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Non-linear spectroscopy of rubidium in hollow-core fibres

Christopher Perrella

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

[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.

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[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.
This thesis describes developments in atomic vapour loading in hollow core photonic crystal fibre (HC-PCF) for fabrication of atomic vapour loaded photonic microcells (PMC). These developments have been targeted at addressing some of the issues associated with loading atomic vapours in confined waveguiding geometries such as increased dephasing and physio-chemical wall absorptions. Atomic vapour loaded HC-PCF and PMC’s have applications in laser metrology, coherent optics and magneto optical spectroscopy. State of the art HC-PCF have been fabricated for loading with atomic vapour including both photonic bandgap (PBG) guiding and inhibited coupling (IC) hypocycloidal core shape Kagome HC-PCF. Record loss of 70 dB/km has been achieved in IC hypocycloid core shape Kagome HC-PCF in the spectral region centred at 800 nm. This fibre retains excellent single mode propagation combined with large core and increased optical bandwidth in comparison with specialist PBG HC-PCF optimised for operation around 800 nm. Aluminosilicate sol-gel coatings have been developed and successfully applied to the inner core wall of HC-PCF’s to reduce the atomic vapour surface interaction. Confining atomic vapours in micron scaled HC-PCF results in increased dephasing rates because of the frequent atom wall collisions. Anti relaxation coating materials have been applied to the inner core wall and the longitudinal relaxation time has been measured in coated and uncoated fibres utilising a magneto optical technique. Additionally sub Doppler transparencies are investigated in anti relaxation coated and uncoated HC-PCF.
Having a strong interaction between hot alkali vapour and light is important for Rydberg physics and nonlinear optics. In optical fibres, the tight confinement by the core offers high laser intensities for even moderate powers. With the invention of hollow-core photonic crystal fibres (HC-PCFs), the interaction of alkali atoms with high intensities over long distances could be studied. In the past, setups combining alkali atoms and a HC-PCF have suffered from long waiting times until atoms are evenly distributed along the fibre. The large size of the used vacuum chambers entailed technical challenges when trying to heat the system uniformly. In this thesis, an all-glass vapour cell incorporating a hollow-core fibre is presented. Its compact size allows fast and precise control of the temperature. Measurements have shown that this cell drastically reduces the time required for equilibration to only hours. Furthermore, the cell’s all-glass design permits unobstructed optical access from all sides. The optical access can be used to record the fluorescence emitted by atoms along the inside of the fibre core. In a first experiment, the frequency-dependent fluorescence along the fibre position was monitored while alkali vapour was diffusing out of the fibre. The atomic density was successfully retrieved using a numerical model based on the optical Bloch equations. Promising technical improvements of the cell design using fibre tapers are discussed, which could lead to an all-fibre vapour cell with fibre-integrated light delivery. The high laser intensity inside a HC-PCF can easily exceed the linear spectroscopy regime. Staying below the saturation intensity inside a waveguide might not always be easy—or even desired. However, established numerical software packages, such as ElecSus, are limited to the weak-probe regime. For that reason, an open-source software based on the Lindblad master equation was developed in this work. The software allows predicting the absolute value of the electric susceptibility for many experimental parameters of hot alkali vapours, including the isotope ratio, temperature or magnetic field strength. Most distinctly, it includes the laser intensity and effects such as optical pumping and transit-time broadening. To verify the model accuracy, spectra of 87Rb-filled cells were measured in a magnetic field of 0.6 T. Entering the hyperfine Paschen-Back regime, individual atomic resonances can be addressed, allowing further benchmarking of the model based on open and closed transitions. Comparing the experimental data with simulated spectra, very good agreement for three different beam diameters and for laser intensities over a range of more than five orders of magnitude was found. Finally, Rydberg states of rubidium—with their multitude of possible transitions—are discussed as a possible source of light in the terahertz (THz) region. In contrast to some other existing sources, THz from hot atomic vapour is expected to be very narrowband. In one report, the successful generation of THz in a four-level-system in hot rubidium vapour was observed. Here, a very similar setup was employed, but unfortunately no THz emission was detected. It is unclear if the reason is of fundamental or experimental nature. One potential route to enhance the THz signal strength at the detector is to channel the signal as much as possible by guiding it inside a waveguide. Appropriate waveguides were investigated, which in contrast to optical frequencies use polymers as a material. Using polymers, waveguides using the 3D printing technique were fabricated and guidance of THz light was observed.
