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Журнал технической физики

Каскадная генерация второй гармоники с половинным порядком периодической ориентации

В.Ю. Мыльников, Н.С. Аверкиев, Г.С. Соколовский

Published Jan 1, 2020• 0 citations
Publisher

Abstract

We theoretically demonstrate cascaded fourth and second harmonic generation in a periodically-poled nonlinear crystal with a half-order of phase-matching period. We consider a cascaded process in which four photons of the fundamental harmonic firstly convert into an intermediate photon of the fourth harmonic, which parametrically decays into two photons of the second harmonic at the second stage. Phase-matching for this cascaded nonlinear conversion is provided by using an asymmetric periodical poling. We use the quantum spatial Heisenberg equations to describe light propagation and conversion inside the nonlinear crystal. With this approach, we calculate second and fourth harmonic average number of photons as a function of the nonlinear crystal length.

DOI
10.21883/os.2020.11.50175.199-20

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We theoretically demonstrate cascaded fourth and second harmonic generation in a periodically-poled nonlinear crystal with a half-order of phase-matching period. We consider a cascaded process in which four photons of the fundamental harmonic firstly convert into an intermediate photon of the fourth harmonic, which parametrically decays into two photons of the second harmonic at the second stage. Phase-matching for this cascaded nonlinear conversion is provided by using an asymmetric periodical poling. We use the quantum spatial Heisenberg equations to describe light propagation and conversion inside the nonlinear crystal. With this approach, we calculate second and fourth harmonic average number of photons as a function of the nonlinear crystal length.
Abstract We demonstrate theoretically cascaded fourth and second harmonics generation in a periodically oriented nonlinear crystal with a half-order of the periodical orientation. We consider a cascaded process in which four photons of the fundamental harmonic are initially converted into an intermediate photon of the fourth harmonic, which parametrically decays at the second stage into two photons of the second harmonic. We show that fractional phase matching can be achieved by using an asymmetric periodically poled grating. To describe the propagation and transformation of light inside a nonlinear crystal, the quantum spatial Heisenberg equations have been applied, using which the average numbers of photons of the fourth and second harmonics have been calculated as functions of the nonlinear crystal length.
We propose an adiabatic scheme for cascaded fourth-harmonic generation (FHG) in chirped dual-period quasi-phase-matched (QPM) crystals, where adiabatic evolution is introduced into the multi-step frequency conversion process. By engineering two chirped QPM gratings for the fundamental-to-second-harmonic ( FF → SH ) and second-to-fourth-harmonic ( SH → FH ) interactions, efficient harmonic generation is achieved through a continuous adiabatic passage in phase-mismatch space, rather than discrete phase-matching conditions in conventional QPM schemes. This approach enables robust and tunable energy transfer among harmonics, allowing controllable generation of second- and fourth-harmonic signals. Numerical results show a maximum FHG efficiency of 94% and a tunable bandwidth of ∼13 nm under realistic conditions. The tunability originates from the controlled shift of QPM resonances along the propagation direction.
We report that the optical orbital angular momentum (OAM) is conserved in the two-step cascaded nonlinear processes through quasi-phase-matched (QPM) in a dual-structure periodically poled LiTaO!(PPLT) crystal. Because of the different reciprocal vectors provided by the different structures in the crystal, collinear second harmonic (SH) beam with an OAM of l2 and third harmonic (TH) beam with an OAM of l3 are simultaneously generated by the input fundamental beam with an OAM of l1. The OAM conversion law (l2 = 2l1, l3 = 3l1) holds well in this nonlinear process. Our results provide an effective way to obtain three beams (fundamental beam, SH beam, TH beam) with different OAMs at the same time, which can be used to enhance the capacity of optical communication.
A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 10 3 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.
We report on efficient generation of continuous-wave squeezed light and second harmonics with a periodically poled MgO:LiNbO(3) (PPMgLN) crystal which enables us to utilize the large nonlinear optical coefficient d(33). We achieved the squeezing level of -7.60+/-0.15 dB at 860 nm by utilizing a subthreshold optical parametric oscillator with a PPMgLN crystal. We also generated 400 mW of second harmonics at 430 nm from 570 mW of fundamental waves with 70% of conversion efficiency by using a PPMgLN crystal inside an external cavity.