Large Kerr nonlinearities are of significant interest for photonic quantum information processing and can be applied towards developing deterministic single photon sources and implementing deterministic quantum logic gates. The required strong nonlinear interaction between photons can be achieved by enhancing the light-matter interaction using alkali vapors interacting with a tightly confined optical mode area, such as that of a waveguide. The guiding properties of the waveguide over long distances helps in achieving a long interaction length while simultaneously increasing the atom-photon coupling by maintaining a small mode area over the entire guiding length. In this thesis, we explore the large effective Kerr nonlinearities (γ) of Rubidium vapor interacting with the optical mode of hollow-core photonic band-gap fibers (Rb-PBGF) and the tightly confined evanescent field of air-clad silicon nitride waveguides (Rb-chip). We have demonstrated efficient and noiseless frequency conversion at low power levels leveraging the large γ for the Rb-PBGF system. As proof of principle, we have also generated spectrally bright quantum correlated photon-pairs in a warm Rb vapor-cell and have proposed an extension of the experiment in the Rb-PBGF platform, which could lead to near unit photon pair generation efficiency. During the course of the study, we have also significantly improved the Rb-PBGF system by attaining an almost continuous operation lasting for several hours (1000X longer) while generating optical depths (OD's) ~ 50, which are required to attain single photon-level nonlinearity in the Rb-PBGF system. Developing on-chip CMOS [complimentary metal-oxide-semiconductor] compatible Rb vapor based platforms can not only achieve large γ, but also significantly reduce the footprint of the device and make it more compatible with other chip-based platforms. In our present work, we have demonstrated the feasibility of this approach by achieving extended interaction time-scales for, air-clad silicon nitride waveguides coated with aluminum oxide and evanescently coupled to warm Rb vapor. The thin aluminum oxide layer has greatly helped in preventing corrosive damage caused by Rb to the nitride surface that have improved the operational lifetimes by a factor > 100. During the course of our study, we realized the limitations of known platforms towards generating tunable sources of single photons at near-visible wavelengths; a wavelength regime at which great progress has been made for quantum storage and detection. Towards the end of this work, we have experimentally demonstrated a broadband frequency comb at near-visible wavelengths by engineering anomalous dispersion for higher order waveguide modes in silicon nitride microresonators. The underling process of parametric wave-mixing being the same for frequency combs and correlated photon-pair generation; our work opens new possibilities for developing spectrally narrow single photon sources in the near-visible which are compatible with highly nonlinear alkali vapor based systems like the Rb-PBGF and Rb-chip platform.
Large Kerr nonlinearities are of significant interest for photonic quantum information processing and can be applied towards developing deterministic single photon sources and implementing deterministic quantum logic gates. The required strong nonlinear interaction between photons can be achieved by enhancing the light-matter interaction using alkali vapors interacting with a tightly confined optical mode area, such as that of a waveguide. The guiding properties of the waveguide over long distances helps in achieving a long interaction length while simultaneously increasing the atom-photon coupling by maintaining a small mode area over the entire guiding length. In this thesis, we explore the large effective Kerr nonlinearities (γ) of Rubidium vapor interacting with the optical mode of hollow-core photonic band-gap fibers (Rb-PBGF) and the tightly confined evanescent field of air-clad silicon nitride waveguides (Rb-chip). We have demonstrated efficient and noiseless frequency conversion at low power levels leveraging the large γ for the Rb-PBGF system. As proof of principle, we have also generated spectrally bright quantum correlated photon-pairs in a warm Rb vapor-cell and have proposed an extension of the experiment in the Rb-PBGF platform, which could lead to near unit photon pair generation efficiency. During the course of the study, we have also significantly improved the Rb-PBGF system by attaining an almost continuous operation lasting for several hours (1000X longer) while generating optical depths (OD's) ~ 50, which are required to attain single photon-level nonlinearity in the Rb-PBGF system. Developing on-chip CMOS [complimentary metal-oxide-semiconductor] compatible Rb vapor based platforms can not only achieve large γ, but also significantly reduce the footprint of the device and make it more compatible with other chip-based platforms. In our present work, we have demonstrated the feasibility of this approach by achieving extended interaction time-scales for, air-clad silicon nitride waveguides coated with aluminum oxide and evanescently coupled to warm Rb vapor. The thin aluminum oxide layer has greatly helped in preventing corrosive damage caused by Rb to the nitride surface that have improved the operational lifetimes by a factor > 100. During the course of our study, we realized the limitations of known platforms towards generating tunable sources of single photons at near-visible wavelengths; a wavelength regime at which great progress has been made for quantum storage and detection. Towards the end of this work, we have experimentally demonstrated a broadband frequency comb at near-visible wavelengths by engineering anomalous dispersion for higher order waveguide modes in silicon nitride microresonators. The underling process of parametric wave-mixing being the same for frequency combs and correlated photon-pair generation; our work opens new possibilities for developing spectrally narrow single photon sources in the near-visible which are compatible with highly nonlinear alkali vapor based systems like the Rb-PBGF and Rb-chip platform.
