About

Kimani Toussaint is the Senior Associate Dean for Research and Strategic Initiatives at Brown University and holds the Thomas J. Watson Sr. Professor of Science title. He is also the Director of the Brown Center for Digital Health. His research interests encompass multiphoton microscopy, biophotonics, plasmonic nanoantennas, nanophotonics, polarimetry, singular optics, multiphoton lithography, and nano- and microfabrication. Toussaint has been recognized for his contributions to engineering and science, including receiving the SPIE Diversity Outreach Award and a $1.5 million grant from the Chan Zuckerberg Initiative. He has been actively involved in advancing health innovation, neurotechnology, and neural engineering, and has contributed to discussions on pulse oximeters and digital health technologies.

Research topics

• Materials science
• Computer Science
• Physics
• Artificial Intelligence
• Optics
• Mathematics
• Acoustics
• Quantum mechanics
• Optoelectronics
• Mathematical analysis

Selected publications

• Demonstration of space-time wave packet carrying orbital angular momentum across scattering media

  APL Photonics · 2025-01-01 · 3 citations

  articleOpen accessSenior author

  In this article, we experimentally generate a vortex space-time (ST) wave packet carrying orbital angular momentum (OAM) and confirm that the OAM of the beam survives after propagating either in the presence of a line obstruction or across a thin scattering medium. A comparison of our findings with both Bessel and standard Gaussian beams carrying OAM reveals that ST-OAM wave packets exhibit a significantly higher mean Pearson’s correlation coefficient compared to these other beams. In addition, we employ a standard 3-mm thick glass microscope slide as a waveguide and find that the OAM belonging to ST wave packets remains protected after propagation. Our findings have potential utility in communication applications, sensing, imaging, and optical trapping.

  PublisherOA PDFDOI

• Light sheet optical tweezers as a force transducer for biological tissues

  2025-03-19

  articleSenior author

  The dynamic changes in tissue mechanics are crucial in distinguishing between healthy and diseased biological states. Atomic force microscopy (AFM) currently serves as the gold standard for local stiffness measurement. However, AFM has limitations such as potential damage from its sharp tip, low repeatability, and high force noise. Optical tweezers (OT) is an alternative technique capable of indenting cells with piconewton forces, capturing the reversible elastic responses. Despite these advantages, traditional OT requires high-power continuous wave lasers and exhibits low throughput, as it can only trap one indenting probe at a time. Here, we introduce a novel approach using ultra-low power light sheet-based optical tweezers with a femtosecond pulsed laser source. This innovation allows for the simultaneous indentation of bovine tendon tissues at multiple locations, facilitating the capture of both local and average elastic properties.

  PublisherDOI

• Robust optical communications using the coherence-rank of partially coherent light

  2025-01-24

  articleSenior author

  In optical communications, logical bits (0 and 1) encoded in light’s degrees-of-freedom (DoFs) are confounded in scattering channels. This scattering may affect a single DoF, e.g., a rotation in polarization, or DoFs may be coupled, e.g., polarization may be coupled to spatial modes, leading to a bit error in both scenarios. Adaptive techniques that probe the channel may be used to prescribe an optical field with properties that counteract the scattering. However, these techniques are ineffective if the channel changes stochastically bit-to-bit. In this work, we utilize partially coherent light to circumvent the deleterious effects of a channel that rapidly and randomly scatters one or two DoFs, including operations that couple DoFs. Partially coherent fields in which two DoFs are relevant can be described by a 4×4 coherence matrix, the rank of which (the coherence-rank) is invariant to unitary channels. In our experiments, we use channels that stochastically change bit-to-bit, unitarily scattering the polarization and spatial DoFs leading to scrambled information transfer when bits are encoded in just one of these DoFs. The scattering from these channels is subverted when data is encoded in the coherence-rank. These results offer a new method of using partially coherent light for optical communications through scrambling channels.

  PublisherDOI

• Harnessing Ultrafast Optical Pulses for 3D Microfabrication by Selective Tweezing and Immobilization of Colloidal Particles in an Integrated System

  Advanced Photonics Research · 2025-05-01

  articleOpen accessSenior authorCorresponding

  3D Microfabrication Microfabrication using nano- to micron-sized blocks has transformative potential for next-gen electronics, optoelectronics, and materials. Traditional methods are limited by scalability and precision. In article number 2500003, Kimani C. Toussaint Jr and co-workers introduce STIC, a single-laser system for precise colloidal manipulation and immobilization using femtosecond lasers, enabling efficient 3D assembly and imaging, advancing scalable, high-precision microstructure fabrication.

