Improving photon pair generation in silica nanofibers through PMMA/DR1 nonlinear coating optimization

EPJ Web of Conferences 287, 06012 (2023) EOSAM 2023 https://doi.org/10.1051/epjconf/202328706012 Improving photon pair generation in silica nanofibers through PMMA/DR1 nonlinear coating optimization Abderrahim Azzoune*,1, Laurent Divay2, Christian Larat2, and Sylvie Lebrun3 1 Ecole Militaire Polytechnique, Laboratoire Systèmes Lasers, BP17, 16111, Bordj-El-Bahri, Algiers, Algeria Thales Research and Technology, 91767, Palaiseau Cedex, France 3 Université Paris-Saclay, Institut d’Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127, Palaiseau, France 2 Abstract. We report on the use of PMMA/DR1 coating to enhance the efficiency of photon pair generation in silica nanofibers. The coating improves the second-order nonlinear susceptibility of the nanofibers, leading to improved photon pair generation efficiency. We investigate the effect of varying the nonlinear optical properties of the composite material, and we characterize the photon pair generation efficiency of the coated silica nanofibers. Our modelling results show a significant enhancement in photon pair generation efficiency by a factor of 1000 compared to a bare silica nanofiber. 1 Introduction Photon pair generation is an important process for various quantum technologies such as quantum cryptography, quantum communication, and quantum computing. The generation of correlated photon pairs by Spontaneous Parametric Down-Conversion (SPDC) can be performed in bulk or periodically polarized crystals such as lithium niobate [1] or in semiconductor waveguides. One of the disadvantages of these techniques is the difficulty of insertion into fiber networks due to coupling losses. The use of silica optical fibers allows to get rid of these coupling problems. However, since silica is a centrosymmetric material, it does not exhibit second order nonlinearity in its solid form, so it must be artificially created. All-fiber sources based on the SPDC process in periodically polarized silica fibers have been proposed [2] but due to technical difficulties, the interaction lengths are limited to a few centimeters. Silica nanofibers have the advantage that they can be inserted into fiber networks with a minimum of coupling losses, and the control of their geometry offers many possibilities for controlling light propagation. They are identified as promising platforms for efficient photon pair generation due to their high surface area, low loss, and compatibility with a wide range of optical fibers. However, their inherently low second-order nonlinear susceptibility limits their efficiency for photon pair generation. PMMA/DR1 coated silica nanofibers have been studied for enhancing the second-order nonlinear susceptibility of the composite material. The PMMA/DR1 composite material has been shown to exhibit a second-order nonlinear susceptibility of 27±5 pm/V [3]. The combination of the high nonlinear optical properties of the DR1 chromophore and the strong adhesion of PMMA to silica surfaces makes the * PMMA/DR1 composite an attractive coating material for silica nanofibers. The enhancement in second-order nonlinear susceptibility is attributed to the strong electric field confinement and the high surface-to-volume ratio of the nanofibers, which can increase the effective nonlinear coefficient of the composite material. The nonlinear optical properties of PMMA/DR1 coated silica nanofibers can also be tuned by adjusting the thickness of the coating, the concentration of the DR1 chromophore, and the fiber dimensions. In this work, we demonstrate that coating silica nanofibers with a thin layer of PMMA/DR1 composite material significantly enhances their second-order nonlinear susceptibility and photon pair generation efficiency. We investigate the effect of composite material’s nonlinear optical properties and show that PMMA/DR1-coated silica nanofibers exhibit significant improvement in photon pair generation efficiency compared to bare silica nanofibers, mainly due to the higher nonlinearity of the PMMA/DR1 composite material. 2 Theory and modeling We present a novel source of correlated photon pairs based on SPDC in a silica nanofiber. The correlated photons are generated through surface dipole and bulk multipole nonlinearities. Its principle is illustrated in Figure 1. Since silica is an isotropic material, phase matching by birefringence is not allowed in a silica nanofiber. Therefore, we opted for modal phase matching. It is verified using a pump photon in the 𝑇𝑀01 mode (or the 𝐻𝐸21 mode, depending on the phase matching) at wavelength 𝜆p (i.e., frequency 𝜔p) to generate photon Corresponding author: © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 287, 06012 (2023) EOSAM 2023 https://doi.org/10.1051/epjconf/202328706012 pairs at both wavelengths 𝜆s and 𝜆i (i.e., frequencies 𝜔s and 𝜔i) in the fundamental 𝐻𝐸11 mode such that 𝜔p = 𝜔s + 𝜔i for energy conservation. The 𝑇𝑀01 – 𝐻𝐸11 phase matching is more efficient than the 𝐻𝐸21 – 𝐻𝐸11 interaction due to the efficient nonlinear overlap between the 𝑇𝑀01 and 𝐻𝐸11 modes. Additionally, the 𝑇𝑀01 mode is easier to generate due to its simpler spatial transverse structure. Therefore, we exclusively consider the 𝑇𝑀01 – 𝐻𝐸11 interaction for photon pair generation, utilizing a 𝑇𝑀01 pump mode at 775nm to generate photon pairs in the 𝐻𝐸11 mode around 1550nm. We focus solely on the photon pair generation initiated by quantum vacuum fluctuations in the absence of an initial wave at 𝜔s or 𝜔i. PMMA/DR1 configuration generates more photon pairs due to its higher spectral density and wider phase matching curves. Fig. 1. SPDC principle in a PMMA/DR1-coated silica nanofiber. Blue indicates the silica core material, while orange represents the PMMA/DR1 nonlinear coating at the surface of the nanofiber. Fig. 2. Photon pair generation spectral densities in the 𝐻𝐸11 mode for a 100μm long silica nanofiber in (a) silica-air and (b) silica-PMMA/DR1 configurations. The photon pair generation spectral density per pump mode in a 100μm long silica nanofiber can be calculated for the non-degenerate case as follows, #$ " 𝐺(𝐿, 𝜔! ) = 𝑔" 𝐿" sinc " 234 " 5 − 𝑔" 𝐿7 We have indeed experimentally deposited PMMA/DR1 on µm range diameter nanofibers with a thickness of ~100nm. The coating thickness was found to decrease linearly with fiber diameter, which may limit the thickness reachable on smaller fibers with a single layer coating. Thicker deposits can however be made on smaller fibers with multilayer coatings. (1) Here, 𝑔" ≈ 𝜌" 𝑎" (𝜔% ), where 𝑎(𝜔% ) represents the square root of the power at the pump frequency 𝜔% , and Δ𝛽 is the phase mismatch. We assume that 𝜌, which is the efficiency of the sum frequency generation process (i.e., the reverse process of SPDC), is constant over the wavelength range considered [4]. 4 Conclu (...truncated)