Researchers have calculated perfect optical tweezers for manipulating atoms, molecules or even living cells.
A special calculation method was developed to determine the perfect waveform to manipulate small particles in the presence of a disordered environment. This makes it possible to hold, move or rotate individual particles inside a sample – even if they cannot be touched directly. The tailor-made light beam becomes a universal remote control for everything small. Microwave experiments have already demonstrated that the method works.
Calculating the Optimal Wave
To achieve this, the particle and its disordered environment are first illuminated with various waves and the way in which the waves are reflected is measured. This measurement is carried out twice in quick succession. “Let’s assume that in the short time between the two measurements, the disordered environment remains the same, while the particle we want to manipulate changes slightly,” says Stefan Rotter. “Let’s think of a cell that moves, or simply sinks downwards a little bit. Then the light wave we send in is reflected a little bit differently in the two measurements.” This tiny difference is crucial: With the new calculation method developed at TU Wien, it is possible to calculate the wave that has to be used to amplify or attenuate this particle movement.
“If the particle slowly sinks downwards, we can calculate a wave that prevents this sinking or lets the particle sink even faster,” says Stefan Rotter. “If the particle rotates a little bit, we know which wave transmits the maximum angular momentum – we can then rotate the particle with a specially shaped light wave without ever touching it.”
Successful Experiments with Microwaves
Kevin Pichler, also part of the research team at TU Wien, was able to put the calculation method into practice in the lab of project partners at the University of Nice (France): he used randomly arranged Teflon objects, which he irradiated with microwaves – and in this way he actually succeeded in generating exactly those waveforms which, due to the disorder of the system, produced the desired effect.
“The microwave experiment shows that our method works,” reports Stefan Rotter. “But the real goal is to apply it not with microwaves but with visible light. This could open up completely new fields of applications for optical tweezers and, especially in biological research, would make it possible to control small particles in a way that was previously considered completely impossible.”
The manipulation of small objects with light has become an indispensable tool in many areas of research ranging from physics to biology and medicine. Here we demonstrate how to implement micro-manipulation at the optimal level of efficiency for targets of arbitrary shape and inside complex environments such as disordered media. Our approach is to design wave-fronts in the far-field that have optimal properties in the near-field of the target such as to apply to it the strongest possible force, pressure or torque as well as to achieve the most efficient focus at the target position. Free of any iterative optimization, our approach only relies on a simple eigenvalue problem established from the scattering matrix of the system and its dependence on the target parameters. To illustrate this theoretical concept, we perform a proof-of-principle experiment in the microwave regime, which fully confirms our predictions.
They presented a general framework for optimal micromanipulation with targets of arbitrary shape and in arbitrarily complex environments. They successfully tested this concept experimentally and envision it to be a key for breaking the barrier imposed by disordered media on the applicability of optical tools for manipulating objects inside of them. Ultimately, our work may serve as a guidepost towards a new generation of micro-manipulation experiments with wave-front shaping protocols that continuously operate at the optimal level based on a real-time monitoring of a system’s scattering matrix.
SOURCES- Arxiv, TU Wein
Written By Brian Wang, Nextbigfuture.com