arXiv subjects

Robert Thomas

Publications and source records attributed to Robert Thomas.

5 recordsLinked to original sources

arXiv2026

Phonon Trapping Lateral Field Excited Suspended Bulk Acoustic Wave Resonators (XBARs)

Film bulk acoustic wave resonators (FBARs) underpin modern wireless communication by enabling compact, high-performance RF filters in modern smartphones. Traditionally, these FBAR devices work with quasi-plane waves of sound where the transverse extent of the acoustic field $\gg$ the acoustic wavelength ($\lambda_a$). On the other hand, strong modal confinement is needed for achieving the interaction strengths necessary for building efficient microwave to optical signal transducers (MW-OT) around an FBAR opto-mechanical cavity platform. While MW-OTs have traditionally been engineered around sub-{\mu}m scale optomechanical cavities, bulk acoustic wave approaches have inherent advantages in phonon injection efficiency, optical power handling and manufacturability. A key limitation of the FBAR geometry is that the acoustic field is confined under the metal electrode which makes it challenging to engineer the small mode-volume, high quality factor optical cavities which are critical for achieving high transduction efficiency. Here, we consider lateral field excited suspended overtone bulk acoustic wave resonators (XBARs) as an alternative bulk wave platform for MW-OT, which overcome this limitation, and outline the requirements needed for building efficient MW-OT around this geometry. As a first step towards viability, we fabricate a small mode-volume phonon trapping acoustic microresonator by shaping the piezoelectric layer into a spherical lens and show an improvement in modal confinement and quality factor ($\approx$ 4$\times$).

physics.optics↗

arXiv2025

Engineering cm-scale true push-pull electro-optic modulators in a suspended GaAs photonic integrated circuit platform by exploiting the orientation induced asymmetry of the Pockels $r_{41}$ coefficient

Electro-optic modulators (EOMs) underpin a wide range of critical applications in both classical and quantum information processing. While traditionally the focus has been on building these devices in materials with large Pockels coefficient (mainly ferroelectric insulators like lithium niobate), there is a need to engineer EOMs in a semiconductor platform with a view towards device stability (in radiation-hard environments), manufacturability (wafer size and foundry compatibility) and integration (with active electronics and quantum confined structures). Here, we demonstrate true push-pull EOMs in a suspended GaAs photonic integrated circuit (PIC) platform by exploiting the orientation induced asymmetry of the Pockels $r_{41}$ coefficient, and folding the two arms of a cm-scale Mach-Zehnder interferometer (MZI) modulator along two orthogonal crystal axes. Our work also shows the potential of incorporating ideas from micro-electro-mechanical systems (MEMS) in integrated photonics by demonstrating high-performance active devices built around cm-scale suspended waveguides with sub-${\mu}$m optical mode confinement.

physics.optics↗

arXiv2023

Quantifying and mitigating optical surface loss in suspended GaAs photonic integrated circuits

Understanding and mitigating optical loss is critical to the development of high-performance photonic integrated circuits (PICs). Especially in high refractive index contrast compound semiconductor (III-V) PICs, surface absorption and scattering can be a significant loss mechanism, and needs to be suppressed. Here, we quantify the optical propagation loss due to surface state absorption in a suspended GaAs photonic integrated circuits (PIC) platform, probe its origins using X-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry (SE), and show that it can be mitigated by surface passivation using alumina ($Al_{2}O_{3}$). We also explore potential routes towards achieving passive device performance comparable to state-of-the-art silicon PICs

physics.optics↗

arXiv2022

Redefining Relationships in Music

AI tools increasingly shape how we discover, make and experience music. While these tools can have the potential to empower creativity, they may fundamentally redefine relationships between stakeholders, to the benefit of some and the detriment of others. In this position paper, we argue that these tools will fundamentally reshape our music culture, with profound effects (for better and for worse) on creators, consumers and the commercial enterprises that often connect them. By paying careful attention to emerging Music AI technologies and developments in other creative domains and understanding the implications, people working in this space could decrease the possible negative impacts on the practice, consumption and meaning of music. Given that many of these technologies are already available, there is some urgency in conducting analyses of these technologies now. It is important that people developing and working with these tools address these issues now to help guide their evolution to be equitable and empower creativity. We identify some potential risks and opportunities associated with existing and forthcoming AI tools for music, though more work is needed to identify concrete actions which leverage the opportunities while mitigating risks.

cs.CY↗

arXiv2020

Night-to-Night Variability of Sleep Electroencephalography-Based Brain Age Measurements

Objective Brain Age Index (BAI), calculated from sleep electroencephalography (EEG), has been proposed as a biomarker of brain health. This study quantifies night-to-night variability of BAI and establishes probability thresholds for inferring underlying brain pathology based on a patient's BAI. Methods 86 patients with multiple nights of consecutive EEG recordings were selected from Epilepsy Monitoring Unit patients whose EEGs reported as being within normal limits. BAI was calculated for each 12-hour segment of patient data using a previously described algorithm, and night-to-night variability in BAI was measured. Results The within-patient night-to-night standard deviation in BAI was 7.5 years. Estimates of BAI derived by averaging over 2, 3, and 4 nights had standard deviations of 4.7, 3.7, and 3.0 years, respectively. Conclusions Averaging BAI over n nights reduces night-to-night variability of BAI by a factor of the square root of n, rendering BAI more suitable as a biomarker of brain health at the individual level. Significance With increasing ease of EEG acquisition including wearable technology, BAI has the potential to track brain health and detect deviations from normal physiologic function. In a clinical setting, BAI could be used to identify patients who should undergo further investigation or monitoring.

q-bio.NC↗