TFO Lab

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Research
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Thin-Film Optoelectronics (TFO) Lab

The lab conducts research and development on materials, components and systems utilizing thin-film technologies for applications in optics and electronics. The lab is focused on three thin-film technologies: 1) organic and hybrid organic/inorganic semiconductors for sensing and imaging, 2) optical metasurfaces for compact, wide-angle imaging, and 3) integrated photonics for LIDAR imaging and free-space optical communications. In addition to these core technologies, the group has embarked on several new applied imaging projects including: a) camera systems for python hunting in the Everglades, b) drone-based remote sensing, c) coherent fiber bundles for wide-angle infrared imaging, and d) multispectral thermal imaging.  

We are looking for motivated graduate students to join our team! Students must meet government requirements on a project-dependent basis prior to joining the team. Contact Prof. Renshaw for more information.

TFO Group Photo May 2021

News & Info

  • Zhao’s Volumetric Imaging Efficiency (VIE) paper published in Opt. Exp.
  • Jen’s python detection work published in App. Opt. was featured in local [1,2] and national news [1,2]
  • Robert presents thermal contrast enhancement using multiple thermal bands at SPIE DCS[link]
  • Cesar wraps up his senior design and research thesis to graduate with honors!
  • May 16: TFO summer party celebrates an end to COVID isolation
  • COVID happened = 2020 🙁
  • Cesar wins best poster at the UofA Winter School of Optics
  • Cesar wins Distinguished Undergraduate Research Award
  • Sajad and Pooya present VLC Transmitter at MobiCom/LIoT
  • Sajad presents PICO Array development at CLEO 
  • The TFO group expands: welcome new team members!
Sajad wins best poster at 2019 IEEE Summer Topical Symposium
  • Cesar receives Most Outstanding Summer Researcher from AFRL. 
  • Zhao’s JAP paper receives editor’s pick and is featured in Scilights.
  • Sajad wins best poster at the IEEE Summer Topical meeting for “Photonic Integrated Circuit Outcoupling (PICO) Arrays for Free-Space Optical Communications.”
  • Sajad is selected for a summer internship at IMEC-Florida. 
  • Sajad wins best poster at the Industrial Affiliates Symposium for “Camera/Inverse-Camera System for Free-Space Optical Communications.”
  • Cesar is accepted for a summer internship at AFRL. 
  • Sajad presents an Inverse-Camera System for Optical Wireless Communications at Frontiers in Optics 
  • Dr. Renshaw presents a switchable organic photodiode at OSA Imaging Congress 
  • Dr. Renshaw awarded AFRL Summer Faculty Fellowship 
  • Zhao presents a poster on curved sensor fabrication at SPIE Defense and Commercial Sensing 
  • Angstrom deposition tool is installed; first films grown and OPD/OLED devices fabricated
  • TFO Lab is completed! 

Research

Volumetric Imaging Efficiency (VIE)

Major investments are pouring into a wide variety of optical materials and lens technologies such as metamaterials, free-form surfaces, printed or molded optical glass and plastics, gradient index materials and manufacturing curved image sensors. These all seek to achieve the same objective – make optical systems (and usually imaging systems) perform better, cost less or become smaller. But which technology stands to provide the best improvement or the best bang for your research buck? We developed the VIE metric in an attempt to answer that question from a technology-agnostic perspective. The VIE is a measure of the resolution density of an imaging optic compared against the most dense system possible based on fundamental limits of diffraction. 

We collected >2800 lens designs and showed an empirical limit to the VIE of conventional imaging systems – using bulk optics imaging onto flat sensors. The limiting VIE decreases exponentially with FOV. We show examples of bulk lenses imaging onto curved sensors and metasurface lenses that surpass conventional systems by ~100x. These technologies particularly excel in wide angle applications where conventional lenses grow to become very bulky. 

