Seminar on non-conservative optical forces in speckle fields by Laura Pérez García from UNAM, Faraday, 26 jun 18

Non conservative optical forces of speckle fields generated with a SLM
Seminar by Laura Pérez García from the Universidad National Autónoma de México (UNAM).

Speckle patterns arise when a highly coherent light source impinges on a rough surface or when it propagates through an inhomogeneous media. This phenomenon appeared after the invention of the laser in the 70’s and, initially was considered as a feature to avoid in optical setups since it limits the imaging resolution. However, speckle patterns can give information about the process that generates it and also can be incorporated by researchers in astronomy, surface characterization, biology, medicine and chemical processes [1, 2, 3]. In particular, speckle has been used in the last years in the area of optical micromanipulation to study the interaction of colloidal particles in random potentials[4, 5]. It is important the use of speckle patterns since it has a wide range of characteristic lengths, optical vortexes and intrinsic robustness to misalignment.

We’ve studied speckle patterns generated by a spatial light modulator (SLM), emphasizing in the intensity distribution, its spatial properties and the dynamical properties of particles subjected to these fields. Specifically, I studied the dynamical behavior of 1.54μm and 1μm spherical polystyrene particles embedded in deionized water in the presence of a speckle light field. We generated the speckle pattern using a 532 nm-wavelength laser which impinged on an SLM, which projected random values for each pixel, and then redirected to an optical micromanipulation system. It is important to mention that, by varying the optical resolution of the system with a diaphragm, we allowed the interference between all the wavefronts.

We analyzed the particle’s trajectories in the overdamped regime as an approximation for the particle dynamics. We didn’t assume the existence of a scalar potential, so we can study the nonconservative nature of the optical forces[6]. Additionally, the mean squared displacement was calculated and com- pared with free diffusion, we observed different regimes, owing to the spatial features in the speckle patterns used.

  1.  J.C. Dainty. Laser speckle and related phenomena. Topics in Applied Physics. Springer-Verlag, 1984.
  2.  J.W. Goodman. Speckle Phenomena in Optics: Theory and Applications. Roberts & Company, 2007.
  3. H.J. Rabal and R.A. Braga. Dynamic Laser Speckle and Applications. Optical Science and Engineering. CRC Press, 2008.
  4. Florian Evers, Christoph Zunke, Richard D L Hanes, J ̈org Bewerunge, Imad Ladadwa, Andreas Heuer, Stefan U. Egelhaaf, Giorgio Giovanni Volpe, Giorgio Giovanni Volpe, and Sylvain Gigan. Particle dynamics in two-dimensional random-energy landscapes: Experiments and simulations. Physical Review E – Statistical, Nonlinear, and Soft Matter Physics, 88(2):3936, 2014.
  5. Giorgio Volpe, Giovanni Volpe, and Sylvain Gigan. Brownian motion in a speckle light field: tunable anomalous diffusion and selective optical manipulation. Scientific Reports, 4:3936, 2014.
  6. Pinyu Wu, Rongxin Huang, Christian Tischer, Alexandr Jonas, and Ernst Ludwig Florin. Direct measurement of the nonconservative force field generated by optical tweezers. Physical Review Letters, 103(10):4–7, 2009.

Place: Faraday room, Fysik Origo, Fysik
Time: 26 June, 2018, 15:00

Seminar on Langevin equation in the small mass limit by Jan Wehr from the University of Arizona, Nexus, 21 Jun 18

Langevin equation in the small mass limit: higher order approximations
Seminar by Jan Wehr from the University of Arizona, Tucson (AZ), USA.

Abstract: We study the Langevin equation describing the motion of a particle of mass m in a potential and/or magnetic field, with state-dependent drift and diffusion.  We develop a hierarchy of approximate equations for the position degrees of freedom that achieve accuracy of order m^{k/2} over finite time intervals for any positive integer k.  This extends the previous work in which effective equations for the position variables were derived in the limit when the mass goes to zero.  The work was done jointly with Jeremiah Birrell.

Place: Nexus, meeting room, Fysik Origo, Fysik
Time: 21 June, 2018, 11:00

Viridiana Carmosa Sosa visits the Soft Matter Lab. Welcome!

