Devices For Quantum Photonic Integrated Circuits

Nowdays,people's daily life is filled with various electronic devices,ranging trom computers to tablets.Especially,almost everyone has at least one mobile phone,and cannot live without it.As the quantity of information is explosively growing,integrated electronics gets developed rapidly,together with the emerging bottleneck.For exam-ple,when the electronic chips are so densely integrated that the width of circuits only allows one electron to pass,the quantum tunneling effect cannot be neglected.Fortu-nately,photonic integrated circuits that encodes based on photons get rising attentions and provide solutions to the problems in integrated electronics,thanks to their multiple encoding degrees of freedom,high response rate,fast transport speed,good parallelism,compatibility and so on.However,in order to meet the needs of information processing with larger quantity,faster processing rate and lower power consumption,exploiting quantum photonic integrated circuits that are based on quantum mechanics become the best choice.In fact,early in 1900s,when Planck and Einstein discovered the concept of quantum starting from photons,photonics and quantum mechanics had been entangled.Similar to integrated electronic circuits,functional photonic integrated circuits are consisted of various components,such as photon sources,waveguides,detectors,mod-ulators,logic gates and so on.The thesis focuses on the design of functional devices for quantum photonic integrated circuits,including linear optical devices that can ma-nipulate photons,and nonlinear-optics-based components that can generate quantum photons.The contents of this thesis are listed as follows:(1)We proposed a polarization rotator inspired by stimulated Raman adiabatic passage model from quantum optics,which can work as Pauli X gate in polarization-encoded quantum information processing.The two orthogonal modes in signal waveg-uide and the oblique mode in ancillary waveguide form a A-type three-level system.By controlling the width of signal waveguide and the gap between two waveguides,adia-batic conversion between two orthogonal modes can be realized in the signal waveg-uide.With such adiabatic passage,polarization conversion is completed within 150?m length,with the efficiencies over 99%for both conversions between horizontal polar-ization and vertical polarization.In addition,such a polarization rotator is quite robust against fabrication error,allowing a wide range of tolerances for the rotator geometric parameters.Our work is not only significative to photonic simulations of coherent quan-tum phenomena with engineered photonic waveguides,but also enlightens the practical applications of these phenomena in optical device designs.(2)Based on adiabatic mode conversion as well,we propose an integrated plas-monic absorber to absorb the stray light in photonic integrated circuits(PICs),whose working principle is to induce metal and utilize the absorption loss of surface plasmon polaritons.With this 40?m-long absorber,the absorption efficiency can be over 99.8%at 1550nm,with both the reflectivity and transmittance reduced to less than 0.1%.Ben-efiting from adiabatic mode conversion,its performance is insensitive to incident wave-length with bandwidth larger than 300nm,and is robust against surrounding environ-ment and temperature.Besides,the use of metal enables it to be very compact and beneficial to thermal dissipation.Such device may find various applications in PICs,to eliminate the residual strong pump laser or stray light.As for quantum photon generation,nonlinear optical interaction is needed.For ex-ample,second-order nonlinear optical effects(second harmonic generation,parametric down conversion,etc.)could generate related/entangled photon pairs;Or,third-order nonlinear optical effects(four-wave mixing,etc.)could relate photons that were not related.For silicon-based nonlinear devices,the lowest order nonlinear optical effect is third-order,resulting in high power consumption.Therefore,we focus on second-order nonlinear optical materials,such as aluminum nitride and lithium niobate.Both of them have very high second-order nonlinear coefficients,and low linear propagation loss.Most importantly,their fabrication technique is CMOS-compatible,making them be great candidates to replace silicon.(1)We propose to use an integrated aluminum nitride waveguide with engineered width variation to achieve optical frequency conversions based on the ?(2)nonlinear effect on a photonic chip.We show that in an adiabatically tapered waveguide,the fre-quency conversion has a much broader bandwidth,and the efficiency within the band-width is almost constant,which is favorable for short pulses.This simple but efficient design is not only more fault-tolerant,but also enables the engineering of the opera-tional bandwidth.We demonstrate an "area law" both analytically and numerically for the frequency conversion,which can be used as a general design guideline for integrated nonlinear optical devices.With our approach,high-efficiency and wavefront-keeping conversion for short pulses becomes possible on a photonic chip,which can be applied in scalable on-chip information processing,allowing the extension of the on-chip com-b source to multiple octaves and control of the carrier envelope offset of a frequency comb.(2)We demonstrate efficient,phase-matched second harmonic generation in thin-film lithographically-defined lithium niobate waveguides with sub-micron dimension-s.Integrated thin-film lithium niobate platform has recently emerged as a promis-ing candidate for next-generation,high-efficiency wavelength conversion systems that allow dense packaging and mass-production.Both modal phase matching in fixed-width waveguides and quasiphase matching in periodically grooved waveguides are theoretically proposed and experimentally demonstrated.Our low-loss(?3.0dB/cm)nanowaveguides possess normalized conversion efficiencies as high as 41%W-1cm-2.