Room-temperature solid-state maser

Abstract

The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser’s birth1,2. The maser’s relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic3 and free-electron4 masers require vacuum chambers and pumping; and solid-state masers5, although they excel as low-noise amplifiers6 and are occasionally incorporated in ultrastable oscillators7,8, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers9 incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser6, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser’s pumping mechanism exploits spin-selective molecular intersystem crossing10 into pentacene’s triplet ground state11,12. When configured as an oscillator, the solid-state maser’s measured output power of around −10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser3, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature.

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Figure 1: Maser pumping scheme (Jablonski diagram).
Figure 2: Anatomy of the maser.
Figure 3: Maser response in the time domain.
Figure 4: Frequency response of maser action.

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Acknowledgements

M.O. thanks J. Sherwood, R. Ferguson and C. Langham for advice on the growth of molecular crystals, for help in cutting and polishing the crystalline p-terphenyl boule and for designing the metal components of the microwave resonator, respectively. M.O. and J.D.B. thank C. Kay for discussions and guidance on pulsed electron paramagnetic resonance spectroscopy. This work was supported by the NMS Pathfinder Metrology Programme and by the Engineering and Physical Sciences Research Council.

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J.D.B. performed initial pulsed electron paramagnetic resonance experiments with M.O., and analysed the resultant data. Both J.D.B. and M.O. carried out independent electromagnetic modelling of the TE01δ-mode microwave resonator using different software. M.O. grew the pentacene:p-terphenyl crystal, designed and constructed the microwave circuitry and optical systems, performed the final experiments and analysed the resultant data. N.M.A. initiated the original work on high-Q cavities. N.M.A. and M.O. wrote the paper.

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Correspondence to Mark Oxborrow or Neil M. Alford.

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The authors declare no competing financial interests.

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Oxborrow, M., Breeze, J. & Alford, N. Room-temperature solid-state maser. Nature 488, 353–356 (2012). https://doi.org/10.1038/nature11339

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