纳米光学力学国际研究所
Optomechanics and Optical Manipulation
Opto-mechanics studies mechanical action of light on material bodies, e.g. micro and nano-scale particles, and, apart from it fundamental significance, takes it towards variety practical applications. Following J.K Maxwell’s theoretical prediction, the first experimental evidence on mechanical action on matter, induced by light, was provided by P. N. Lebedev at the very end of 19thcentury. The next major step, revolutionizing the field, was done by A. Ashkin in 70s, who proposed and demonstrated the ability to trap and manipulate micron particles with the help of focused laser beams. This investigation started the era of ‘optical tweezers’. Further exploration of this technique made it one of the most frequently used tools in several niches of bio-physics and bio-medicine, as it offers unique abilities of non-invasive control over living objects’ transport.
Recently, advances in opto-electronic and nano-technologies boosted the development of Opto-mechanics, which provides us with cutting edge abilities in manipulation and control over mechanical motion on nano-scale. For example, holographic optical tweezers enable simultaneous manipulation of hundreds of particles; tractor beams provide additional degree of freedom by attracting objects to a source of illumination, and other systems, aiming to provide ultimate on demand control over complex systems.
One of the major goals, to be archived in the field, is the ability to control nano-scale objects – this niche of Opto-mechanics is usually referred by the name Nano-opto-mechanics. The significant reduction of object’s dimensions to the nanometre range requires novel approaches and involves large span of novel physical phenomena. Several proposed and already demonstrated solutions in the field rely on the employment of auxiliary nanostructures, enabling focusing optical field way beyond the diffraction limit. As the result, severe enhancement of optical forces could be obtained.
The main research focus of ‘Nano-opto-mechanics’ Laboratory nowadays is on exploring abilities of nanostructures to tailor and manipulate opto-mechanical phenomena on nano-scale. Several research projects aim to investigate the impact of the near fields, reconfigurable on demand by structured environment, such as metamaterials and metasurfaces. The far going goal is to develop a tool, enabling on demand control over mechanical motion of complex objects on nano-scale. Several objectives were already achieved, e.g. properly designed metal surface and incident illumination provided us with an ability of achieving optical attraction (‘tractor beam’) owning to directional excitation of surface plasmon wave. Furthermore, optical binding in asymmetric dimers was demonstrated and provided us with a tool of controlling trajectory of a bigger particle, while the smaller one is completely static. The light wave takes the mechanical recoil and prevents the violation of the 3rd Newton’s law. Those few examples indicate the very promising direction of the Field’s development and motivate additional theoretical, numerical and experimental multidisciplinary studies with very promising potential applications.
Optical Invisibility
Since 2006 quite considerable attention is drawn towards realization of cloaking devices, suggesting placing an object into carefully designed camouflage cover.
Among the large span of existing proposals, periodic shells with alternating refractive index enables incident wave bending around the object and without introducing scattering channels. Another proposal, coined by the name of ‘Carpet Cloak’, relaxes some tight constrains on perfect cloaking conditions and just emulate an object by a flat surface, e.g. hides things under the carpet.
Our original proposal on cloaking takes another approach to invisibility and requires just scattering suppression from an object. Foe majority of applications weaker condition of scattering suppression is sufficient and, in contrary to perfect cloaking schemes, does not require extremely complex realizations. Our device utilizes layered metal dielectric composite, operating at the regime of vanishing dielectric permittivity along the layers. This property controls radius of curvature of scattering waves, namely maintain the same profile as of the incident wave. It was shown, that the wave front of transmitted wave does not depend on the size or shape of an object, embedded inside the layered structure. It means, that it is undetectable by an observer, and the invisibility application is achieved this way (7% wave front distortion was estimated in the worst case).
All-dielectric covers were also investigated. The main impact of this approach is that it does not supper from additional losses, always present in metal-based structures. Our numerical routine utilizes stochastic optimization algorithms and shows 2-fold scattering suppression from bigger-than-a-wavelength object. For this application only 4 layers of dielectric were used.
