Title
Design and Characterization of an Electrically-Injected, Solid State, Acoustoelectric Surface Acoustic Wave Phonon Laser
Abstract
The evolution of lasers from bulk optical systems to compact, integrated sources has had profound impacts in both commercial applications such as communications and information processing as well as experimental platforms such as quantum computing and precision metrology. Surface acoustic waves (SAWs) occupy a comparably important position in modern technology, forming the basis of RF filters in billions of mobile devices as well as platforms for chemical and biological sensing, microfluidics, and quantum phononics. Notably, coherent SAWs are arguably more straightforward to generate than coherent light, achievable by driving an interdigital transducer (IDT) with an electrical RF source. The IDT has found extensive use in commercial and experimental applications alike. However, this paradigm has intrinsic limitations, including high-frequency scaling limits, the requirement of an external RF source, and bandwidth constraints, which have prevented SAW-based devices from broader implementation. Just as integrated lasers unlocked new frontiers in photonics, new, scalable, IDT-free sources of coherent SAWs are needed to continue advancing phononic technology.
When mobile charge carriers are free to interact with the quasistatic electric field that accompanies an acoustic wave propagating in a piezoelectric medium, a phenomenon called the acoustoelectric effect occurs. While the acoustic wave loses energy to the electrons in an unbiased medium, the electrons may give energy to the acoustic wave if they are moving with a sufficiently fast drift velocity, effectively amplifying the acoustic wave. Since its discovery in 1952, this effect was studied as a potential source of amplification and generation of coherent acoustic waves. However, early efforts to implement it were limited in success due as material science was still in its infancy at the time, leaving the acoustoelectric effect closer to a scientific curiosity than a practical technology.
Modern semiconductor growth technology has enabled the production of pristine thin films of III-V compound semiconductors such as InGaAs with high electron mobility. The potential of heterogeneously integrating these films onto strong piezoelectric substrates such as lithium niobate, combining the desirable characteristics of both materials, has reinvigorated interest in the acoustoelectric effect. Indeed, this approach has been used for the better part of the last decade to create exceptionally-performing acoustic amplifiers, and has demonstrated strong nonlinear mixing.
In this thesis, an acoustoelectric-based SAW laser is both theoretically described and experimentally characterized. The device consists of an acoustoelectric amplifier heterostructure interposed between two acoustic distributed Bragg reflectors that form a Fabry-Perot cavity. When the thermally-populated acoustic modes of the resonator achieve sufficient round-trip gain to overcome the intrinsic losses, the system undergoes self-oscillation, resulting in sustained, high-power, coherent acoustic radiation. Below the lasing threshold of 36 V, it acts as a resonant amplifier with an end-to-end RF gain of 19.6 dB, and is able to dynamically enhance the effective quality factor of the resonator by over 100%. Above threshold, the device exhibits a strong increase in radiation output up to -6.1 dBm accompanied by linewidth narrowing from the MHz regime to 77 Hz at a center frequency of about 1 GHz. As this demonstration is the first of its kind, there are many avenues for improvement. We discuss some of these potential improvements, and provide projections for this technology in the near term using rigorous analytical and FEM modeling.
Please email Jini at jini@optics.arizona.edu or Alex at ajwendt@arizona.edu for a Zoom link.
