Birefringence phase-matched direct third-harmonic generation in a ridge optical waveguide based on a KTiOPO₄ single crystal

Abstract

Birefringence phase-matched third-harmonic generation at 1594 nm is performed for the first time in a KTiOPO4 single crystal micrometric ridge waveguide. The energy conversion efficiency reaches 3.4% for a pump energy as low as 2 µJ over a pulse duration of 15 ps at a repetition rate of 10 Hz. Strong agreements between theory and experiments for both phase-matching and conversion efficiency is obtained, which let us envision future triple photon generation quantum experiments.

Demonstrating quantum properties of triple photons generated by $$\chi ^3$$ processes

“…The experiments were carried out using a TOPAS Optical Parametric Generator to deliver the pump beam, with a pulse duration of 15 ps, a repetition rate of 10Hz, and a wavelength that is tunable around 1600 nm. By measuring the Third-Harmonic (TH) intensity as a function of the fundamental wavelength, we found that the phase-matching was achieved at λ ω = 1594 nm [6]. The calculated value is λ ω = 1594.2 nm from Eq.…”

“…By this way, the pump and triplet waves can exhibit the same propagation modes so that the overlap will be optimal. KTP ridge waveguides are fabricated using a technique based on precise diamond blade dicing [6]. A picture of such a waveguide is shown in Fig.…”

“…The transverse average section was found to be of about 38 µm 2 , non-constant along the ridge axis: it corresponds to an average square waveguide of side d = 6.17 µm. We performed a preliminary validation of this technology by achieving high efficiency third-harmonic generation (THG: ω + ω + ω → 3ω) in the waveguide depicted in Fig.8 [6]. THG is particularly interesting since it is the exact reverse of TPG that is degenerated in energy, i.e.…”

“…1 (a), using a phase-matched bulk KTiOPO 4 crystal [4]. More recently, we proposed a waveguide approach to boost the generation efficiency [5,6]. TPG has also been an active field of research for many groups around the world [7][8][9][10][11][12][13][14][15].…”

“…Then the design of an optimal TPG requires to find a material with a high effective nonlinear coefficient, low spatial walk-off angles, and allowing phase-matching at the targeted wavelengths. We identified the biaxial crystal KTiOPO 4 (KTP) as the good platform, under two different technologies: a bulk crystal [4] and a ridge optical waveguide crystal [6]. Then the strategy is to perform a birefringence phase-matching in both cases.…”