Ultrasonic subwavelength acoustic focusing and imaging using a 2D membrane metamaterial

Lani, Shane W.

Title:

Ultrasonic subwavelength acoustic focusing and imaging using a 2D membrane metamaterial

dc.contributor.advisor	Sabra, Karim G.
dc.contributor.advisor	Degertekin, F. Levent
dc.contributor.author	Lani, Shane W.
dc.contributor.committeeMember	Leamy, Michael
dc.contributor.committeeMember	Ruzzene, Massimo
dc.contributor.committeeMember	Hunt, William D.
dc.contributor.department	Mechanical Engineering
dc.date.accessioned	2016-05-27T13:09:50Z
dc.date.available	2016-05-27T13:09:50Z
dc.date.created	2015-05
dc.date.issued	2015-03-09
dc.date.submitted	May 2015
dc.date.updated	2016-05-27T13:09:50Z
dc.description.abstract	A metasurface or 2D metamaterial composed of a membrane array can support an interesting acoustic wave field. These waves are evanescent in the direction normal to the array and can propagate in the immersion fluid immediately above the metasurface. These waves are a result of the resonant membranes coupling to the fluid medium and propagate with a group and phase speed lower than that of the bulk waves in the surrounding fluid. This work examines and utilizes these evanescent surface waves using Capacitively Micromachined Ultrasonic Transducers (CMUT) as a specific example. CMUT arrays can generate and detect membrane displacement capacitively, and are shown to support the surface waves capable of subwavelength focusing and imaging. A model is developed that can solve for the modes of the membrane array in addition to transiently modeling the behavior of the array. It is found that the dispersive nature of the waves is dependent on the behavior of the modes of the membrane array. Two-dimensional dispersion analysis of the metasurface shows evidence of four distinct frequency bands of surface wave propagation: isotropic, anisotropic, directional band gap, and complete band gap around the first resonant frequency of the membrane. Some of the frequencies in the partial band gap show concave equifrequency contours capable of negative refraction. The dispersion and modal properties are also examined as to how they are affected by basic array parameters. Potential applications of this wave field are examined in the context of subwavelength focusing and imaging. Several methods of acoustic focusing are used on an array consisting of dense grid of membranes and several membranes spatially removed from the structure. Subwavelength acoustic focusing to a resolution of λ/5 is shown in simulations and verified with experiments. An imaging test is also performed in which a subwavelength defect is localized. This fundamental work in characterizing the waves above the membrane metasurfaces is expected to have impact and implications for transducer design, resonant sensors, 2D acoustic lenses, and subwavelength focusing and imaging.
dc.description.degree	Ph.D.
dc.format.mimetype	application/pdf
dc.identifier.uri	http://hdl.handle.net/1853/54867
dc.publisher	Georgia Institute of Technology
dc.subject	Metamaterial
dc.subject	Acoustic focusing
dc.subject	CMUT
dc.subject	Membrane metasurface
dc.title	Ultrasonic subwavelength acoustic focusing and imaging using a 2D membrane metamaterial
dc.type	Text
dc.type.genre	Dissertation
dspace.entity.type	Publication
local.contributor.advisor	Sabra, Karim G.
local.contributor.advisor	Degertekin, F. Levent
local.contributor.corporatename	George W. Woodruff School of Mechanical Engineering
local.contributor.corporatename	College of Engineering
relation.isAdvisorOfPublication	746faa42-e7e0-4510-969d-e841de5920da
relation.isAdvisorOfPublication	9afd92fe-bb98-4c83-b8ac-fc596320991a
relation.isOrgUnitOfPublication	c01ff908-c25f-439b-bf10-a074ed886bb7
relation.isOrgUnitOfPublication	7c022d60-21d5-497c-b552-95e489a06569
thesis.degree.level	Doctoral