Faster and more accurate geometrical-optics optical force calculation using neural networks published in ACS Photonics

Focused rays scattered by an ellipsoidal particles (left). Optical torque along y calculated in the x-y plane using ray scattering with a grid of 1600 rays (up, right) and using a trained neural network (down, right). (Image by the Authors of the manuscript.)
Faster and more accurate geometrical-optics optical force calculation using neural networks
David Bronte Ciriza, Alessandro Magazzù, Agnese Callegari, Gunther Barbosa, Antonio A. R. Neves, Maria A. Iatì, Giovanni Volpe, Onofrio M. Maragò
ACS Photonics, 2022
doi: 10.1021/acsphotonics.2c01565
arXiv: 2209.04032

Optical forces are often calculated by discretizing the trapping light beam into a set of rays and using geometrical optics to compute the exchange of momentum. However, the number of rays sets a trade-off between calculation speed and accuracy. Here, we show that using neural networks permits one to overcome this limitation, obtaining not only faster but also more accurate simulations. We demonstrate this using an optically trapped spherical particle for which we obtain an analytical solution to use as ground truth. Then, we take advantage of the acceleration provided by neural networks to study the dynamics of an ellipsoidal particle in a double trap, which would be computationally impossible otherwise.

Press release on Tunable critical Casimir forces counteract Casimir-Lifshitz attraction

An illustration of microscopic gold flakes on surface. (Image by F. Schmidt.)
The article Tunable critical Casimir forces counteract Casimir-Lifshitz attraction has been featured in the News of the University of Gothenburg (in English and in Swedish), SISSA-International School of Advanced Studies in Trieste, Italy, Heinrich-Heine-Universität Düsseldorf, and Friedrich-Schiller-Universität Jena.

The study, published in Nature Physics and co-written by researchers at the Soft Matter Lab of the Department of Physics at the University of Gothenburg, demonstrate that tunable repulsive critical Casimir forces can be used to counteract stiction, i.e., the tendency of tiny parts of micro- and nanoelectromechanical devices to stick together, which is caused by the Casimir-Lifshitz interaction.

The study is featured also in Phys.org, NanoWerk.

Here the links to the press releases:
Casimir vs Casimir – using opposing forces to improve nanotechnology (GU, English)
https://www.gu.se/nyheter/casimir-vs-casimir-klaschande-krafter-kan-forbattra-nanotekniken (GU, Swedish)
Casimir vs Casimir – usare forze opposte per migliorare le nanotecnologie (SISSA, Italian)
Casimir vs Casimir – using opposing forces to improve nanotechnology (SISSA, English)
Nano-Bauteile clever voneinander lösen (Heinrich-Heine-Universität Düsseldorf)
Clever method for separating nano-components (Friedrich-Schiller-Universität Jena)
Clever method for separating nano-components (Phys.org)
Clever method for separating nano-components (NanoWerk)

Tunable critical Casimir forces counteract Casimir-Lifshitz attraction published in Nature Physics

Gold flake suspended over a functionalized gold-coated substrate. (Image by F. Schmidt.)
Tunable critical Casimir forces counteract Casimir-Lifshitz attraction
Falko Schmidt, Agnese Callegari, Abdallah Daddi-Moussa-Ider, Battulga Munkhbat, Ruggero Verre, Timur Shegai, Mikael Käll, Hartmut Löwen, Andrea Gambassi and Giovanni Volpe
Nature Physics (2022)
arXiv: 2202.10926
doi: 10.1038/s41567-022-01795-6

Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occurring even at zero temperature. Their generalization at finite temperature and in material media are referred to as Casimir-Lifshitz forces. These forces are typically attractive, leading to the widespread problem of stiction between the metallic parts of micro- and nanodevices. Recently, repulsive Casimir forces have been experimentally realized but their reliance on specialized materials prevents their dynamic control and thus limits their further applicability. Here, we experimentally demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to actively control microscopic and nanoscopic objects with nanometer precision. We demonstrate this by using critical Casimir forces to prevent the stiction caused by the Casimir-Lifshitz forces. We study a microscopic gold flake above a flat gold-coated substrate immersed in a critical mixture. Far from the critical temperature, stiction occurs because of dominant Casimir-Lifshitz forces. Upon approaching the critical temperature, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. This experimental demonstration can accelerate the development of micro- and nanodevices by preventing stiction as well as providing active control and precise tunability of the forces acting between their constituent parts.

