# Large-area photonic circuits for terahertz detection and beam profiling

## TL;DR

This paper introduces a new on-chip photonic system for efficient and accurate terahertz detection and beam profiling using thin-film lithium niobate.

## Contribution

The work presents an integrated photonic architecture in thin-film lithium niobate that enables large-area terahertz detection and beam profiling without domain inversion.

## Key findings

- The device uses a double array of up to 18 terahertz antennas in a Mach–Zehnder interferometer to boost interaction efficiency.
- Quasi-phase-matching is achieved through a periodic terahertz near-field pattern, allowing custom frequency response tuning.
- The system enables terahertz beam profile reconstruction and operates effectively with diverse beam settings.

## Abstract

Deployment of terahertz communication and spectroscopy systems relies on the availability of low-noise and fast detectors, with plug-and-play capabilities. However, most current technologies are stand-alone, discrete components. They are often slow or susceptible to temperature drifts and require tight beam focusing to maximize the signal-to-noise of the detector. Here, we demonstrate an integrated photonic architecture in thin-film lithium niobate that addresses these challenges by exploiting the electro-optic modulation induced by a terahertz signal onto an optical beam at telecom frequencies. Leveraging on the low optical losses provided by this platform, we integrate a double array of up to 18 terahertz antennas within a Mach–Zehnder interferometer, considerably extending the device collection area and boosting the interaction efficiency between the terahertz signal and the optical beam. We show that the double array coherently builds up the probe modulation through a mechanism of quasi-phase-matching, driven by a periodic terahertz near-field pattern, circumventing physical inversion of the crystallographic domains. This provides means to fully custom-tailor the frequency response of the device, limit it to a desired frequency band and effectively suppress out-of-band signals. The large detection area ensures correct operation with diverse terahertz beam settings. Furthermore, we show that the antennas act as pixels that allow reconstruction of the terahertz beam profile impinging on the detector area. Our on-chip design in thin-film lithium niobate overcomes the detrimental effects of two-photon absorption and fixed phase-matching conditions, which have plagued previously explored electro-optic detection systems, especially in the telecom band, paving the way for more advanced on-chip terahertz systems.

We demonstrate a large-area, integrated photonic architecture in thin-film lithium niobate relying on the electro-optic modulation of a telecom beam to perform field-resolved terahertz detection and beam profiling