μm where optical-to-THz conversion is not limited by self-absorption. These results shed new light on the employment of thin film lithium niobate platforms for the development of new integrated broadband THz emitters and detectors. This may also open the door for further control (e.g., polarization, directivity, and spectral selectivity) of the process in nanophotonic structures, such as nanowires and metasurfaces, realized in the thin film platform. We illustrate this potential by numerically investigating optical-to-THz conversion driven by localized surface phonon−polariton resonances in sub-wavelength lithium niobate rods.

Efficient Cherenkov-Type Optical-to-Terahertz Conversion of Femtosecond Oscillator Pulses

“…In Figure 1a, the optical (c) axis of LiNbO 3 is in the plane of the crystal layer and perpendicular to the propagation direction of the pump light. The polarization of the laser beam is along the c-axis, which is the optimal polarization for optical rectification in LiNbO 3 [25]. Second-order nonlinear polarization P NL induced in the crystal via optical rectification is also directed along the c-axis.…”

Numerical investigation of high-order harmonic generation by gold nanorod arrays in hybrid nanostructures: effect of number of gold nanorod

“…They are used in a variety of applications, including imaging [22], frequency conversion [23][24][25], ultrafast lasers [26], and modulators [27]. Various frequencies, including THz, can be generated using nonlinear optics [28,29].…”

Enhanced Terahertz Phonon Polariton in Lithium Niobate Chip

“…[12] In addition, phonon polariton generation in LN has demonstrated its potential in substantially enhancing optical nonlinearity and THz generation. [16,17] Despite recent advancements in THz phonon polariton generation, its efficiency remains relatively low. For the chip-based THz application, A rectangular LN film is typically employed as a generation media and dielectric waveguide, and an ultra-short laser pulse normal incident on its surface.…”