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.
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.
Alkali vapours, such as rubidium, are being used extensively in several important fields of research such as slow and stored light nonlinear optics quantum computation, atomic clocks and magnetometers. Recently, there is a growing effort towards miniaturizing traditional centimetre-size vapour cells. Owing to the significant reduction in device dimensions, light–matter interactions are greatly enhanced, enabling new functionalities due to the low power threshold needed for nonlinear interactions. Here, taking advantage of the mature platform of silicon photonics, we construct an efficient and flexible platform for tailored light–vapour interactions on a chip. Specifically, we demonstrate light–matter interactions in an atomic cladding waveguide, consisting of a silicon nitride nano-waveguide core with a rubidium vapour cladding. We observe the efficient interaction of the electromagnetic guided mode with the rubidium cladding and show that due to the high confinement of the optical mode, the rubidium absorption saturates at powers in the nanowatt regime. Alkali vapours are increasingly useful in photonic research and metrology applications, and they provide a useful test bed for investigating light–matter interaction. Stern et al. integrate silicon nitride waveguides with alkali vapours to study light–matter interactions on a chip-scale platform.
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.
The demonstration of slow, fast, and stopped light1 has helped generate considerable interest in quantum interference effects. In addition to producing intriguing physical phenomena, these effects will be valuable for future optical communications. Buffers using slow-light modules will be important elements in alloptical data networks. Cryptography systems may also be expected to use quantum interference effects for photon creation and detection. But if slow-light buffers and quantum cryptography are to be practical in real-world applications, wemust create miniaturized, stand-alone quantum interference modules. This is the aim of research involving on-chip atomic vapor cells integrated with optical waveguides Slow-light effects are a product of a medium with large, frequency-dependent gain or absorption features that then give rise to large dispersion characteristics. Our work employs quantum interference effects in atomic vapor to create large dispersion. At present, atomic vapor can provide excellent dispersion characteristics, but it usually requires a large system footprint. In classical optical setups, which beam a laser through a transparent vapor cell, there is also a constant trade-off between focal depth and beamwidth. By using on-chip hollow-core optical antiresonant reflecting optical waveguides (ARROWs), we are able to direct light down vapor-filled cores, maintaining large optical intensities over long distances. This is particularly beneficial for nonlinear optical effects that can be exploited for single-photon generation and detection. In addition, ARROW-based platforms also require only a small footprint (1×1cm). ARROWs are constructed by growing alternating layers of silicon oxide and silicon nitride around a sacrificial core on a silicon wafer. The alternating dielectric materials with specific thicknesses confine and guide light through antiresonance. After etching out the sacrificial core, we are left with a hollow channel into which we can introduce various gases and liquids for optical experiments. Solid waveguides etched into the silicon on Figure 1. A rubidium (Rb)-loaded ARROW atomic vapor cell.
The exceptional nonlinearity of alkali-metal vapors enables highly efficient nonlinear optical processes even at relatively low optical intensities. However, such processes have traditionally relied on centimeter-scale vapor cells. Here, we utilize a versatile chip-scale Rb vapor platform to generate coherent blue and mid-IR light in continuous-wave mode by means of resonant four-wave mixing. Optimized optical overlap with the atomic medium enables blue light generation of $\sim$20 μW over a very short interaction length, while maintaining a directly measured linewidth of $\sim$1 MHz, which is presently limited by the measurement apparatus. Comparison with a conventional glassblown vapor cell further shows that the micromachined platform can achieve higher coherent blue-light generation efficiency despite its substantially shorter interaction length. Moreover, an anodically bonded Si window enables to detect coherent mid-IR emission with collected powers of $\sim$50 nW. We further characterize the temperature dependence and input-power scaling of the blue emission, confirming efficient nonlinear conversion within these compact vapor cells. This chip-scale platform provides a versatile foundation for a range of nonlinear optical functions, from precise wavelength references and quantum light sources to next-generation quantum sensors.