J. Eur. Opt. Society-Rapid Publ. 2025, 21, 38 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025033 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
The temperature-dependent in noncollinear SPDC for generating biphotons with single cycles
Haibo Lin * and Jinbao Wang Institute of Mechanical & Electrical Technology, Taizhou Vocational & Technical College, Taizhou 318000, PR China
Received 24 June 2025 / Accepted 11 July 2025
Abstract. The influence of temperature on generating single cycle biphotons by noncollinear spontaneous parametric down-conversion( SPDC) in chirped quasi-phase-matching( QPM) periodically poled lithium niobate( PPLN) crystal have been studied in this paper. Numerical simulation shows that characteristics of the biphotons are mainly determined by the phase matching function which is dependent on the angle of the noncollinear SPDC and the temperature of the nonlinear crystal. Moreover, the angle plays a more significant role in noncollinear SPDC. The smaller angle and lower temperature can produce better phase matching effect while the angle and temperature changes within an appropriate range. The count rate of the biphotons will decrease, the bandwidth of biphotons will be compressed and the temporal width is widened for a certain appropriate angle in noncollinear SPDC while temperature is increased. However, as long as the angle and temperature are suitable, the biphotons generated by SPDC in the chirped QPM PPLN crystal are ultrashort pulses with single cycles.
Keywords: Chirped QPM, Single cycles, Biphotons, Sellmeier equation, Noncollinear.
1 Introduction
In recent decades, generation ultrashort optical pulses with single or even subcycle, has been regarded as a basic optical topic, shows great progresses in the range of THz, visible and extreme ultraviolet band [ 1 – 5 ] as ultrafast optical pulses, with few-, single- or sub-cycle, they are used for coherent population transfer, because they can easily access to the electric ground states of many molecules. Especially in the multi-level system, the single- and sub-cycle pulses show more suitable application for transferring population due to their ultrabroad spectra [ 6 ]. Various techniques for generation ultrafast pulse have been proposed and studied in both theory and experiment [ 7 – 12 ]. And the bandwidths of the experimental pulses have reached several or even tens of THz [ 13 ]. Among these excellent works, the spontaneous parametric down conversion( SPDC) is a primary and convenient method for generating biphotons with entanglement [ 14 – 18 ].
The SPDC process mainly concerns the interaction between the pump light and the nonlinear crystal. The crystal is precisely designed for SPDC, such as periodically poled lithium niobate( PPLN) in this paper. As the pump light with frequency x p incidentsintothiscrystal, itcan be split into two photons named as signal and idler lights
* Corresponding author: linhb @ tzvtc. edu. cn with lower frequencies x s and x i, respectively. Thetwo simultaneously generated photons show wide broadband spectra and high degree of entanglement based on the squeezed quantum states. Generally, the broadband spectra could be produced by a thin nonlinear crystal. And the spectra bandwidth is inversely proportional to the length of crystal during SPDC [ 19, 20 ]. However, the effect produced by shortening the crystal length to increase the spectral width is limited. The bandwidth of the biphotons can be made wider by other methods, such as the more perfect phase-matching condition [ 21 ], the better phase compensation [ 22 ], or chirped quasi-phase-matched crystals [ 23 ], etc.
The collinear SPDC is an ideal model for the generation of ultrafast biphotons with single cycles. However, in experiments, two factors need to be considered: the effect of temperature on the crystal length and the effect of angle on the noncollinear SPDC. Reference [ 24 ] focused on the thermal expansion and contraction effect of temperature on nonlinear crystals, which directly determines the effect of temperature on the grating periods of the QPM PPLN crystal. Reference [ 25 ] focused on the effect of angle on the SPDC, and found that a single cycles ultrashort pulse can be generated with tiny angle.
In this paper, we focus on the chirped QPM PPLN crystal, it is subjected to the influence of thermal expansion and contraction in SPDC process, which affects the optical path of pulse propagation in the crystal [ 26 ], and this directly
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