High-Efficiency Multilevel Volume Diffraction Gratings inside Silicon

Abstract

Silicon (Si)-based integrated photonics is considered to play a pivotal role in multiple emerging technologies, including telecommunications, quantum computing, and lab-chip systems. Diverse functionalities are either implemented on the wafer surface (“on-chip”) or recently within the wafer (“in-chip”) using laser lithography. However, the emerging depth degree of freedom has been exploited only for single-level devices in Si. Thus, monolithic and multilevel discrete functionality is missing within the bulk. Here, we report the creation of multilevel, high-efficiency diffraction gratings in Si using three-dimensional (3D) nonlinear laser lithography. To boost device performance within a given volume, we introduce the concept of effective field enhancement at half the Talbot distance, which exploits self-imaging onto discrete levels over an optical lattice. The novel approach enables multilevel gratings in Si with a record efficiency of 53%, measured at 1550 nm. Furthermore, we predict a diffraction efficiency approaching 100%, simply by increasing the number of levels. Such volumetric Si-photonic devices represent a significant advance toward 3D-integrated monolithic photonic chips.

Multilevel diffraction gratings inside silicon towards spectral filtering

“…2(a)) and field intensity is observed with an increasing number of layers. This suggests that consecutive grating levels enhance the propagation of ±1 orders while suppressing the zeroth order, a phenomenon further elaborated in our previous work [9].…”

“…The Talbot effect, a near-field diffraction phenomenon, can be effectively leveraged in a multilevel design to enhance diffraction efficiency in gratings with low refractive index contrast [9,11]. This effect is characterized by the self-imaging of a periodic grating at specific intervals, known as the Talbot distances, c [12]:…”

“…By arranging multiple gratings in a stack, where each layer is separated by either exact or half multiples of the Talbot length [9], the diffracted waves from one layer undergo constructive interference with those from subsequent levels. This constructive interference stems from the coherent superposition of wavefronts at these specific distances, leading to a significant amplification of the diffracted light intensity [9]. In this multilayer system, each layer serves to reinforce the diffraction pattern established by its predecessors.…”

“…These results are obtained through finite-difference time-domain (FDTD) simulations, taking into account a 50% duty cycle, 3 μm period, 10 μm length of individual grating levels, and a refractive index contrast of |Δn|=2×10 −3 . Here, a laterally shifted design [9] is used, with the center-to-center distance between the grating levels set to half the Talbot distance c/2 = 20.1 μm. The methodology used in the FDTD computations and a definition of the combined first-order diffraction efficiency are detailed elsewhere [9].…”

“…In our previous work, we introduced an innovative volumetric multilevel diffraction grating in Si with period of Λ = 8 μm for the 1.55 µm wavelength [9], the conventional telecommunications wavelength. Our design utilized the Talbot self-imaging effect to boost the theoretical first-order diffraction efficiency (DE) from around 5% to 95%.…”