Broadband Tunable Photon-Pair Generation and Spectrum Measurement Based on Noncritical Lithium Niobate Crystals

TL;DR

This paper presents a broadband, tunable photon-pair source based on noncritical phase matching in an x-cut LiNbO$_3$ crystal, pumped by a continuous-wave $532\ \mathrm{nm}$ laser to generate near-infrared photon pairs across $800$–$1600\ \mathrm{nm}$ with CAR$>20\ \mathrm{dB}$. The phase-matching relies on temperature-tunable Δk(T) under NCPM, enabling continuous wavelength agility while preserving strong two-photon correlations, evidenced by a $g^{(2)}(\tau)$ width of about $0.5\ \mathrm{ns}$. The authors demonstrate practical utility by performing CO gas absorption spectroscopy with coincidence measurements and show how optical-path-length changes map to coincidence delays, achieving high linearity ($R^2\approx0.9984$). They further analyze absorption details via post-processing to extract spectral features beyond individual photon bandwidths. The work establishes LN-based, temperature-tunable, broadband photon-pair generation with potential applications in broadband spectroscopy, metrology, and quantum interfaces in the NIR.

Abstract

Photon pairs play a vital role in modern science, driving extensive research into their generation. Yet, the narrow phase-matching bandwidth of conventional crystals has largely confined studies to specific wavelengths, leaving research on broadband tunable sources underexplored. Here, we employ a non-critical phase-matched lithium niobate (LN) crystal to generate widely tunable photon pairs. The generated near-infrared (NIR) photon pairs exhibit a high coincidence-to-accidental ratio (CAR > 20 dB) and are tunable across the 800-1600 nm range. We further showcase the utility of NIR photon pairs in spectroscopy by detecting carbon monoxide (CO) gas absorption. This approach will facilitate the design of advanced LN-based photonic experiments.

Figures (5)

• Figure 1: Schematic of the experimental setup. L: lens. WPG: waveplate group, consisting of a half-wave plate (HWP) followed by a quarter-wave plate (QWP). M: mirror. DM: dichroic mirror, with the cut-on wavelength of $1100nm$. LPF: longpass filter. The SPDC Part shows the generation of photon pairs and a temperature control system to monitor the temperature of LN crystal, not shown in the figure. The Gas Cell Part shows the path of the beam, which undergoes multiple reflections within the gas cell before exiting through an aperture in a concave mirror for collection.
• Figure 2: The schematic representation of the photon-pair characterization setup. (a) Dependence of the center wavelength of the photon pairs on the LN temperature. (b) The spectral bandwidth distribution of VIS photon pairs generated at different temperatures. The upper axis represents the wavelength of NIR photons calculated through energy conservation.
• Figure 3: (a) The CAR curves obtained from several temperatures at equal intervals. The horizontal axis represents the power input to the crystal, and the vertical axis CAR represents the signal-to-noise ratio of photon pairs. (b) The pattern obtained by normalizing the coincidence curve collected from TCSPC. For ease of observation, the peak position is set as $\tau=0$ on the horizontal axis.
• Figure 4: (a) The original CO absorption rates for coincidence measurement and single NIR channel measurement. The wavelength range of $1.56-1.59\mu m$, which shows the most obvious CO absorption, is selected. (b) The CO absorption curve after deep learning post-processing, the vertical label "Norm. Abs. Intensity" refers to the normalized absorption intensity.
• Figure 5: The relation between optical delay and the induced time-delay shifts. The curves were directly obtained from TCSPC at optical delays of $0$, $43.8\ mm$, $89.2\ mm$, $132.9\ mm$, $175.5\ mm$, respectively, and normalized on the right axis for clarity. The dashed line indicates the linear relation between optical delay and time-delay.