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v3.3.5
Opponent: Dr De Rossi Alfredo, Thales Research and Technology, Frankrike
Supervisor: Marcin Swillo, Tillämpad fysik; professor Srinivasan Anand, Tillämpad fysik
This thesis investigates counter-propagating spontaneous parametric downconversion(CPSPDC) in III-V semiconductor waveguides, wherein photonpairs emerge in opposing directions. These unconventional geometries affordenhanced spectral purity, improved photon pair separation, and extendedcoherence times, albeit at the cost of a substantially larger phase mismatch,which must be compensated. We demonstrate CPSPDC in periodicallypoled KTP with a measured efficiency of 10−10 pairs per pump photon,verified through high-visibility Hong-Ou-Mandel interference. The requiredpoling period to quasi-phase-match the CPSPDC interaction is 433 nm,in contrast to the 10.15 μm period required to phase-match the analogousco-propagating SPDC interaction. However, we demonstrate linewidths of0.34 nm, an order of magnitude smaller than the 3.5 nm exhibited by theco-propagating crystal.
Building upon this foundation, we advance toward the realization of anonlinear platform based on Ga0.51In0.49P transferred onto SiO2, enablingthe exploration of second-order nonlinear processes in subwavelength-scalewaveguides. Surface-induced second-harmonic generation (SHG) serves bothas a diagnostic tool for interface quality and as a mechanism to access polarizationmodes inaccessible via bulk nonlinearities. These advances enabledthe realization of ultra-thin waveguides suitable for twin-photon generationin reverse-engineered systems. Fundamental-mode to fundamental-modeSHG conversion from 1420 nm to 1520 nm is demonstrated in 100 nm-thickwaveguides, with operational bandwidth tailored through waveguide thicknessengineering.
The core achievement is the experimental realization of out-of-planepumped CPSPDC in Ga0.51In0.49P subwavelength waveguides. We demonstratephoton pair generation rates on the order of 200 pairs/s/mW in 1 mmlongwaveguides, slightly smaller the 313 pairs/s/mW demonstrated in PPKTP,yet with a footprint a million times smaller and linewidths under 0.3nm. The cornerstone of this work is the demonstration of full individualcontrol over the generated photon pair wavelengths and broadband operationby fine-tuning the waveguide cross-section and pump incidence angle.To illustrate the breadth of spectral tunability, we implement a 250 nmthickGa0.51In0.49P waveguide to generate CPSPDC photon pairs with 250nm of spectral separation, one at 1550 nm and the other at 1300 nm, correspondingto the wavelengths at which optical fibers exhibit minimal loss andiiidispersion, respectively. Furthermore, we increase the spectral separationby an additional 80 nm by tilting the pump by 5°. Degenerate CPSPDCis demonstrated, yielding a counter-propagating twin-photon source tunableacross the optical communications band, is achieved by compensatingthe momentum mismatch via pump tilting, demonstrating a novel phasematchingapproach and shifts toward degeneracy exceeding 601 nm.
To overcome the phase-matching limitations of traditional geometries,we introduce and fabricate amplitude-and-phase-matched structures (APMS),which require precise control of the crystallographic orientation within theIII-V waveguide. Theoretical efficiency gains exceeding 13 orders of magnitudeunder ideal conditions are demonstrated. The feasibility of domaininvertedGa0.51In0.49P via native oxide self-bonding has been experimentallyvalidated. These results establish III-V semiconductor waveguides notmerely as compact, efficient sources of non-classical light, but as a versatileplatform poised to shape the next generation of tunable, integrated quantumphotonic devices.