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Few-Photon Nonlinear Optics In Photonic Bandgap Fibers

Vivek Venkataraman

Published Aug 20, 2012• 0 citations
OpenAlex

Abstract

The ability to control light with light at ultralow powers has been a major avenue of research in photonics with applications to optical communications, computation, and signal processing. Such light-by-light scattering is achieved in a medium with a strong light-matter interaction, and for the development of quantum information networks it is important to demonstrate such effects near the single-photon level. Alkali-metal vapors such as rubidium (Rb) enable strong light-matter interactions due to the large cross section per atom and well-defined energy level structure, while the use of optical fibers offers the advantage of possible integration with modern optical communication systems. Hollow-core photonic band-gap fibers (PBGFs) can combine both these technologies such that both the atoms and the optical fields are transversely confined to a region that is a few wavelengths in size, which offers the prospect of exploring few-photon nonlinear interactions. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We demonstrate large signal amplification (>100) and frequency conversion using a four-wave mixing process with only microwatts of pump power. This is, to our knowledge, the largest gain observed at such low power. We perturb the coherence of this four-wave mixing to demonstrate all-optical modulation at unprecedented bandwidths (~300 MHz) for an atomic-vapor system, with an energy density of only tens of pho- tons per atomic cross-section, comparable to that achieved in more elaborate setups based on cold-atomic clouds. We then demonstrate an enhancement of several orders of magnitude in degenerate two-photon absorption in our RbPBGF system over that achieved in bulk vapor cells in a focused beam geometry. This allows us to directly measure two-photon absorption from a beam by detecting its intensity on a photodiode. Further, employing a near-resonant, non-degenerate two-photon transition in Rb, we demonstrate all-optical intensity modulation with just a few photons (<20), or only a few attojoules of energy, at relatively large bandwidths (~50 MHz) for such a sensitive scheme. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Finally, we produce relatively large cross-phase shifts of a few milliradians on a meter beam with <20 signal photons by tuning slightly away from resonance on the same non-degenerate two-photon transition. This corresponds to a phase shift of 0.3 milliradian per photon, with a fast response time of <5 ns. This represents, to our knowledge, the largest such nonlinear phase shift induced in a single-pass through a room temperature medium. Our Rb-PBGF system can thus potentially be employed to realize weak-nonlinearity based quantum computation and quantum non-demolition measurement of photon number. Through these experiments, we show the potential of a Rb-PBGF system for exploring quantum nonlinear optics at ultralow powers. Moreover, our system is simpler and easier to control and manipulate than setups based on cold atomic clouds and/or high-finesse cavities, and holds promise for integration with fiber-optic communication networks.

