Spatially-resolved coherence of organic molecular spins at room-temperature

TL;DR

The paper demonstrates spatially resolved, room-temperature ODMR of molecular spins in pentacene-doped p-terphenyl across thin-films, micro-crystals, and nano-crystals. By combining wide-field imaging with Hahn-echo, Rabi, and Ramsey measurements, it reveals pronounced disorder-induced variability in thin films (contrast and $T_2$ vary across the film) versus the more uniform coherence in crystalline substrates, including edge-enhanced coherence in micro-crystals. Nano-crystals preserve nearly bulk-like coherence and contrast, suggesting viable high-proximity sensing with minimal coherence loss, while enabling future nanoscopic sensing and optomechanical applications. These results inform material choice and geometry for molecular spin sensors and highlight the trade-offs between film-based and crystal-based deployment for quantum sensing at the nanoscale.

Abstract

Molecular spins are a versatile platform for quantum sensing. Not only are the spin-bearing molecules themselves widely tunable, they are also capable of being used as sensors as crystals, films and in solution. Using thin-films offers the advantages of high doping ratios and the ability to control the thickness with nanometre precision, however they also introduce disorder to the system. High proximity sensing can also be realised by using micro- and nano-crystals, however in many solid-state systems this leads to a reduction in coherence. In this paper we combine room-temperature optically detected coherent control of molecular spins and microscopy to image the coherence properties of both thin-films and micro-crystals of pentacene doped p-terphenyl. In thin-films we find large amounts of variation in both the contrast and coherence times, leading to a variability in the magnetic field sensitivity of approximately 7.6 %. Applying the technique to micro-crystals shows much lower sensitivity variability (1.3 %), and we find no evidence of coherence loss toward the edge of the crystal. Finally we perform optically-detected coherent control on a nano-crystal, showing minimal loss in coherence and contrast compared to the bulk crystal, with a coherence time of 1.09 μs and a contrast of 25 %.

Figures (4)

• Figure 1: Optically detected magnetic resonance with organic triplets. a) Schematic of the setup used for spatially resolved optically detected magnetic resonance, including a microwave source, optical excitation, emission, optics for imaging and a camera used to perform spatially resolved measurements. b) Energy structure for pentacene, with a singlet ground and excited state. The lowest lying triplet is an excited state which can be populated via intersystem crossing. The triplet states are spin-1 and have microwave addressable transitions. A field-frequency sweep of the $|T_x\rangle \leftrightarrow |T_z\rangle$ transition can be seen in c) and d) for the crystal and film respectively.
• Figure 2: Spatially resolved coherent control of a thin-film. a) Histogram of the Hahn-echo measurement contrast across the film. b) Histogram of the coherence times, $T_2$, measured across the film using an optically detected Hahn-echo. c) Histogram of the sensitivity scale factors found by calculating the impact of contrast and coherence time variation in the film. d) Spatial map of the deviation in measurement contrast. e) Spatial map of the deviation in coherence time. f) Spatial map of the deviation in sensitivity scale factor.
• Figure 3: Spatially resolved coherent control of a micro-crystal. a) A photoluminescence image of the micro-crystal measured. b) Histogram of the coherence times measured within the same micro-crystal. c) Histogram of the measurement contrasts measured within the same micro-crystal. d) Histogram of the sensitivity scale factors measured within the same micro-crystal. e)Average coherence time as a function of radial distance ($R$) from the centre of the micro-crystal with errors represented by the shaded area. The radial distance is illustrated in figure a). f. Coherence time, $T_2$, mapped for the micro-crystal. The data has been plotted as deviation from the mean to highlight variation. g) Spatially resolved measurement contrast (C) mapped as deviations for the micro-crystal. h) Sensitivity scale factor, $\eta_\mathrm{rel}$, mapped as deviations across the micro-crystal.
• Figure 4: Optically detected magnetic resonance of a bulk and nano-crystal. a) Pulse sequence used for optically detected Rabi oscillations. b) Rabi oscillations measured on the bulk crystal with contrasts up to $30\,$%. c) Rabi oscillations measured on the nanocrystal with contrasts up to $25\,$%. d) Pulse sequence used for optically detected Hahn echo. e) Hahn echo measured on the bulk crystal with $1.14 \pm 0.09\,\mu$s coherence time and up to $35\,$% contrast. c) Hahn echo measured on the nano-crystal with $1.09 \pm 0.10\,\mu$s coherence time and up to $25\,$% contrast. g) Pulse sequence used for optically detected Ramsey fringes. h) Ramsey fringes measured on the bulk crystal with $T_2^* = 408 \pm 24\,$ns and up to $30\,$% contrast. i) Ramsey fringes measured on the nano-crystal with $365 \pm 17\,$ns coherence time and up to $20\,$% contrast. All measurements were performed on the $|T_x\rangle \leftrightarrow |T_z\rangle$ transition under ambient conditions.