Volumetric Channel Density of various lenses plotted along with the fundamental limits based on diffraction-limited imaging onto a flat sensors (blue circles) and onto a curved sensor (red squares)
Volumetric Channel Density of various lenses plotted along with the fundamental limits based on diffraction-limited imaging onto a flat sensors (blue circles) and onto a curved sensor (red squares)
VIE vs FOV for >2800 lenses. Efficiency of multi-metasurface (MMS) lenses are plotted as curved lines. Short focal length MMSs with 2 or more surfaces can surpass the limits of conventional lenses
VIE vs FOV for >2800 lenses. Efficiency of multi-metasurface (MMS) lenses are plotted as curved lines. Short focal length MMSs with 2 or more surfaces can surpass the limits of conventional lenses
VIE vs Field-of-view for library of conventional lenses using bulk optics. VIE decreases exponentially when imaging onto flat sensors (numbered points); imaging onto a curved surface enables high efficiency at wide angles (crescents)
VIE vs Field-of-view for library of conventional lenses using bulk optics. VIE decreases exponentially when imaging onto flat sensors (numbered points); imaging onto a curved surface enables high efficiency at wide angles (crescents)
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Organic Photodetectors for
Curved Image Sensors

A simple lens focuses to a curved surface due to Petzval field curvature. However, modern imagers rely on flat image sensors that evolved from wafer-based semiconductor fabrication technology. Consequently, imager design focuses on flattening this field to improve image quality – resulting in stringent trade-offs between lens complexity, field-of-view (FOV) and image quality. This project is developing non-planar focal plane arrays similar to image sensors found in biological imaging systems such as the human eye. These sensor arrays could lead to the next generation of imaging systems and enable dramatic improvements in compact, wide FOV imagers.

We are developing photodetectors based on organic molecules and polymers. These materials have strong optical absorption that is controllable across the visible and near-infrared spectrum via the molecular composition of the photodetector. The strong absorption allows fabrication of devices with thin active regions (typically < 100 nanometers) while their soft, Van der Waals bonding allows deposition onto any type of substrate including flexible and formable plastics. 

Utilization of curved image surfaces enables reduction in imager volume and improved resolution. However, modern sensors are flat and fabrication technologies for curving sensors are not mature yet.
Utilization of curved image surfaces enables reduction in imager volume and improved resolution. However, modern sensors are flat and fabrication technologies for curving sensors are not mature yet.
(top) Exploded view of a wide-angle imager utilizing a monocentric lens and hemispherically curved image sensor based on organic photodiodes (OPDs). (bottom) Circuit-diagram for vertically-stacked anti-polar diode array, top view and cross-section of a cross-hatch OPD array.
(top) Exploded view of a wide-angle imager utilizing a monocentric lens and hemispherically curved image sensor based on organic photodiodes (OPDs). (bottom) Circuit-diagram for vertically-stacked anti-polar diode array, top view and cross-section of a cross-hatch OPD array.
High-resolution curved interconnects fabricated on polymer substrates. Interconnects survive 100 degree deformations but fail when deformed to 148 degrees of a spherical cap.
High-resolution curved interconnects fabricated on polymer substrates. Interconnects survive 100 degree deformations but fail when deformed to 148 degrees of a spherical cap.
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Curved Image Relays based on
MWIR/LWIR coherent fiber bundles

The DARPA SCENICC program demonstrated viability of curved image relays to enable compact, wide-angle imaging systems in the visible. We are collaborating with experts in chalcogenide glass growth and fiber drawing (all at CREOL) to manufacture high-resolution and large format coherent fiber bundles for thermal imagers in the midwave (MWIR) and longwave (LWIR) infrared bands. We are fabricating these novel optics to relay curved images to flat sensors to enable a new breed of compact, wide-angle and high-resolution thermal imagers. 

