[Paper Review] Demonstration of second harmonic generationin gallium phosphide nano-waveguides

Aravind P. Anthur, Haizhong Zhang+5 more|arXiv (Cornell University)|Jan 17, 2020

Photonic and Optical Devices62 references35 citations

TL;DR

This paper demonstrates efficient second harmonic generation (SHG) in gallium phosphide (GaP) nano-waveguides using modal phase matching, achieving visible light emission at ~655 nm with low-power continuous-wave pumping in the O-band (1310 nm). The key contribution is experimental validation of mode-dependent SHG through precise control of waveguide width and pump wavelength, with measured external conversion efficiency of 1.34 × 10⁻⁴ % W⁻¹cm⁻² and near-ideal quadratic scaling with pump power.

ABSTRACT

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using low power continuous-wave pump in the optical communication O-band. Our structures utilize modal phase matching, such that lower order eigenmodes of the pump are phase matched to higher order eigenmodes of the second harmonic. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Motivation & Objective

• To develop an integrated photonic platform based on gallium phosphide (GaP) for efficient nonlinear frequency conversion between telecom and visible wavelengths.
• To address the challenge of bridging the gap between quantum photonics (visible band) and optical fiber communication (telecom band) using compact, on-chip nonlinear waveguides.
• To demonstrate second harmonic generation (SHG) in GaP nano-waveguides using modal phase matching with low-power continuous-wave pumps.
• To validate the theoretical model of SHG in GaP waveguides by measuring and confirming the crystal axis orientation and its impact on mode coupling.

Proposed method

• Theoretical modeling of SHG in GaP waveguides using coupled-mode equations with phase mismatch Δβmnl(ω) = βm(ω) + βn(ω) − βl(2ω), incorporating the nonlinear susceptibility tensor χ(2) with d36 = 50 pm/V.
• Experimental measurement and confirmation of the GaP crystal axis orientation via SHG under normal incidence, with tilt angle θ = 15° relative to the [111] direction.
• Fabrication of GaP nano-waveguides on sapphire substrates with SiO2 cladding using electron-beam lithography and reactive ion etching, with variable top widths from 300–340 nm.
• Use of a continuous-wave Ti-Sapphire laser at 1311.8 nm (160 mW at fiber output) as pump source, with SHG signal detected via amplified photodiode and lock-in amplifier.
• Measurement of SHG efficiency as a function of input pump power and wavelength, with log-linear fitting to confirm quadratic dependence.
• Modeling of loss effects using an effective interaction length Leff derived from propagation losses (αω ≈ 0.2 dB/μm for pump, higher at 2ω), accounting for discrepancies between theory and experiment.

Experimental results

Research questions

• RQ1 Can second harmonic generation be efficiently achieved in GaP nano-waveguides using modal phase matching with low-power continuous-wave pumping?
• RQ2 How does the waveguide width and pump polarization influence the phase-matched wavelength and mode coupling in GaP waveguides?
• RQ3 To what extent do fabrication-induced non-uniformities and propagation losses affect the experimental SHG conversion efficiency compared to theoretical predictions?
• RQ4 What is the role of crystal axis orientation in determining the overlap integrals and SHG efficiency in GaP waveguides?

Key findings

• The experimental SHG output power scales quadratically with input pump power, with a slope of 2.3 (H polarization) and 2.5 (V polarization) on log scale, confirming the expected second-order nonlinear process.
• Phase matching bandwidth was experimentally measured at ~1.2 nm (FWHM), broader than the simulated ~0.2 nm, likely due to waveguide non-uniformities.
• The external normalized conversion efficiency was measured at 1.34 × 10⁻⁴ % W⁻¹cm⁻² for H-polarized pump at 1303.1 nm in a 340 nm wide waveguide, with 773 pW of second harmonic power at 160 mW pump power.
• Theoretical internal normalized conversion efficiency was calculated at 356 % W⁻¹cm⁻², indicating significant room for improvement through reduced coupling and propagation losses.
• Waveguide width tuning enabled phase-matched SHG across different mode combinations, with experimental phase-matched wavelengths closely matching simulations.
• Propagation losses were estimated at 2 dB/mm for the pump, with higher losses expected at the second harmonic due to λ⁻² dependence of roughness scattering.