Cryogenic electro-optic modulation in titanium in-diffused lithium niobate waveguides

TL;DR

This study characterizes electro-optic modulators in lithium niobate waveguides at cryogenic temperatures, revealing significant changes in operation voltage and wavelength properties crucial for quantum photonics applications.

Contribution

It provides the first detailed cryogenic characterization of various electro-optic components in lithium niobate waveguides, essential for integrated quantum optics at low temperatures.

Findings

Operation voltage increases significantly at cryogenic temperatures.

Wavelength shifts occur in directional couplers and polarisation converters when cooled.

Broadband modulation is preserved at cryogenic temperatures.

Abstract

Lithium niobate is a promising platform for integrated quantum optics. In this platform we aim to efficiently manipulate and detect quantum states by combining superconducting single photon detectors and modulators. The cryogenic operation of a superconducting single photon detector dictates the optimisation of the electro-optic modulators under the same operating conditions. To that end, we characterise a phase modulator, directional coupler, and polarisation converter at both ambient and cryogenic temperatures. The operation voltage $V_{\pi/2}$ of these modulators increases due to the decrease of the electro-optic effect by 74% for the phase modulator, 84% for the directional coupler and 35% for the polarisation converter below 8.5$\,\mathrm{K}$. The phase modulator preserves its broadband nature and modulates light in the characterised wavelength range. The unbiased bar state of the directional coupler changed by a wavelength shift of 85$\,\mathrm{nm}$ while cooling the device down to 5$\,\mathrm{K}$. The polarisation converter uses periodic poling to phasematch the two orthogonal polarisations. The phasematched wavelength of the used poling changes by 112$\,\mathrm{nm}$ when cooling to 5$\,\mathrm{K}$