We are investigating microstructured and step-index fibers for MWIR imaging bundles. (top) Imaging fiber bundle for visible fibers used by S. Karbasi, et al., to create compact, wide FOV imagers. (bottom) Silica-based MWIR imaging fiber bundle designs based on anti-resonant reflective optical waveguides (ARROW).
We are investigating microstructured and step-index fibers for MWIR imaging bundles. (top) Imaging fiber bundle for visible fibers used by S. Karbasi, et al., to create compact, wide FOV imagers. (bottom) Silica-based MWIR imaging fiber bundle designs based on anti-resonant reflective optical waveguides (ARROW).
Fiber pre-form and drawing process using tabletop tower in collaboration with Dr. Kaufman and Dr. Tan
Fiber pre-form and drawing process using tabletop tower in collaboration with Dr. Kaufman and Dr. Tan
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Python Detection System

Burmese pythons are invading the everglades and destroying the natural ecosystem. We are working with the Florida Fish and Wildlife Commission (FFWC) to develop a near-infrared python detection system comprised of a multi-camera array and automated detection algorithms. The system is in development and we are actively working with the FFWC and contracted hunters to test the system effectiveness. 

Preliminary data collection for proof-of-concept
Preliminary data collection for proof-of-concept
Hyperspectral characterization of pythons and background samples typically found in the everglades
Hyperspectral characterization of pythons and background samples typically found in the everglades
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Multi-Metasurface Lenses

Metasurface lenses use sub-wavelength, nanostructured scattering features to to control light in new ways. For example, flat lenses mimic the phase delay of a conventional, bulk lens by engineering the effective index to give the same profile across a planar surface. Flat singlets, doublets and achromats have already been fabricated and demonstrated in laboratory camera systems. Challenges abound related to scattering efficiency, dispersive characteristics and scaling challenges. We are working to address MS scaling by combining bulk lenses for primary power and MS lenses for aberration correction. This work includes modelling and simulation, optical design, nanofabrication in the cleanroom and optical test and characterization. Multiscale modelling and simulation spans full-wave FDTD simulations in Lumerical to geometric ray tracing in Zemax with aid from inverse design optimization for design down to individual meta-atom placements across the surface.  

2-MS aberration corrector improves performance of a lens
2-MS aberration corrector improves performance of a lens
MMS2
Full-wave simulations of a phase discontinuity at the border between Fresnel zones
Full-wave simulations of a phase discontinuity at the border between Fresnel zones
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Imaging-Based Beam Steering

We have introduced a new beam-steering mechanism that utilizes the passive mapping between spatial and angular coordinates provided by an imaging optic. This can provide high-resolution beam steering over a wide field-of-regard with no moving parts in a compact and low-power system; overcoming many of the problems inherent in conventional beam-steering approaches using moving mirrors, phased-arrays or spatial light modulators.  

Schematic, model and experimental setup using an organic LED display coupled to a 50mm focal length photographic lens to demonstrate imaging-based beam steering (IBBS).
Schematic, model and experimental setup using an organic LED display coupled to a 50mm focal length photographic lens to demonstrate imaging-based beam steering (IBBS).
(top) Beam profile measurements near the lens and sufficiently far that we observe the microstructured irradiance pattern of the 200micron spot on the OLED display. (bottom) Experimentally measured beam locations across the lenses 30 degree FOV yield RMS pointing error of ~5mrad. Beam radius verse distance from the transmitter shows ~1.25mrad beam divergence.
(top) Beam profile measurements near the lens and sufficiently far that we observe the microstructured irradiance pattern of the 200micron spot on the OLED display. (bottom) Experimentally measured beam locations across the lenses 30 degree FOV yield RMS pointing error of ~5mrad. Beam radius verse distance from the transmitter shows ~1.25mrad beam divergence.
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People

Faculty Advisor
C. Kyle Renshaw
C. Kyle Renshaw
Graduate Students
Undergrads

Publications

J. Hewitt, O. Furxhi, C.K. Renshaw and R. Driggers “Detection of Burmese pythons in the near infrared vs. visible band” Applied Optics, 60(17), pp. 5066-5073 (2021)

Z. Ma, K.H. Shih, C. Lopez-Zelaya and C.K. Renshaw “Volumetric Imaging Efficiency: the Fundamental Limit to Compactness of Imaging Systems” Optics Express, 29(3), pp. 3173-3192 (2021)

C.Y. Zhang, J.X. Li, A. Belianinov, Z. Ma, C.K. Renshaw and R.M. Gelfand “Nanoaperture fabrication in ultra-smooth single-grain gold films with helium ion beam lithography” Nanotechnology, 31(46), A. 465302 (2020)

