# Interferometric autocorrelation in the ultra-violet utilizing spontaneous parametric down-conversion

## TL;DR

This paper introduces a novel method using spontaneous parametric down-conversion to perform autocorrelation measurements of ultra-short UV pulses, overcoming limitations of traditional second harmonic generation techniques.

## Contribution

It demonstrates the first direct measurement of ultra-short UV pulses within a UV enhancement cavity using SPDC-based autocorrelation in the infrared.

## Key findings

- Successfully measured 176 fs UV pulses at 390 nm
- Utilized n-th order SPDC emission for autocorrelation
- Achieved high-efficiency detection of near-infrared photons

## Abstract

Autocorrelation is a common method to estimate the duration of ultra-short laser pulses. In the ultra-violet (UV) regime it is increasingly challenging to employ the standard process of second harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal for UV pulses in the infrared. Our method utilizes the $n^{\text{th}}$ order emission of the SPDC process, which occurs for low pumping powers proportional to the $n^{\text{th}}$ power of the UV intensity. Thus, counting $2n$ down-converted photons directly yields the $n^{\text{th}}$-order autocorrelation. The method, now with detection of near infrared photons with high efficiency, is applied here to the first direct measurement of ultra-short UV-pulses (approximately $176\, \text{fs}$, center wavelength $390\, \text{nm}$) inside a UV enhancement cavity.