Solid-state Maser Research Articles

Tailoring Coherent Microwave Emission from a Solid-State Hybrid System for Room-Temperature Microwave Quantum Electronics.

Quantum electronics operating in the microwave domain are burgeoning and becoming essential building blocks of quantum computers, sensors, and communication devices. However, the field of microwave quantum electronics has long been dominated by the need for cryogenic conditions to maintain delicate quantum characteristics. Here, a solid-state hybrid system, constituted by a photo-excited pentacene triplet spin ensemble coupled to a dielectric resonator, is reported for the first time capable of both coherent microwave quantum amplification and oscillation at X band via the masing process at room temperature. By incorporating external driving and active dissipation control into the hybrid system, efficient tuning of the maser emission characteristics at ≈9.4GHz is achieved, which is key to optimizing the performance of the maser device. The work not only pushes the boundaries of the operating frequency and functionality of the existing pentacene masers but also demonstrates a universal route for controlling the masing process at room temperature, highlighting opportunities for optimizing emerging solid-state masers for quantum information processing and communication.

N-Heteroacenes as an Organic Gain Medium for Room-Temperature Masers.

The development of future quantum devices such as the maser, i.e., the microwave analog of the laser, could be well-served by the exploration of chemically tunable organic materials. Current iterations of room-temperature organic solid-state masers are composed of an inert host material that is doped with a spin-active molecule. In this work, we systematically modulated the structure of three nitrogen-substituted tetracene derivatives to augment their photoexcited spin dynamics and then evaluated their potential as novel maser gain media by optical, computational, and electronic paramagnetic resonance (EPR) spectroscopy. To facilitate these investigations, we adopted an organic glass former, 1,3,5-tri(1-naphthyl)benzene to act as a universal host. These chemical modifications impacted the rates of intersystem crossing, triplet spin polarization, triplet decay, and spin-lattice relaxation, leading to significant consequences on the conditions required to surpass the maser threshold.

Enhanced quantum sensing with room-temperature solid-state masers.

Quantum sensing with solid-state electron spin systems finds broad applications in diverse areas ranging from material and biomedical sciences to fundamental physics. Exploiting collective behavior of noninteracting spins holds the promise of pushing the detection limit to even lower levels, while to date, those levels are scarcely reached because of the broadened linewidth and inefficient readout of solid-state spin ensembles. Here, we experimentally demonstrate that such drawbacks can be overcome by a reborn maser technology at room temperature in the solid state. Owing to maser action, we observe a fourfold reduction in the electron paramagnetic resonance linewidth of an inhomogeneously broadened molecular spin ensemble, which is narrower than the same measured from single spins at cryogenic temperatures. The maser-based readout applied to near zero-field magnetometry showcases the measurement signal-to-noise ratio of 133 for single shots. This technique would be an important addition to the toolbox for boosting the sensitivity of solid-state ensemble spin sensors.

Perspective on room-temperature solid-state masers

The first solid-state masers to operate at room-temperature and ambient air-pressure were recently demonstrated using optically pumped spin-triplet states as the gain medium. In this Perspective, we briefly review the previous state-of-the-art in cryogenic solid-state masers and then discuss the development of the room-temperature solid-state maser: from the organic pentacene pulsed maser to the diamond nitrogen-vacancy continuous-wave maser. We characterize the operation of these masers as coherent microwave sources and ultra-low noise amplifiers before outlining how they can be adapted to act as model systems in which to explore room-temperature cavity quantum electrodynamics. After discussing challenges facing current embodiments of the room-temperature solid-state maser, we explore how they might be addressed or by-passed altogether through the development of alternative materials and masing mechanisms. Finally, we speculate on how the advent of masers that can operate in ambient conditions might lead to novel applications in metrology and quantum technologies.

