Silicon Waveguides Create Infrared and Telecom Photon Pairs

Scientists are increasingly focused on developing efficient sources of correlated photon pairs spanning the mid-infrared and telecom bands for advances in diverse technologies.

Abhishek Kumar Pandey, Deepak Jain, both from the Optics and Photonics Centre at the Indian Institute of Technology Delhi, and Catherine Baskiotis from the De Vinci Research Centre, present a quantitative study exploring silicon waveguides for generating these photon pairs through Spontaneous Four-Wave Mixing.

This research is significant as it demonstrates the potential of all-solid Silicon On Insulator waveguides, offering a potentially lower-cost and more versatile alternative to current methods, and proposes designs capable of producing correlated pairs bridging the 3m-4m mid-infrared region and the telecom C-band. Through validated modelling and simulations, the team identified realistic operating conditions achieving a photon pair generation probability of 0.05 per pulse, with designs optimised for atmospheric key distribution and sensitive gas detection of methane and nitrogen dioxide. Generating linked photons across different wavelengths has long been a difficult task for many technologies. Now, a new approach utilising silicon waveguides offers a flexible and cost-effective solution for creating these essential light sources. These designs promise efficient photon pair generation, bridging mid-infrared and telecom bands for applications like atmospheric sensing and secure communication. Scientists are increasingly focused on generating correlated photons, pairs of light particles exhibiting a quantum link, spanning the mid-infrared (MIR) and telecom wavelengths. These sources hold considerable promise for a range of technologies, yet current methods often rely on expensive and inflexible platforms. Researchers have investigated the potential of spontaneous four-wave mixing (SFWM) within all-solid silicon on insulator (SOI) waveguides, presenting designs capable of producing correlated photon pairs bridging the 3m-4m MIR band and the telecom C-band. Detailed simulations performed targeted a photon pair generation probability of 0.05 per pulse, using a pump laser with a wavelength between 2100nm and 2210nm and a pulse duration of 5ps. These calculations identified practical operating conditions, including 2cm-long straight waveguides and intra-modal SFWM using the fundamental TE00 mode, requiring a pump peak power between 9.2mW and 32mW. A primary design, designated wCOM, achieves a signal wavelength of 3.905m, falling within a region of high atmospheric transparency, alongside an idler wavelength in the telecom C-band, making it particularly suitable for atmospheric quantum key distribution. Two further designs, wCH4 and wNO2, adapt designs for precise detection of methane and nitrogen dioxide gases, respectively, with signal wavelengths of 3265nm and 3461nm. In particular, wCOM attains a signal/idler wavelength separation of 2364nm, exceeding the previously reported value of approximately 1125nm achieved using SFWM in similar SOI waveguides. Beyond quantum key distribution, these sources could enable new sensing and imaging techniques. Correlated photons could be used in “sensing with undetected light” schemes, where detection in the visible or near-infrared range reveals information about a target in the MIR. Since the MIR region is a unique fingerprint for many gases, including nitrogen dioxide and methane, photon-level sensing promises highly precise measurements of even very low concentrations. Realistic operational conditions were identified, including 2cm-length straight waveguides and intra-modal Four Wave Mixing utilising the fundamental TE00 mode, requiring a pump peak power between 9.2mW and 32mW. A design designated wCOM achieved a signal wavelength of 3.905m, positioned within an atmospheric transparency window, alongside an idler in the Telecom C-band, advantageous for atmospheric Key Distribution applications. Further designs, wCH4 and wNO2, created designs to detect specific gases, achieving signal wavelengths of 3265nm and 3461nm respectively. Calculating the probability of photon pair generation per pulse involved modelling the Joint Spectral Density, assuming negligible losses within the silicon waveguide and a narrow spectral extension of the pump. The factor defining the two-dimensional joint spectral density expresses as a product of three components, accounting for group indices, waveguide length, and pump pulse energy. The factor accounts for phase mismatch and is calculated using the difference in wavevector components, while addresses phase-matching conditions and dispersion effects. Once the one-dimensional Spectral Density of Probability of photon pair generation was computed, integration over all possible collected frequencies provided the Probability of photon Pair Generation per Pulse. Validating this formula, the research team modelled a photonic crystal fibre using experimental data, considering a fused silica core with a refractive index computed through standard Sellmeier coefficients. At a core radius of 0.965m, consistent with a reported core diameter of approximately 2m, the team simulated the characteristics of the fundamental guided mode. By multiplying the calculated probability of photon pair generation per pulse by the pump repetition rate of 80MHz, the Photon Pair Generate Rate was determined, and an attenuation factor was applied to account for optical path losses. The resulting theoretical photon pair rate demonstrates the feasibility of this approach for correlated photon pair sources. Simulating efficient mid-infrared photon pair generation in silicon-on-insulator waveguides A detailed examination of spontaneous four-wave mixing (SFWM) within all-solid silicon-on-insulator (SOI) waveguides underpinned this work.

