Overtone Rabi oscillation of optically polarized triplet electron spins and nuclear hyperpolarization in powder

TL;DR

This work demonstrates, for the first time, coherent overtone Rabi oscillations of optically polarized triplet electron spins in powder at room temperature and uses the overtone transition to achieve nuclear hyperpolarization via ISE. The authors develop a comprehensive theoretical treatment of overtone transitions in axially symmetric ZFS systems, deriving the resonance condition, nutation, and lineshape as functions of orientation. They verify the theory experimentally in pentacene-doped p-terphenyl and NV− centers in microdiamonds, observing overtone EPR with narrow linewidths and quantifying overtone Rabi frequencies. Leveraging overtone DNP, they report a 2600× enhancement of 1H NMR polarization at 0.2 T, reaching 0.183% polarization, and discuss how higher microwave power and systems with larger ε could further improve performance.

Abstract

We report coherent overtone Rabi oscillations of optically-polarized triplet electron spins and nuclear hyperpolarization in powder samples at room temperature. The strong dependence of the single-quantum resonance on the orientation of the zero-field splitting (ZFS) interaction is overcome by coherently driving the significantly narrower overtone transition. Analytical formulas for the overtone lineshape and nutation functions for axially symmetric ZFS interactions are derived. Overtone Rabi oscillations are observed in pentacene-doped \textit{p}-terphenyl and NV$^-$ centers in microdiamonds. For the former, overtone triplet dynamic nuclear polarization using the integrated solid effect leads to $^1$H spin polarization of $0.183\pm0.005$\% at a magnetic field of 0.2~T. The $^1$H NMR polarization is enhanced by a factor of 2600 with respect to thermal equilibrium and reaches a large portion of the randomly oriented microcrystals.

Figures (10)

• Figure 1: Magnetic-field dependence of the energies of the three states $|1 \rangle,|0 \rangle,| -1 \rangle$ of an electron spin in the triplet state. Vertical arrows with the same length indicate single quantum (dotted line), DQ (dashed line), and half-field or overtone (solid line) transition at a given, common microwave frequency.
• Figure 2: (a) EPR echo pulse sequence used in the measurement. $\tau$ is the echo time and set to 1 $\mu$s. For the Rabi measurement, the pulse length of $\pi/2$ is swept, and the microwave frequency is 11.6 GHz. (b) Powder EPR spectra of pentacene doped $p$-terphenyl. The spectrum from 0.22 T to 0.5 T (inset) was obtained with accumulation over 100 times, and the pulse lengths for $\pi/2$ and $\pi$ were adjusted to maximize the single-quantum EPR signal. The spectrum ranging from 0.2 T to 0.22 T was obtained with 5,000 scans with the pulse lengths set to maximize the overtone EPR signal. (c) Experimental Rabi oscillations (spin echo amplitude as a function of the width of the first microwave pulse) measured at 0.39 T (dotted line in the inset figure in (b)) corresponding to the single quantum transition (black circles) and at 0.207 T (dashed line in (b)) for the overtone transition (red circles). The oscillation signal for single quantum transition was obtained with accumulation over 100 times, whereas that for overtone transition was obtained with accumulation over 5,000 times. Solid lines are fitted curves of a decaying sinusoidal function ($a\exp(-\Gamma t)\sin(\omega_{\mathrm{nut}} t)$). The signal intensity obtained for the overtone transition is scaled by a factor of 100. The $\pi$ pulse length in these measurements were 175 ns for the single quantum transition and 585 ns for the overtone transition.
• Figure 3: (a) An EPR spectrum of NV$^-$ centers in microdiamond powder measured at 11.6 GHz. In magnetic fields ranging from 0.22 T to 0.5 T data were obtained with the lengths of the $\pi/2$ and the $\pi$ pulses set to maximize the single-quantum EPR signal with 100 times signal averaging (inset). The spectrum ranging from 0.19 T to 0.22 T was obtained with 1,000 times signal averaging and the pulse lengths adjusted to maximize the overtone EPR signal. (b) Experimental Rabi oscillations for single-quantum transition (black circles) at 0.361 T (dotted line in the inset figure in (b)) and overtone transition with $\bm{B}_{1} \perp(\parallel) \bm{B}_{0}$ (red(blue) circles) at 0.196 T (dashed line in (b)). The oscillation signal for single quantum transition at 0.361 T was obtained with accumulation over 100 times, whereas that for overtone transitions at 0.196 T was obtained with accumulation over 1,000 times. The solid lines are fitted curves with a decaying sinusoidal function. The $\pi$ pulse length in these measurement were 146 ns for the single quantum transition, 391 ns for the overtone transition with $\bm{B}_{1} \perp \bm{B}_{0}$, and 488 ns for overtone transition with $\bm{B}_{1} \parallel \bm{B}_{0}$.
• Figure 4: Orientational dependence of polarization $P_{1,-1}$ between the overtone states for pentacene doped in $p$-terphenyl ($B=0.207$ T) and NV$^-$ center ($B=0.196$ T).
• Figure 5: (a) Pulse sequence for the ISE followed by $^1$H NMR detection using MSE sequence. A laser pulse is used for photoexcitation of the triplet electron spins. Then, a microwave pulse is applied together with a magnetic-field sweep. The ISE pulse sequence is repeated before the enhanced $^1$H magnetization is detected by a MSE sequence. The definitions of the symbols used in the figure are as follows. $\omega_{\mathrm{nut}}$: microwave Rabi nutation frequency, $t_{\mathrm{MW}}$: microwave-pulse width, $B_{\mathrm{sweep}}$: field-sweep width, $R$: ISE repetition rate, and N: ISE repetition number. (b-d) Optimization of the ISE pulse sequence for pentacene-doped $p$-terphenyl (PHPT) through adjustment of (b) the field-sweep width $B_{\mathrm{sweep}}$ sweep, (c) the microwave intensity $\omega_{\mathrm{nut}}$, and (d) the microwave duration $t_{\mathrm{MW}}$. The values of the fixed parameters are indicated in each graph.