Performance analysis of diamond-based masers

Masers, the microwave analog of lasers, are an important class of devices that can be used to generate highly coherent and stable microwave signals as well as quantum-limited amplification of microwave photons. Recently, the interest in such devices has increased, probably owing to experiments demonstrating the successful operation at room temperature of solid-state maser employing defects in diamond. These defects, called nitrogen vacancy (NV) centers, are comprised of a negatively charged substitutional nitrogen that replaces one of the diamond crystal lattice’s carbons and a vacancy adjacent to the excluded carbon atom. The performance of diamond-based masers, in terms of gain, noise, bandwidth, frequency of operation, and saturation, greatly depends upon the specific diamond crystal composition and its geometry in the respective microwave device. The currently available data on these issues are scarce and sometimes conflicting. It is, therefore, important to provide additional experimental data, complemented by theoretical analysis, to further optimize the required diamond material and thus enhance the capabilities of diamond-based maser technology. The latter is currently limited to operate only as an oscillator at very small bandwidths, with low saturation power. Here, we provide experimental results on a set of important parameters affecting diamond maser operation, such as the population and linewidth of the electron spin energy levels of the NVs under light illumination, as well as their relaxation times for several different diamond material compositions. These results are then used to point out which diamond material compositions and crystal geometry may be best suitable for maser-type applications.

Physics of the sapphire whispering-gallery-mode solid-state MASER oscillator

The Cryogenic Sapphire Oscillator (CSO) is currently the best available technology that can provide a relative frequency stability better than 10−15 with integration times between 1 s and 10,000 s. But, the CSO remains a complex instrument that requires multiple loop controls to achieve the best frequency stability. The possibility to use the sapphire resonator in a self-sustained MASER oscillator presents an elegant alternative to the CSO. Here, sustaining the amplification is achieved through the interaction between a high-Q factor whispering gallery mode and the paramagnetic Fe3+ ions, which are present in small concentration in the sapphire crystal. The Fe3+ ion exhibits three energy states enabling to realize a self-sustaining solid-state maser. Although, this principle has been already experimentally demonstrated few years ago, its development as a truly usable ultra-stable source has not yet been completed, mainly due to the lack of control of the complex physical phenomena involved. This paper complements the previous theoretical work based on the rate equations model. Here we derive the full quantum equations describing the evolution of the Fe3+ ions inside the sapphire lattice and submitted to a pump and a maser signal. The influence of the ions concentration and spin-spin relaxation time will be pointed out.

Solid-state pulsed microwave emitter based on Rydberg excitons.

We theoretically demonstrate how the cuprous oxide Cu2O could be used as a gain medium in a solid-state maser. By taking advantage of radiative microwave transitions between highly excited Rydberg states, one can achieve population inversion and masing in a wide range of wavelengths. In the pulsed emission regime, the considered excitonic system is characterized by intricate and rich dynamics, which are investigated numerically, taking into account several key features of the medium, such as strong Stark shift of energy levels and the presence of the Rydberg blockade effect.

Dynamically Steered Maser Action of Rydberg Excitons in Cu2O

The Rydberg excitons in Cu2O are a promising candidate for realization of solid state maser. The highly excited states, unique to this crystal, provide the necessary energy level transitions for emission of microwaves. The achievement of population inversion is facilitated by exceptionally long lifetimes of excitons and their strong coupling to external field. Moreover, due to the strong Stark shift of excitonic energy levels, one can tune the emission wavelength on demand.

Proposal of tunable Rydberg exciton maser.

We propose a continuous-mode solid-state maser based on Cu2O, where ensemble of highly excited Rydberg exciton states serves as a gain medium. We show that the system is highly tunable with an external electric field, allowing for a wide range of emission frequencies within terahertz range. Numerical simulations of the system dynamics are performed to optimize the conditions for efficient masing and estimate the emission power.

Continuous-wave room-temperature diamond maser.

