Modification of phonon processes in nanostructured rare-earth-ion-doped crystals

Abstract

Nano-structuring impurity-doped crystals affects the phonon density of states and thereby modifies the atomic dynamics induced by interaction with phonons. We propose the use of nanostructured materials in the form of powders or phononic bandgap crystals to enable or improve persistent spectral hole-burning and coherence for inhomogeneously broadened absorption lines in rare-earth-ion-doped crystals. This is crucial for applications such as ultra-precise radio-frequency spectrum analyzers and optical quantum memories. As an example, we discuss how phonon engineering can enable spectral hole burning in erbium-doped materials operating in the convenient telecommunication band, and present simulations for density of states of nano-sized powders and phononic crystals for the case of Y2SiO5, a widely-used material in current quantum memory research.Rare-earth-ion-doped crystals have been studied for decades because of their unique spectroscopic properties [1][2][3] that arise since their 4f -electrons do not participate in chemical bonding. At cryogenic temperatures they can offer narrow linewidths [4][5][6] together with the possibility to spectrally tailor their broad inhomogeneous absorption lines [2]. These properties have led to many applications, including optical quantum memories [7][8][9], signal processing [10,11], laser stabilization [12][13][14], as well as ultra-precise radio frequency spectrum analyzers [15,16]. Quantum memory implementations in rareearth-ion-doped (REI-doped) crystals, such as those based on electromagnetically-induced transparency [17]), atomic frequency comb [18], or controlled reversible inhomogeneous broadening [19], crucially rely on long coherence and spin-state lifetimes to achieve high efficiency and long storage time.Operating REI-doped crystals at low temperatures generally improves material properties. Yet, even at temperatures below 2 K, spin-lattice relaxation, i.e. thermalization of spins via interaction with phonons, still restricts lifetimes of spin states, reducing the ability to spectrally tailor the material. Furthermore, by contributing to spectral diffusion [20,21] and two-phonon elastic scattering processes [22], lattice vibrations limit coherence times. We propose the use of nano-structured materials to overcome these limitations. This is achieved by tailoring the phonon density of states [23] to restrict phonon processes, an approach not limited to REI-doped materials but also applicable to other impurity-doped materials such as color centers in diamond [24]. The result is an improved performance for all applications based on spectral hole burning, or that require long coherence times, by providing long-lived spin states while simultaneously suppressing phonon-driven decoherence.We note that the modification of population dynamics between REI crystal field levels in small powders, possibly related to phonon restriction, has been observed in the context of understanding luminescence dynamics [25, 26] -but not yet to improve spectral hole burning or optical coheren...

Effects of fabrication methods on spin relaxation and crystallite quality in Tm-doped Y3Al5O12 powders studied using spectral hole burning

“…In contrast to REI doped bulk materials, powders offer low cost and rapid prototyping. Furthermore, the understanding of powders constitutes a first step towards nanofabrication of devices from these materials [4–7]. However, despite much effort [8–11], producing monodisperse powders with properties comparable to those of bulk materials remains challenging.…”

All-optical control of long-lived nuclear spins in rare-earth doped nanoparticles

“…Other hybrid structures for quantum technologies have been proposed like nanodiamonds deposited on an active substrate 23 or interacting with plasmons in metallic particles 24 . Furthermore, nanoparticles could also enable photon and phonon density of states engineering to create bandgaps and achieve longer optical and spin population and coherence lifetimes 25 – 27 .…”

Modification of relaxation dynamics in Tb3+:Y3Al5O12 nanopowders

“…We modeled this effect by calculating the VDOS [19], assuming that each mode contributes a Lorentzian with a width ∆ν, for 10 5 particles with different particle diameter following a Gaussian distribution. We obtained the mean diameter and the standard deviation of this distribution from the SEM images of our samples.…”

“…Whereas a bulk crystal has a Debye VDOS (a continuous function that increases with the square of the vibrational frequency), the distribution becomes discrete in small crystallites, exhibiting gaps and even a cutoff below which no phonons are supported. Furthermore, phononic crystals-nano-machined, periodic structures-can feature engineered frequency band-gaps where vibrations are forbidden [19]. These approaches to phonon engineering could potentially benefit applications in the field of quantum information, in particular quantum memories, since the absence of phonons could enhance spin-population lifetimes as well as optical coherence times.…”