A diffraction-compensating 0-25 ns free space terahertz delay line for coherent quantum control

TL;DR

This paper introduces a novel diffraction-compensating free space terahertz delay line capable of providing up to 25 ns delay, improving pulse control for quantum and optical experiments in the THz range.

Contribution

The authors present a scalable, diffraction-compensating delay line design for THz frequencies that maintains beam quality over long delays and is adaptable to other wavelengths.

Findings

Achieved up to 25 ns delay with good beam profile preservation.

Insertion loss per delay stage is approximately 3 dB.

The design is compact, enclosed in less than 0.5 m^3.

Abstract

Free space delay lines provide pulses of variable time spacing for optical experiments such as pump-probe spectroscopy and coherent quantum control, including spin and photon echo techniques. However, in the terahertz (THz) region of the spectrum, beam divergence due to diffraction limits the useful length of traditional free space delay lines. We present a novel double-folded variable delay line for light in the frequency range 0.24-1.2 THz, which incorporates a symmetric arrangement of lenses whose spacing can be adjusted to compensate for diffraction at each delay. Scalable for use in other wavelength regimes, the design relays an input Gaussian beam waist to the output with up to 25 ns (~8 m) total delay and is enclosed in a desiccated volume of <0.5 m^3. The delay line can deliver two or three pulses with relative amplitudes controlled via variable spacing silicon etalon beam splitters. Beam profiles of a 0.24 THz beam show good agreement with calculations at long delays, with insertion loss per delay stage of~3 dB.