Optics of Metamaterials and Metasurfaces
This area gives absolutely new spin for the fields of metamaterials and opto-mechanics by interfacing between them. Historically, one of the key objectives of metamaterial research was achieving negative index of refraction by artificial structuring. Since then, the scope is changing and new applications come into consideration. One of the approaches is to utilize unique optical properties of metamaterials towards opto-mechanical manipulation. In particular, we currently study light scattering and optical forces on small objects, situated in the close proximity, or embedded inside metamaterials, based on metal dielectric layers on vertically aligned metal nanorods. Unit cells of both structures have deep subwavelength nature. One of the recent proposals shows the ability of complete scattering suppression from objects, situated inside layered structures with effectively vanishing dielectric permittivity along the layers. Tailoring scattering patterns in this way makes objects to be invisible to an external observer, and gives rise to variety of intriguing and practical applications. Another outcome of the ability to control scattering with metamaterials is directly related to optical forces. We recently showed that ‘tractor beams’ could be straightforwardly realized with the help of metamaterials by utilizing their ability to enhance optical density of states. ‘Tractor beams’ attract particles to illumination sources by overcoming radiation pressure and provide additional degree of freedom to manipulation techniques. The picture above demonstrates the realization of ‘tractor bema’ – extraordinary waves inside nanorod metamaterial are excited via scattering from a big particle, situated on top of the layer. Small particle inside (blue sphere), being situated in the field of highly confined extraordinary wave exhibits attraction.
Another research topic deals with metasurfaces for photo-voltaic applications. Antireflection coatings based metasurfaces are investigated for enhancing photo-absorption and, as the result, for improving efficiencies of thin solar cells.
We showed that nano-structured film, made of periodic array of holes, enables suppression of scattering in the broad spectral range, in contrary to common intuition suggesting narrowband operation. The effect is based on carefully designed frequency-dependent phase shift, mimicking broadband quarter wave plate. This approach, in fact, relies on absolutely new concept, and gives rise to a new family of anti-reflection coatings.
All-dielectric Nano-photonics
There is a common belief, seemingly relying on seminal textbook of Landau and Lifshitz, that there is impossible to achieve magnetic responses at high, e.g. optical frequencies. The reasoning was microscopic and related to the nature of spin and orbital interactions of light with electrons – its inefficiency, in fact. Hence the permeability of optical materials is almost equal to 1. However, it was recently shown, that multipole moments in particles made of high refractive index materials, e.g. silicon or germanium, could indeed exhibit magnetic properties. This discovery started the era of ‘All-dielectric Nanophotonics’, having the studies of novel types of light-matter interactions, light harvesting and concentration, novel type of magnetic metasurfaces, detection of magnetic transitions in atoms, and others as primary objectives.
Our Lab investigates all-dielectric systems by applying approaches of configuring multipole spectrum of complex geometries. Effects of induced bi-anisotropy, enhancement of magnetic moments, and harvesting of magnetic fields and their concentration. Those basic building blocks are aimed to be employed for large scale metasurfaces for photo-voltaic applications.
Opto-mechanical tools could also contribute to development of all-dielectric devices. Multipole moments in particles could be utilized for directional scattering and, as the result, tailor characteristics of mechanical forces. For example, an interplay between electric and magnetic dipolar responses could lead to a unique beaming of scattered radiation. This additional flexibility in control of macroscopic structural responses was shown to enable above mentioned ‘tractor beams’, side control motion, levitation and others.
Nonlinear Optics
Relatively new branch of opto-mechanics explores emergent nonlinear effects, directly related, or mediated by mechanical motion of particles on nano-scale. Nonlinear phenomena are starting to kick in once traditional CW trapping sources are replaced by impulse illumination. Under certain circumstances, utilization of short pulses enable achieving lower trapping powers and mitigates heating effects. On the other hand, high peak intensity gives rice to effects of nonlinear generation. Furthermore, traditional system with a single trap could exhibit 2 trapping centres once short pulses are used instead of CW. Optical attractors, based on vortex solitons could be also achieved. At the moment, the area of nonlinear opto-mechanics just started to develop, but already holds a great promise for delivering fundamental breakthroughs and applications.