About

John Fourkas is the Millard Alexander Professor of Chemistry at the University of Maryland, with a background in physical chemistry, having earned his B.S. and M.S. degrees from the California Institute of Technology and his Ph.D. from Stanford University. His research focuses on ultrafast nonlinear optical spectroscopy of liquids, the dynamics of nanoconfined liquids, nonlinear optical microscopy, and nontraditional approaches to micro- and nanofabrication. He investigates chemical and physical processes at liquid/solid interfaces, developing spectroscopic tools to probe interface-specific dynamics, and employs molecular dynamics simulations to gain molecular-level insights. His work also encompasses applications of multiphoton absorption for high-resolution imaging and nanofabrication, as well as controlling triplet state dynamics to influence photophysical and photochemical processes. Additionally, he explores multicolor lithography techniques to achieve resolutions comparable to EUV lithography using multiple wavelengths of visible light. In cellular biophysics, he uses multiphoton fabrication to create substrates that study cell behavior, discovering mechanisms such as esotaxis, where cell interaction with topography influences actin polymerization. His contributions extend to advancing understanding and control of complex photochemical events, with a focus on developing innovative optical and nanofabrication techniques.

Research topics

• Chemistry
• Nanotechnology
• Physical chemistry

Selected publications

• Multicolor lithography in thin films

  2026-04-08

  articleSenior author
  PublisherDOI

• Nanotopographic Control of Actin Waves and Growth Cone Navigation in Developing Neurons

  bioRxiv (Cold Spring Harbor Laboratory) · 2025-05-21

  preprintOpen access

  The development of axons and dendrites (neurites) in a neural circuit relies on the dynamic interplay of cytoskeletal components, especially actin, and the integration of diverse environmental cues. Building on prior findings that actin dynamics can serve as a primary sensor of physical guidance cues, this work investigates the role of nanotopography in modulating and guiding actin waves and neurite-tip dynamics during early neural circuit development. Although actin dynamics is well known to contribute to pathfinding in wide axonal tips, typically referred to as growth cones, we also observe dynamic actin remodeling throughout neurites and at other, narrower, neurite tips. We find that actin-wave speeds do not change significantly in the first two weeks of neurite development on flat substrates, but decrease over the same period in neurites on nanoridges. The ability of nanoridges to guide actin waves and the neurite-tip direction also decreases as neurites mature, both for narrow tips and wide growth cones. This change in responsiveness to physical guidance cues with neuronal maturation may impact the regenerative capacity of developing neural cells that are inserted into mature brains.

  PublisherOA PDFDOI

• The mechanisms of endothermic triplet energy transfer in photochemical systems

  Chemical Physics Reviews · 2025-08-28 · 6 citations

  articleOpen accessSenior author

  Short-range triplet energy transfer (TET) is a photophysical phenomenon that is central to important photochemical and photophysical processes. For instance, Dexter-type energy transfer facilitates the extraction of dark triplet excitons in electroluminescent systems and sensitizes metastable triplet states that drive photochemical reactions or undergo triplet–triplet annihilation to achieve upconversion. The rate of TET is expected to be vanishingly small when the energy gap between the donor and the acceptor is several times larger than the magnitude of thermal energy. However, recent studies have shown that TET in such “endothermic” cases can be surprisingly efficient. In this review, we present a detailed account of experimental and theoretical work that sheds light on the mechanisms that influence the rate of “uphill” TET. We show that well-understood factors that are often not considered, such as molecular flexibility and low-frequency vibrations, may play a major role in facilitating endothermic TET. We further provide insights regarding how the prevalence of endothermic TET in certain systems can be understood from a kinetic, rather than a thermodynamic, perspective. This review will be relevant for scientists who seek to exploit endothermic TET to design and use novel donor–acceptor systems.

