# Experimental generation of phase wraps for subwavelength phase structures in Bose-Einstein condensate with two-dimensional optical lattice

## TL;DR

This paper demonstrates an experimental method to generate and measure sub-wavelength phase structures in a Bose-Einstein condensate using two-dimensional optical lattices, revealing new ways to study optical lattice properties and topological defects.

## Contribution

The study introduces a novel technique for creating sub-wavelength phase wraps in BECs through phase modulation with optical lattices, including analysis of different lattice configurations and validity conditions.

## Key findings

- Successful generation of phase wraps with multiple 2π jumps
- Measurement of phase wraps in momentum space via time-of-flight imaging
- Identification of conditions for the validity of the Kapitza-Dirac regime

## Abstract

We report an experimental demonstration of engineering phase wraps for sub-wavelength structure in a Bose-Einstein condensate (BEC) with two-dimensional optical lattices. A short lattice pulse is applied on BEC working in the Kapitza-Dirac (or Raman-Nath) regime, which corresponds to phase modulation imprint on matter wave. When the phase modulation on matter wave is larger than $2\pi$ in a lattice cell, there appears phase wraps with multiple $2\pi$ jumps, generating the sub-wavelength phase structure. The phase wraps for sub-wavelength structure are measured in momentum space via the time-of-flight absorption image, which corresponds to converting phase information into amplitude. %Two different kinds of two-dimensional optical lattice are studied, which show the different subwavelength phase structure. Moreover, we identify an additional condition for the validity of Kapitza-Dirac regime, which relies crucially on the lattice configurations. This scheme can be used for studying the property of optical lattices and topological defects in matter wave.