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arXiv:2605.01997

Nonlinear Frequency Translation in Micromachined Rb Vapor Cells

Heleni Krelman, Ori Nefesh, Liron Stern

physics.opticsquant-phSubmitted May 3, 2026• Announced May 5, 2026• 0 citations
arXivHTMLPDF

Abstract

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 $μ$W 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.

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We demonstrate highly efficient generation of coherent 420 nm light via up-conversion of near-infrared lasers in a hot rubidium vapor cell. By optimizing pump polarizations and frequencies we achieve a single-pass conversion efficiency of 260% per Watt, significantly higher than in previous experiments. A full exploration of the coherent light generation and fluorescence as a function of both pump frequencies reveals that coherent blue light is generated close to (85)Rb two-photon resonances, as predicted by theory, but at high vapor pressure is suppressed in spectral regions that do not support phase matching or exhibit single-photon Kerr refraction. Favorable scaling of our current 1 mW blue beam power with additional pump power is predicted.

arXiv:0911.0812•Optics Express•Jul 27, 2010•102 citations
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 μμμW 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.
The parametric four-wave mixing (4WM) process responsible for the generation of coherent blue light in alkali vapors is a self-seeded process which starts by population inversion and amplified spontaneous emission (ASE). Lately, attention has been turned toward frequency up- and down-conversion in alkali vapors, using relatively low pump powers with CW diode lasers. In this Letter, we investigate the interplay between ASE and 4WM in rubidium (Rb) vapors by studying the mid-infrared (mid-IR) radiation emitted in the forward direction at 5.23 μm. We show that the ASE can be suppressed by 4WM in the CW regime. Thus, we demonstrate the generation of coherent mid-IR light at 5.23 μm in hot Rb vapors via parametric 4WM.
Electro-optic frequency combs (EO combs) enable high spectral and temporal resolution in atomic, molecular, and cavity-based spectroscopy that is at the heart of many quantum sensing applications. However, producing EO combs in the visible range remains challenging due to the limited availability of key components such as narrow-linewidth lasers and electro-optic phase modulators. Rubidium (Rb) is a highly nonlinear material that supports various four-wave mixing (4WM) processes, which are utilized for spectral translation and the generation of squeezed light. Here, we show the spectral translation of a 780 nm EO comb to 420 nm (a spectral separation of 330 THz) via a 4WM process in Rb vapor. We demonstrate the capability to extend the output comb bandwidth and signal-to-noise ratio by varying input power levels. Through this coherent process, we can generate frequency-agile combs in the blue, including a 1 kHz spaced comb with over one million individual comb teeth, of relevance for spectroscopic measurement of narrow-linewidth features such as electromagnetically induced transparency. Leveraging this approach across different 4WM interactions in Rb should allow for further wavelength conversion and the generation of correlated frequency combs, paving the way for sub-shot-noise (squeezed-light) measurements.
We have generated a stable and continuously tunable single frequency (420 nm) coherent blue light (CBL) by the cavity-enhanced four-wave mixing process in Rb vapor. With the bow-tie-type ring cavity, the output power was 3.3 mW, two orders of magnitude higher than that of the CBL generated by the single-pass four-wave mixing process. The measured power stabilities of the 420 nm CBL operated at 2.5 mW and 1.5 mW for 1000 s were 2.89% and 1.88%, respectively. Under the scanning pump laser frequency, the continuously tunable range of the generated CBL reached 1.68 GHz without mode hopping. The measured beam qualities of the generated laser were M x 2 = 1.117 and M y 2 = 1.179 . The excellent performance of the 420 nm CBL was characterized by velocity transfer spectroscopy with 780 nm probe laser. This high quality short wavelength coherent light is potentially applicable to photon-atom interactions in quantum optics and fundamental research fields.
Diode-pumped alkali lasers, possessing high efficiency and narrow linewidth, can provide feasible solutions for wavelength ranges difficult to reach by commercial lasers. In this study, we investigate a generation of coherent blue light (CBL) via four-wave mixing (FWM)-based up-conversion processes in cesium (Cs) vapor. A bichromatic pumping scheme with 852- and 917-nm lasers drives the Cs atoms to the 6 D 5/2 excited level, followed by cascaded decay of 6 D 5/2 → 7 P 3/2 → 6 S 1/2 , producing 456-nm CBL under phase matching conditions. The fluorescence in multiple bands from blue to near- and far-infrared in the FWM process is demonstrated under different experimental conditions. To optimize the experimental parameters, we investigate the dependence of 456-nm CBL on the vapor temperature, frequency, and intensity of the two pump lasers. A maximum power of 2.94 mW is achieved with pump powers of 430 mW (for 852 nm) and 470 mW (for 917 nm). The corresponding conversion efficiency is 1.5%/W, three-fold higher than those in previous studies. Our results can contribute to fundamental research on atom—photon interactions and quantum metrology.
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.
We demonstrate highly efficient generation of coherent 420 nm light via up-conversion of near-infrared lasers in a hot rubidium vapor cell. By optimizing pump polarizations and frequencies we achieve a single-pass conversion efficiency of 260% per Watt, significantly higher than in previous experiments. A full exploration of the coherent light generation and fluorescence as a function of both pump frequencies reveals that coherent blue light is generated close to (85)Rb two-photon resonances, as predicted by theory, but at high vapor pressure is suppressed in spectral regions that do not support phase matching or exhibit single-photon Kerr refraction. Favorable scaling of our current 1 mW blue beam power with additional pump power is predicted.