The optical manipulation of small particles represents an important tool and a very flexible method for the investigation of processes at the micro- and nano-scale [1–5], as well as for technological development of driving micromachines [6–11]. Their features enable applications in several fields ranging from cell surgery and microinjection to microrheology. Taking advantage of complex structures of the light beam, many objects can be manipulated through multiple traps configurations [12], and their spinning and orbital motion can be simultaneously controlled through light beams with the proper phase profile [13–17]. Nevertheless, the use of Gaussian beams in simplified geometries enables easier and wider applications. In this view, the fabrication of complex micro- and nano-objects that undergo multiple motions when illuminated with light can offer several opportunities for mechanics at small scale.

Rotation at the microscale has been largely investigated [18]. Microrheology has gained several advantages from these studies, with applications in several fields ranging from hydrodynamics to biology. Also, micromechanics and microrobotics can achieve great advances in developing micromotors and microrotors with remote control. Birefringent particles were first investigated, since their spinning can be easily controlled by the optical torque induced by polarized light [19,20]. However, the magnitude of the torques that can be exerted by light to birefringent particles is small.

Recent investigations have demonstrated that spherical nanoparticles (NPs) with eccentric core-shell geometry can be used to generate considerable optical torques and then rotation rates, opening opportunities for nanorheology and biophotonics [21]. Indeed, core-shell microparticles (MPs) exhibit exceptional properties resulting from the proper combination of the inner and outer materials as well as from the size and geometry of the structures [22–28].

Recently, we have developed a method to generate core-shell MPs composed of a polymeric liquid crystal core and a shell of gold nanoparticles (AuNPs) [29]. Our approach relies on the combination of two processes occurring during UV exposure of reactive mesogenic emulsion: (1) photopolymerization of mesogenic droplets (which constitute the solid cores) created in water; (2) photoreduction of chloroauric ions and precipitation of AuNPs that aggregate on the surface of the polymeric core in a raspberry-like structure. In our previous work, a chiral mesogen has been exploited as polymeric core [29], which results in spherical MPs with a spherulitic cholesteric arrangement [30]. Such configuration has demonstrated opportunities for photonic applications as Bragg onion resonators and the AuNPs’ shell improves lasing performances of the dye-doped MPs [29].

Here the method is applied to nematic reactive mesogens (RMs) droplets, to create core-shell MPs with an anisotropic core, both in the shape and in the optical properties. Indeed, based on the bipolar supramolecular arrangement of the nematic director inside the droplets, the core acquires an ellipsoidal shape. We demonstrate the improved performances introduced by the Au nanoshell in the optomechanical behavior of these MPs. Rotational dynamics of the nematic core-shell MPs induced by a circularly polarized Gaussian beam has been investigated and compared with the bare polymeric nematic MPs (NMPs). While simple spinning is observed for the latters, the formers undergo more complex motion. Peculiarities, motivations, and advantages are discussed.

The MPs are prepared via light-induced polymerization of nematic RMs droplets in pure water (bare NMP) or in a chloroauric acid aqueous solution (AuNMPs). Due to the RMs’ hydrophobicity, nematic droplets can be obtained by emulsification process in water. The uniaxial nematic phase has a symmetry axis defined by the so-called nematic director n and causes the formation of topological point and line defects in confined geometries (such as droplets). Indeed, the parallel orientation of the molecules at the water interface promotes an internal bipolar configuration of the nematic director with two points defects at the opposite poles of the droplets. Depending on the balancing of the bulk elasticity and interfacial free energy, the nematic droplets in the isotropic fluid are expected to acquire an ellipsoidal or tactoid shape [31]. The subsequent UV light exposure contextually induces the polymerization of the core, which preserves the droplet’s configuration and the precipitation of AuNPs.

