Photon propagation control on laser-written photonic chips enabled by composite waveguides

Femtosecond laser direct writing(Fs LDW)three-dimensional(3D)photonic integrated circuits(PICs)can realize arbitrary arrangement of waveguide arrays and coupling devices.Thus,they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation.However,the propagation constantβis limited to a narrow range in single-mode waveguides by solely changing the processing parameters,which greatly hinders the design of Fs LDW PICs.This study proposes a composite waveguide(CWG)method to increase the range ofβ,where a new single-mode composite waveguide comprises two adjacent circular waveguides.As a result,the photon propagation can be controlled and the variation range ofβcan be efficiently enlarged by approximately two times(Δβ~36 cm-1).With the CWG method,we successfully realize the most compact Fs LDW directional couplers with a 9μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array.Thus,the study represents a step further toward the fine control of the coupling between waveguides and compact integration of Fs LDW PICs.

On-chip single-photon chirality encircling exceptional points

Exceptional points(EPs),which are typically defined as the degener-acy points of a non-Hermitian Hamiltonian,have been investigated in various physical systems such as photonic systems.In particular,the intriguing topological structures around EPs have given rise to novel strategies for manipulating photons and the underlying mechanism is especially useful for on-chip photonic applications.Although some on-chip experiments with the adoption of lasers have been reported,EP-based photonic chips working in the quantum regime largely re-main elusive.In the current work,a single-photon experiment was proposed to dynamically encircle an EP in on-chip photonic waveg-uides possessing passive anti-parity-time symmetry.Photon coinci-dences measurement reveals a chiral feature of transporting single photons,which can act as a building block for on-chip quantum de-vices that require asymmetric transmissions.The findings in the cur-rent work pave the way for on-chip experimental study on the physics of EPs as well as inspiring applications for on-chip non-Hermitian quantum devices.

On-chip beam rotators,adiabatic mode converters,and waveplates through low-loss waveguides with variable cross-sections

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss,in order to provide more degrees of freedom in manipulating propagating light.Here,we report a new method for femtosecond laser writing of optical-fiber-compatible glass waveguides,namely spherical phase-induced multicore waveguide(SPIM-WG),which addresses this challenging task with three-dimensional on-chip light control.Fabricating in the heating regime with high scanning speed,precise deformation of cross-sections is still achievable along the waveguide,with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions.We observed that these waveguides have high refractive index contrast of 0.017,low propagation loss of 0.14 dB/cm,and very low coupling loss of 0.19 dB coupled from a single-mode fiber.SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles,or manipulate the polarization state of propagating light for target wavelengths.We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric nonuniform modes;examples include circular,elliptical modes,and asymmetric modes from ppKTP(periodically poled potassium titanyl phosphate)waveguides which are generally applied in frequency conversion and quantum light sources.Created inside optical glass,these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths.The compatibility with optical fiber also paves the way toward packaged photonic integrated circuitry,which usually needs input and output fiber connections.