Nonlinear Crystal Waveguide Research Articles

Fabrication limits of waveguides in nonlinear crystals and their impact on quantum optics applications

Waveguides in nonlinear materials are a key component for photon pair sources and offer promising solutions to interface quantum memories through frequency conversion. To bring these technologies closer to every-day life, it is still necessary to guarantee a reliable and efficient fabrication of these devices. Therefore, a thorough understanding of the technological limitations of nonlinear waveguiding devices is paramount. In this paper, we study the link between fabrication errors of waveguides in nonlinear crystals and the final performance of such devices. In particular, we first derive a mathematical expression to qualitatively assess the technological limitations of any nonlinear waveguide. We apply this tool to study the impact of fabrication imperfections on the phasematching properties of different quantum processes realized in titanium-diffused lithium niobate waveguides. Finally, we analyse the effect of waveguide imperfections on quantum state generation and manipulation for few selected cases. Studying the impact of fabrication errors on the waveguide widths, we find that the presence of correlated noise plays a major role in the degradation of the phasematching and we suggest different possible strategies to reduce the impact of fabrication imperfections.

Tunable single- and dual-wavelength SHG from diode-pumped PPKTP waveguides.

A compact, all-room-temperature, widely tunable, continuous wave laser source in the green spectral region (502.1-544.2 nm) with a maximum output power of 14.7 mW is demonstrated. This was made possible by utilizing second-harmonic generation (SHG) in a periodically poled potassium titanyl phosphate (PPKTP) crystal waveguide pumped by a quantum-well external-cavity fiber-coupled diode laser and exploiting the multimode-matching approach in nonlinear crystal waveguides. The dual-wavelength SHG in the wavelength region between 505.4 and 537.7 nm (with a wavelength difference ranging from 1.8 to 32.3 nm) and sum-frequency generation in a PPKTP waveguide is also demonstrated.

Experimental Demonstration of a Hybrid-Quantum-Emitter Producing Individual Entangled Photon Pairs in the Telecom Band.

Quantum emitters generating individual entangled photon pairs (IEPP) have significant fundamental advantages over schemes that suffer from multiple photon emission, or schemes that require post-selection techniques or the use of photon-number discriminating detectors. Quantum dots embedded within nanowires (QD-NWs) represent one of the most promising candidate for quantum emitters that provide a high collection efficiency of photons. However, a quantum emitter that generates IEPP in the telecom band is still an issue demanding a prompt solution. Here, we demonstrate in principle that IEPPs in the telecom band can be created by combining a single QD-NW and a nonlinear crystal waveguide. The QD-NW system serves as the single photon source, and the emitted visible single photons are split into IEPPs at approximately 1.55 μm through the process of spontaneous parametric down conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide. The compatibility of the QD-PPLN interface is the determinant factor in constructing this novel hybrid-quantum-emitter (HQE). Benefiting from the desirable optical properties of QD-NWs and the extremely high nonlinear conversion efficiency of PPLN waveguides, we successfully generate IEPPs in the telecom band with the polarization degree of freedom. The entanglement of the generated photon pairs is confirmed by the entanglement witness. Our experiment paves the way to producing HQEs inheriting the advantages of multiple systems.

Theoretical Analysis of Intra-Cavity Second-Harmonic Generation of Semiconductor Lasers by a Periodically Poled Nonlinear Crystal Waveguide

General time-domain traveling wave rate equations are employed for the simulation of the intra-cavity second harmonic generation (IC-SHG) of semiconductor lasers by a periodically poled nonlinear crystal waveguide. A 1060 nm high power, single-mode, ridge waveguide semiconductor laser is used in the simulation. The SHG crystal is a MgO-doped periodically poled lithium niobate with a single-mode ridge waveguide. Comparisons are made between the computed and experimental results of single-pass SHG for the purposes of model validation. The design of an IC-SHG green laser is further compared to the single-pass SHG laser in terms of the SHG output power, conversion efficiency, temperature tolerance, and high speed modulation capability. It is theoretically found that by using the IC-SHG configuration, SHG power and efficiency can be improved with a relatively short nonlinear crystal.