The maser-the microwave progenitor of the optical laser-has been confined to relative obscurity owing to its reliance on cryogenic refrigeration and high-vacuum systems. Despite this, it has found application in deep-space communications and radio astronomy owing to its unparalleled performance as a low-noise amplifier and oscillator. The recent demonstration of a room-temperature solid-state maser that utilizes polarized electron populations within the triplet states of photo-excited pentacene molecules in a p-terphenyl host paves the way for a new class of maser. However, p-terphenyl has poor thermal and mechanical properties, and the decay rates of the triplet sublevel of pentacene mean that only pulsed maser operation has been observed in this system. Alternative materials are therefore required to achieve continuous emission: inorganic materials that contain spin defects, such as diamond and silicon carbide, have been proposed. Here we report a continuous-wave room-temperature maser oscillator using optically pumped nitrogen-vacancy defect centres in diamond. This demonstration highlights the potential of room-temperature solid-state masers for use in a new generation of microwave devices that could find application in medicine, security, sensing and quantum technologies.

Whispering gallery mode resonators in microwave physics and technologies

We review the main results of the development of whispering gallery mode (WGM) resonators and their unique applications due to their quasi-optical functionality. Several types of advanced WGM resonators are proposed by the authors. The theoretical results are described for the resonators with an analytical solution of the electromagnetic problems. Special emphasis is given to the interaction of moving charged particles and waves of cylindrical resonators. Important aspects are described concerning the developed sapphire resonators, for which an exact solution can only be found by using specially designed computer program products. A separate section of the paper is devoted to application aspects of the WGM resonators. In particular, it describes advanced solutions for overcoming the problems of measuring the small microwave (MW) surface impedance of unconventional superconductors in the form of large-area thin films and small samples under study. In addition, a demonstration of accurate complex permittivity measurements of small volumes of lossy liquids is provided. Special emphasis is given to highly stable MW signal sources, namely Ka-band transistor-based feedback oscillator and solid-state maser WGM oscillators. Recently obtained results are presented of experimental studies of the auto-oscillatory system developed on the basis of the WGM resonator with relativistic electron beam.

Proposal for a room-temperature diamond maser.

The application of masers is limited by its demanding working conditions (high vacuum or low temperature). A room-temperature solid-state maser is highly desirable, but the lifetimes of emitters (electron spins) in solids at room temperature are usually too short (∼ns) for population inversion. Masing from pentacene spins in p-terphenyl crystals, which have a long spin lifetime (∼0.1 ms), has been demonstrated. This maser, however, operates only in the pulsed mode. Here we propose a room-temperature maser based on nitrogen-vacancy centres in diamond, which features the longest known solid-state spin lifetime (∼5 ms) at room temperature, high optical pumping efficiency (∼106 s−1) and material stability. Our numerical simulation demonstrates that a maser with a coherence time of approximately minutes is feasible under readily accessible conditions (cavity Q-factor ∼5 × 104, diamond size ∼3 × 3 × 0.5 mm3 and pump power <10 W). A room-temperature diamond maser may facilitate a broad range of microwave technologies.

Masers made easy

The technological potential of masers — the microwave equivalents of lasers — has been thwarted by their impractical operating conditions. A solid-state maser that works at room temperature may change that. See Letter p.353 The maser is the microwave-frequency precursor of the now ubiquitous laser — or 'optical maser', as it was once known. But it has had little technological impact compared with the laser, in large part because of inconvenience: masers typically require vacuum and/or low-temperature operating conditions. Overcoming these obstacles would pave the way for significant maser-based innovations, including drastically more sensitive measurements across a range of scientific disciplines, from molecular biology to radio astronomy. And, if the history of the laser is any indication, applications not yet dreamt of. Mark Oxborrow, Jonathan Breeze and Neil Alford have now developed a solid-state room-temperature maser, based on an organic molecular crystal, that should enhance the potential of the maser as a tool for science and technology.

Room-temperature solid-state maser

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 birth. The maser's relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic and free-electron masers require vacuum chambers and pumping; and solid-state masers, although they excel as low-noise amplifiers and are occasionally incorporated in ultrastable oscillators, 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 spectrometers 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 maser, 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 crossing into pentacene's triplet ground state. 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 maser, 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.

