اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدMotion Induced by Light: Photokinetic Effects in the Rayleigh Limit
46
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Structured beams of light can move small objects in surprising ways. Particularly striking examples include observations of polarization-dependent forces acting on optically isotropic objects and tractor beams that can pull objects opposite to the direction of the lights propagation. Here we develop a theoretical framework in which these effects vanish at the leading order of light scattering theory. Exotic optical forces emerge instead from interference between different orders of multipole scattering. These effects create a rich variety of ways to manipulate small objects with light, so-called photokinetic effects. Applying this formalism to the particular case of Bessel beams offers useful insights into the nature of tractor beams and the interplay between spin and orbital angular momentum in vector beams of light, including a manifestation of orbital-to-spin conversion.
قيم البحث
اقرأ أيضاً
Understanding the mechanics of detrimental convective instabilities in drying polymer solutions is crucial in many applications such as the production of film coatings. It is well known that solvent evaporation in polymer solutions can lead to Raylei
gh-Benard or Marangoni-type instabilities. Here we reveal another mechanism, namely that evaporation can cause the interface to display Rayleigh-Taylor instabilities due to the build-up of a dense layer at the air-liquid interface. We study experimentally the onset time ($t_p$) of the instability as a function of the macroscopic properties of aqueous polymer solutions, which we tune by varying the polymer concentration ($c_0$), molecular weight and polymer type. In dilute solutions, $t_p$ shows two limiting behaviors depending on the polymer diffusivity. For high diffusivity polymers (low molecular weight), the pluming time scales as $c_0^{-2/3}$. This result agrees with previous studies on gravitational instabilities in miscible systems where diffusion stabilizes the system. On the other hand, in low diffusivity polymers the pluming time scales as $c_0^{-1}$. The stabilizing effect of an effective interfacial tension, similar to those in immiscible systems, explains this strong concentration dependence. Above a critical concentration, $hat{c}$, viscosity delays the growth of the instability, allowing time for diffusion to act as the dominant stabilizing mechanism. This results in $t_p$ scaling as $( u/c_0)^{2/3}$.
Materials of which the optical response is determined by their structure are of much interest both for their fundamental properties and applications. Examples range from simple gratings to photonic crystals. Obtaining control over the optical propert
ies is of crucial importance in this context, and it is often attempted by electro-optical effect or by using magnetic fields. In this paper, we introduce the use of light to switch and tune the optical response of a structured material, exploiting a physical deformation induced by light itself. In this new strategy, light drives an elastic reshaping, which leads to different spectral properties and hence to a change in the optical response. This is made possible by the use of liquid crystalline networks structured by Direct Laser Writing. As a proof of concept, a grating structure with sub-millisecond time-response is demonstrated for optical beam steering exploiting an optically induced reversible shape-change. Experimental observations are combined with finite-element modeling to understand the actuation process dynamics and to obtain information on how to tune the time and the power response of this technology. This optical beam steerer serves as an example for achieving full optical control of light in broad range of structured optical materials.
The breakup pathway of Rayleigh fission of a charged drop is unequivocally demonstrated by first of its kind, continuous, high-speed imaging of a drop levitated in an AC quadrupole trap. The experimental observations consistently exhibited asymmetric
, sub-critical Rayleigh breakup with an upward (i.e. opposite to the direction of gravity) ejection of a jet from the levitated drop. These experiments supported by numerical calculations show that the gravity induced downward shift of the equilibrium position of the drop in the trap cause significant, large amplitude shape oscillations superimposed over the center-of-mass oscillations. The shape oscillations result in sufficient deformations to act as triggers for the onset of instability well below the Rayleigh limit (a subcritical instability). At the same time, the center-of-mass oscillations which are out of phase with the applied voltage, lead to an asymmetric breakup such that the Rayleigh fission occurs upwards via the ejection of a jet at the pole of the deformed drop. As an important application, it follows from corollarial reasoning that the nanodrop generation in electrospray devices will occur, more as a rule rather than as an exception, via asymmetric, subcritical Rayleigh fission events of micro drops due to inherent directionality provided by the external electric fields.
For dielectric multilayered metamaterials, the effective-parameter representation is known to be insensitive to geometrical features occurring at deeply subwavelength scales. However, recent studies on periodic and aperiodically ordered geometries ha
ve shown the existence of certain critical parameter regimes where this conventional wisdom is upended, as the optical response of finite-size samples may depart considerably from the predictions of standard effective-medium theory. In these regimes, characterized by a mixed evanescent/propagating light transport, different classes of spatial (dis)order have been shown to induce distinctive effects in the optical response, in terms of anomalous transmission, localization, enhancement, absorption and lasing. Here, we further expand these examples by considering a quasiperiodic scenario based on a modified-Fibonacci geometry. Among the intriguing features of this model there is the presence of a scale parameter that controls the transition from perfectly periodic to quasiperiodic scenarios of different shades. Via an extensive parametric study, this allows us to identify the quasiperiodicity-induced anomalous effects, and to elucidate certain distinctive mechanisms and footprints. Our results hold potentially interesting implications for the optical probing of structural features at a resolution much smaller than the wavelength, and could also be leveraged to design novel types of absorbers and low-threshold lasers.
We experimentally demonstrate that a new nanolens of designed plasmonic subwavelength aperture can focus light to a single-line with its width beyond the diffraction limit that sets the smallest achievable line width at half the wavelength. The measu
rements indicate that the effect of the near-field on the light focused is negligible in the intermediate zone of 2 < kr < 4 where the line-width is smaller than the limit. Thus, as a verification of theoretical prediction, the fields focused are radiative and with a momentum capable of propagating to the far zone as concerned by the limit.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
