Quantum Emitters in Hexagonal Boron Nitride Have Spectrally Tunable Quantum Efficiency

Revealing the properties of novel solid-state quantum emitters is crucial for a number of applications, ranging from quantum optics to biology. Quantum emitters in 2D-materials have recently been discovered and defects in... [ view full abstract ]

Revealing the properties of novel solid-state quantum emitters is crucial for a number of applications, ranging from quantum optics to biology. Quantum emitters in 2D-materials have recently been discovered and defects in hexagonal boron nitride (hBN) has proven to efficiently supply single photons at room temperature with narrow emission lines and great photostability.¹

These quantum emitters have been studied at different emission wavelengths and operating temperatures. Various mechanisms to create defects or modify the emission wavelength have been investigated. Nevertheless, up to date, details of the emitters’ level structure remain elusive.

In this contribution,² we present a study of the energy level structure using photoluminescence excitation spectroscopy on single quantum emitters (Fig. 1a). We varied the excitation wavelength using an optical parametric oscillator, and monitored the emission spectrum and intensity through a fiber-coupled spectrometer and APDs in a HBT-configuration. The samples were hBN nanoflakes, which we drop casted on clean silicon wafers. Figure 1b-c show SEM of different hBN nanoflakes at 35° and 80° incidence, respectively.

As an example, Fig. 1d show the photoluminescence for excitation wavelengths 530-620 nm. Two distinct peaks are visible, at 656 and 676 nm. The saturation curves (Fig. 1e) for the two main emission lines, using three different wavelengths, indicate that the saturation intensity and the saturation count rate vary with excitation wavelength. From fits to the data we calculate the relative quantum efficiencies (Figure 1f, 656 nm – orange, 676 nm - black). Our results show that bright single photon emission with high quantum efficiency is highly dependent on matching the excitation wavelength to the individual emitter. This is a strong indication that the level scheme is complex and cannot be described by simple two or three level systems.

The excitation dependence of the emission thus allow us to gain further insight to the internal level scheme and demonstrate how to distinguish different emitters both spatially as well as in terms of their photon correlations.

[1] Tran et al. Nature Nanotech. 2015, 11, 37-41

[2] Schell et al. arXiv:1706.0830