Playing with Active Matter on ArXiv

One exemplar of the HEXBUGS used in the experiment. (Image by the Authors of the manuscript.)
Playing with Active Matter
Angelo Barona Balda, Aykut Argun, Agnese Callegari, Giovanni Volpe
arXiv: 2209.04168

In the last 20 years, active matter has been a very successful research field, bridging the fundamental physics of nonequilibrium thermodynamics with applications in robotics, biology, and medicine. This field deals with active particles, which, differently from passive Brownian particles, can harness energy to generate complex motions and emerging behaviors. Most active-matter experiments are performed with microscopic particles and require advanced microfabrication and microscopy techniques. Here, we propose some macroscopic experiments with active matter employing commercially available toy robots, i.e., the Hexbugs. We demonstrate how they can be easily modified to perform regular and chiral active Brownian motion. We also show that Hexbugs can interact with passive objects present in their environment and, depending on their shape, set them in motion and rotation. Furthermore, we show that, by introducing obstacles in the environment, we can sort the robots based on their motility and chirality. Finally, we demonstrate the emergence of Casimir-like activity-induced attraction between planar objects in the presence of active particles in the environment.

Presentation by A. Callegari at SPIE-ETAI, San Diego, 23 August 2022

Simplified sketch of the neural network used for the simulations of intracavity optical trapping. (Image by A. Callegari.)
Simulating intracavity optical trapping with machine learning
Agnese Callegari, Mathias Samuelsson, Antonio Ciarlo, Giuseppe Pesce, David Bronte Ciriza, Alessandro Magazzù, Onofrio M. Maragò, Antonio Sasso, Giovanni Volpe
Submitted to SPIE-ETAI
Date: 23 August 2022
Time: 13:40 (PDT)

Intracavity optical tweezers have been proven successful for trapping microscopic particles at very low average power intensity – much lower than the one in standard optical tweezers. This feature makes them particularly promising for the study of biological samples. The modeling of such systems, though, requires time-consuming numerical simulations that affect its usability and predictive power. With the help of machine learning, we can overcome the numerical bottleneck – the calculation of optical forces, torques, and losses – reproduce the results in the literature and generalize to the case of counterpropagating-beams intracavity optical trapping.

Soft Matter Lab members present at SPIE Optics+Photonics conference in San Diego, 21-25 August 2022

The Soft Matter Lab participates to the SPIE Optics+Photonics conference in San Diego, CA, USA, 21-25 August 2022, with the presentations listed below.

Giovanni Volpe is also co-author of the presentations:

A. Callegari and F. Schmidt won the Poster Prize at the 729. WE Heraeus Seminar on Fluctuation-induced Forces

Gold flake suspended over a functionalized gold-coated substrate. (Image by F. Schmidt.)
Agnese Callegari and Falko Schmidt share one of the three Poster Prizes of the 729. WE-Heraeus-Seminare on Fluctuation-induced Forces.

The two complementary posters focused on the experimental and theoretical/numerical aspects of a system constituted by a micron-sized gold flake suspended in a solution of water-lutidine at critical concentration above a gold-coated substrate. The dynamic of such a system is driven by the interplay of Casimir-Lifshitz forces and critical Casimir forces, which, under convenient circumstances, are the keystone to prevent stiction.

The other two Poster Prizes were awarded to Ariane Soret ( University of Luxembourg, with the poster: Forces Induced by Quantum Mesoscopic Coherent Effects) and Fred Hucht (University of Duisburg-Essen, with the poster: The Square-Lattice Ising Model on the Rectangle).

The Poster Prizes recipients’ names were announced during the closing session on 17 February. Each prize consisted in 100 EUR, which in the case of Agnese and Falko will be shared equally between the two. Andrea Gambassi, who made the announcement on the behalf of the organizers, amusingly mentioned the custom of equally sharing the Nobel Prize.

