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Optical and Quantum Sensing and Precision Metrology II

Room-temperature microwave MASERs with LED pumping: a step towards practical realization

Juna Sathian, Sophia Long, Lisa Lopez, Bethan Ford, François Balembois, Wern Ng, Daan M. Arroo, Neil McN. Alford, Hamdi Torun

Published Mar 13, 2024• 0 citations
Publisher

Abstract

Room-temperature MASERs (Microwave Amplification by Stimulated Emission of Radiation) are coherent sources that exploit stimulated emission to amplify electromagnetic waves at microwave frequencies. Here we report the first demonstration of an LED-pumped maser based on photo-excited triplet states of pentacene in p-terphenyl. The LED light is brightness enhanced and guided using a Cerium-doped Yttrium Aluminium Garnet luminescent crystal that uses light recycling to reach record luminance and power levels. We observe stable maser emission at 1.45 GHz using LED pumping at a much lower pump threshold power. This new maser is cost-effective, safe, and environmentally friendly, and hence ensures a wide variety of potential uses, including more sensitive magnetic resonance body scanners, quantum optical coherence tomography, advanced quantum computer components, portable atomic clocks, and better radio astronomy devices for deep space exploration.

DOI
10.1117/12.3003423

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Room-temperature MASERs (Microwave Amplification by Stimulated Emission of Radiation) are coherent sources that exploit stimulated emission to amplify electromagnetic waves at microwave frequencies. Here we report the first demonstration of an LED-pumped maser based on photo-excited triplet states of pentacene in p-terphenyl. The LED light is brightness enhanced and guided using a Cerium-doped Yttrium Aluminium Garnet luminescent crystal that uses light recycling to reach record luminance and power levels. We observe stable maser emission at 1.45 GHz using LED pumping at a much lower pump threshold power. This new maser is cost-effective, safe, and environmentally friendly, and hence ensures a wide variety of potential uses, including more sensitive magnetic resonance body scanners, quantum optical coherence tomography, advanced quantum computer components, portable atomic clocks, and better radio astronomy devices for deep space exploration.
Through their ability to achieve cryogenic levels of noise performance while operating at room temperature, optically-pumped, solid-state (OPSS) masers show great promise as quantum sensors, oscillators, and amplifiers. We here demonstrate maser oscillation in a microwave cavity containing a crystal of pentacene-doped para-terphenyl (ptc:ptp) pumped by a single, chip-scale LED. Here, unlike previous work, the size of the pump source does not dominate the size of the maser system as a whole. This miniaturization is achieved through invasive optical pumping in the form of a waveguide, the tip of which is embedded into the maser crystal. Using experimental measurements combined with microwave and optical simulations, we find that our approach offers at least a factor-of-2 enhancement in cooperativity over end-on optical excitation. We use our simulations to define a figure of merit for maser pumping efficiency, and conclude that there remains significant headroom to improve the performance of ptc:ptp masers through improved optical design.
We present in this work a quasi-continuous-wave (cw) pentacene maser operating at 1.45 GHz in the Earth's magnetic field at room temperature with a duration of approximately 4 ms and an output power of up to $\ensuremath{-}25$ dBm. The maser is optically pumped by a cerium-doped yttrium-aluminum-garnet (YAG) (Ce:YAG\mathrm{Ce}:\mathrm{YAG}Ce:YAG) luminescent concentrator (LC), the wedge-shaped output of which is embedded inside a 0.1% pentacene-doped para-terphenyl (Pc:Ptp) crystal. The pumped crystal is located inside a ring of strontium titanate (STO) that supports a ${\mathrm{TE}}_{01\ensuremath{\delta}}$ mode of high magnetic Purcell factor. Combined with simulations, our results indicate that cw operation of pentacene masers at room temperature is perfectly feasible so long as excessive heating of the crystal is avoided.
