Enhanced zero-phonon line emission from an ensemble of W centers in circular and bowtie Bragg grating cavities

Abstract

Color centers in silicon have recently gained considerable attention as single-photon sources and as spin qubit-photon interfaces. However, one of the major bottlenecks to the application of silicon color centers is their low overall brightness due to a relatively slow emission rate and poor light extraction from silicon. Here, we increase the photon collection efficiency from an ensemble of a particular kind of color center, known as W centers, by embedding them in circular Bragg grating cavities resonant with their zero-phonon-line emission. We observe a ≈5-fold enhancement in the photon collection efficiency (the fraction of photons extracted from the sample and coupled into a single-mode fiber), corresponding to an estimated ≈11-fold enhancement in the photon extraction efficiency (the fraction of photons collected by the first lens above the sample). For these cavities, we observe lifetime reduction by a factor of $\approx 1.3$ . For W centers in resonant bowtie-shaped cavities, we observed a ≈3-fold enhancement in the photon collection efficiency, corresponding to a ≈6-fold enhancement in the photon extraction efficiency, and observed a lifetime reduction factor of $\approx 1.1$ . The bowtie cavities thus preserve photon collection efficiency and Purcell enhancement comparable to circular cavities while providing the potential for utilizing in-plane excitation methods to develop a compact on-chip light source.

Keywords: Purcell factor; color center; enhanced emission; quantum emitter; silicon defect.

Figure 1:

A single W center dipole in the center of a Bragg grating cavity. (a, b) Top-view of circular Bragg grating (CBG) and bowtie grating cavities. White circles represent the etched trenches, and the gray region represents unetched silicon (a) schematic of a CBG (bullseye) cavity with seven rings. 2Λ is the central disk radius, w is the trench width, and Λ is the grating period. (b) Schematic of a bowtie cavity, with a cavity enclosure angle of 90°. (c) Schematic of the cross-sectional view of a partially etched bullseye cavity on SOI with a trench depth of 140 nm and a silicon device layer thickness of 220 nm. The red arrow represents a W center dipole pointed in the [111] direction at 45° with respect to the cavity plane. (d) Finite Element Method (FEM) simulation of a W center dipole placed in the center of a bullseye cavity at 45° with respect to the cavity plane. Plot showing the Purcell factor (F _p) and extraction efficiency (η) into a 0.9 numerical aperture (NA) objective for a dipole placed in the center of a partially etched bullseye cavity resonant at ZPL. The inset shows the same curves for a 90° h-bowtie cavity, with the dipole oriented perpendicular to the horizontal axis of the cavity and at 45° with respect to the cavity plane. (e) Normalized electric field intensity of the resonant mode in the XY plane. Black rings represent the circular gratings, and the white circles represent the etched trenches of the bullseye cavity.

Figure 2:

Fabricated partially etched circular Bragg grating (bullseye) cavities containing W centers. (a) Top view of the scanning electron micrograph (SEM) of bullseye cavities with varying parameters. (b) SEM of a bullseye cavity cross section, showing seven partially etched rings around the central disk. The cross section was created by a focused ion beam (FIB), where platinum (Pt) was deposited before ion milling to reduce charging effects.

Figure 3:

Different segments in the W center PL spectrum and their associated resonant cavities. (a) Photoluminescence spectrum of a W center ensemble in silicon on insulator at 10 K. ZPL: zero-phonon line. FPR: first phonon replica. PSB: phonon sideband. (b) Supercontinuum laser reflectivity spectra of bullseye cavity resonant with the ZPL (red), bowtie cavity resonant with the ZPL (black), and bullseye cavity resonant with the FPR (orange).

Figure 4:

W center photoluminescence (PL) enhancement in resonant cavities. (a) PL spectra showing the zero-phonon-line (ZPL) enhancement in the bullseye and bowtie cavities (orange and red curves respectively), as well as first-phonon-replica (FPR) enhancement (green and violet, respectively), compared to the emission from W centers in unpatterned silicon-on-insulator (blue curve). (b) A near-field, near-infrared image of the ZPL resonant bullseye cavity when pumped with a 635 nm continuous-wave laser in the center of the cavity. The annular shape shows strong PL scattering from the first few rings of the cavity. (c) Same as (b) for a bowtie cavity.

Figure 5:

Zero-phonon-line (ZPL) intensity as a function of pump power for bullseye cavity resonant with the ZPL (black diamond markers), bowtie cavity resonant with the ZPL (yellow squares), and unpatterned (plain) silicon on insulator (SOI) (green circles) under 1 MHz pulsed laser excitation.

Figure 6:

Time-resolved photoluminescence (PL) decay trace. PL zero-phonon line (ZPL) decay traces for W center ensembles in unpatterned silicon on insulator and bullseye and bowtie cavities resonant with the ZPL. Faster decays are apparent for the two cavities. Inset: same curves plotted on a linear scale for the first 300 ns. Curve fit on the first 120 ns of the lifetime trace using a biexponential decay function.