Radiative Force from Optical Cycling on a Diatomic Molecule

We demonstrate a scheme for optical cycling in the polar, diatomic molecule strontium monofluoride (SrF) using the $X^2{\Sigma }^{+}\to A^2{\Pi }_{1/2}$ electronic transition. SrF’s highly diagonal Franck-Condon factors suppress vibrational branching. We eliminate rotational branching by employing a quasicycling $N=1\to N^{\prime }=0$ type transition in conjunction with magnetic field remixing of dark Zeeman sublevels. We observe cycling fluorescence and deflection through radiative force of an SrF molecular beam using this scheme. With straightforward improvements our scheme promises to allow more than ${10}^5$ photon scatters, possibly enabling the direct laser cooling of SrF.

Figure 1

Relevant electronic and vibrational structure in SrF. Solid upward lines indicate laser-driven transitions in the experiment at the wavelengths ${\lambda }_{v,v^{\prime }}$. Solid wavy lines indicate spontaneous decays from the $A(v^{\prime }=0)$ state with FCFs $f_{0v}$ as shown. Dashed wavy lines indicate spontaneous decays from the $A(v^{\prime }=1)$ state populated by the proposed second vibrational repump laser (dashed vertical arrow), with FCFs $f_{1v}$ as shown.

Figure 2

(a) Relevant rotational energy levels, splittings, parity, and magnetic $g$ factors in the SrF cycling scheme. The relevant cycling transitions are shown as vertical arrows. (b) Laser (vertical lines) and molecular (dashed curve) spectra for addressing all HFS sublevels. Molecular lines are shown with a power broadened linewidth of 10 MHz and are equally weighted for clarity. For an EOM modulation index of $m=2.6$ and frequency of 42.5 MHz, there is a laser sideband $<10\mathrm{MHz}$ detuned from each molecular line.

Figure 3

Signals demonstrating cycling fluorescence in SrF. LIF signal from the $v_{00}$ pump laser in the pump region with $B=0\mathrm{G}$, with $B=7\mathrm{G}$, and with both the $v_{10}$ repump and $B=7\mathrm{G}$. The addition of the magnetic field $B$ results in a $\sim 3.5\times \mathrm{\text{enhancement}}$ in signal due to remixing from dark Zeeman sublevels. The addition of the $v_{10}$ repump results in another $\sim 3.5\times \mathrm{\text{enhancement}}$, indicating pumping to and from the $X(v=1)$ vibrational level as a result of optical cycling.

Figure 4

Radiative force deflection of the SrF beam. Positional LIF from the $v_{00}$ probe laser with no lasers in the pump region (unperturbed), with $v_{00}$ pump and $v_{10}$ repump lasers in pump region (deflected), and with pump laser but no repump or $v_{10}$ cleanup laser (depleted). Without the repump and cleanup lasers, nearly complete depletion of the $v=0$ sublevel is observed. Addition of these lasers results in nearly full recovery of the LIF signal amplitude, indicating optical pumping to and repumping from the $v=1$ level. The pump and repump lasers also cause deflection of the SrF beam due to radiative force. This plot shows raw data, with no rescaling applied. Inset: Dependence of the number of scattered photons on the interaction time in the pump region. Points are data and the line is a fit, showing that $N_{\mathrm{scat}}$ increases linearly with interaction time.