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The ability to control light with light at ultralow powers has been a major avenue of research in photonics with applications to optical communications, computation, and signal processing. Such light-by-light scattering is achieved in a medium with a strong light-matter interaction, and for the development of quantum information networks it is important to demonstrate such effects near the single-photon level. Alkali-metal vapors such as rubidium (Rb) enable strong light-matter interactions due to the large cross section per atom and well-defined energy level structure, while the use of optical fibers offers the advantage of possible integration with modern optical communication systems. Hollow-core photonic band-gap fibers (PBGFs) can combine both these technologies such that both the atoms and the optical fields are transversely confined to a region that is a few wavelengths in size, which offers the prospect of exploring few-photon nonlinear interactions. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We demonstrate large signal amplification (>100) and frequency conversion using a four-wave mixing process with only microwatts of pump power. This is, to our knowledge, the largest gain observed at such low power. We perturb the coherence of this four-wave mixing to demonstrate all-optical modulation at unprecedented bandwidths (~300 MHz) for an atomic-vapor system, with an energy density of only tens of pho- tons per atomic cross-section, comparable to that achieved in more elaborate setups based on cold-atomic clouds. We then demonstrate an enhancement of several orders of magnitude in degenerate two-photon absorption in our RbPBGF system over that achieved in bulk vapor cells in a focused beam geometry. This allows us to directly measure two-photon absorption from a beam by detecting its intensity on a photodiode. Further, employing a near-resonant, non-degenerate two-photon transition in Rb, we demonstrate all-optical intensity modulation with just a few photons (<20), or only a few attojoules of energy, at relatively large bandwidths (~50 MHz) for such a sensitive scheme. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Finally, we produce relatively large cross-phase shifts of a few milliradians on a meter beam with <20 signal photons by tuning slightly away from resonance on the same non-degenerate two-photon transition. This corresponds to a phase shift of 0.3 milliradian per photon, with a fast response time of <5 ns. This represents, to our knowledge, the largest such nonlinear phase shift induced in a single-pass through a room temperature medium. Our Rb-PBGF system can thus potentially be employed to realize weak-nonlinearity based quantum computation and quantum non-demolition measurement of photon number. Through these experiments, we show the potential of a Rb-PBGF system for exploring quantum nonlinear optics at ultralow powers. Moreover, our system is simpler and easier to control and manipulate than setups based on cold atomic clouds and/or high-finesse cavities, and holds promise for integration with fiber-optic communication networks.
Photonic structures such as photonic bandgap fibers and high confinement nanowaveguides have proven to be excellent platforms for studying nonlinear optical interactions tailored towards applications in spectroscopy, quantum communication, quantum computation protocols, optical clockwork and precision frequency metrology. This thesis discusses our approach towards exploiting these high confinement media for demonstrating novel few photon nonlinear optical interactions such as two-photon absorption and cross-phase modulation in hot atomic vapors (Rubidium) confined inside hollow-core photonic bandgap fibers (PBGF), generating ultra-broadband optical frequency combs that utilize cascaded parametric four-wave mixing and mode-locked femtosecond pulses in silicon nitride nanowaveguides. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We observe 25% all-optical modulation with <20 photons, i.e., a few attojoules of energy, and large cross-phase shifts of 0.3 mrad per photon with a response time <5 ns in Rb-filled PBGF. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Using high quality factor silicon nitride optical microcavities we show that the gain from the four-wave mixing process can lead to optical parametric os- cillation, allowing for the generation of multiple new wavelengths as wide as an octave of wavelengths. Next we show that by dispersion engineering the waveguide dimensions, we can generate combs by pumping at 1064 nm. The advantage of this platform is that we can independently tune the free spectral range (FSR) and the dispersion. We exploit this property of the silicon nitride microresonator platform to generate microcombs with various FSRs such as 20, 40-, 80-,100-GHz. Next we go on to characterize the spectral and temporal dynamics of the microresonator based combs and demonstrate that such parametric frequency combs can generate modelocked ultra-short pulses.
Low-light-level optical nonlinearities are of significant interest for performing operations such as single-photon switching and quantum non-demolition measurements on single-photons. To evoke strong nonlinearities from singlephotons, one can enhance the matter-photon interaction using strongly nonlinear materials such as alkali vapors in combination with an appropriate geometry such as a waveguide, which provides a long interaction length while maintaining a small light mode area. We demonstrate for the first time that such a system can be experimentally realized by loading rubidium vapor inside a hollow-core photonic band-gap fiber. Using the technique of light-induced atomic desorption in this geometry, we have generated optical depths greater than 1000. As a proof of principle, we demonstrate electromagnetically induced transparency (EIT) with control powers 1000 times lower than those used for hot vapor cells in a focused beam geometry. Working with such a high aspect ratio geometry requires us to identify and measure the various sources of decoherence via spectroscopy of desorbed atoms in the fiber. Using such techniques, we also estimate the temperature of the desorbing atoms inside the fiber. The desorption mechanism is studied, and we show that pulsed desorption beams of the right amplitude and duration can be used for generating precisely controlled optical depths. Finally, we investigate the use of various buffer gas techniques for increasing the effective transverse path of the atoms as they move across the fiber in order to reduce their ground state decoherence and map this effect as a function of buffer gas pressure.
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.
We demonstrate extremely efficient four-wave mixing with gains greater than 100 at microwatt pump powers and signal-to-idler conversion of 50% in Rb vapor confined to a hollow-core photonic band-gap fiber. We present a theoretical model that demonstrates such efficiency is consistent with the dimensions of the fiber and the optical depths attained. This is, to our knowledge, the largest four-wave mixing gain observed at such low total pump powers and the first demonstrated example of four-wave mixing in an alkali-metal vapor system with a large (approximately 30 MHz) ground state decoherence rate.