Viridiana Carmosa Sosa studied her bachelor and master degree in Physics in the National Autonomous University of Mexico. In those years, she was working with optical tweezers, structured laser beams, and cavitation bubbles. Nowadays, she is a PhD student at Sapienza University of Rome under the supervision of Roberto Di Leonardo, where she uses two-photon polymerization to fabricate microstructures that allow her to study the dynamics of active and non-active matter at the micron scale.

She will spend a week at the Soft Matter Lab to work together with Alessandro Magazzù on a joint project.

Francesco Patti visits the Soft Matter Lab. Welcome!

Francesco Patti is a PhD student in Physics at the University of Messina (started in October 2017). His master’s degree thesis was about “Theoretical study of the interaction between E.M. radiation and chiral nanomaterials” (July 2017) and now he is a visiting student at the Soft Matter Lab where he will work on modeling of optical forces in liquids and vacuum as well as modelling of passive and active stochastic systems“ (June-July 2018).

Antonio A. R. Neves visits the Soft Matter Lab. Welcome!

Antonio Alvaro Ranha Neves is a Visiting Professor from the Federal University of ABC in Brazil. His visiting position is financed through a FAPESP-ERC grant. He will visit us for 4 months from May 12, 2018, to September 12, 2018.

He works mainly with optical tweezers studying optical forces with both experimental and theoretical tools.

He obtained his Ph.D. in physics in 2006, at the State University of Campinas (Brazil). From 2006 to 2012, he worked as a postdoctoral researcher at the National Nanotechnology Laboratories of the Nanoscience Institute in Lecce (Italy), within the Soft-matter division. Since 2012, he is a professor at the Federal University of ABC (Brazil), accredited in the graduate program of Nanoscience and Advanced Materials.

His main research interest is in the field of light-matter interaction, with a special focus on the applications of optical tweezers as well as linear and multi-photon spectroscopy as well. His current line of research is the study of bull sperm motility with optical tweezers, and starting the characterization of thermal properties of metallic nanoparticles in optical traps.

Seminar on photophoretic forces by Ayan Banerjee from IISER-Kolkata, Nexus, 20 Mar 18

Photophoretic forces: A new enabler for robust single fiber-based optical traps in air
Seminar by Ayan Banerjee from the Indian Institute of Science Education and Research (IISER), Kolkata, India

Abstract: Photophoretic forces, which are derived from the momentum exchange of absorbing particles with surrounding fluid molecules, are especially useful for trapping particles in air, where their very large magnitude (about five orders more than optically induced dipole forces) successfully balances gravity. Thus, particles levitate in the direction of gravity, while in the transverse direction, they are trapped by a restoring force emanating from the rotation of the particles around the trapping beam axis. Photophoretic forces thus enable the use of single optical fibers for stable three dimensional traps. In this talk, I shall describe our efforts to develop such single fiber based traps, where we find that a single mode fiber is not necessarily the most efficacious in terms of trapping. Robust trapping is achieved when the off-axis intensity of the trapping beam is high, so that rather unexpectedly, we observe that a single multi-mode fiber allows much stronger trapping in general, and especially in the radial direction compared to a single mode finer. We are able to trap particles at extremely low laser powers (around 5 mW) in air, and can manipulate printer toner particles of diameter less than 20 microns at translation velocities of 5 mm/s in our multi-mode fiber trap. Particles can be manipulated by merely changing the power in the trapping beam, which accentuates the power and promise of this technique as a possible candidate for single fiber-based hand held tweezers for confining and even spectroscopically analysing aerosols or pathogens present in the air.

Bio: Ayan Banerjee has been working in the field of optics and spectroscopy for the last 22 years. He obtained his Ph.D in physics from the Indian Institute of Science, Bangalore, following which he was a research scientist at General Electric Global Research, Bangalore, India. Since 2009, he has been an associate professor of physics at the Indian Institute of Science Education and Research (IISER), Kolkata. His research interests span a wide range of subjects in optics and spectroscopy. At IISER, he has set up an optical tweezers lab to study diverse problems in a truly interdisciplinary mode of research.

Place: Nexus, meeting room, Fysik Origo, Fysik
Time: 20 March, 2018, 14:00