title: Dynamic axial control over optically levitating particles in air with an electrically-tunable variable-focus lens.
abstract: Efficient delivery of viruses, proteins and biological macromelecules into a micrometer-sized focal spot of an XFEL beam for coherent diffraction imaging inspired new development in touch-free particle injection methods in gaseous and vacuum environments. This paper lays out our ongoing effort in constructing an all-optical particle delivery approach that uses piconewton photophoretic and femtonewton light-pressure forces to control particle delivery into the XFEL beam. We combine a spatial light modulator (SLM) and an electrically tunable lens (ETL) to construct a variable-divergence vortex beam providing dynamic and stable positioning of levitated micrometer-size particles, under normal atmospheric pressure. A sensorless wavefront correction approach is used to reduce optical aberrations to generate a high quality vortex beam for particle manipulation. As a proof of concept, stable manipulation of optically-controlled axial motion of trapped particles is demonstrated with a response time of 100ms. In addition, modulation of trapping intensity provides a measure of the mass of a single, isolated particle. The driving signal of this oscillatory motion can potentially be phase-locked to an external timing signal enabling synchronization of particle delivery into the x-ray focus with XFEL pulse train.
authors.affiliation: Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia; The State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China; Equal contribution.
authors.forename: Wenguo
authors.initials: W
authors.lastname: Zhu
authors.affiliation: Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia.
authors.forename: Niko
authors.initials: N
authors.lastname: Eckerskorn
authors.affiliation: Research School of Engineering, The Australian National University, Canberra ACT 0200, Australia.
authors.forename: Avinash
authors.initials: A
authors.lastname: Upadhya
authors.affiliation: Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia; The State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China.
authors.forename: Li
authors.initials: L
authors.lastname: Li
authors.affiliation: Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia; andrei.rode@anu.edu.au.
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Rode
authors.affiliation: Research School of Engineering, The Australian National University, Canberra ACT 0200, Australia; Equal contribution; steve.lee@anu.edu.au.
authors.forename: Woei Ming
authors.initials: WM
authors.lastname: Lee
keywords: (140.7010) Laser trapping; (350.4855) Optical tweezers or optical manipulation