Linear propagation characteristics of periodically segmented waveguides

Engineering of Zeno Dynamics in Integrated Photonics

Frequent observations to a quantum system modify its coherent evolution through the Zeno effect and Zeno dynamics. Generally, the measurement process destroys the evolution environment of the monitored system, making repeated observations remain a challenge. Here, using the quantum analogy experiments, we realize and engineer the Zeno effect and Zeno dynamics in optical waveguide arrays, where the optical modes correspond to distinct quantum states, and the temporal evolution is mapped into the spatial propagation. We propose a new, extensible experimental strategy for realizing an optical analog of stroboscopic measurements, which are performed by the build-in, on-demand segmented waveguide portions. The weak-to-strong stroboscopic measurements are realized, where the monitored system undergoes a transition from free evolution to optical Zeno freezing. Setting the measurements in the strong regime, the optical Zeno effect and optical Zeno dynamics are successfully generated, and their relationship is demonstrated in optics. We then propose a novel quantum Zeno slicing approach, which allows us to dynamically engineer the Hilbert space of the monitored system. This generic approach is verified by generating a series of Zeno subspaces with different measurement projectors, based on the quantum-optical analogy. The complexity of light dynamics is largely increased, providing full control of the propagation via steering Zeno dynamics. Our results pave the way for manipulation of quantum states by harnessing Zeno dynamics in integrated photonics.

Diffraction-less propagation beyond the sub-wavelength regime: a new type of nanophotonic waveguide

Sub-wavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the propagation of light. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size. Here, we present a new nanophotonic waveguide grating concept that exploits phase-matching engineering to suppress diffraction effects for a period three times larger than those with SWG approaches. This long-period grating not only facilitates fabrication, but also enables a new diffraction-less regime with additional degrees of freedom to control light propagation. More specifically, the proposed phase-matching engineering enables selective diffraction suppression, providing new tools to shape propagation in the grating. We harness this flexible diffraction control to yield single-mode propagation in, otherwise, highly multimode waveguides, and to implement Bragg filters that combine highly-diffractive and diffraction-less regions to dramatically increase light rejection. Capitalizing on this new concept, we experimentally demonstrate a Si membrane Bragg filter with record rejection value exceeding 60 dB. These results demonstrate the potential of the proposed long-period grating for the engineering of diffraction in nanophotonic waveguides and pave the way for the development of a new generation of high-performance Si photonics devices.

Segmented waveguide arrays: deriving discrete diffraction relations in a square lattice photonic crystal

Segmented strip-loaded waveguide arrays are investigated within a rigorous square lattice photonic crystal model. We derive a full multiband discrete diffraction approach for near-axial injection in the direction of a lattice vector. We obtain an effective waveguide array picture, with quasi-linear dependence on the segmentation ratio in a simplified single-band scheme. Our results are validated by beam deviation experiments. Such a diffraction framework allows for efficient shaping of the phase map in waveguide arrays and enriches the engineering toolkit of photonic crystals with the in-plane free propagation structures of discrete photonics.

Sub-wavelength grating for enhanced ring resonator biosensor

While silicon photonic resonant cavities have been widely investigated for biosensing applications, enhancing their sensitivity and detection limit continues to be an area of active research. Here, we describe how to engineer the effective refractive index and mode profile of a silicon-on-insulator (SOI) waveguide using sub-wavelength gratings (SWG) and report on its observed performance as a biosensor. We designed a 30 μm diameter SWG ring resonator and fabricated it using Ebeam lithography. Its characterization resulted in a quality factor, Q, of 7 · 10³, bulk sensitivity Sb = 490 nm/RIU, and system limit of detection sLoD = 2 · 10^-6 RIU. Finally we employ a model biological sandwich assay to demonstrate its utility for biosensing applications.

Non-reciprocal transmission in photonic lattices based on unidirectional coherent perfect absorption

A method for realizing asymmetric (one-way) transmission of discretized light in modulated, linear, and purely passive optical lattices is suggested, which exploits the idea of unidirectional coherent perfect absorption. The system consists of a linear photonic lattice of coupled resonators or waveguides, side coupled to a chain of lossy elements, in which light can avoid the occupation of the dissipative sites when propagating in one way, but not in the opposite one. Non-reciprocity requires modulation of the resonator/waveguide parameters, realizing a dissipative optical Aharonov-Bohm diode with non-reciprocal behavior.

Straight multimode interference phased array structure using periodic segmented waveguide phase array

A new design of a multimode interference (MMI) phased array structure is proposed. This design is based on replacing the arrayed delay arms between the first MMI power splitter and the second MMI combiner by periodic segmented waveguides. This allows a straight structure without curved waveguides and thus reduces greatly the size of the structure. A design example of an 8 channel multiplexer is presented showing a size reduction by a factor of 23 when compared with conventional design, while keeping nearly the same performance.

Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs

A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi guided modes are calculated for the case of self-standing membranes as well as for silicon-on-insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasiguided modes above the light line depend in a nontrivial way on structure parameters, mode index, and wave vector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.

Stable and efficient bloch-mode computational method for one-dimensional grating waveguides

We present a stable and efficient method for the Bloch-mode computation of one-dimensional grating waveguides. The approach uses the Fourier modal method and the S-matrix algorithm to remove numerical instabilities. The use of perfectly matched layers provide a high accuracy. Numerical results obtained for different lamellar grating waveguides and for both TE and TM polarizations illustrate the performance of the approach.