Room-temperature solid-state masers offer exceptional frequency selectivity and ultra-low noise for weak-signal detection. However, their reliance on bulky metallic resonators has significantly hindered integration, miniaturization, and extension to lower frequencies. Here, we demonstrate the first on-chip radio-frequency maser operating at room temperature, exploiting optically pumped triplet states of pentacene. The device produces stimulated emission at 106.62 MHz and enables ultra-sensitive microwave magnetic-field detection with a sensitivity of (∼10 fT/Hz\sim 10\,\rm{fT/\sqrt{Hz}}∼10fT/Hz​), functioning simultaneously as a local oscillator and a sensor. By actively controlling microwave dissipation, we achieve efficient regulation of the maser output, revealing a key mechanism for tuning emission in open cavity-free systems. This work extends pentacene-based masers into the radio-frequency regime and establishes a highly integrated on-chip architecture for room-temperature masers, offering a new pathway toward portable quantum devices.
We experimentally demonstrate the temporary removal of thermal photons from a microwave mode at 1.45 GHz through its interaction with the spin-polarized triplet states of photo-excited pentacene molecules doped within a p-terphenyl crystal at room temperature. The crystal functions electromagnetically as a narrowband cryogenic load, removing photons from the otherwise room-temperature mode via stimulated absorption. The noise temperature of the microwave mode dropped to 50_{-32}^{+18} K (as directly inferred by noise-power measurements), while the metal walls of the cavity enclosing the mode remained at room temperature. Simulations based on the same system's behavior as a maser (which could be characterized more accurately) indicate the possibility of the mode's temperature sinking to ∼10 K (corresponding to ∼140 microwave photons). These observations, when combined with engineering improvements to deepen the cooling, identify the system as a narrowband yet extremely convenient platform-free of cryogenics, vacuum chambers, and strong magnets-for realizing low-noise detectors, quantum memory, and quantum-enhanced machines (such as heat engines) based on strong spin-photon coupling and entanglement at microwave frequencies.
Though the performance of room-temperature masers has improved over the last decade, relatively little attention has been paid to the optics used to pump the maser's gain medium. In this work, we investigate a novel multi-blade optical ``injector'' that permits more effective and more scaleable pumping. The reported work encompasses an interdisciplinary mix of conceptualization, simulation, crystal growth, fabrication, and microwave engineering. Our gain medium is pentacene dissolved as a solid solution with para-terphenyl (Pc:PTP) molecular crystal. We accurately determine this pentacene's molecular absorption cross-section as a function of wavelength. Ray-tracing is then used to assess how different designs of waveguide inject light into the Pc:PTP crystal. A multi-blade injector made of high-refractive-index glass (namely Ohara S-TIH6) is predicted to pump it more completely and uniformly than previous designs. Upon hand-fabricating such an injector and Bridgman-growing a crystal of 0.1% Pc:PTP over it, an experimental maser oscillator using this combined injector-crystal assembly is demonstrated. The performance and scaleability of multiblade injection vis-a-vis alternative strategies is analyzed.
Coherent sources exploiting the stimulated emission of non-equilibrium quantum systems, i.e. gain media, have proven indispensable for advancing fundamental research and engineering. The operating electromagnetic bands of such coherent sources have been continuously enriched for increasing demands.Nevertheless, for a single bench top coherent source, simultaneous generation of radiation in multiple bands, especially when the bands are widely separated, present formidable challenges with a single gain medium. Here, we propose a mechanism of simultaneously realizing the stimulated emission of radiation in the visible and microwave bands, i.e. lasing and masing actions, at ambient conditions by utilizing photoexcited singlet and triplet states of the pentacene molecules that are doped in p-terphenyl. The possibility is validated by the observed amplified spontaneous emission (ASE) at 645 nm with a narrow linewidth around 1 nm from the pentacene-doped p-terphenyl crystal used for masing at 1.45 GHz and consolidated by a 20 fold lower threshold of ASE compared to the reported masing threshold. The overall threshold of the pentacene-based multiband coherent source can be optimized by appropriate alignment of the pump-light polarization with the pentacene's transition dipole moment. Our work not only shows a great promise on immediate realization of multiband coherent sources but also establishes an intriguing solid-state platform for fundamental research of quantum optics in multiple frequency domains.