  PublisherDOI

• Identifying efficiency-loss pathways in triplet–triplet annihilation upconversion systems

  Physical Chemistry Chemical Physics · 2025-01-01 · 4 citations

  articleOpen accessSenior author

  Triplet-triplet annihilation upconversion (TTA-UC) systems have been studied extensively recently, and have been proposed for use in a wide range of applications. Identification of the dominant mechanisms of upconversion-efficiency loss (UEL) will assist in the development of efficient TTA-UC systems. In this work, we combine experiments and kinetic analysis to study UEL. We identify exciplex formation and reverse triplet energy transfer (TET) as the two most important UEL mechanisms in the model TTA-UC system of platinum octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA). Based on spectral analysis and time-resolved photoluminescence experiments, we show that exciplex formation is a potent UEL pathway in the PtOEP-DPA system. We demonstrate that prolonged sensitizer phosphorescence arises from reverse TET from annihilator triplet states, and that the reverse TET is likely facilitated by thermal population of low-frequency vibrational states in the sensitizer and the annihilator. Additionally, we demonstrate how the rate constants for reverse TET and exciplex formation can be estimated based on knowledge of a few key parameters and the experimental value of the optimum sensitizer concentration.

  PublisherOA PDFDOI

• Molecular Fingerprints of Ice Surfaces in Sum Frequency Generation Spectra: A First-Principles Machine Learning Study

  JACS Au · 2025-02-07 · 10 citations

  articleOpen access

  Understanding the molecular-level structure and dynamics of ice surfaces is crucial for deciphering several chemical, physical, and atmospheric processes. Vibrational sum-frequency generation (SFG) spectroscopy is the most prominent tool for probing the molecular-level structure of the air–ice interface as it is a surface-specific technique, but the molecular interpretation of SFG spectra is challenging. This study utilizes a machine-learning potential, along with dipole and polarizability models trained on ab initio data, to calculate the SFG spectrum of the air–ice interface. At temperatures below ice surface premelting, our simulations support the presence of a proton-ordered arrangement at the Ice Ih surface, similar to that seen in Ice XI. Additionally, our simulations provide insight into the assignment of SFG peaks to specific molecular configurations where possible and assess the contribution of subsurface layers to the overall SFG spectrum. These insights enhance our understanding and interpretation of vibrational studies of environmental chemistry at the ice surface.

  PublisherDOI

• Suppressing collective cell motion with bidirectional guidance cues

  Physical review. E · 2025-02-25 · 2 citations

  article

  In natural environments, cells move in the presence of multiple physical and chemical guidance cues. Using a model system for such guided cell migration, Dictyostelium discoideum (Dicty), we investigate how chemical and physical signals compete in guiding the motion of cell groups. In Dicty cells, chemical signals can lead to collective streaming behavior, in which cells follow one another head-to-tail and aggregate into clusters of ∼10^{5} cells. We use experiments and numerical simulations to show that streaming and aggregation can be suppressed by the addition of a physical guidance cue of comparable strength to the chemical signals, parallel nanoridges. The bidirectional character of physical guidance by ridges is a determining factor in the suppression of streaming and aggregation. Thus, combining multiple types of guidance cues is a powerful approach to trigger or explain a broad range of collective cell behaviors.

  PublisherDOI

• Nanotopographic control of actin waves and growth cone navigation in developing neurons

  Frontiers in Cell and Developmental Biology · 2025-09-10 · 2 citations

  articleOpen access

  The development of axons and dendrites (neurites) in a neural circuit relies on the dynamic interplay of cytoskeletal components, especially actin, and the integration of diverse environmental cues. Building on prior findings that actin dynamics can serve as a primary sensor of physical guidance cues, this work investigates the role of nanotopography in modulating and guiding actin waves and neurite-tip dynamics during early neural circuit development. Although actin dynamics is well known to contribute to pathfinding in wide axonal tips, typically referred to as growth cones, we also observe dynamic actin remodeling throughout neurites and at other, narrower, neurite tips. We find that actin-wave speeds do not change significantly in the first 2 weeks of neurite development on flat substrates, but decrease over the same period in neurites on nanoridges. The ability of nanoridges to guide actin waves and the neurite-tip direction also decreases as neurites mature, both for narrow tips and wide growth cones. This change in responsiveness to physical guidance cues with neuronal maturation may impact the regenerative capacity of developing neural cells that are inserted into mature brains.

