[Paper Review] A stable 2 W continuous-wave 261.5 nm laser for cooling and trapping aluminum monochloride

Jamie Shaw, Stephan Hannig+1 more|arXiv (Cornell University)|Sep 2, 2021

Solid State Laser Technologies46 references26 citations

TL;DR

This paper presents a stable, 2 W continuous-wave deep-ultraviolet (DUV) laser at 261.5 nm using two cascaded cavity-enhanced doubling stages with LBO and CLBO crystals, frequency-quadrupling a 1046 nm external cavity diode laser. The system achieves 2.75 W output with 19% conversion efficiency and enables the first spectroscopy of the A1Π |v′=0, J′=1⟩ hyperfine structure in aluminum monochloride (AlCl), revealing partly resolved hyperfine features with a linewidth of ~50 MHz.

ABSTRACT

We present a high-power tunable deep-ultraviolet (DUV) laser that uses two consecutive cavity enhanced doubling stages with LBO and CLBO crystals to produce the fourth harmonic of an amplified homebuilt external cavity diode laser. The system generates up to 2.75 W of 261.5 nm laser light with a ~2 W stable steady-state output power and performs second harmonic generation in a largely unexplored high intensity regime in CLBO for continuous wave DUV light. We use this laser to perform fluorescence spectroscopy on the $X^1\Sigma\leftarrow A^1\Pi$ transition in a cold, slow beam of AlCl molecules and probe the $A^{1} \Pi|v'=0,~J'=1>$ state hyperfine structure for future laser cooling and trapping experiments. This work demonstrates that the production of tunable, watt-level DUV lasers is becoming routine for a variety of wavelength-specific applications in atomic, molecular and optical physics.

Motivation & Objective

• To develop a high-power, tunable, continuous-wave deep-ultraviolet laser at 261.5 nm for laser cooling and trapping of aluminum monochloride (AlCl).
• To demonstrate stable watt-level DUV output in a largely unexplored high-intensity regime using CLBO for second harmonic generation.
• To enable precision spectroscopy of the A1Π state hyperfine structure in AlCl using a cold, slow molecular beam.

Proposed method

• Frequency quadrupling of a 1046 nm homebuilt external cavity diode laser (ECDL) via two cascaded resonant enhancement cavities.
• First stage uses a 25 mm LBO crystal for Type I non-critical phase-matched doubling to 523 nm, with a circulating power of ~220 W.
• Second stage employs a 15 mm CLBO crystal for further doubling to 261.5 nm, with optimized mode-matching and temperature control.
• Analog auto-locking circuits stabilize both enhancement cavities over a >100 GHz tuning range.
• Laser output is stabilized using a commercial wavemeter and a Fabry-Perot cavity referenced to a HeNe laser for sub-MHz frequency resolution.
• Fluorescence spectroscopy is performed on a cold, slow AlCl molecular beam using a 0.1 mW/cm² DUV beam to minimize power broadening.

Experimental results

Research questions

• RQ1 Can a stable, high-power continuous-wave DUV laser at 261.5 nm be generated using cascaded cavity-enhanced frequency doubling with LBO and CLBO crystals?
• RQ2 What is the performance and stability of the 261.5 nm output over extended operation, particularly in the high-intensity regime of CLBO?
• RQ3 Can the A1Π state hyperfine structure of AlCl be resolved using this laser system?
• RQ4 What is the linewidth and resolution of the observed hyperfine features in the AlCl A1Π state?
• RQ5 How does UV-induced degradation affect long-term performance at high crystal intensities?

Key findings

• The system achieves a maximum output power of 2.75 W at 261.5 nm with a stable steady-state power of ~2 W over 13 hours.
• The total conversion efficiency from 1046 nm to 261.5 nm is ~19%, demonstrating high efficiency in cascaded frequency doubling.
• The laser exhibits a linewidth estimate of ~2 MHz in the visible and ~2 MHz in the DUV, consistent with high coherence and stability.
• The first spectroscopy of the A1Π |v′=0, J′=1⟩ hyperfine structure in AlCl is performed, revealing a partly resolved spectrum with a narrowest feature at ~180 MHz and a linewidth of ~50 MHz.
• UV-induced degradation is observed as a recoverable power drop at high intensities, consistent with prior reports in pulsed systems using CLBO.
• The ground state hyperfine structure is unresolved, while the excited state structure is only partly resolved, indicating that unresolved hyperfine structure limits resolution more than the natural linewidth.