We demonstrate supermode-based second harmonic generation in an integrated nonlinear interferometer made of linear and nonlinear directional couplers. We use a fully-fibered pump shaper to demonstrate second harmonic generation pumped by the symmetric or anti-symmetric fundamental spatial modes. The selection of the pumping mode and thus of a specific SHG spectral profile is achieved through the selection of the fundamental wavelength and via a robust phase setting scheme. We use two methods: either post-selecting or actively setting the pumping mode. Such modal phase matching paves the way for classical and quantum applications of coupled nonlinear photonic circuits, where multimode excitation, encoding and detection are a route for multiplexing and scaling up light-processing.
We demonstrate supermode-based second harmonic generation in an integrated nonlinear interferometer made of linear and nonlinear directional couplers. We use a fully-fibered pump shaper to demonstrate second harmonic generation pumped by the symmetric or anti-symmetric fundamental spatial modes. The selection of the pumping mode and thus of a specific SHG spectral profile is achieved through the selection of the fundamental wavelength and via a robust phase setting scheme. We use two methods: either post-selecting or actively setting the pumping mode. Such modal phase matching paves the way for classical and quantum applications of coupled nonlinear photonic circuits, where multimode excitation, encoding and detection are a route for multiplexing and scaling up light-processing.
The availability of photon states with well-defined temporal modes is crucial for photonic quantum technologies. Ever since the inception of generating photonic quantum states through pulse pumped spontaneous parametric processes, many exquisite efforts have been put on improving the modal purity of the photon states to achieve single-mode operation. However, because the nonlinear interaction and linear dispersion are often mixed in parametric processes, limited successes have been achieved so far only at some specific wavelengths with sophisticated design. In this paper, we resort to a different approach by exploiting an active filtering mechanism originated from interference fringe of nonlinear interferometer. The nonlinear interferometer is realized in a sequential array of nonlinear medium, with a gap in between made of a linear dispersive medium, in which the precise modal control is realized without influencing the phase matching of the parametric process. As a proof-of-principle demonstration of the capability, we present a photon pairs source using a two-stage nonlinear interferometer formed by two identical nonlinear fibers with a standard single mode fiber in between. The results show that spectrally correlated two-photon state via four wave mixing in a single piece nonlinear fiber is modified into factorable state and heralded single-photons with high modal purity and high heralding efficiency are achievable. This novel quantum interferometric method, which can improve the quality of the photon states in almost all the aspects such as modal purity, heralding efficiency, and flexibility in wavelength selection, is proved to be effective and easy to realize.
English: In this project has been designed and implemented a setup to produce the nonlinear process of the second harmonic generation (SHG) in Bragg reflection waveguides (BRW). Starting with the adjustment of a tunable single frequency laser at 1550 with an external cavity configuration. Two diferent types of SHG have been observed, we measured their spectrum and its shifting caused by temperature variations. Finally, it has also been observed the spatial shape of the Bragg mode guided along the BRW, which is the responsible for reaching the phase-matching condition between all the waves involved in this non-linear process. The results obtained in this project will be used in a future experiments, specially, in the implementation of a monolithic integrated source of entangled photon pairs generated by using the spontaneous parametric down-conversion (SPDC) process.
The second harmonic generation (SHG) technology based on quasi-phase matching in periodically poled lithium niobate (PPLN) has reached a high level of maturity. However, the generated second harmonic signal is often overwhelmed by pump light, making direct application challenging. In this paper, we have designed and fabricated, for the first time, to the best of our knowledge, an integrated second harmonic generation device with on-chip wavelength division demultiplexing (WDDM). The device achieves a second harmonic conversion efficiency of 1200% W −1 cm −2 and provides approximately 20 dB attenuation in the telecom C-band near 1550 nm, while introducing about 4 dB insertion loss in the visible light near 775 nm, significantly improving the signal-to-noise ratio. The WDDM section employs a cascaded linearly tapered waveguide design, which offers broadband coupling efficiency. This result shows promise as a foundation for fully integrated quantum photonic chips.
Second-harmonic generation in multimode laser beams is studied experimentally. A multimode laser beam derived from a laser oscillating simultaneously in many longitudinal modes that have the same transverse profile (TEM00) is focused into a nonlinear crystal. Noncritical phase matching is utilized to generate the second harmonic. The number of longitudinal modes is systematically varied from 1 to 7 and the dependence of the efficiency of second-harmonic generation on the number of modes in the fundamental beam is studied. Experimental results clearly demonstrate the increasing degree of photon bunching in the fundamental beam as the number of modes increases. Experimental results are discussed and compared with the predictions of theoretical models.
Long ago, it was predicted that stimulating two-photon processes such as two-photon absorption (TPA) and second harmonic generation (SHG) with entangled photons produced by parametric down-conversion (PDC) could result in higher efficiencies compared to classical illumination [1]. This was experimentally demonstrated for SHG [2] and TPA [3], where, however, the classical comparison was performed by adding losses to the PDC state rather than employing a true classical reference. In addition, these results considered spatially single-mode PDC radiation and were conducted at low photon fluxes, where the PDC modes are, on average, populated by less than one photon. This resulted in an extremely low SHG/TPA signal, considered of limited practical use for metrological applications (such as two-photon fluorescence microscopy or SHG imaging - see, e.g., [4]). A recent theoretical investigation [5] highlighted that a proper analysis of quantum-enhanced two-photon processes requires a fully spatiotemporal model of the nonlinear interactions. It further showed that highly multimode (in space and time) entangled PDC photons can provide a quantum advantage compared to an equivalent classical pulse, even in regimes well beyond the “one-photon-per-mode” limit, considered the boundary of quantum behavior.
Nonlinear interaction between orbital angular momentum (OAM) modes has interesting effects and tremendous applications in both classical and quantum realms. Here we propose a strategy of using perfect vortices, which not only overcomes the spatial mode matching problem of nondegenerate OAM modes, but also reveals the OAM-only influence on the momentum matching by tuning the OAMs of the fundamental wave. We use perfect vortices of degenerate and nondegenerate OAM modes to clearly demonstrate the flexible control of OAM modes on nonlinear interaction by carefully exploring second-harmonic generation (SHG) in periodically poled crystal. We generate second-harmonic perfect vortices with the OAM order up to 40. Based on the observed phase-matching temperature shift caused by OAMs, we also use the SHG nonlinear process to distinctly confirm the OAM-only-induced subluminal effect. Our results pave the way toward unveiling the underlying physics behind nonlinear process of OAM modes and building high-capacity quantum links between systems operating at various wavelengths. The inverse process of SHG we developed, perfect-vortex-based spontaneous parametric down-conversion, may provide a promising solution for high-dimensional OAM entangled source.