  PublisherOA PDFDOI

• Three-color lithography for high resolution fabrication

  2024-04-09

  articleSenior author

  Three-color lithography (3CL) is a technique for fabricating high-resolution nanopatterns using visible light. This technique has been benchmarked in one promising photoresist by fabricating arrays of posts in a three-color material. The smallest posts are ~130nm in diameter, and the best pitches are around 180nm. If the 3CL deactivation step is not used, any features that are fabricated this close together merge due to proximity effects. The benefit of 3CL over other techniques, such as multi-patterning and extreme UV lithography, is that 3CL uses inexpensive optics and light sources, and the setup is simpler because high vacuum is not necessary.

  PublisherDOI

• Chiral Electrokinetic Phenomena in Single Nanopores

  Electroanalysis · 2024-08-14 · 1 citations

  articleOpen access

  Abstract The arrangement of solvent molecules and ions at solid–liquid interfaces determines electrochemical properties that are important in separations platforms, sensing technologies, and energy‐storage systems. Here we show that single glass and polymer pores in contact with propylene carbonate (PC) solutions of LiClO 4 exhibit an effective surface potential that is modulated by the enantiomeric excess of the solvent. In particular, electrochemical and electrokinetic measurements of ionic transport through glass pipettes and polymer pores reveal that the effective surface potential is significantly lower in solutions prepared using enantiomerically pure PC than in solutions prepared using racemic PC. Both pore systems became positively charged in all racemic solutions examined in the range of LiClO 4 concentrations between 1 mM and 100 mM, whereas solutions in ( R )‐(+)‐PC induced a positive surface potential only at concentrations above ~5 mM. The effective surface potential is quantified through asymmetry in current–voltage curves and zeta‐potential measurements. Vibrational sum‐frequency‐generation experiments on LiClO 4 solutions in racemic and enantiomerically pure PC indicate that the surface lipid‐bilayer‐like region in the former is more strongly organized than in the latter, dictating the favorable positions for lithium and perchlorate ions in each case. The more ordered molecular packing in the racemic liquid leads to accumulation of lithium ions on the outside of the bilayer, creating a higher effective positive charge. Our results highlight the extreme sensitivity of the interfacial potential on molecular organization of the solvent, and the relatively unexplored role that chirality can play in electrokinetic phenomena.

  PublisherOA PDFDOI

• Multicolor lithography, a promising approach to large-area nanopatterning

  2024-04-10

  articleSenior author

  Multicolor approaches to resolution enhancement in visible-light lithography hold the promise to afford resolution on the scale of tens of nm for large-area nanofabrication at a cost that is attractive for medium- to small-volume manufacturing applications, as well as in the research laboratory. In this talk I will review the history of multicolor lithography and discuss some of the latest advances in materials and methods.

  PublisherDOI

Recent grants

• SNM: Three-Color Photolithography for Scalable, Large-Area, Low-Cost Nanomanufacturing

  NSF · $1.5M · 2014–2019

• Nonlinear-Optical Spectroscopy and Microscopy of Confined Liquids

  NSF · $217k · 2005–2007

• New Nonlinear Optical Approaches for Probing the Microscopic Nature of Liquids and Solutions

  NSF · $478k · 2018–2023

• CRC: Molecular-Level Structure and Dynamics at Solid-Liquid Interfaces

  NSF · $2.5M · 2006–2012

• Nonlinear Optical and Simulation Studies of Perturbed Liquids

  NSF · $482k · 2015–2019

Frequent coauthors

• Richard A. Farrer

  Colorado State University Pueblo

  68 shared

• Wolfgang Losert

  University of Maryland, College Park

  57 shared

• Linjie Li

  Lishui Central Hospital

  42 shared

• Christopher N. LaFratta

  39 shared

• Matt J. Hourwitz

  University of Maryland, College Park

  34 shared

• Tommaso Baldacchini

  Edwards Lifesciences (United States)

  32 shared

• Edo Waks

  32 shared

• N. R. Aluru

  Walker (United States)

  32 shared

Education

• PhD, Chemistry

  Stanford University

  1991

• MS, Chemistry

  California Institute of Technology

  1986

• BS, Chemistry

  California Institute of Technology

  1986

Awards & honors

• Dean’s Award for Excellence in Teaching (2023)
• Regents Faculty Award for Research, Scholarship and Creative…
• Camille Dreyfus Teacher-Scholar Award (1999)
• Alfred P. Sloan Research Fellow (1998)
• Research Corporation Cottrell Scholar Award (1997)