Broadband Quantum Photon Source in Step-Chirped Periodically Poled Lithium Niobate Waveguide

TL;DR

This work demonstrates a 6.82 mm step-chirped PPLN waveguide on LNOI that simultaneously supports broadband SHG and SPDC via tailored quasi-phase matching. The SHG response achieves an average efficiency of about 54%/W/cm^2 across 1510–1620 nm with a bandwidth exceeding 110 nm, while SPDC pumped at 775–785 nm yields photon-pair spectra with up to 99 THz full bandwidth and brightness up to 20 GHz/mW/nm. The results show the platform's potential for broadband quantum light sources and quantum metrology, enabling wide spectral coverage and high brightness in compact, chip-scale devices. These characteristics open avenues for high-rate quantum imaging, frequency multiplexing, and possible Hong-Ou-Mandel interference experiments with spectrally engineered photon pairs.

Abstract

Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate~(CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation~(SHG) and spontaneous parametric down-conversion~(SPDC). The SHG achieves an average efficiency of 54.4\%/W/cm$^2$ over the first-harmonic wavelength range of 1510~nm-1620~nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775~nm, 780~nm, and 785~nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99~THz~(846~nm) and 20~GHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.

Figures (4)

• Figure 1: (a) Cross-section of the ridge waveguide fabricated on a $h_2=600$ nm thick $x$-cut LNOI wafer. (b) The mode profiles of TE$_{00}$ of $\lambda_p=775$ nm (top) and $\lambda_s=1550$ nm (bottom). (c) Simulated effective refractive indices $n^\text{eff}$ of TE$_{00,s}$ (blue) and TE$_{00,p}$ (red) with $\lambda_s\in[1480~\text{nm}, 1640~\text{nm}]$. (d) Poling periods to satisfy QPM between $\lambda_{p}$ and $\lambda_{s}$ with $\lambda_s\in[1480~\text{nm}, 1640~\text{nm}]$. (d) Design of the step-chirped PPLN. Upward (downward) arrows indicate the sign of the nonlinear susceptibility $+(-)$. (e) PFM phase image of the periodical poling region. (f) SEM images of the fabricated waveguide.
• Figure 2: (a) The experimental setup for characterizing the SHG process. (b) Experimental (red solid line) and simulation (red dashed line) results of the normalized SHG efficiency.
• Figure 3: (a) The setup for pumping CPPLN waveguide to generate photon pairs. (b) The setup for measuring the spectrum of the signal photon. (c) The setup to measure the coincidence of signal and idler photons, where the signal photon is filtered across different channels. (d) Measured spectrum of signal photons with the pump wavelength set at 775 nm, 780 nm and 785 nm, respectively. (e) Coincidences of photon pairs by setting the wavelength of pump light at 775 nm. The blue line represents the spectrum measured by spectrometer (shown in (d)). The black frames represent the raw coincidence counts, while purple bars represent the coincidence counts after subtracting the accidental coincidences.
• Figure 4: (a) Setup for measuring the pair generation rate (PGR) using a fiber beam splitter (FBS). (b) Setup for measuring the PGR using a coarse wavelength division multiplexer (CWDM) and an FBS.(c)–(g) Measured PGR and coincidence-to-accidental ratio (CAR) under different pump wavelengths and measurement configurations.