Saturable absorption in NV-doped diamond studied by femtosecond Z-scan

TL;DR

This work addresses the nonlinear optical absorption of NV-doped diamond under ultrafast illumination and shows that saturable absorption at 1032 nm arises from a mixed defect landscape, including H2 (NVN−) centers, rather than NV centers alone. Using open-aperture Z-scan with 230 fs pulses and corroborating linear spectroscopy, the authors quantify saturable absorption parameters and employ a two-level model to interpret the data. They report for the highly doped sample an effective linear absorption of about 6.52 cm−1 and a saturation intensity near 40 GW cm−2, highlighting the crucial role of ancillary defects in governing the nonlinear response. The findings emphasize the need to account for various defect species when designing diamond-based nonlinear and quantum photonic devices, with implications for ultrafast imaging and defect-engineered quantum technologies.

Abstract

We investigate nonlinear optical absorption in diamond crystals containing high densities of nitrogen vacancy (NV) centers using open-aperture Z-scan measurements with 230 fs laser pulses at 1032 nm, within the transparency window of diamond. While high-purity electronic-grade diamond exhibits third-order nonlinear absorption, NV-doped samples display pronounced saturable absorption that strengthens with increasing defect concentration. Linear transmission spectroscopy reveals that, in addition to NV centers, the crystals host significant populations of H2 (NVN-) defect complexes whose absorption band partially overlaps the excitation wavelength. By correlating spectroscopic data with nonlinear measurements and modeling the response using an effective two-level system, we show that the observed saturation cannot be attributed solely to NV centers but arises from the combined contribution of NV-related and H2 defects. For the highly doped sample, we determine an effective linear absorption coefficient of alpha0 = 6.52 cm-1 and a saturation intensity of Is = 40.0 GW/cm2. These findings highlight the critical role of the complex defect landscape in governing the nonlinear optical response of NV-doped diamond and underscore the necessity of accounting for ancillary defect species in the design of diamond-based nonlinear and quantum photonic devices.

Figures (5)

• Figure 1: Relative absorbance spectra of the HCNV crystal measured with MCNV crystal in the reference channel of the spectrometer with marked zero-phonon lines (ZPL) of NV, H2 and H3 point defects. In the inset ZPL of $\rm NVN^-$ center at 986 nm. The feature near 840 nm is an artefact caused by spectrometer malfunction in this spectral region.
• Figure 2: Schematic of the open-aperture (OA) Z-scan setup used to study nonlinear absorption.
• Figure 3: Normalized transmittance measured by the open-aperture (OA) Z-scan method for pure diamond, EGSC (upper row), and low concentration NV-doped diamond, MCNV (lower row), at various laser beam intensities $I_0$. The solid lines represent the fits of Eq. (\ref{['eq:ZScanOA_NAbs']}) to the experimental data. The upper-right figure shows the nonlinearity parameter $\Delta \Psi_0$ as a function of the laser beam intensity $I_0$. The determined nonlinear absorption coefficient at $\lambda=1032$ nm is $\beta=4.9(3) \times 10^{-15}$ m/W.
• Figure 4: (a) -- Normalized transmittance measured by the open-aperture (OA) Z-scan method for diamond with a high concentration of NV color centers (HCNV), for various laser beam intensities $I_0$. The solid lines represent the fit of Eq. (\ref{['eq:ZScanOA_SatAb']}) to the experimental data. (b, c) –- dependence of the linear absorption coefficient $\alpha_0$ and the saturation intensity $I_s$, respectively, obtained from the fit, on the intensity $I_0$.
• Figure 5: (a) Schematic representation of point-defect centers modeled as effective two-level systems with resonance frequency $\omega_{\mathrm{ba}}$, interacting with a nonresonant laser field of photon energy $\hbar\omega$. The parameters $T_1$ and $T_2$ denote the effective population relaxation time of the excited state $b$ and the coherence decay (dephasing) time of the induced dipole moment, respectively. (b) Absorbance spectra measured using the spectrophotometer (solid black) and reconstructed using Eq. (\ref{['eq: Isat od Delta']}) (solid red), with the coherence decay time $T_2$ inferred from combined closed-aperture (CA) and open-aperture (OA) Z-scan measurements. The vertical black dashed line indicates the excitation laser frequency, while the colored dashed lines denote the resonance frequencies of the considered defect centers.