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Abstract
Coupling to longitudinal modes of thin spherical shells, under Gaussianbeam illumination, was explored with a theoretical method based on Fourieroptics analysis and vector spherical harmonics and was scrutinized with an experimental setup. For the theory part, the illumination frequency band was fixed between 100–600 GHz and the outer spherical shell radius of curvature and thickness are 7.5 mm and 0.5 mm, respectively. The shell material was either the lossless cornea or an aqueous effective media representing the cornea. Six different beamtarget strategies were introduced being potential candidates for maximum coupling. Two dispersiontuned beam ensembles with strongly frequencydependent phase center location have been created with a fixed incident beam 1/e radius and radius of curvature called forward strategies. These computations of different alignments were continued with four beam ensembles of frequencyinvariant phase center, constructed from fits to experimental data, oriented at four different axial locations with respect to the spherical shell center of curvature, they are called reverse strategies. Coupling efficiency for all strategies was calculated for different targets including perfect electrical conductor (PEC) sphere, PEC core covered by a cornea lossfree layer and cornea. All scattering strategies contrasted to scattering from equivalent planar targets as a reference with maximum coupling. The results show that, under an ideal calibration, forward strategies are a closer approximation to the planewave condition for the cornea. An experimental setup was assembled to explore the simulation approach in a frequency range between 220 GHz to 330 GHz. Two different quartz samples with permittivity of 4.1 were mounted on a water core, acting for a cornea. The first and second quartz radius and thickness were 7.5 mm and 0.5 mm and 8 mm and 1 mm, respectively. An adequate agreement between theory and experiment was confirmed. A particle optimisation swarm algorithm was applied to extract the thickness and permittivity of quartz from the measured backscattered field for reverse strategies.
Original language  English 

Pages (fromto)  36993722 
Number of pages  24 
Journal  Biomedical Optics Express 
Volume  13 
Issue number  7 
Early online date  2 Jun 2022 
DOIs  
Publication status  Published  1 Jul 2022 
MoE publication type  A1 Journal articlerefereed 
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Dive into the research topics of 'Vector spherical harmonic analysis and experimental validation of spherical shells illuminated with broadband, millimeter wave Gaussian beams : applications to corneal sensing'. Together they form a unique fingerprint.Projects
 1 Finished

AGRUM: Assessment of the Graft Rejection Using Millimeter Waves
Taylor, Z., Baggio, M., Tamminen, A., Grigorev, R., Lamberg, J., AlaLaurinaho, J., Nefedova, I., Masyukov, M., Rezapoor, P., Kiviharju, P. & Vaynshteyn, S.
01/01/2020 → 31/12/2022
Project: Academy of Finland: Other research funding