Effects of modulation strength in guided-mode resonant subwavelength gratings at normal incidence

A comparative study of the reflection spectral resonances in weakly and strongly modulated subwavelength gratings is presented. The effects of strong modulation in resonant subwavelength gratings have been largely ignored in the literature. We show that the spectral stability of resonances as a function of angle of incidence around normal can be greatly enhanced with strongly modulated gratings while the desirable narrow line-width associated with weakly modulated gratings is still maintained.

Experimental observation of longitudinal modulation of mode fields in periodically segmented waveguides

We present what is to our knowledge the first experimental observation by fluorescent visualization, of the evolution of the mode field profile in a periodically segmented waveguide. The experimental observations are then compared with the numerical results obtained by a finite-difference beam propagation method. Good agreement between experimental and numerical results is observed.

Ray analysis of parabolic-index segmented planar waveguides

A ray analysis of periodically segmented waveguides with parabolic-index variation in the high-index region is presented. We carried out the analysis using ray transfer matrices, which is convenient to implement and which can be extended to study different types of graded-index segmented waveguide. Results of this ray tracing approach clearly illustrate the waveguiding properties and the existence of stable and unstable regions of operation in segmented waveguides. We also illustrate the tapering action exhibited by segmented waveguides in which the duty cycle varies along the length of the waveguide. This analysis, although restricted to multimode structures, provides a clear visualization of the waveguiding properties in terms of ray propagation in segmented waveguides.

Evanescent field sensors with periodically segmented waveguides

An evanescent field-sensing configuration, based on a periodically segmented waveguide, is proposed and analyzed. It has advantages over existing configurations that relate to its simple realization (in single-step lithography). The sensitivity and operating point can be controlled easily with geometric parameters alone, which facilitates realization of more complex sensing arrays.

Kronig-Penney model for periodically segmented waveguides

Physical insight into the propagation characteristics of periodically segmented waveguides can be obtained by viewing the guiding layer as a thin slab of an idealized infinite medium with periodic layers along the direction of propagation. One can analyze the properties of this infinite medium using a formal analogy with the Kronig-Penney model used to describe the propagation of electron wave functions in a periodic crystal potential. The model correctly gives the effective refractive index of the layer, gives insight into the electric field profiles at different points on the dispersion curve, and leads to a qualitative explanation of why the optical loss diminishes at both large and small duty cycles.

Adiabatically tapered periodic segmentation of channel waveguides for mode-size transformation and fundamental mode excitation

We report modeling, fabrication, and characterization of tapered waveguides, using periodically segmented annealed proton exchange in LiNbO(3). For a taper transforming the 1/e full width (intensity) mode size from 6.0 microm x 4.4 microm to 2.0 microm x 1.3 microm, 0.4-dB excess loss was observed. The large, slightly elliptical mode can ease coupling to single-mode fibers and circular free-space beams. The same taper structure was used to excite a highly multimoded (13 modes) waveguide with more than 95% of the power stably launched into the lowest-order mode. This function is useful for nonlinear waveguide devices, such as difference-frequency generators, in which modes at highly disparate wavelengths interact.

Spot size transformer with a periodically segmented waveguide based on InP

We report what is to our knowledge the first optimization of a spot size transformer by use of a periodically segmented waveguide for photonic integrated circuits based on InP. It is shown that a nonlinear variation of the segmentation ratio along the propagation direction allows one to make an adiabatic transition of a short length. A simple method based on the study of a beveled edge transition is presented for the optimization of such a device. Internal losses in the transition region as low as 1.2 dB are obtained between a coupling waveguide and a simple heterostructure waveguide, with a very high transition spot size expansion factor.

Propagation characteristics of planar segmented waveguides with parabolic index segments

We have analyzed the linear propagation characteristics of planar periodic segmented waveguides with infinitely extended parabolic refractive-index segments in the transverse direction, using the matrix formulation of Gaussian beam propagation. Modal propagation and the variation of effective index as a function of duty cycle and the period of segmentation have been studied. Conditions for the existence of guided modes in such a segmented waveguide are presented.

Periodically segmented waveguides in Ti:LiNbO(3)

Periodically segmented waveguides with periods of few tens of micrometers and nonconnected segments are studied experimentally in Ti:LiNbO(3). The near-field patterns were found to expand with decreasing duty cycle but to be independent of the segmentation period. The measured segmentation losses vary between 0.5 and 2.5 dB/cm, depending on the period and the duty cycle. These results agree with a simple numerical model, based on two loss mechanisms: waveguide segmentation and surface relief related to Ti indiffusion.

Proton-exchanged periodically segmented waveguides in LiNbo(3)

We report on the characterization of single-mode and multimode proton-exchanged periodically segmented waveguides in lithium niobate. Variation of effective indices of different modes for different annealing times as a function of duty cycle and period of segmentation is presented. It is shown that the proton-exchanged periodically segmented waveguide can be modeled by an equivalent z-invariant depth-independent graded-index waveguide with a Gaussian index distribution. Although the depth of the equivalent waveguide is independent of duty cycle, the peak refractive-index change increases almost linearly with the duty cycle and saturates at large duty cycles. The results should benefit the design of linear and nonlinear devices that are based on segmented waveguides.