Spin bath maser in a cryogenically cooled sapphire whispering gallery mode resonator

Abstract

We report the observation of a mechanism of maser generation in an ensemble of intercoupled, inhomogeneously broadened two-level systems, enhanced by high quality factor electromagnetic cavity modes. In this form of population inversion, an inseparable quantum system leads to cavity-enhanced stimulated emission arising from interactions within an ensemble of two-level systems, as opposed to a traditional ensemble of noninteracting identical three-level systems. The effect is observed in a cryogenically cooled whispering gallery mode sapphire resonator containing dilute Fe 3+ impurity ions. These ions exhibit strong spin-lattice interaction, leading to both electron spin resonance broadening and phonon mediated spin-spin coupling. The maser effect is due to a |1/2 → |3/2 energy transition in an electron spin angular momentum observed at zero external magnetic field. Both continuous and oscillating regimes are observed with corresponding thresholds both in detuning frequency and incident power.

Ultrahigh cooperativity interactions between magnons and resonant photons in a YIG sphere

“…From figure 4, we can see that both the doublet pairs demonstrate a gyrotropic response when interacting with the magnon resonances, i.e., one mode interacts more than its doublet pair. This is a common occurrence in spin ensemble systems and has been observed in paramagnetic systems such as Fe 3+ in sapphire [26,27,40]. This asymmetric interaction strength for doublet pairs has also been observed in ferromagnets by Krupka et al [41,42] and predicted by Rameshti et al [24], with the latter stating that g n,m=n > g n,m=−n , where a different notation to that used here is employed, in which m = −n, .…”

“…In order to investigate this regime we use common microwave spectroscopy techniques [5,23,[25][26][27][28] to directly observe the mode splitting caused by the magnon interaction to determine the coupling values. Similar systems have been extensively utilised not only in the field of spintronics to investigate the interaction between microwave photons and paramagnetic spin ensembles [25][26][27][28][29], but also to realise optical comb generation [30], ultra-low threshold lasing [31] measurement of optomechanical systems [32,33], and extremely stable cryogenic sapphire oscillator clock technology [34,35]. To date, exciting internal, highly-confined photonic modes in a YIG sphere has only recently been demonstrated in the optical regime using Whispering Gallery Modes (WGM) [10,11] but has never before been achieved in the microwave domain.…”

Collective behavior ofCr3+ions in ruby revealed by whispering gallery modes

“…This concentration is approximately two orders of magnitude larger than the concentration of Fe 3+ impurities in [27] (150 ppb), and the value of g is also approximately an order of magnitude larger. In addition, the losses associated with the Cr 3+ ESR (∆ω spins /2π = 9 MHz) are three times less than those associated with the Fe 3+ case (∆ω spins /2π = 27 MHz) [27,31]. In the present case, g > ∆ω spins , ∆ω W GM (∆ω W GM /2π = Q/f res = 1.6 kHz), satisfying the conditions for strong coupling.…”

“…It is due to this strong atom-field coupling that superradiance can occur as it is this interaction from which the collective action of the ensemble is derived, and large ion concentrations contribute to this (as g = g 0 √ N ). This explains why a superradiant ESR, and hence a four SHO model, is observed in the Cr 3+ case and not in the previously reported Fe 3+ case [27,31], which did not satisfy the conditions of strong coupling.…”

Single-photon level study of microwave properties of lithium niobate at millikelvin temperatures

“…Depending on material losses, this technique can detect ion ensembles with concentrations down to few parts per billion at multiple frequencies in the X and K u bands (5 − 25 GHz). Additionally WGMs are a nice tool to observe various nonlinear phenomena related to dielectrics and spin ensembles such as four-wave mixing and masing [22,23]. Advantages of the WGM technique are high filling factor (due to the fact that the microwave cavity is simultaneously a host for the spin ensemble) and high quality factors.…”