This research explored the potential of SFWM to bridge the 3m-4m mid-infrared (MIR) band with the telecom C-band, rather than relying on costly platforms typically used for correlated photon pair generation. Simulations quantitatively assessed the probability of photon pair generation per pulse, targeting a value of 0.05, once a pump wavelength range of 2100nm-2210nm was selected. Simulations employed straight waveguides, 2cm in length, utilising intra-modal four-wave mixing with the fundamental TE00 mode to achieve realistic operating conditions. Accurate modelling of SFWM necessitated a multi-step approach beginning with guided modes modelling, implemented via COMSOL Multiphysics’s Wave Optics Module. Sellmeier’s equations defined the refractive indices of the silicon core and fused silica cladding, while the buried oxide (BOX) layer shared the refractive index of SiO2. By computing effective indices, propagation constants, and mode profiles, the foundation for subsequent nonlinear parameter calculations was established. The nonlinear coefficient associated with SFWM required careful determination, represented by the equation = , incorporating the pump wavelength alongside the overlap integral for the pump, signal, and idler modes. Here, the overlap integral, calculated as a multi-dimensional integral over the waveguide cross-section, quantified the spatial interaction between these modes. At this stage, energy and momentum conservation conditions were applied, ensuring phase-matching for efficient photon pair generation. Calculating the probability of photon pair generation per pulse (PGP) demanded a specific model derived under assumptions of minimal loss within the silicon waveguide and a narrow spectral extension of the pump. Inside this model, the Joint Spectral Density (JSD), representing the probability of generating correlated photons within specific spectral ranges, calculated as a product of three factors, accounting for various physical parameters. Experimental validation of the PGP calculation utilised previously reported results from the literature, confirming the model’s accuracy and predictive capability. Silicon photonics enables efficient generation of mid-infrared photon pairs for sensing and secure communication Once a technology remains confined to the laboratory, its potential impact feels theoretical. However, recent advances in silicon photonics are steadily dissolving that barrier, bringing practical mid-infrared (MIR) light sources closer to reality. Researchers have now demonstrated a method for generating paired photons spanning the important 3-4m MIR band alongside telecom wavelengths, using a comparatively simple and scalable platform, silicon on insulator waveguides. For years, creating sources operating in this spectral region proved difficult, demanding materials with limited versatility or complex fabrication processes. The significance extends beyond simply achieving a broader wavelength range. These correlated photon pairs hold promise for atmospheric sensing, particularly in detecting trace gases like methane and nitrous oxide, where absorption features fall within this MIR band. The designs presented offer a path towards secure communication via quantum key distribution through the atmosphere, a challenge hampered by signal loss. Current designs necessitate precise control over waveguide dimensions and pump laser characteristics, potentially limiting widespread adoption. A key limitation resides in the relatively low probability of photon pair generation per pulse, although the reported value represents a substantial improvement over previous silicon-based approaches. Further work must focus on enhancing this efficiency, perhaps through novel waveguide geometries or pump schemes. Exploring the integration of these sources with detectors remains a vital step. In the end, the field could move towards compact, integrated devices capable of performing complex spectroscopic analyses or establishing secure communication links, moving beyond the need for bulky and expensive laboratory setups. 👉 More information 🗞 Quantitative study of Silicon Waveguides for the Generation of Quantum Correlated Photon Pairs Bridging Mid-Infrared and Telecom Bands 🧠 ArXiv: https://arxiv.org/abs/2602.16464 Tags: