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arXiv:2107.14501

Narrow and broadband single-photon sources using customised-tapered waveguides

Harrison R. Greenwood, Mohammed F. Saleh

Published Jul 30, 2021• 0 citations
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Abstract

In this paper, we present a thorough investigation for a spontaneous parametric four-wave mixing process in third-order nonlinear waveguides with various continuous tapering patterns. It has been previously shown that these devices can quasi-phase-match the four-wave-mixing process and enhance its conversion efficiency by orders of magnitude. By altering the tapering profile curve we found that these devices can enable single-photon sources with either narrow or broadband spectral widths at on-demand frequencies. Using our model, we were also able to identify the waveguide length at which the single-photon spectral purity is maximised.

DOI
10.48550/arxiv.2107.14501

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In this paper, we present a thorough investigation for a spontaneous parametric four-wave mixing process in third-order nonlinear waveguides with various continuous tapering patterns. It has been previously shown that these devices can quasi-phase-match the four-wave-mixing process and enhance its conversion efficiency by orders of magnitude. By altering the tapering profile curve we found that these devices can enable single-photon sources with either narrow or broadband spectral widths at on-demand frequencies. Using our model, we were also able to identify the waveguide length at which the single-photon spectral purity is maximised.
In this paper, we present a thorough investigation for a spontaneous parametric four-wave mixing process in third-order nonlinear waveguides with various continuous tapering patterns. It has been previously shown that these devices can quasi-phase-match the four-wave mixing process and enhance its conversion efficiency by orders of magnitude. By altering the tapering profile curve we found that these devices can enable single-photon sources with either narrow or broadband spectral widths at on-demand frequencies. Using our model, we were also able to identify the waveguide length at which the single-photon spectral purity is maximized.
Quasi-phase-matching periodically-tapered waveguides (PTWs) can enable efficient on-demand third-order parametric interactions using right combinations of the tapering period and modulation amplitude [1]. Similar to periodically-poled ferroelectric crystals, this new technique eliminates the stringent constraints imposed by conventional methods on the frequencies, mode profiles, and polarisations of the interacting photons. An example of PTWs is the sinusoidally-tapered fibres that have been exploited in manuiplating supercontinuum generation and modulation instability [2]. In this work, I have developed a rigorous quantum model to investigate spontaneous four-wave mixing (SFWM) inside these tapered waveguides [3]. The right combinations between the modulation amplitude Δd and tapering period ΛTthat enhances the expected number of photons (N̂s) at the targeted wavelengths are portrayed in Fig. 1(a), for fibres with same number of periods M. The values of (N̂s) are normalised to the case when Δd = 0, to quantify the enhancement in photon-pairs generation using the PTW-technique in comparison to uniform fibres. For only M = 50, (N̂s) is remarkably enhanced by 35 dB. The output spectrum of the photon-pairs is featured as a narrow sinc-function with very weak sidelobes that are significantly diminished for large number of periods, as depicted in Fig. 1(b). The 2D representation of the spectrum as a function of the photon-pairs wavelengths (λs, λi) is shown in panel (c). In this plot, the pump is assumed to be a monochromatic at a frequency satisfies the energy conservation. The corresponding (N̂s) for a Gaussian-pulse pump source with an input energy 1 nJ and a full-width-half-maximum 4 ps is portrayed in Fig. 1(d). Using the Schmidt decomposition analysis, the spectral-purity is 0.74. This shows the ability of the PTW-technique in producing highly-efficient relatively-pure single photons at any on-demand frequencies without applying any bandpass filters. This work will also open a new direction of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in third-order nonlinear guided structures.
A periodically tapered waveguides technique is an emerging potential route to establish quasi-phase-matching schemes in third-order nonlinear materials for efficient on-demand parametric interactions. In this paper, I investigate this method in enhancing spontaneous photon-pair emission in microstructured fibres and planar waveguides with sinusoidally varying cross sections. To study this process for continuous and pulsed-pump excitations, I have developed a general robust quantum model that takes into account self- and cross-phase modulations. The model shows a great enhancement in photon-pair generation in waveguides with a small number of tapering periods that are feasible via the current fabrication technologies. I envisage that this work will open a new area of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in nonlinear guided structures.
Integrated quantum photonics benefits from photon-pair sources that generate photons directly in waveguides, where they can be efficiently collected, routed, and manipulated. Here, we investigate spontaneous four-wave mixing (SFWM) in a fused tapered-fibre microcoupler formed from two single-mode fibres, and report four contributions. First, we observe SFWM photon pairs in this multimode device: its elliptical central region supports three spatial mode profiles, each with two polarisations, giving six guided modes, and pumping it near 800 nm yields two photon pairs at 648/1048 nm and 660/1021 nm. Second, because several allowed SFWM processes produce similar wavelengths and wavelength alone does not identify their origin, we distinguish the processes by combining phase-matching calculations and selection rules with two independent polarisation measurements: the dependence of the coincidence rate on pump polarisation and polarisation tomography of the generated photons. Third, within the present geometrical model, we find that the 660/1021 nm pair is generated by co-polarised pump photons in a process occurring entirely within a single higher-order mode, whereas the 648/1048 nm pair is generated by orthogonally polarised pump photons in an intermodal process coupling a fundamental and a higher-order mode; the signal photons from the two processes have similar output polarisations, while the idler photons are nearly orthogonal. Fourth, extending the analysis beyond the measured operating point, we theoretically identify pairs of simultaneous SFWM processes driven by a common pump that could generate either polarisation entanglement or composite spatial-polarisation entanglement, depending on the pump wavelength. More generally, this work provides a practical strategy for identifying intermodal SFWM processes in multimode waveguides when spectral information alone is insufficient.
Third-order nonlinear processes require phase matching between the interacting fields to achieve high efficiencies. Typically in guided-wave χ(3)χ^{(3)}χ(3) platforms this is achieved by engineering the dispersion of the modes through the transverse profile of the device. However, this limits the flexibility of the phase matching that can be achieved. Instead, we analyze four-wave mixing in a pair of asymmetric waveguides and show that phasematching may be achieved in any χ(3)χ^{(3)}χ(3) waveguide by coupling of a nondegenerate pump from an adjacent waveguide. We demonstrate the additional flexibility that this approach yields in the case of photon-pair generation by spontaneous FWM, where the supermode dispersion may be modified to produce pure heralded single photons -- a critical capability required for example by silicon platforms for chip-scale quantum photonics.