M. Teng, A. Honardoost, Y. Alahmadi, S. S. Polkoo, K. Kojima, H. Wen, C.K. Renshaw , P. LiKamWa, G. F. Li, S. Fathpour, R. Safian and L. M. Zhuang “Miniaturized silicon photonics devices for integrated optical signal processors” IEEE/OSA Journal of Lightwave Technology, vol. 38, pp. 6-17, January 2020 (INVITED). (2020)

Z. Ma and C.K. Renshaw “Organic photodetectors with frustrated charge transport for small-pitch image sensors” Journal of Applied Physics, 126(4), 045501 (2019)

S.S. Polkoo and C.K. Renshaw “Imaging-based beam steering for free-space optical communication” Applied Optics, 58(13), pp. D12-D21 (2019)

C. K. Renshaw and S. R. Forrest “Excited State and Charge Dynamics of Hybrid Organic/Inorganic Junctions. I. Theory” Phys. Rev. B 90, 045302 (2014) (2014)

A. Panda, C. K. Renshaw, A. Oskooi, K. Lee, and S. R. Forrest “Excited State and Charge Dynamics of Hybrid Organic/Inorganic Junctions. II. Experiment” Phys. Rev. B 90, 045303 (2014) (2014)

B. E. Lassiter, C. K. Renshaw and S. R. Forrest “Understanding Tandem Organic Photovoltaic Cell Performance” J. Apply. Phys. 113, 214505 (2013) (2013)

C. K. Renshaw, J. D. Zimmerman, B. E. Lassiter, and S. R. Forrest “Photoconductivity in Organic Photovoltaics” Phys. Rev. B 86, 085324 (2012) (2012)

J. D. Zimmerman, X. Xiao, C. K. Renshaw, S. Wang, V. V. Diev, M. E. Thompson, and S. R. Forrest “Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells” Nano Lett. 12, 4366 (2012) (2012)

C. K. Renshaw, C. Schlenker, M. E. Thompson, and S. R. Forrest “Reciprocal Carrier Collection in Organic Photovoltaics” Phys. Rev. B 84, 045315 (2011) (2011)

N. Li, K. Lee, C. K. Renshaw, X. Xiao, and S. R. Forrest “Improved Power Conversion Efficiency of InP Solar Cells Using Organic Window Layers” Appl. Phys. Lett. 98, 053504 (2011) (2011)

K. Lee, K. Shiu, J. D. Zimmerman, C. K. Renshaw, and S. R. Forrest “Multiple Growths of Epitaxial Lift-off Solar Cells from a Single InP Substrate” Appl. Phys. Lett. 97, 101107 (2010) (2010)

B. E. Lassiter, R. R. Lunt, C. K. Renshaw, and S. R. Forrest “Structural Templating of Multiple Polycrystalline Layers in Organic Photovoltaic Cells” Optics Express 18, No. 103, A444 (2010) (2010)

G. Wei, S. Wang, K. Renshaw, M. E. Thompson, and S. R. Forrest “Solution-Processed Squaraine Bulk Heterojunction Photovoltaic Cells” ACS Nano 4, No. 4, 1927 (2010) (2010)

C. K. Renshaw, X. Xu, and S. R. Forrest “A Monolithically Integrated Organic Photodetector and Thin Film Transistor” Org. Elect. 11, 175 (2010) (2010)

M. S. Arnold, J. D. Zimmerman, C. K. Renshaw, X. Xu, R. R. Lunt, C. M. Austin, and S. R. Forrest “Broad Spectral Response Using Carbon Nanotube/Organic Semiconductor/C60 Photodetectors” Nano Lett. 9, No. 9, 3354 (2009) (2009)

S. Ghosh, A. R. Bhagwat, C. K. Renshaw, S. Goh, and A. L. Gaeta “Low-Light-Level Optical Interactions with Rubidium Vapor in a Photonic Band-Gap Fiber” Phys. Rev. Lett. 97, 023603 (2006) (2006)