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Journal of Nonlinear Optical Physics & Materials

The single-cycle biphotons generated by SPDC in chirped QPM PPKTP crystal

Wang Jinbao, Lin Haibo

Published Feb 18, 2024• 0 citations
Publisher

Abstract

In this paper, we have studied the process of monochromatic pump light in linearly chirped Periodically Poled [Formula: see text] (PPKTP) nonlinear crystal to generate single-cycle biphotons under Quasi-Phase-Matching (QPM) conditions through a Spontaneous Parametric Down-Conversion (SPDC). A detailed analysis of the phase mismatching function reveals that in PPKTP crystals, for the proper frequency range, the phase mismatching [Formula: see text] is stable and relatively close to 0 due to the relatively stable refractive index, and therefore has a good phase matching effect. Even when the chirp rate becomes small, it is able to produce single-cycle biphotons due to the near-perfect phase matching. Of course, as the chirp rate increases, the frequency response range becomes wider, the temporal width is narrower, and the number of cycles is smaller.

DOI
10.1142/s0218863524500061

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2017•0 citations
In this paper, we have studied the process of monochromatic pump light in linearly chirped Periodically Poled [Formula: see text] (PPKTP) nonlinear crystal to generate single-cycle biphotons under Quasi-Phase-Matching (QPM) conditions through a Spontaneous Parametric Down-Conversion (SPDC). A detailed analysis of the phase mismatching function reveals that in PPKTP crystals, for the proper frequency range, the phase mismatching [Formula: see text] is stable and relatively close to 0 due to the relatively stable refractive index, and therefore has a good phase matching effect. Even when the chirp rate becomes small, it is able to produce single-cycle biphotons due to the near-perfect phase matching. Of course, as the chirp rate increases, the frequency response range becomes wider, the temporal width is narrower, and the number of cycles is smaller.
We analysis the noncollinear Spontaneous Parametric Down Conversion (SPDC) and compare the biphotons generated by the chirped Quasi-Phase-Matching (QPM) between the Periodically Poled Lithium Niobate (PPLN) and Periodically Poled KTiOPO4 (PPKTP) crystals. Due to the chirping of the crystals, the frequency response range of the biphotons would be greatly increased. For nonlinear SPDC, angular variation is limited (less than 0.06° in this paper), and the angle would narrow the frequency response range of the biphotons. We compare the effect of angle in PPLN crystals and PPKTP crystals for biphotons. Both the two crystals with chirped QPM, the single-cycle biphotons can be generated during noncollinear SPDC within a suitable angle range, which is favorable for wider applications in experiments.
We study the temperature-dependent type II spontaneous parametric down-conversion (SPDC) in the linearly chirped quasi-phase-matching (QPM) periodically poled lithium niobate (PPLN) crystal. The SPDC process can generate the biphotons with the single-cycle level. It shows that the chirp rate of the poled grating periods is the main influence factor for the phase matching function, frequency response range, Fedorov parameter and the count rate of the biphotons. Meanwhile, the temperature plays an important role for the type of e→o+ee\rightarrow o+ee→o+e, the higher temperature makes the frequency response range be wider, Fedorov parameter and count rate be higher. However, the appropriate chirp rate and temperature can help the type II SPDC generate the single cycle biphotons in chirp QPM PPLN.
Parametric down-conversion of light (PDC) in a quasi-phase-matched (QPM) nonlinear crystal can be spectrally very broad when the spatial frequency of periodical poling is linearly chirped. The present day technology makes possible the fabrication of chirped QPM crystals phase-matched for PDC in all optical spectrum. In the low-gain regime this allows for generation of biphotons with the correlation time of the order of a single optical cycle. A natural next step is the study of the PDC light produced by such a crystal in the high-gain regime, where PDC is widely used as a source of squeezed states of light. Up to now only a classical treatment of this regime was undertaken. In the present work we demonstrate a possibility to generate single-cycle squeezing in a chirped QPM nonlinear crystal with squeezing spectrum covering almost all the optical octave.
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.
Quantum entanglement of two photons created by spontaneous parametric downconversion (SPDC) can be used to probe quantum optical phenomena during a single cycle of light. Harris [Opt. Express 98, 063602 (2007)] suggested using ultrabroad parametric fluorescenc generated from a quasi-phase-matched (QPM) device whose poling period is chirped. In the Harris's original proposal, it is assumed that the photons are collinearly generated and then spatially separated by frequency filtering Here, we alternatively propose using noncollinearly generated SPDC. In our numerical calculation, to achieve 1.2 cycle temporal correlation for a 532 nm pump laser, only 10% -chirped device is sufficien when noncollinear condition is applied, while a largely chirped (50%) device is required in collinear condition. We also experimentally demonstrate an octave-spanning (790-1610 nm) noncollinear parametric fluorescenc from a 10% chirped MgSLT crystal using both a superconducting nanowire single-photon detector and photomultiplier tube as photon detectors. The observed SPDC bandwidth is 194 THz, which is the largest width achieved to date for a chirped QPM device. From this experimental result, our numerical analysis predicts that the bi-photon can be compressed to 1.2 cycles with appropriate phase compensation.
Summary form only given. In aperiodically poled quasi-phase-matching (QPM) crystals, parametric down-conversion (PDC) can generate very broadband pulsed squeezed light. This is because each part of the crystal will generate squeezed light at a different frequency [1]. But the emission will be thus inhomogeneously broadened, and one requires special measures to eliminate the chirp accompanying the pulses. This can be done by placing a group-velocity dispersion material after the crystal [2] or by specially designing the nonlinear grating [3]. If the bandwidth of squeezed light is large enough, the squeezing and photon-number correlations, after the chirp is removed, will be “single-cycle”, with the correlation time comparable to one optical cycle [4].In this work, the first results on such broadband squeezing are demonstrated. In the 5-mm-long aperiodically poled MgO:LiNbO3 crystal we use, the QPM grating period changes from 7.38 μm to 8.91 μm. With this device, we generated bright twin beams through collinear frequency-nondegenerate high-gain PDC. The phase chirp was eliminated through the design of the nonlinear grating. The frequency bandwidth was measured to be 17 THz, a factor of 15 larger than in a periodically poled crystal. The correlation time was measured using non-phase matched sum frequency generation from the signal and idler beams. The dependence of the sum frequency intensity on the time delay between the signal and idler beams is shown in Fig.1. Fine-resolution scanning (b) shows a narrow peak, demonstrating a correlation time of 90 fs. It can be further reduced by using more advanced grating designs.