[Paper Review] A Cold-Strontium Laser in the Superradiant Crossover Regime

Matthew A. Norcia, James K. Thompson|arXiv (Cornell University)|Oct 22, 2015

Cold Atom Physics and Bose-Einstein Condensates3 citations

TL;DR

This paper demonstrates a cold-strontium laser operating on the 7.5 kHz linewidth dipole-forbidden 3P1–1S0 transition in 88Sr, achieving quasi-steady-state lasing via atomic repumping. By tuning between the bad-cavity (superradiant) and good-cavity regimes, the laser exhibits a pulling coefficient of 0.09(2), confirming atomic coherence dominance and reducing frequency sensitivity to cavity length fluctuations by up to an order of magnitude.

ABSTRACT

Recent proposals suggest that lasers based on narrow dipole-forbidden transitions in cold alkaline earth atoms could achieve linewidths that are orders of magnitude smaller than linewidths of any existing lasers. Here, we demonstrate a laser based on the 7.5 kHz linewidth dipole forbidden $^3 $P$_1$ to $^1 $S$_0$ transition in laser-cooled and tightly confined $^{88}$Sr. We can operate this laser in the bad-cavity regime, where coherence is primarily stored in the atoms, or continuously tune to the more conventional good-cavity regime, where coherence is primarily stored in the light field. We show that the cold-atom gain medium can be repumped to achieve quasi steady-state lasing, and demonstrate up to an order of magnitude suppression in the sensitivity of laser frequency to changes in cavity length, the primary limitation for the most frequency stable lasers today.

Motivation & Objective

• To develop a laser based on a narrow dipole-forbidden transition in cold alkaline earth atoms for ultra-narrow linewidths.
• To achieve quasi-steady-state lasing through repumping of the atomic gain medium.
• To demonstrate operation in the superradiant (bad-cavity) regime, where laser frequency is set by atomic transition, not cavity resonance.
• To reduce laser frequency sensitivity to cavity length fluctuations, a key limitation in state-of-the-art frequency-stable lasers.
• To explore the crossover between bad-cavity and good-cavity regimes by tuning atomic linewidth via repumping.

Proposed method

• Laser-cooled 88Sr atoms are tightly confined in a 1D optical lattice within a high-finesse cavity (finesse = 24,000, κ = 2π × 160 kHz).
• The lasing transition is the 7.5 kHz-wide 3P1 (mJ=0) to 1S0 transition at 689 nm, driven via optical pumping and repumping at 688/689 nm.
• Repumping is achieved via incoherent excitation through the 3P1, mJ=–1 and 3S1 states, with additional lasers for population recovery from 3P0 and 3P2.
• The pulling coefficient P = Δfl/Δfc is measured by heterodyning the laser output with a frequency-stabilized 689 nm source while scanning the cavity frequency.
• Laser linewidth is characterized via power spectra from heterodyne detection, with frequency alignment across trials to suppress low-frequency noise.
• The system is tuned across the good- and bad-cavity regimes by varying the effective atomic linewidth via repump beam power and Rayleigh scattering from the 461 nm MOT beam.

Experimental results

Research questions

• RQ1 Can quasi-steady-state lasing be achieved on a narrow dipole-forbidden transition in cold 88Sr atoms using repumping?
• RQ2 To what extent is the laser frequency determined by the atomic transition rather than the cavity resonance in the bad-cavity regime?
• RQ3 How does the pulling coefficient P = Δfl/Δfc vary across the crossover between bad-cavity and good-cavity operation?
• RQ4 What is the linewidth of the emitted light, and how does it compare to the natural linewidth and repumping-induced broadening?
• RQ5 Can the laser achieve reduced sensitivity to cavity length fluctuations, enabling improved frequency stability?

Key findings

• The laser achieves quasi-steady-state operation with up to 60,000 atoms each emitting ~35 photons before repumping-induced heating terminates lasing.
• A pulling coefficient of P = 0.09(2) is measured at low repump power (w = 2π × 14 kHz), confirming operation deep in the bad-cavity regime with coherence primarily stored in the atoms.
• At high effective atomic linewidth (2γ′⊥ ≈ 2π × 3 MHz from 461 nm Rayleigh scattering), the pulling coefficient increases to P = 0.97(3), indicating a transition to the good-cavity regime where coherence is stored in the light field.
• The measured linewidth is 6.0(3) kHz (Lorentzian) and 4.7(3) kHz (Gaussian), narrower than the natural 7.5 kHz atomic linewidth, indicating frequency narrowing due to laser synchronization.
• The laser exhibits a 10-fold reduction in sensitivity to cavity length fluctuations in the bad-cavity regime, a key advantage for frequency stability.
• The system demonstrates a path toward sub-Hz linewidth lasers using the even narrower 1S0–3P0 transition in 87Sr, with potential for robust, lab-external frequency standards.