Optically detected magnetic resonance (ODMR) and optical T1 relaxation measurements are two important methods to assess the quality of NV centers for quantum sensing. These methods also serve as the basis of sensing methodologies enabled by NV.  Learn more on the methods here.

Quantum sensing with diamond requires that NV centers have suitable spin properties.  Assessing the spin properties of NV centers requires instrumentation beyond a standard fluorescent microscope. Characterization instrumentation at Adámas consists of several components: (i) an inverted microscope for fluorescence imaging, (ii) a light source, (iii) a photodetector, (iv) an arbitrary waveform generator (AWG), (v) a signal generator, (vi) a power amplifier, (vii) microwave antenna, (viii) an oscilloscope, (ix) and a PC equipped with remote control software.  Fig. 1 shows the instrument chain currently in use at Adámas.

 For optical detection, we utilize the Olympus IX71 microscope. The microscope offers the flexibility of mounting samples on a movable stage and enables observation of fluorescence. A pE-300 white LED (CoolLED Ltd.) provides optical illumination. This light source offers broadband illumination, but a 562/40 nm bandpass filter (Semrock) provides maximal excitation of NV centers.  The light source is controllable via computer and/or TTL trigger control for light pulsing. The microscope also provides various camera/detector positions, one position collects fluorescence light through a fiber optic into the detector. To translate light intensity into an electrical signal, we use the APD440A (ThorLabs) avalanche photodiode (APD). A MDO4104C oscilloscope (Tektronix) captures the electrical signal output of the APD. The MDO4104C has a 1 GHz bandwidth and up to 5 GS/s sampling rate. This ensures fast response and high-resolution signal capture. The oscilloscope is a key component that provides robust noise reduction via averaging and contributes to the accuracy and reliability of data acquisition in our experimental setup. Fig. 2 shows the fully assembled setup in operation.

For microwave stimulation in optically detected magnetic resonance (ODMR) measurements, we use a SRS SG384 (Stanford Research Systems Inc.) signal generator. It is known for stability and high-frequency output up to 4 GHz. It provides substantial microwave output up to 16 dBm (40 mW) that can be fed directly into the antenna or (optionally) through an amplifier before the antenna. We opt for the latter case, and utilize a MiniCircuits ZHL-2W-63-S+ to amplify the microwave output up to 2 W. The delivery of microwave energy to the sample is facilitated through a near-field “sniffing” antenna. The near-field antennas come in various sizes and types (strip line or loop) as a set. We typically use a loop antenna with a 4 mm opening. With loop antennas, the samples are placed at the peak of the microwave field (B1 field) and are easily accessible for optical illumination and detection on the microscope stage. A black box covering around the sample shields the measurement from ambient light and internal reflections.

For continuous wave ODMR (CW-ODMR), only the function generator is necessary.  More advanced pulsed measurement require the use of the AWG as a unified logical instrument that synchronizes excitation and detection devices on the same time scale. The AWG serves as both a trigger source for the oscilloscope and as a pulse generator for both the light source during T1 relaxation measurements and microwaves for pulsed ODMR experiments. A PC and software written in MATLAB provides remote operation of the entire instrument configuration.

The microscope configuration currently provides the greatest flexibility in measurement capability, but these components can be integrated into a portable benchtop size for field measurements as shown in Fig. 3.