The pure RMs mixture is obtained by extracting the solvent (propylene glycol monomethyl ether acetate, PGMEA) from the Licrivue RMS03-001C (Merck, Germany) solution using vacuum evaporation at 90°C overnight. NMP emulsions are prepared by adding 0.4% (mass fraction) of the nematic RMs to ultrapure (Type I) water. The glass vial containing the mixture is then heated at 100°C and shaken at 22 r/min for 30 s in a laboratory vortex mixer in order to obtain particles with a micrometric size, suitable for the present study. By properly setting the frequency and time of shaking, submicrometric particles can be also obtained. The AuNPs shell on the NMPs is obtained by mixing the emulsion with an equivalent volume of water containing chloroauric acid (HAuCl4, Sigma-Aldrich) in order to achieve a final concentration of 0.25 mmol/L. Afterwards, the emulsions are polymerized using a UV lamp (Mega Electronics mod. LV202-E) with light intensity 2mWcm−2 and emission band centered at 350 nm. To promote homogeneous reactions, the UV curing is performed under dry nitrogen flux at 25°C by placing a large section vial between the two fluorescent tubes of the UV lamp for 3 h [29]. After polymerization, most of the particles lay down at the vial bottom. In order to eliminate polymerization residuals, the supernatant is replaced by pure water. The suspension is sonicated, helping to prevent MP clustering. The suspension of core-shell AuNMPs acquires a gray coloration, as expected due to the AuNPs aggregation on the NMP surface.

Scanning electron microscopy (SEM) measurements are performed using an FEI-Philips ESEM-FEG Quanta 200F apparatus operating at a range of 5–20 kV with a working distance of 6–15 mm.

In Figs. 1(a) and 1(b), the SEM images of the NMPs and AuNMPs are reported, respectively. The image in Fig. 1(b) clearly shows the presence of AuNPs over the polymeric MPs’ surface, forming a raspberry-like structure. A large heterogeneity of the MPs’ size, whose diameter ranges from 1 to 20 μm, is observed in both cases and is due to the microemulsification process. The mean diameter is 5±1μm, as estimated from the distribution recovered by SEM investigations [29].

Transmission electron microscopy (TEM) has been carried out to investigate the shape of the MPs and the distribution of the AuNPs. The AuNMPs are first embedded in an epoxy resin (Araldite, Fluka) and successively cut into 100-nm-thick sections by means of a diamond knife. The ultrathin sections are collected on copper grids and are then examined with a Zeiss EM10 TEM at 80 kV acceleration voltage. In Figs. 1(c) and 1(d), central sections of AuNMPs are reported; they make evidence of their ellipsoidal shape and allow evaluating the length of the two axes. For both particles in Fig. 1(c), the short-to-long axis ratio is about 0.75, while for the particle in Fig. 1(d), it is 0.82. An evaluation of this ratio for both kinds of spheroidal NMPs (i.e., with and without the AuNPs shell) shows that its value is independent from the AuNPs’ coverage and ranges from 0.75 to 0.95. Figures 1(c) and 1(d) also clearly show that AuNPs aggregate around the polymeric core. The TEM image in Fig. 1(e) shows a section of an AuNMP, which has been sliced close to the edge of the particle: the image displays a larger section of the shell that visualizes the distribution of the AuNPs over the NMPs’ surface. Clusters of AuNPs, of different shapes and sizes, are almost uniformly distributed around the polymeric core. A magnification of the shell section is reported in Fig. 1(f), where both spherical AuNPs (see the arrow in the figure) and clusters can be clearly seen, of about 10 nm and 30–50 nm in size, respectively.

Optical polarizing transmission microscopy has been carried out by a Zeiss Axioscope to investigate the optical features of bare and core-shell NMPs. Between crossed polarizers, both kinds of MPs appear bright when the axes of the ellipse are not parallel to the polarizers’ one [i.e., at 45°, as in Fig. 2(a)]. This demonstrates that NMPs are birefringent, with the optical axis parallel to the major axis of the ellipsoid, as expected, considering the bipolar schematic representation of the molecular director within the polymeric NMPs [31].

