Laser-cooled atoms inside a hollow-core photonic-crystal fiber

We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of $~30$,000 rubidium atoms, which creates a medium with an optical depth of $~180$ inside the fiber. Compared to our earlier study [1] this represents a sixfold increase in the maximum achieved optical depth in this system.

Figure 1

(a) The schematics of the experimental setup. (b) Scanning electron microscope (SEM) image of a cleaved hollow-core photonic-crystal fiber, Model HC-800-02, from Blaze Photonics. (c) Detail of the photonic-crystal region with the hollow core in the center. Manufacturer’s specifications for (d) losses of guided mode propagating in the fiber as a function of wavelength and (e) near-field intensity distribution of the guided mode.

Figure 2

(a) Fiber assembly. (b) The anchoring of the fiber inside the mount.

Figure 3

The loading procedure. (a) Atoms collected in a MOT above the fiber. (b) Absorption image of the atoms in the magnetic-funnel area above the fiber. (c) Once near the fiber tip, the atoms are transferred into a red-detuned dipole trap inside the fiber. (d) Contour plot of the dipole trap potential above the fiber tip resulting from the diverging beam emerging from the fiber tip (located at the origin). The contour labels correspond to a 10-mK-deep trap inside the fiber resulting from $~25$ mW of 802-nm trap light inside the fiber.

Figure 4

Hollow-beam atomic waveguide. (a) Schematics of the optics used for the hollow-beam generation. (b) CCD image of the hollow-beam intensity distribution about 1 mm above the fiber face. (c) Fluorescence image of the freely expanding atomic cloud 20 ms after its release from the optical molasses. (d) Fluorescence image of the atomic cloud guided by the blue-detuned hollow beam 20 ms after the optical molasses beams are turned off. (e) Absorption image of the atoms collected in the hollow-beam guide $~1$ mm above the fiber tip. Here, the hollow beam is intersected by a blue-detuned Gaussian beam focused by a cylindrical lens into a sheet.

Figure 5

Atoms inside the fiber. (a) Transmission of the fiber as a function of probe frequency with constant dipole trap (broad red data curve centered at $~60$ MHz) and with modulated dipole trap (narrower black data curve centered at 0 MHz). (b) Modulation scheme for probe and dipole trap. The probe is broken into $~100$ short pulses that interact with the atoms when the dipole trap is off. (c) Detected optical depth of the atomic cloud inside the fiber as a function of the dipole trap modulation frequency.

Figure 6

Frequency scan over the $D_2$ line of ${}^{87}\mathrm{Rb}$ for $~26$ 000 atoms loaded inside the fiber. The atoms are optically pumped into the $F=2$ state and then probed with linearly polarized light over the three transitions accessible from this state: $F=2\to F^{\prime }=1$ (left), $F=2\to F^{\prime }=2$ (middle), and $F=2\to F^{\prime }=3$ (right). The different observed optical depths agree well with a number of loaded atoms of $N_{\mathrm{at}}~30\hspace{0.5em}000$ when the relative strengths of these transitions are taken into account.

Figure 7

(a) Calibration of the number of atoms inside the fiber by counting the number of photons required to transfer the amount of atoms required to cause ${\mathcal{D}}^{\mathrm{opt}}=1$ on a given probe transition. (b) With ${\gamma }_1\approx {\gamma }_2$, fitting an exponential (dashed line) to the measured data (red dots with error bars) yields $\eta \approx 0.42$ as the value of the prefactor from Eq. (3).

Figure 8

Atoms inside the fiber. (a) Doppler shift of the falling atom cloud observed by using two probes propagating through the fiber in opposite directions. (b) Lifetime of the atoms inside the fiber. Inset: A qualitative sketch of the potential along the fiber axis experienced by the atoms as they load into the fiber-guided dipole trap.

Figure 9

Temperature estimate from a TOF measurement. (a) The time sequence diagram: After the atoms are released from the trap and then recaptured, the trap is modulated for the ${\mathcal{D}}^{\mathrm{opt}}$ measurement. (b) Optical depth of the recaptured cloud as a function of the release time. The dashed red line represents a fit based on the Gaussian cloud expansion model.