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Behaviour of Objects in Structured Light Fields and Low Pressures
Flajšmanová, Jana ; Čižmár, Tomáš (oponent) ; Marago, Onofrio (oponent) ; Zemánek, Pavel (vedoucí práce)
A deeper understanding of behaviour of optically trapped particles reveals underlying physical phenomena arising from the light-matter interaction. We present an explanation of the enhancement of the pulling force acting on optically bound particles in the structured optical field, so--called tractor beam. It is demonstrated that the motion of two optically bound objects in a tractor beam strongly depends on their mutual distance and spatial orientation, which adds an extra flexibility to our ability to control matter with light.\newline Subsequently, the thesis is focused on the optical levitation of a particle in a vacuum. We propose a novel methodology for a characterization of properties of a weakly nonlinear Duffing oscillator represented by an optically levitated nanoparticle. The method is based on averaging recorded trajectories with defined initial positions in the phase space of nanoparticle position and momentum and provides us with the oscillator parameters directly from the recorded motion. Our innovative approach is compared with the commonly used power--spectral--density fitting, and exploiting numerical simulations, we show its applicability even at lower pressures where the nonlinearity starts to play a~significant role.
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Cooling of thermal motion of optically levitated nanoobjects
Zemánková, Tereza ; Flajšmanová, Jana (oponent) ; Jonáš, Alexandr (vedoucí práce)
The master's thesis deals with optical levitation of dielectric nanoparticles and cooling of their thermal motion. By focusing two counter-propagating laser beams, the particle can be stably trapped between the foci of the two beams. By subsequently applying an external electric field to the optically trapped charged particle and properly adjusting the feedback loop, it is possible to remove energy from the particle, reduce its position variance, and thus cool the particle thermal motion. The thesis is divided into three main chapters. The first discusses the theoretical introduction to optical trapping, describes the dynamics of the trapped particle, and connects it to the experimental section. A schematic of the experimental setup, preparation of particles for experiments, detection of the particle position, instructions on how to properly tune the optical setup, and calibration of the data to SI units are described. In the second part, various methods of cooling the thermal motion of an optically levitating particle are presented. Experiments performed with a single captured particle are compared with the theoretical model. In a laser beam with circular profile, the particle was cooled in one axis and the elliptical profile of the beam allowed cooling the thermal motion of the particle in two axes. In the third part, the trapping and interaction of two levitating particles, the formation of normal modes and their subsequent cooling are discussed. The experimentally obtained data are compared with theoretical models.
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Dynamics of Microparticles Optically Trapped in Vacuum
Svak, Vojtěch ; Čižmár, Tomáš (oponent) ; Marago, Onofrio (oponent) ; Brzobohatý, Oto (vedoucí práce)
A microparticle levitating in vacuum only by optical forces constitutes a mechanical system which is extremely well isolated from its environment, including its sources of noise. This unique feature provides the system with outstanding sensitivity on any change of surrounding conditions. We introduce a unique experimental set-up for trapping in vacuum which we built the Institute of scientific instruments of CAS in Brno. Subsequently we provide an experimental study of mechanical effect of circularly polarized light which, contrary to linearly polarized light, generates non-conservative contribution to the optical force field. We follow by presenting observation of optical binding of two particles in free space in vacuum which has never been realised before and show how the inter-particle interaction can be tuned and characterized. In the end we introduce a promissing method for optical force field estimation based on particles stochastic trajectory analysis.
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Behaviour of Objects in Structured Light Fields and Low Pressures
Flajšmanová, Jana ; Čižmár, Tomáš (oponent) ; Marago, Onofrio (oponent) ; Zemánek, Pavel (vedoucí práce)
A deeper understanding of behaviour of optically trapped particles reveals underlying physical phenomena arising from the light-matter interaction. We present an explanation of the enhancement of the pulling force acting on optically bound particles in the structured optical field, so--called tractor beam. It is demonstrated that the motion of two optically bound objects in a tractor beam strongly depends on their mutual distance and spatial orientation, which adds an extra flexibility to our ability to control matter with light.\newline Subsequently, the thesis is focused on the optical levitation of a particle in a vacuum. We propose a novel methodology for a characterization of properties of a weakly nonlinear Duffing oscillator represented by an optically levitated nanoparticle. The method is based on averaging recorded trajectories with defined initial positions in the phase space of nanoparticle position and momentum and provides us with the oscillator parameters directly from the recorded motion. Our innovative approach is compared with the commonly used power--spectral--density fitting, and exploiting numerical simulations, we show its applicability even at lower pressures where the nonlinearity starts to play a~significant role.
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Optical binding of polystyrene particles in tractor beam
Damková, Jana ; Chvátal, Lukáš ; Oulehla, Jindřich ; Ježek, Jan ; Brzobohatý, Oto ; Zemánek, Pavel
The motion of a particle illuminated by a laser beam is usually driven by the photon flow due\nto the radiation pressure and therefore for particle trapping, one has to employ gradient forces. But in a tractor beam, objects are illuminated by the uniform light intensity and even so they can be pulled against the beam propagation. There have been developed several techniques how to create such a tractor beam. In our case, the tractor beam is created by two identical Gaussian beams that interfere under the defined angle. It creates the\nstanding wave, where in the transversal plane the particle is trapped by means of the gradient\nforce, but in the total beam propagation direction, the particle manipulation is driven by the non-conservative force. It is remarkable that this force can for the specific combinations of\nparameters pull the micro-particle against the beam propagation. This kind of behavior is\nbecause of the particle scattering where the majority of the incident photons is scattered in the forward direction and, based on the principle of action and reaction, the transfer of\nmomentum leads to a backward movement of the object. The pushing and pulling force is\nsensitive to the polarization of the laser beam, its incident angle and the particle size so this\ntechnique can be used for example for sorting of objects of different sizes.
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Optický transport a třídění pomocí tažného paprsku
Brzobohatý, Oto ; Karásek, Vítězslav ; Šiler, Martin ; Chvátal, Lukáš ; Čižmár, Tomáš ; Zemánek, Pavel
Realizovali jsme experimentálně novou metodu třídění suspenze koloidních částic v samostatném širokém laserovém svazku. Třídění se provádí realizací tzv. tažného paprsku slabě fokusovaným laserovým svazkem, jenž je pod malým úhlem zpětně odražen na dielektrickém zrcadle. V tomto uspořádání je příčná poloha vertikálně zachycených částic závislá na směru lineární polarizace svazku. Otočení polarizace o 90 stupňů změní znaménko optické síly působící podél osy z na částice zvolených vlastností a tím i směr pohybu částice. Tento způsob třídění poskytuje překvapivě efektivní metodu pro pasivní roztřídění řádově desítek částic najednou pouhou změnou polarizace.
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