Comparative UV-NIR spectroscopy is carried out on both kinds of NMPs, collected on glass substrates. Reflectance spectra of the two specimens are acquired by a Cary 5E spectrophotometer equipped with an integrating sphere. The reflectance spectra in Figs. 2(c) and 2(d) show a noticeable difference in the investigated spectral range that can be clearly ascribed to the presence of AuNPs shells. Indeed, the bare NMPs exhibit an almost flat reflectance over the whole spectral range, while the AuNMPs’ spectrum has a clearly different trend, with a substantial increase in the red-NIR region (above 600 nm) due to the Au shell. Moreover, the evident depletion around 550 nm could be associated to the plasmonic resonance of the AuNPs embedded in the polymeric surface [29].

When an MP is illuminated by a tightly focused laser beam, optical trapping and manipulation can be performed thanks to the optical force Fopt originating from the exchange of linear momentum between the light beam and the particle [1]. However, in the case of structured light beams and/or birefringent or absorbing particles, angular momentum can be also transferred, generating a torque that induces a rotational motion of the particle [20,32]. For particles size larger than the laser wavelength, a ray optics approximation can be considered to discuss the optical trapping condition and the induced dynamical behavior. In such an approach, the light beam is decomposed into individual rays that propagate in straight lines inside the media. Each ray is reflected and refracted at the particle interface according to Snell’s law, with reflection and transmission amplitudes proportional to the Fresnel coefficients [1]. The linear momentum exchange between the incident ray and the reflected or refracted ones induces a force on the particle. The total optical force is obtained by the sum over all the incident rays. Dielectric particles can be trapped in 3D, providing that their refractive index is larger than the surrounding fluid one. Moreover, the 3D trapping stability also depends on the experimental geometry and focusing condition of the laser beam. When the beam focus is located inside the chamber where the particle suspension is, 3D trapping occurs; conversely, if the beam focus is positioned before or after the chamber and the illuminated area of the sample is larger than the particle size, the particle is trapped in a bidimensional plane perpendicular to the beam axis, the so-called 2D trapping [33].

In the case of absorbing or reflecting particles, a large radiation pressure is generally produced, and the particles are pushed out from the irradiated region [34,35], preventing the optical trapping.

Rotational motion is produced by the transfer of angular momentum from the light to the particles, in the form of spin angular momentum (SAM), related to the light polarization (circular), or orbital angular momentum (OAM), associated with the light wavefront shape [13,18].

Particles features also allow this transfer: absorbing particles or particles with anisotropic shape, like the ellipsoidal ones, or with anisotropic optical properties (like birefringence), can experience a torque due to SAM transfer. In these cases, spinning is typically induced, while light with OAM (Laguerre–Gauss, Bessel beams, etc.) generates orbital motion (Ref. [18] and references therein). In all cases, the spin and orbital velocities result from the balance between the optical and the viscous torques and forces [18,19].

Based on their anisotropic shape and optical properties, both bare and core-shell NMPs are expected to rotate. The present study is aimed at comparing the rotational motions produced in the same experimental condition for both kinds of NMPs, addressing the role of their different optical properties (see Fig. 2).

A standard optical tweezers setup, based on Gaussian laser beams, is employed, as shown in Fig. 2(b). In order to investigate the optomechanical behavior of the AuNMPs in the spectral regions where they display the main differences with respect to the bare NMPs, two continuous wave laser sources are used. An argon-ion laser (Innova 90c, Coherent) at λ=514nm has been chosen to probe how the plasmonic absorption of the AuNPs [Fig. 2(d)] affects the optical trapping and the rotational dynamics. A Nd:YAG solid-state source (Ventus HP, Laser Quantum) with lasing wavelength at 1064 nm is used to explore the optomechanics in the spectral region where the AuNMPs exhibit a larger reflectivity [Fig. 2(d)]. A 60× microscope objective (with numerical aperture, NA=0.85) focuses the Gaussian beam on the 200 μm thick sample chamber, containing the NMPs dispersed in water. A quarter-wave plate (λ/4), placed before the objective imposes the circular polarization state of the beam. A beam expander before a dichroic mirror controls the laser focus position with respect to the sample chamber.

The sample position is adjusted by a three-axis translation stage. A fiber illuminator and a charge-coupled device (CCD) camera are used to illuminate and image the sample.