EPR and development of quantum electronics

A role of electron paramagnetic resonance in development of quantum electronics is discussed. Basic principles and history of masers are briefly described. Spin-levels of paramagnetic ions in crystals as a very suitable object for active media of solid-state masers (called as EPR-masers) and physical processes in EPR-masers (population inversion of energy states) are analyzed. This analysis demonstrates a significant role of relaxation processes in multi-level spin-systems for efficient maser action. In this context peculiarities of spin-lattice and spin-spin cross relaxation processes in multi-level systems are analyzed. Development of EPR-masers and their application in radioastronomy and far-space communication systems are briefly described.

High-power solid-state sapphire whispering gallery mode maser

We present new results on a cryogenic solid-state maser frequency standard, which relies on the excitation of whispering gallery (WG) modes within a doped monocrystalline sapphire resonator (alpha-Al2O3). Included substitutively within the highest purity HEMEX-grade sapphire crystal lattice are Fe2+ impurities at a concentration of parts per million, an unavoidable result of the manufacturing process. Mass conversion of Fe2+ to Fe3+ ions was achieved by thermally annealing the sapphire in air. Above-threshold maser oscillation was then excited in the resonator at zero applied DC magnetic field by pumping high-Q WG modes coincident in frequency with the electron spin resonance (ESR) energy levels of the Fe3+ spin population. A 2 stage annealing process was undertaken for a sapphire resonator with exceptionally low Fe3+ concentration, resulting in an improvement of 6 orders of magnitude in output power for this particular crystal, and exceeding the previous best implementation of our scheme in another crystal by nearly 20 dB. This represents an output signal 7 orders of magnitude more powerful than a typical commercial hydrogen maser. At this power level, we estimate a limit on the frequency stability of order 1 x 10(-17)/square root(tau) due to the Schawlow-Townes fundamental thermal noise limit.

Theory of a type of quantum amplification: Phase-sensitive amplification by frequency upconversion.

It is shown that the spectroscopic bridge, a recently proposed method to achieve high-efficiency microwave oscillators and phase-sensitive detectors, may be used in a spin multiplet of certain paramagnetic ions to obtain a type of microwave quantum amplification: phase-sensitive amplification by frequency upconversion. This operation of a solid-state maser without population inversion is based on a principle similar to that used for achieving amplification in a transistor (triode). The major differences between this amplification and the parametric amplification, or the usual double-resonance maser action, are discussed.

Solid-state masers without inversion: Theoretical prediction for high-efficiency oscillators or phase-sensitive detectors.

Analytic steady-state solutions of the density-matrix equation are derived by means of a formalism suitable for multilevel spin systems in dilute paramagnetic solids at high temperature and subjected to several strong fields. It is shown that solid-state masers without inversion may be obtained in the presence of one, two, or three quasimonochromatic strong fields at resonance. In the usual cases of two fields, the maser action with or without inversion cannot achieve high-efficiency microwave generation or detection, since for strong emitted fields, the saturating and nonlinear effects are strong too. On the contrary, whenever three frequency-correlated fields fulfill the ``spectroscopic bridge'' conditions, the saturating effects do not occur, and linear effects prevail, even when all these fields are strong. In this case, high-efficiency microwave generations or phase-sensitive detections may be achieved.

Millimeter-wavelength maser amplifiers (review)

During the last 20 years the noise temperature of the best centimeter-range radio receiving systems has been reduced from above 1000~ to values of 15-50~ [1-4]. This significant improvement in sensitivity -by approximately two orders of magnitude -was achieved due to the development and use of low-noise amplifiers, primarily paramagnetic maser amplifiers (PMA) or solid-state masers and semiconducting parametric amplifiers (SPA).

EPR Investigation of the Very High Zero‐Field Splitting of Gd3+ in La2O3 Single Crystals

AbstractSingle crystalsof lanthanum oxide doped with Gd3+ are grown by the flame fusion method and studied by ESR. The very high zero‐field parameter B (0.131 cm−l) is of the same order of magnitude asthe energy quantum of X‐band radiation. This gives an abnormal spectrum which has been resolved by computer. These crystals may find interesting applications in solid state masers.