High-power solid-state sapphire whispering gallery mode maser

Abstract

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.

Single-crystal sapphire resonator at millikelvin temperatures: Observation of thermal bistability in high-Qfactor whispering gallery modes

“…9, 19, and 20͒ and whispering-gallery maser oscillator ͑WHIGMO͒ experiments. [21][22][23][24] Here, we report on the observation of the lowest frequency-temperature turning points for WG mode resonances ever measured, as well as making the observation of a thermal bistability effect in sapphire for this ultralow-temperature regime. We give a model to predict thermal bistability threshold power and show that the effect is dependent on the thermal conductivity of the sapphire.…”

Four-Wave Mixing fromFe3+Spins in Sapphire

“…FWM is an enabling process for both frequency comb generation and many quantum computing and metrology applications. Our system is further suited to these applications due to the extremely low dielectric loss tangent at millikelvin temperature, which persists even at single photon input power [21].The experimental system consists of a cryogenic sapphire resonator-oscillator [22][23][24][25][26][27] as shown in Fig. 1(a).…”

“…The system is cooled to liquid helium temperature and pumped with microwave power to excite WG mode resonances. As a result of the manufacturing process, paramagnetic Fe 3+ are included in the sapphire lattice at a concentration of 150 ppb (∼ 10 16 spins in the lattice) [27]. The crystal field splitting results in an inhomogeneously broadened electron spin resonance (ESR) with 27 MHz linewidth [28] at zero applied DC magnetic field, corresponding to the spin-|1/2 , |3/2 , and |5/2 states of the ion.…”

Linear and nonlinear effects of electron paramagnetic resonance in high-Q cryogenic sapphire microwave resonators

“…The experimental system consists of a cryogenic sapphire resonator-oscillator [22][23][24][25][26][27] as shown in Fig. 2(a).…”

“…As a result of the manufacturing process, paramagnetic Fe 3+ ions are included in the sapphire lattice at a concentration of 150 ppb (∼ 10 16 spins in the lattice). 27 The crystal field splitting results in an inhomogeneously broadened electron spin resonance (ESR) with 27 MHz linewidth 28 at zero applied DC magnetic field, corresponding to the spin-|1/2 , |3/2 , and |5/2 states of the ion. Within the system a complex interaction occurs between the microwave input field, a dilute paramagnetic Fe 3+ spin system, and 27 Al lattice ions, which ultimately results in the production of signal and idler photons equally spaced in frequency, characteristic of degenerate FWM.…”