This thesis reported on atomic spectroscopy for the development of alkaline atomic vapor photonic microcell (PMC). The work is motivated by reproducing the outstanding laboratory environment based performances achieved in the fields of frequency standard and coherent optics in highly compact and stand-alone devices that can be accessible to a wider scientific community or to a commercial market. In our case these future devices will be based a Hollow-core photonic crystal fiber (HC-PCF) filled with a gas phase material to form a PMC, and outstands with an ultra-long long interaction length and micrometric modal area. However, the micrometric scale of the fiber core harboring the atoms raises several technical and scientific challenges. Among the technical challenges, we list the development of efficient process for atom loading inside long hollow fiber with small core diameter, the suppression or mitigation of physio-chemical reactivity of the atoms (i.e. Rubidium) with the fiber core inner-surface silica etc. In parallel, the large surface-to-volume ratio of the fiber-core raises questions like the coherence relaxation dynamics and the nature and effect of the atom-surface interaction. The thesis reports on fiber-core inner surface coating with different materials to mitigate the physio-chemical reactions of the confined atoms with the surface, on tapering large core inhibited coupling Kagome HC-PCF, and splicing technique that ensures low splice loss and no atomic reactivity during the splicing process. In parallel, the thesis reports on a set of spectroscopy experiments to assess the relaxation dynamics of atoms inside HC-PCF and to report on novel sub-Doppler transparencies.
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 present high resolution measurements of Doppler free features within Rubidium (Rb) filled photonic crystal hollow core fibre (HC-PCF). Measurements within a Rb filled fibre with core diameter of 35μm show broadening of only 1 MHz associated with the transit time effect. This low level of additional broadening opens the door to compact frequency standards. We present initial spectroscopy of the narrow Rb 5S→5D two-photon transition within the fibre which we will use to create a compact frequency standard.
This thesis presents the development of photonic microcells for use as the host for coherent optics phenomena and related applications. A photonic microcell consists of a length of hollow-core photonic crystal fibre (HC-PCF) with a gas-filled core that is spliced to conventional optical fibre at either end to seal the gas within the fibre. Towards the goal of demonstrating and assessing the coherence properties of quantum optical effects in photonic microcells, the fabrication of two types of HC-PCF is presented. The established photonic bandgap HC-PCF offers extremely low transmission loss of ~10 dB/km over kilometre distances. However, the fibre has a limited transmission bandwidth of ~50 THz and exhibits modal coupling unfavourable for many applications. Work is presented on the tailoring of this fibre by control and shaping of the core-surround in order to improve its modal properties. A second type of HC-PCF is based on a large-pitch lattice, whose guidance relies on a new mechanism. This fibre exhibits a much improved bandwidth (>1000 THz) and has a relatively higher but still practical loss of ~1 dB/m. The development of photonic microcells at microbar pressure level and with low optical insertion loss is shown, an important step in the improvement of the technology for coherent optics applications which will take advantage of the extreme gas-laser interaction efficiency achieved in HC-PCF. Finally, quantum optical effects are demonstrated in HC-PCF and photonic microcells loaded with both the molecular gas acetylene and atomic vapour rubidium. The observation of electromagnetically induced transparency (EIT) in acetylene-filled HC-PCF represents the first such observation in a molecular gas, while the use of a photonic microcell allows a comparison of many experimental configurations to explore the coherence properties of coherent optical systems in the core of a HC-PCF. Furthermore, EIT is observed unambiguously in a rubidium loaded HC-PCF for the first time, and the anti-relaxation effects of a polymer coating demonstrated in this configuration.