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Physical Review A

High-resolution two-photon spectroscopy of rubidium within a confined geometry

Christopher Perrella, P. S. Light, James D. Anstie, Thomas M. Stace, Fetah Benabid, Andre Nicholas Luiten

Published Jan 17, 2013• 38 citations
Publisher

Abstract

We present two-photon spectroscopy of a thermal rubidium vapor confined to the hollow core of a photonic-crystal fiber. Linewidths as narrow as 10 MHz were observed on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ transition enabling the hyperfine splitting of the excited state to be resolved. Very strong nonlinear absorption ($>90%$) was observed, with substantial absorption maintained over large detunings (9 GHz) from an intermediate state. These attributes make this system ideal for many frequency metrology and quantum optics applications.

DOI
10.1103/physreva.87.013818

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We present two-photon spectroscopy of a thermal rubidium vapor confined to the hollow core of a photonic-crystal fiber. Linewidths as narrow as 10 MHz were observed on the 5S1/2\ensuremath→5D5/25{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}5S1/2​\ensuremath→5D5/2​ transition enabling the hyperfine splitting of the excited state to be resolved. Very strong nonlinear absorption (>90>90%>90) was observed, with substantial absorption maintained over large detunings (9 GHz) from an intermediate state. These attributes make this system ideal for many frequency metrology and quantum optics applications.
In this paper, we present detailed high-resolution spectroscopy of rubidium (Rb) vapor confined within a hollow-core photonic crystal fiber (HC-PCF). We find a very low level of additional frequency broadening associated with this confinement, with spectral features being only 1 MHz broader than the natural linewidth of the excited state. We show that this additional broadening is consistent solely with the atoms' transit across the fiber's optical mode. This low level of decoherence opens the door to a wide variety of applications including compact frequency standards and new types of quantum optical devices based on alkali-metal-loaded HC-PCFs. We highlight the low level of decoherence through observation of electromagnetically induced transparency in the confined vapor.
We show that two-photon absorption (TPA) in rubidium atoms can be greatly enhanced by the use of a hollow-core photonic-band-gap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ transition and observe 11%1 absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit-time effects play an important role in determining the two-photon absorption cross section in a confined geometry.
We demonstrate an optical frequency standard based on rubidium vapor loaded within a hollow-core photonic crystal fiber. We use the 5S(1/2)→5D(5/2) two-photon transition, excited with two lasers at 780 and 776 nm. The sum-frequency of these lasers is stabilized to this transition using modulation transfer spectroscopy, demonstrating a fractional frequency stability of 9.8×10(-12) at 1 s. The current performance limitations are presented, along with a path to improving the performance by an order of magnitude. This technique will deliver a compact, robust standard with potential applications in commercial and industrial environments.
Accessing the 5S1/2→5D5/2\rm 5S_{1/2} \rightarrow 5D_{5/2}5S1/2​→5D5/2​ two-photon transition in Rb using a diode laser system
We report observation of the 5S1/2→5D5/2\rm 5S_{1/2} \rightarrow 5D_{5/2}5S1/2​→5D5/2​ two-photon transition in Rb vapor at 778 nm, using an external cavity diode laser system and a heated vapor cell. The spectra in the two isotopes show well-resolved hyperfine transitions. The peaks are Doppler free, and have a Lorentzian lineshape with a typical linewidth of 2.2 MHz. This linewidth is larger than the natural linewidth of 300 kHz, but is still 5--10 times smaller than the linewidth for single-photon transitions in the D2_22​ line. Since the absolute frequency of this transition is measured with 8 kHz precision, it can form a better secondary reference in the optical regime compared to the D2_22​ line.
[Truncated] This thesis concerns a new photonic technology based on hollow-core photonic-crystal fibre (HC-PCF) that has been loaded with an atomic vapour. It was demonstrated that this technology may have important applications in the fields of metrology and quantum optics. Furthermore, the small size and robustness of HC-PCF allows these devices to potentially be miniaturised for possible commercial and industrial applications. By filling the HC-PCF with an atomic vapour, excellent light-atom coupling can be attained as both the atoms and light are confined within the same volume. The small transverse dimensions of the fibre’s optical mode leads to high intensities at low input powers, enabling efficient driving of non-linear transitions which would otherwise be difficult with free-space optical systems. Additionally, long interaction lengths, obtained through an arbitrary length of HC-PCF, lead to large optical depths on these typically weak non-linear transitions. The non-linear transition employed here is the 5S1/2 → 5D5/2 rubidium (Rb) two-photon transition. This system is of great interest due to its non-linearity, long excited state lifetime, efficient excitation and large optical depth attained within the HC-PCF. A number of methods for loading a HC-PCF with Rb are discussed, along with proposed ideas for making this process more efficient. Spectroscopic techniques were used to investigate the effect of the fibre’s confined geometry upon the atomic spectra. Extensive theoretical modelling was employed to confirm the observed spectra and give insight to the mechanisms behind the observations. Using this experimental and theoretical knowledge, two applications were targeted: optical atomic frequency standards; and cross phase modulation for quantum logic gates.
Recent demonstrations of light-matter interactions with atoms and molecules confined to hollow waveguides offer great promise for ultralow-light-level applications. The use of waveguides allows for tight optical confinement over interaction lengths much greater than what could be achieved in bulk geometries. However, the combination of strong atom-photon interactions and nonuniformity of guided light modes gives rise to spectroscopic features that must be understood in order to take full advantage of the properties of such systems. We use light-induced atomic desorption to generate an optically dense Rb vapor at room temperature inside a hollow-core photonic band-gap fiber. Saturable-absorption spectroscopy and passive slow-light experiments reveal large ac Stark shifts, power broadening, and transit-time broadening, that are present in this system even at nanowatt powers.