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Bulletin of the Russian Academy of Sciences Physics

Optimizing the Parameters of a Periodically Poled LiNbO3 Nanowaveguide Structure for Generating Ultrabroadband Biphotons in the Near-IR Range

O. A. Ermishev, Maksim A. Smirnov, A. F. Khairullin, Арсланов Наркис Мусавирович

Published Dec 1, 2022• 10 citations
Publisher

Abstract

Abstract For quantum biomicroscopy, light sources need to provide a reliable signal at low intensity levels with a high rate generation in the near-infrared region. To develop this light source, we investigated the abilities of generating ultrabroadband two-photon states in the near-infrared in a periodically poled lithium niobate (LiNbO3) nanowaveguide with different geometrical cros-section and pump wavelength. Using modeling and numerical calculations the optimal nanowaveguide parameters providing the maximum width of the spectrum of two-photon states ~180 THz under the pump wavelength about 1.0 μm were determined.

DOI
10.3103/s1062873822120085

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Abstract For quantum biomicroscopy, light sources need to provide a reliable signal at low intensity levels with a high rate generation in the near-infrared region. To develop this light source, we investigated the abilities of generating ultrabroadband two-photon states in the near-infrared in a periodically poled lithium niobate (LiNbO3) nanowaveguide with different geometrical cros-section and pump wavelength. Using modeling and numerical calculations the optimal nanowaveguide parameters providing the maximum width of the spectrum of two-photon states ~180 THz under the pump wavelength about 1.0 μm were determined.
In this paper, we studied the properties of two-photon quantum states of light that emerge due to spontaneous parametric downconversion in a nonlinear waveguide with periodically poles structure. The spectral and entangled properties have been studied mainly for photon pairs whose spectra is located in the near-infrared range. We found that it is possible to select the geometric parameters of the structure of a lithium niobate nanowaveguide in such a way that the two-photon quantum state in the near-IR range will have a spectrum width of about 180THz.
The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100 THz (1.2 μm-2 μm) generation bandwidth with a high efficiency of 13 GHz/mW and excellent noise performance with the coincidence-to-accidental ratio exceeding 10^{5}. We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.
Entangled photon pairs generated by a low-power continuous-wave laser could replicate pulsed laser-based tabletop spectroscopy by taking advantage of the inherent quantum correlations between the photons. Much of the current work in the thin-film lithium niobate platform has focused on infrared wavelengths, leaving shorter wavelengths still a largely unexplored space, particularly for spectroscopy. In this work, we have fabricated periodically poled lithium niobate nanophotonic waveguides for entangled photon generation through spontaneous parametric downconversion with a visible pump (406 nm). Characterization of the waveguided pair source confirms the spectral and temporal correlations of energy-time entangled photons with an on-chip pair generation efficiency of (2.3 ± 0.5) × 1011 pairs/s/mW, brightness of (1.6 ± 0.3) × 109 pairs/s/mW/nm, and two-photon interference visibility greater than 99%. With the same material platform, we have also demonstrated second harmonic generation with on-chip powers up to 30 μW and wavelengths as low as 355 nm, demonstrating lithium niobate's potential for ultraviolet nonlinear photonics and frequency doubling in the UV-A spectral region. Through design of larger cross-section waveguides, we have also explored how variations in the lithium niobate thin film thickness can affect quasi-phase matching. To date, this is the first reported demonstration of periodically poled lithium niobate nanophotonic waveguides for spontaneous parametric downconversion at a fully visible pump wavelength (406 nm) as well as second harmonic generation in the UV (355 nm). Future work towards fully on-chip spectroscopy will explore integrating an on-chip Mach-Zehnder interferometer with the entangled photon source.
Efficient on-chip entangled photon pair generation at telecom wavelengths is an integral aspect of emerging quantum optical technologies, particularly for quantum communication and computing. However, moving to shorter wavelengths enables the use of more accessible silicon detector technology, and opens up applications in imaging and spectroscopy. Here, we present high brightness ((1.6 ± 0.3) × 10 9 pairs/s/mW/nm) visible–near-IR photon pair generation in a periodically poled lithium niobate nanophotonic waveguide. The degenerate spectrum of the photon pairs is centered at 811 nm with a bandwidth of 117 nm when pumped with a spectrally multimode laser diode. The measured on-chip source efficiency of (2.3 ± 0.5) × 10 11 pairs/s/mW is on par with source efficiencies at telecom wavelengths and is also orders of magnitude higher than the efficiencies of other visible sources implemented in bulk crystal or diffused waveguide-based technologies. Further improvements in the brightness and efficiencies are possible by pumping the device with a single-frequency laser, which would also shrink the pair bandwidth. These results represent the shortest wavelength of photon pairs generated in a nanophotonic waveguide reported to date by nearly an octave.
Mid-infrared quantum light sources hold broad application prospects in fields such as gas sensing and infrared thermal imaging. However, currently used mid-infrared quantum entangled light sources primarily rely on bulk periodically poled lithium niobate (PPLN) crystals, which limits brightness and integration. This paper proposes a theoretical scheme based on lithium niobate thin films, in which 1556.9 nm pumping is used to generate entangled photon pairs with a central wavelength of 3113.8 nm. By optimizing the waveguide structure and periodic polarization design, type-II phase matching and group velocity matching are achieved. This enables transverse electric (TE)-polarized pump input to be down converted to generate photon pairs with TE and transverse magnetic (TM) polarizations. Furthermore, by combining a domain arrangement algorithm used for the customized design of polarization direction in PPLN waveguides, precise phase matching is achieved, resulting in a quantum light source with a purity as high as 0.999 and a brightness of 6.18×106\times 10^6×106 cps/mW, which is three orders of magnitude higher than that of the bulk PPLN crystal source. This study provides a promising solution for realizing high-brightness, high-purity on-chip quantum light sources in the mid-infrared band.