  PublisherOA PDFDOI

• Using flat-top light sheet generated by femtosecond-pulsed laser for optical manipulation of microscopic particles

  Optical Engineering · 2025-02-14 · 1 citations

  articleSenior authorCorresponding

  An optical tweezer (OT) platform based on light sheet microscopy with a continuous wave (CW) laser has been developed to trap multiple microscopic dielectric particles. However, the reduced gradient force resulting from the light sheet intensity distribution produces a trap stiffness an order of magnitude lower than its traditional circularly symmetric Gaussian counterpart. As a result, a high laser power on the order of 50 mW is required, which risks phototoxicity for biological applications. In addition, OT using 2D flat-top wavefronts has been shown to provide a more stable trap due to its steep intensity profile. The combination of flat-top beams and light-sheet techniques in OT significantly improves our ability to investigate and manipulate biological systems with exceptional precision and biological safety. Recently, we introduced femtosecond laser–assisted selective holding with ultra-low power (FLASH-UP), which enables the direct trapping of dielectric particles and bacteria using a 2D Gaussian wavefront at average powers sub-1 mW. To further elucidate the capabilities of FLASH-UP, we compare the OT applied to dielectric spheres using 2D flat-top, 1D light sheet, and 1D flat-top light sheet configurations generated by FLASH-UP to its CW counterpart utilizing average powers as low as 1 mW. Our findings demonstrate that FLASH-UP OT consistently generates higher trap stiffness than CW-OT. We propose leveraging flat-top light sheet OT to characterize the local and average mechanical properties of biological specimens.

  PublisherDOI

• Ultrafast optical pulses for advanced 3D microstructure fabrication and assembly

  2025-03-19

  articleSenior author

  Two-Photon Polymerization (TPP) has emerged as an exceptional 3D fabrication tool for tissue engineering and regenerative medicine. Constructs fabricated using TPP can be further enhanced by functional particles or cells to improve their targeted applications. This enhancement can be achieved through the combination of TPP using femtosecond lasers, and Optical Tweezers (OT) using Continuous Wave (CW) laser sources. However, this complicates the optical alignment; additionally, conventional CW-OT requires high intensity which poses risks of photodamage to heat-sensitive components. In this study, we demonstrate an integrated platform using the same ultrafast laser source for fabrication and manipulation of 3D microstructures without inducing thermal damage.

  PublisherDOI

• Harnessing ultrafast optical pulses for 3D microfabrication by selective tweezing and immobilization of colloidal particles in an integrated system

  ArXiv.org · 2025-01-13

  preprintOpen accessSenior author

  Microfabrication using nano- to micron-sized building blocks holds great potential for applications in next-generation electronics, optoelectronics, and advanced materials. However, traditional methods like chemical vapor deposition and molecular beam epitaxy require highly controlled environments and specialized equipment, limiting scalability and precision. To address these challenges, we present a single-laser platform for selective tweezing and immobilization of colloids (STIC) that integrates particle manipulation, assembly, and stabilization in one system. STIC utilizes a femtosecond laser at ultra-low power for precise, contact-free optical manipulation of colloids without material damage. At higher power, the same laser enables two-photon polymerization (TPP) to immobilize colloids securely in their intended positions. Using STIC, we demonstrate the assembly of 3D structures from dielectric beads to patterned arrangements of transition metal dichalcogenides (TMD e.g., MoS2). We also incorporate a TPP-fabricated handle as an intermediate support, significantly enhancing the optical tweezing efficiency of TMDs. The single-laser design eliminates the need for dual-laser systems, simplifying optical alignment, reducing heating damage, and improving efficiency. Additionally, we show that STIC supports direct multiphoton imaging for in situ inspection during fabrication. This work establishes a versatile, scalable optical platform for high-precision microstructure fabrication, offering a pathway to overcome current limitations in micro- and nanomanufacturing.

  PublisherOA PDFDOI

• Smartphone tristimulus colorimetry for skin-tone analysis at common pulse oximetry anatomical sites

  Biophotonics discovery. · 2025-05-19 · 4 citations

  articleOpen accessSenior authorCorresponding

  Significance: Smartphones hold great potential in point-of-care settings due to their accessibility and computational capabilities. This is critical as clinicians increasingly seek to quantify skin-tone, a characteristic that has been shown to impact the accuracy of pulse oximetry readings, particularly for dark skin tones, and, hence, disproportionately affect patient outcomes. Aim: We present a smartphone-based imaging technique for determining individual typology angle (ITA) and compare these results to those obtained using an industry-standard tristimulus colorimeter, particularly for the finger, a common site for pulse oximetry measurements. Approach: We employ a smartphone-based imaging method to extract ITA values from four volunteers across diverse skin-tones. We provide recommendations for minimizing errors caused by ambient light scattering, which can affect skin-tone readings. Results: The smartphone-based ITA (SITA) measurements with camera flash disabled and minimal ambient lighting correlate well with an industry-standard colorimeter without the need for auxiliary adapters and complex calibration. The method presented enables wide-field ITA mapping for skin-tone quantification that is accessible to clinicians. Conclusions: Our findings demonstrate that smartphone-based imaging provides an effective alternative for assessing skin-tone in clinical settings. The reduced complexity of the approach presented makes it highly accessible to the clinical community and others interested in carrying out pulse oximetry across a diversity of skin-tones in a manner that standardizes skin-tone assessment.

