Saturable absorption in diamond nanophotonics

Abstract

Diamond is a leading quantum photonics platform due to its ability to host qubits based on crystal defects such as nitrogen vacancy centres. Fabricating nanophotonic devices from defect-rich diamond, which is central to many quantum sensing technologies, promises to enable enhanced performance and integrability of diamond quantum sensors. Here we demonstrate microdisk cavities fabricated from defect-rich diamond that support optical modes with high quality factor ($Q\sim7\times10^4$ at 1042 nm), and show that they exhibit saturable absorption. Power dependent spectroscopy measurements spanning 979 nm to 1604 nm are used to extract wavelength-dependent absorption coefficients and saturation intensities, which indicate that a hydrogen-related defect is a likely origin of the observed absorption. At 1047 nm, we measure a saturation intensity of 3.3 (1) MW/cm$^2$ and an absorption coefficient of 0.537 (4) cm$^{-1}$. These results provide insight into defect-mediated optical loss in diamond nanophotonics and suggest strategies to harness defect-induced nonlinearities in future diamond photonic devices.

Figures (10)

• Figure 1: Loss in a diamond cavity.(a) A diamond crystal inside a Fabry-Perot cavity. Photons are side-coupled into the cavity at rate $\kappa_{\text{ex}}$, whose linewidth is determined by the total optical cavity loss, $\kappa = \kappa_{\text{ex}} + \kappa_{\text{c}}$. Various mechanisms contribute to the cavity loss rate $\kappa_{\text{c}}$, and in highly doped samples absorption loss from defects is significant. (b) The diamond crystal hosts a variety of different point defects, several of which cause absorption loss ($\kappa_{\text{a}}$) including hydrogen-based defects (H), and nitrogen-based defects like the NV centre and the $\text{N}_2\text{V}$ centre. (c) A scanning electron micrograph reveals the diamond microdisk used to study the absorption dynamics. A fibre-taper waveguide is used to couple light into the microdisk, and changes in the transmission spectrum are used to characterize saturable absorption by the whispering-gallery mode (d). Fitting the transmission spectrum of the mode at 1042.35 nm, we find quality factors of ${Q}_{\text{s}}=71.6\,(6)\times10^3$ and ${Q}_{\text{a}}=76.0\,(5)\times10^3$ for the two dips corresponding to the symmetric and anti-symmetric standing wave modes of the WGM, respectively.
• Figure 2: Fundamental WGMs in a diamond microdisk.(a) A wideband transmission spectrum reveals an array of modes, and we highlight the fundamental TM (TE) modes in red (blue). Eigenmodes of the resonator are simulated using a finite element solver, and insets show the simulated TM and TE mode field distributions (azimuthal mode number $m=18$) inside the microdisk. (b) The simulated eigenfrequencies, marked using 'x' scatter points, align with the measured ones, marked using the dot scatter points. The azimuthal field distribution of two of the modes are shown as insets to (b). Note that the two highest energy TM eigenmode transmission spectra are not shown in (a).
• Figure 3: Power-dependent laser transmission scans in a microdisk.(a) Photons coupled into and out of the microdisk at rate $\kappa_{\text{ex}}$, and the internal cavity loss is power-dependent, $\kappa_{\text{c}}(P)$. (b) By varying the power injected into the WGM cavity, we measure power-dependent transmission spectra for two different modes, each with a different eigenfrequency. The WGM with the higher eigenfrequency (1047 nm) exhibits nonlinear lineshape dependence on $P$, whereas the low eigenfrequency mode shows no power dependence. The increase in transmission contrast and reduction in linewidth of the Lorentzian signifies a reduction in loss.
• Figure 4: Intensity-dependent loss in a microdisk. Internal cavity loss rate as a function of intracavity photon number for WGMs between 979 nm and 1604 nm. Three WGMs at wavelengths 979 nm, 1047 nm, and 1267 nm exhibit nonlinear dependence of loss rate on optical intensity, which we attribute to a saturable absorber. The right plot highlights the intensity-dependent internal cavity loss rate of the 1047 nm and 1267 nm doublet modes, to which we fit a saturable absorber model to extract the change in internal cavity loss and saturation intensity. In the right figure, the $\mathcal{N}_{\text{cav}}$ axis pertains to both wavelengths; however, the conversion between $\mathcal{N}_{\text{cav}}$ and $\langle I\rangle$ is wavelength-dependent, so the $\langle I\rangle$ axis pertains only to the 1047 nm mode. The scatter points are the measured data with fit lines corresponding to \ref{['eq:fitSatAbs']}. See the main text for more details.
• Figure 5: Saturable absorption a diamond microdisk.(a) Saturation of defect absorbers. At low optical intensities (left), most defects are in the ground state. Higher intensities (middle, right) cause the defect population to invert, reducing material absorption loss. (b) Extracted wavelength-dependent linear absorption coefficients and saturation intensities of the saturable absorbers. The zero-phonon lines for the two candidate defects are shown using two vertical blue lines.