[Paper Review] Room temperature "Optical Nanodiamond Hyperpolarizer": physics, design and operation

Ashok Ajoy, Raffi Nazaryan+13 more|PubMed|Nov 26, 2018

Atomic and Subatomic Physics Research4 citations

TL;DR

This paper presents a room-temperature optical nanodiamond hyperpolarizer that achieves >720-fold hyperpolarization of 13C nuclei using low-power optical and microwave irradiation, enabling high-sensitivity NMR and MRI applications. The device leverages nitrogen-vacancy centers in nanodiamonds for efficient spin polarization transfer at ambient conditions, with hyperpolarization retained for minutes and background signals suppressed by over two orders of magnitude.

ABSTRACT

Dynamic Nuclear Polarization (DNP) is a powerful suite of techniques that deliver multifold signal enhancements in nuclear magnetic resonance (NMR) and MRI. The generated athermal spin states can also be exploited for quantum sensing and as probes for many-body physics. Typical DNP methods require the use of cryogens, large magnetic fields, and high power microwave excitation, which are expensive and unwieldy. Nanodiamond particles, rich in Nitrogen-Vacancy (NV) centers, have attracted attention as alternative DNP agents because they can potentially be optically hyperpolarized at room temperature. Here, unraveling new physics underlying an optical DNP mechanism first introduced by Ajoy et al. [Sci. Adv. 4, eaar5492 (2018)], we report the realization of a miniature "optical nanodiamond hyperpolarizer," where ¹³C nuclei within the diamond particles are hyperpolarized via the NV centers. The device occupies a compact footprint and operates at room temperature. Instrumental requirements are very modest: low polarizing fields, low optical and microwave irradiation powers, and convenient frequency ranges that enable miniaturization. We obtain the best reported optical ¹³C hyperpolarization in diamond particles exceeding 720 times of the thermal 7 T value (0.86% bulk polarization), corresponding to a ten-million-fold gain in averaging time to detect them by NMR. In addition, the hyperpolarization signal can be background-suppressed by over two-orders of magnitude, retained for multiple-minute long periods at low fields, and deployed efficiently even to ¹³C enriched particles. Besides applications in quantum sensing and bright-contrast MRI imaging, this work opens possibilities for low-cost room-temperature DNP platforms that relay the ¹³C polarization to liquids in contact with the high surface-area particles.

Motivation & Objective

• To develop a low-cost, room-temperature dynamic nuclear polarization (DNP) platform using nanodiamonds with nitrogen-vacancy (NV) centers.
• To overcome the limitations of conventional DNP methods that require cryogenic cooling, high magnetic fields, and high microwave power.
• To enable practical, miniaturized NMR and MRI systems by hyperpolarizing 13C nuclei in nanodiamonds at ambient conditions.
• To achieve long-lived hyperpolarization with background suppression and efficient polarization transfer to liquid-phase molecules.

Proposed method

• The hyperpolarizer uses 100 nm diameter nanodiamond particles with ~1 ppm NV center density, optically polarized via 800 mW diode lasers in an octagonal array for uniform illumination.
• Microwave frequency sweeps (100 MHz–1 GHz) are generated using dual multiplying DACs (LTC1590) to excite NV electron spins across the ESR spectrum, enabling broad-spectrum DNP enhancement.
• A voltage-controlled oscillator (VCO) and power-combined microwave delivery system (total output <1.5 W) generate low-power microwave irradiation (Rabi frequency ~430 kHz) for efficient polarization transfer.
• A Helmholtz coil generates a weak polarizing field (14 mT) for DNP, with in-situ Hall sensors providing real-time field feedback for precise control.
• The system integrates with a 7T NMR magnet via a mechanical field-cycling stage, enabling rapid sample transfer from hyperpolarization to detection in the magnet’s fringe field.
• Background suppression is achieved by halving the number of nanodiamond particles in successive steps, allowing quantification of polarization transfer to Fmoc-Gly-OH-13C2 in DMSO.

Experimental results

Research questions

• RQ1 Can efficient room-temperature 13C hyperpolarization be achieved in nanodiamonds using only low-power optical and microwave irradiation?
• RQ2 What is the maximum achievable 13C hyperpolarization yield in nanodiamonds under ambient conditions with minimal instrumentation?
• RQ3 How long can hyperpolarization be preserved, and can background signals from the diamond particles be effectively suppressed?
• RQ4 Can the hyperpolarization be efficiently transferred to 13C nuclei in liquid-phase molecules in contact with the nanodiamonds?
• RQ5 What are the optimal microwave sweep parameters and field configurations for maximizing DNP enhancement in this system?

Key findings

• The device achieves a record 720-fold hyperpolarization of 13C nuclei in nanodiamonds at room temperature, corresponding to a 0.86% bulk polarization, which is 10 million times more effective than thermal polarization at 7T.
• Hyperpolarization is retained for over two minutes, enabling extended NMR detection and signal averaging.
• Background signals from the diamond particles are suppressed by more than two orders of magnitude through particle concentration control and signal normalization.
• The system operates with extremely low microwave power (estimated <1.5 W) and low optical power, enabling compact, low-cost, and portable NMR/MRI platforms.
• Polarization transfer to 13C in Fmoc-Gly-OH-13C2 is demonstrated, with the ratio of external to internal 13C nuclei quantified via mass and geometric considerations.
• The hyperpolarization efficiency is field-dependent, with optimal enhancement observed across a broad range of polarizing fields (10–70 mT), validated through dual-range field-sweep experiments with 11% error bars.