Quantum efficiency of the B-center in hexagonal boron nitride

Abstract

B-centers in hexagonal boron nitride (hBN) are gaining significant research interest for quantum photonics applications due to precise emitter positioning and highly reproducible emission wavelengths at 436 nm. Here, we leverage the layered nature of hBN to directly measure the quantum efficiency (QE) of single B-centers. The defects were engineered in a 35 nm flake of hBN using electron beam irradiation, and the local dielectric environment was altered by transferring a 250 nm hBN flake on top of the one containing the emitters. By analyzing the resulting change in measured lifetimes, we determined the QE of B-centers in the thin flake of hBN. Additionally, we propose two approaches to quantify the QE of B-centers in thick flakes of hBN. Our results indicate that B-centers located in thin flakes can exhibit QEs higher than 40 %. Near-unity QEs are achievable under reasonable Purcell enhancement for emitters embedded in thick flakes of hBN, highlighting their promise for quantum photonics applications.

Keywords: B-center; hexagonal boron nitride; quantum efficiency; single photon emitter.

Figure 1:

B-Centre creation. (a) Schematic of the sample and experimental setup. The B-centers were engineered by electron beam irradiation of a 35 nm flake of hBN. Subsequently, a thick top hBN flake was transferred on the top, functioning as a dielectric medium that modifies the radiative decay rates of the emitters. (b) Optical image of hBN flake and height profile (inset) along the pink line. The scale bar corresponds to 50 μm. (c) Schematics of the experimental setup used for the optical measurements. (d) Confocal PL map showing an array of emissions generated by electron beam irradiation. The scale bar corresponds to 5 μm. (e) PL spectrum from a single B-center with a ZPL at 436 nm. The inset is the emission polarization of the B-center. (f) Second-order photon correlation measurement under pulsed laser excitation of the same emitter, confirming it is a single photon emitter.

Figure 2:

Quantum efficiency measurment. (a) Optical image of the transferred thick flake of hBN on top of the 35 nm flake that hosts the B-centers. The inset is a height profile of the thick flake along the blue line. The scale bar is 50 μm. (b) Lifetime measurement from the same emitter performed before (light blue dots) and after (purple dots) transfer of the top hBN flake with the instrument response function (IRF), pink line, performed using an excitation power of 50 μW. (c) Numerical simulation of the radiative decay rate ratio α versus the emitter depth in hBN flake. The shaded area shows the range of possible emitter depths within the 35 nm flake used in our experiments. (d) QE of the measured 7 emitters, calculated for depths ranging from 1 nm to 35 nm below the flake surface. Each curve corresponds to one emitter.

Figure 3:

Calculated QE for each emitter after transferring the top hBN flake. The interval for each emitter represents the uncertainty on its position within the flake.