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arXiv:1605.05083 · Scientific Reports

Parametric down-conversion with nonideal and random quasi-phase-matching

Chun-Yao Yang, Chun Lin, Charlotte Liljestrand, Wei-Min Su, Carlota Canalias, Chih‐Sung Chuu

Published May 13, 2016• 8 citations
arXivHTMLPDF

Abstract

Quasi-phase-matching (QPM) has enriched the capacity of parametric down-conversion (PDC) in generating biphotons for many fundamental tests and advanced applications. However, it is not clear how the nonidealities and randomness in the QPM grating of a parametric down-converter may affect the quantum properties of the biphotons. This paper intends to provide insights into the interplay between PDC and nonideal or random QPM structures. Using a periodically poled nonlinear crystal with short periodicity, we conduct experimental and theoretical studies of PDC subject to nonideal duty cycle and random errors in domain lengths. We report the observation of biphotons emerging through noncritical birefringent-phasematching, which is impossible to occur in PDC with an ideal QPM grating, and a biphoton spectrum determined by the details of nonidealities and randomness. We also observed QPM biphotons with a diminished strength. These features are both confirmed by our theory. Our work provides new perspectives for biphoton engineering with QPM.

DOI
10.1038/srep26079

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Quasi-phase-matching (QPM) has enriched the capacity of parametric down-conversion (PDC) in generating biphotons for many fundamental tests and advanced applications. However, it is not clear how the nonidealities and randomness in the QPM grating of a parametric down-converter may affect the quantum properties of the biphotons. This paper intends to provide insights into the interplay between PDC and nonideal or random QPM structures. Using a periodically poled nonlinear crystal with short periodicity, we conduct experimental and theoretical studies of PDC subject to nonideal duty cycle and random errors in domain lengths. We report the observation of biphotons emerging through noncritical birefringent-phasematching, which is impossible to occur in PDC with an ideal QPM grating, and a biphoton spectrum determined by the details of nonidealities and randomness. We also observed QPM biphotons with a diminished strength. These features are both confirmed by our theory. Our work provides new perspectives for biphoton engineering with QPM.
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.
In this work we study the transverse spatial correlation of the pair of photons generated via the process of spontaneous parametric frequency down-conversion, in periodically poled non-linear crystals illuminated by a pulsed laser beam. It is well known that the two-photon state generated in quasi-phase matching (QPM) configurations depends explicitly on the characteristics of the pump beam, on the crystal modulated non-linearity, and on the detection geometry. This has allowed the development of several techniques for controlling the biphoton spectral and spatial properties. Here we discuss another technique for implementing the spatial entanglement modification in QPM gratings. We show, theoretically and experimentally, that in nearly collinear geometries, the spatial shape modulation of the pump beam allows for the control of the biphoton transverse spatial correlation.
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.
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.
Counter-propagating parametric conversion processes in non-linear bulk crystals have been shown to feature unique properties for efficient narrowband frequency conversion. In quantum optics, the generation of photon pairs with a counter-propagating parametric down-conversion process (PDC) in a waveguide, where signal and idler photons propagate in opposite directions, offers unique material-independent engineering capabilities. However, realizing counter-propagating PDC necessitates quasi-phase-matching (QPM) with extremely short poling periods. Here, we report on the generation of counter-propagating single-photon pairs in a self-made periodically poled lithium niobate waveguide with a poling period on the same order of magnitude as the generated wavelength. The single photons of the biphoton state bridge GHz and THz bandwidths with a separable joint temporal-spectral behavior. Furthermore, they allow the direct observation of the temporal envelope of heralded single photons with state-of-the art photon counters.
Quasi-phase-matching (QPM) technique has been successfully applied in nonlinear optics, such as optical frequency conversion. Recently, remarkable advances have been made in the QPM generation and manipulation of photon entanglement. In this paper, we review the current progresses in the QPM engineering of entangled photons, which are finished mainly by our group. By the design of concurrent QPM processes insides a single nonlinear optical crystal, the spectrum of entangled photons can be extended or shaped on demand, also the spatial entanglement can be transformed by transverse inhomogeneity of domain modulation, resulting in new applications in path-entanglement, quantum Talbot effects, quantum imaging etc. Combined with waveguide structures and the electro-optic effect, the entangled photons can be generated, then guided and phase-controlled within a single QPM crystal chip. QPM devices can act as a key ingredient in integrated quantum information processing.