The Wilhelm and Else Heraeus Foundation is a private institution that supports scientific research and education with an emphasis on physics. It was established in 1963 by Dr. Wilhelm Heinrich Heraeus and his wife Else Heraeus. The Wilhelm and Else Heraeus Foundation is Germany’s most important private institution funding physics.

Flash Talk by F. Schmidt at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 16 February 2022

Title slide of the presentation. (Image by F. Schmidt.)
Casimir-Lifshitz forces vs. Critical Casimir forces: Trapping and releasing of flat metallic particles
Falko Schmidt
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
16 February 2022, 14:50 CET

Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occuring even at zero temperature. Their generalization at finite temperature and in material media are referred to as Casimir-Lifshitz forces. These forces are typically attractive, leading to the widespread problem of stiction between the metallic parts of micro- and nanodevices. Recently, repulsive Casimir forces have been experimentally realized but their use of specialized materials stills means that the system can not be controlled dynamically and thus limits further implementation to real-world applications. Here, we experimentally demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to prevent stiction due to Casimir-Lifshitz forces. We show that critical Casimir forces can be dynamically tuned via temperature, eventually overcoming Casimir-Lifshitz attraction. We study a microscopic gold flake above a flat gold-coated substrate immersed in a critical mixture. Far from the critical temperature, stiction occurs because of Casimir-Lifshitz forces. Upon approaching the critical temperature, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. By removing one of the key limitations to their deployment, this experimental demonstration can accelerate the development of micro- and nanodevices for a broad range of applications.

Flash Talk by A. Callegari at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 14 February 2022

Potential energy landscape for a flake suspended on a patterned substrate. (Image by A. Callegari.)
Theoretical and numerical study of the interplay of Casimir-Lifshitz and critical Casimir force for a metallic flake suspended on a metal-coated substrate
Agnese Callegari
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
14 February 2022, 14:50 CET

Casimir-Lifshitz forces arise between uncharged metallic objects because of the confinement of the electromagnetic fluctuations. Typically, these forces are attractive, and they are the main cause of stiction between microscopic metallic parts of micro- and nanodevices. Critical Casimir forces emerge between objects suspended in a critical binary liquid mixture upon approaching the critical temperature, can be made either attractive or repulsive by choosing the appropriate boundary conditions, and dynamically tuned via the temperature.
Experiments show that repulsive critical Casimir forces can be used to prevent stiction due to Casimir-Lifshitz forces. In a recent work, a microscopic metallic flake was suspended in a liquid solution above a metal-coated substrate [1]. By suspending the flake in a binary critical mixture and tuning the temperature we can control the flake’s hovering height above the substrate and, in the case of repulsive critical Casimir forces, prevent stiction.
Here, we present the model for the system of the metallic flake suspended above a metal-coated substrate in a binary critical mixture and show that repulsive critical Casimir forces can effectively counteract Casimir-Lifshitz forces and can be used to control dynamically the height of the flake above the surface. This provides a validation of the experimental results and a base to explore and design the behavior of similar systems in view of micro- and nanotechnological applications.

References
[1] F. Schmidt, A. Callegari, A. Daddi-Moussa-Ider, B. Munkhbat, R. Verre, T. Shegai, M. Käll, H. Löwen, A. Gambassi and G. Volpe, to be submitted (2022)

Book “Simulation of Complex Systems” published at IOP

Book cover. (From the IOP website.)
The book Simulation of Complex Systems, authored by Aykut Argun, Agnese Callegari and Giovanni Volpe, has been published by IOP in December 2021.

The book is available for the students of Gothenburg University and Chalmers University of Technology through the library service of each institution.
The example codes presented in the book can be found on GitHub.

Links
@ IOP Publishing

@ Amazon.com

Citation 
Aykut Argun, Agnese Callegari & Giovanni Volpe. Simulation of Complex Systems. IOP Publishing, 2022.
ISBN: 9780750338417 (Hardback) 9780750338431 (Ebook).