  PublisherOA PDFDOI

• Evaluation of a polarization-sensitive, dual-wavelength wearable photoplethysmography sensor across a range of skin tones

  Biophotonics discovery. · 2025-11-13

  articleOpen accessSenior authorCorresponding

  Significance: High-quality photoplethysmography (PPG) signals are essential for accurate extraction of cardiovascular metrics such as heart rate, heart rate variability, and perfusion index (PI). However, signal degradation for individuals with dark skin tones can compromise PPG quality and pose challenges for equitable sensing. Aim: We developed a dual-wavelength, polarization-sensitive PPG device to assess PI across a range of skin tones. Approach: To evaluate the impact of polarization on PPG signal quality, we recorded PI for co-polarized (polarized illumination and parallel-aligned polarized detection) and cross-polarized conditions (polarized illumination and orthogonally aligned polarized detection) at 655 and 940 nm in participants representing light, medium, and brown skin tone categories. Skin tone classification was based on the individual typology angle values derived from the CIE L*b* color space measurements. Results: ) relative to light from the co-polarized illumination condition, although the magnitude of the improvement decreases with lighter skin tone, indicating a possible interaction between skin tone and polarization. This improvement was consistent across all skin tones. Conclusions: Our results suggest that the cross-polarized condition improves PPG signal quality by reducing the influence of superficial scattering and enhancing deeper vascular signals. This approach may be especially beneficial for individuals with darker skin tones and offers a promising path toward more robust and inclusive physiological monitoring using PPG-based technologies.

  PublisherOA PDFDOI

• Harnessing Ultrafast Optical Pulses for 3D Microfabrication by Selective Tweezing and Immobilization of Colloidal Particles in an Integrated System

  Advanced Photonics Research · 2025-03-31 · 1 citations

  articleOpen accessSenior authorCorresponding

  Microfabrication using nano‐ to micron‐sized building blocks holds a great potential for applications in next‐generation electronics, optoelectronics, and advanced materials. However, traditional methods like chemical vapor deposition and molecular beam epitaxy require highly controlled environments and specialized equipment, limiting scalability and precision. To address these challenges, a single‐laser platform is presented for selective tweezing and immobilization of colloids (STIC) that integrates particle manipulation, assembly, and stabilization in one system. STIC utilizes a femtosecond laser at ultra‐low power for precise, contact‐free optical manipulation of colloids without material damage. At higher power, the same laser enables two‐photon polymerization (TPP) to immobilize colloids securely in their intended positions. Using STIC, the assembly of 3D structures from dielectric beads to patterned arrangements of transition metal dichalcogenides (TMDs e.g., MoS 2 ) is demonstrated. Also a TPP‐fabricated handle as an intermediate support is incorporated which significantly enhances the optical tweezing efficiency of TMDs. The single‐laser design eliminates the need for dual‐laser systems, simplifying optical alignment, reducing heat damage, and improving efficiency. Additionally, it is shown that STIC supports direct multiphoton imaging for in situ inspection during fabrication. This work establishes a versatile, scalable optical platform for high‐precision microstructure fabrication, offering a pathway to overcome current limitations in micro‐ and nanomanufacturing.

  PublisherOA PDFDOI

Recent grants

• CAREER (IDBR): Novel Quantitative Second-Harmonic Generation Microscope for Fundamental Study of Collagen Fibers in the Eye

  NSF · $620k · 2010–2015

• Optimizing the Nonlinear Optical Response from Nanoantenna Arrays for Frequency Up-Conversion for Solar Cells

  NSF · $450k · 2010–2013

• Collaborative Research: A Digital Manufacturing Platform to Democratize Biological Tissue Access Using Smart Two-Photon Polymerization

  NSF · $513k · 2021–2024

• Minority Postdoctoral Research Fellowship for FY 2005

  NSF · $120k · 2005–2007

• Eager: Demonstration of Space-Time Surface Plasmon Polaritons

  NSF · $150k · 2020–2022

Frequent coauthors

• Zixi Lin

  Brown University

  48 shared

• Mbaye Diouf

  45 shared

• Joshua A. Burrow

  43 shared

• Krishangi Krishna

  42 shared

• Mitchell Harling

  41 shared

• Barbara L. McFarlin

  University of Illinois Chicago

  32 shared

• Ayman F. Abouraddy

  28 shared

• Amy J. Wagoner Johnson

  28 shared

Awards & honors

• SPIE Diversity Outreach Award (2026)
• 2025 Hazeltine Innovation Awards
• Chan Zuckerberg Initiative Grant ($1.5M) (2024)