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

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 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 ( approximately 8 m) total delay and is enclosed in a desiccated volume of <0.5 m3. The delay line can deliver two or three pulses with relative amplitudes controlled via variable spacing silicon etalon beam splitters. Profiles of a 0.24 THz beam show good agreement with calculations at long delays, with insertion loss per delay stage of approximately 3 dB.

Pulsed EPR spectrometer with injection-locked UCSB free-electron laser

“…The beam splitter is made of two parallel silicon wafers. Transmission is controlled in the range of 10-100% by changing the distance of the two wafers while the polarization of both reflection and transmission remains the same [7]. The delay line is made by a 2 m long translation stage.…”

“…The delay line is made by a 2 m long translation stage. In order to compensate diffraction of 240 GHz waves, the delay stage also includes several lenses on an independent translation stage [7]. Through the quasi-optical block, the FEL pulses are propagated as a free-space fundamental Gaussian mode.…”