An ultrafast diamond nonlinear photonic sensor

Technical Summary: An Ultrafast Diamond Nonlinear Photonic Sensor

Problem Statement
The development of next-generation nanometer-scale quantum devices requires sensing technologies capable of measuring electric and magnetic fields, as well as temperature, with high spatio-temporal resolution. While color centers in diamond, particularly nitrogen-vacancy (NV) centers, are established for quantum sensing via optically detected magnetic resonance (ODMR), conventional ODMR techniques are limited by nanosecond time resolution. Conversely, existing electro-optic (EO) sensors based on the Pockels effect offer sub-picosecond time resolution but are constrained by the diffraction limit of light, preventing nanometer-scale spatial resolution. Furthermore, pure diamond lacks a second-order nonlinear susceptibility ($\chi^{(2)}$) due to its spatial inversion symmetry, rendering it inert to the Pockels effect unless defects are introduced. There remains a critical need for a sensing technique that simultaneously achieves nanometer spatial resolution and ultrafast (femtosecond) time resolution without requiring metallic contacts or microwave excitation.

Methodology
The authors developed an ultrafast diamond nonlinear photonic sensor by integrating scanning probe microscopy (SPM) with an optical pump-probe technique. The core of the system is a diamond nanotip probe containing NV centers, fabricated via laser cutting and Ga+ ion focused ion beam (FIB) milling on a high-purity (100)-oriented CVD diamond. NV centers were introduced at a shallow depth (<40 nm) via 14N+ ion implantation (30 keV) followed by annealing, creating a mixture of negatively charged (NV–) and neutral (NV0) states. The presence of NV defects breaks the spatial inversion symmetry of the diamond lattice, enabling a non-zero second-order nonlinear susceptibility ($\chi^{(2)}$) and allowing the Pockels effect to occur.

The experimental setup utilizes a 10-fs near-infrared laser pulse (795 nm, 1.56 eV) as both the pump and probe. The pump pulse excites the sample, while the probe pulse interacts with the diamond NV tip. The system employs a reflective (Schwarzschild) objective lens to minimize dispersion and a self-sensing cantilever to detect atomic forces, allowing the diamond tip to contact the sample surface. The sensor operates by measuring the change in anisotropic reflectivity of the diamond tip, which is proportional to the local surface electric field via the Pockels effect. This configuration allows for the monitoring of local electric field dynamics as a function of time delay ($t$).

Key Contributions and Results
The study demonstrates the successful application of this diamond NV probe for electro-optic sampling with spatio-temporal resolutions of $\le$500 nm and $\le$100 fs, respectively.

1. Validation on Semiconductors: The sensor was first tested on an n-type GaAs wafer. Macroscopic measurements confirmed a positive EO response with a picosecond relaxation time, consistent with upward band bending and surface field screening by photogenerated carriers. When using the NV tip, the signal amplitude decreased (consistent with a probing area of $\approx$800 nm), but a positive EO signal with a $\approx$0.5 ps relaxation time was still observed, confirming the probe's ability to detect surface electric fields.
2. Nanoscale Sensing of 2D Materials: The technique was applied to a transition metal dichalcogenide (WSe2) sample containing both monolayer (1 ML) and bulk regions.
   • Monolayer WSe2: The probe detected a negative EO signal, indicating p-type character due to surface oxidation, with a single exponential relaxation time of $\tau \approx 0.2$ ps.
   • Bulk WSe2: The probe revealed complex dynamics distinct from macroscopic measurements. Initially, a negative signal (p-type surface) was observed, which transitioned to a positive signal (n-type/intrinsic bulk behavior) at $t \approx 1$ ps. The dynamics were characterized by a double exponential decay: an initial fast component ($\tau_+ \approx 0.3$ ps) attributed to intervalley scattering (K to $\Lambda$ valleys) and carrier diffusion out of the surface region, and a slower component ($\tau_- \approx 2.2$ ps) attributed to carrier trapping by surface defect states.
3. Mechanism Analysis: The authors analyzed the carrier density evolution using the Boltzmann transport equation. They concluded that the observed ultrafast dynamics reflect the interplay between intraband/intervalley scattering and trapping at surface defects, which are sensitive to the local electric field.

Significance and Claims
The paper claims that this technique breaks the spatial limit of conventional pump-probe techniques while maintaining ultrafast time resolution, offering a new horizon for sensing advanced nano-materials. The authors state that the method provides an excellent information limit with respect to time ($\le$100 fs) and space ($\le$500 nm), enabling the observation of carrier screening dynamics at the surface of bulk transition metal dichalcogenides that are inaccessible to conventional macroscopic measurements.

The authors modestly note that while the current sensitivity is comparable to conventional NV-based electrometers, future improvements in sensitivity (e.g., reducing the number of NV centers to a single center) could potentially achieve spatial resolutions down to $\approx$10 nm. They further suggest that because the NV probe is sensitive to spins and temperature, this approach could eventually provide additional degrees of freedom for detecting magnetic and thermal fields. The work establishes a foundation for using diamond nonlinear photonic sensors in materials science and nanotechnology, particularly for investigating power device materials, topological insulators, and layered 2D materials.