The longitudinal position of the laser beam focus with respect to the sample chamber is adjusted to provide a 2D trapping of the NMPs close to the bottom chamber surface. The optical force has a component perpendicular to the beam axis, mainly induced by the gradient of the intensity beam profile [force f in Fig. 2(b)] and a weaker axial component (scattering force). The latter is generally responsible for the repulsive force on metallic particles, which could prevent their trapping [35,36]. Nevertheless, given the distance between the focal point and the lower chamber substrate, it does not push the AuNMPs away from the illuminated region, although it is expected to be more effective with respect to the bare NMPs.

The beam radius at the sample plane is estimated to be about 7 μm for the argon laser (514 nm) and about 10 μm for the Nd:YAG (1064 nm).

Rotational motion has been investigated by means of optical tweezers [Fig. 2(b)]. Birefringent dielectric MPs can be easily trapped due to a gradient force; moreover, also SAM transfer from light to particles is allowed, which produces a reaction torque [18,37]. Then, when they are trapped by circularly polarized light into the region of highest intensity (i.e., the axis of the Gaussian beam), they undergo a continuous spinning. The exerted optical torque [20] induced by the change of the beam’s photon polarization state σ, traveling within the birefringent particle, is τop=(σin−σout)Pω,(1)where σin is the incoming polarization, σout is the outgoing polarization, P is the beam power on the particle, and ω is the light angular frequency.

The trapped particle reaches a steady state when the optical torque τop and the rotational viscous torque τvis balance each other. In the simplified spherical geometry τvis=KΩ=8πR3ηΩ, where η is the kinematic viscosity of the surrounding medium, R is the average radius of the particle, and Ω=2πν is its angular velocity.

For a nonspherical particle also a shape-induced birefringence contribution has to be considered; the torque arises due to the asymmetric scattering forces when the principal axis of the particle is not parallel to the light’s electric field [38].

Bare NMPs are weakly trapped on axis with the laser beam, for both visible and near-IR wavelengths. Spin rotation occurs clockwise or counterclockwise, according to the handedness of the circularly polarized light.

In Fig. 3(a), a frames sequence (extracted from video 1, see Visualization 1) is reported, which shows the spinning of an NMP over a period of 3.3 s. The investigated NMP has a mean radius of 2.8±0.2μm and is illuminated with a left circularly polarized laser source, at 1064 nm, with a power density of 1.7mW/μm2. The major axis of the particle, denoted in blue in Fig. 3(a), lies perpendicular to the beam axis and rotates around it. The graph in Fig. 3(b) shows the position of the particle center of mass (in black) during 20 s (about five full turns) superimposed to the light spot in the trapping plane (in red), demonstrating that the particle is stably trapped at the beam axis position during the rotational motion.

In Fig. 3(c), the rotational frequencies of NMPs with similar mean radius (2.8±0.2μm) are reported versus the incident beam intensity for both the 514 nm and 1064 nm lasers. The linear trends of the frequency ν observed for both wavelengths agree with the literature [20], ν=τop2πK.(2)

Higher rotational frequency is observed for the longer radiation wavelength in the same range of power density, according to Eq. (1). The slopes of the best fit lines, as reported in Fig. 3(c), are 0.104 and 0.057μm2(mWs)−1 at 1064 and 514 nm wavelengths, respectively. These values are in good agreement with the expectation of Eqs. (1) and (2), since all experimental parameters are the same (particle size, surrounding fluid, etc.), except for the wavelengths. The frequencies observed for our birefringent NMPs are compatible with those of particles having comparable birefringence (Δn∼0.1), as vaterite (i.e., 300–400 Hz at 600mW/μm2 light intensity [39]) when accounting for the different experimental conditions, i.e., moderate focusing and lower light intensity.

Under the same visible and near-IR circularly polarized laser irradiation, core-shell AuNMPs undergo different rotational dynamics with respect to the bare NMPs, which can be accounted for by the peculiar optical properties introduced by the metallic NPs shell. First, the equilibrium position relocates from the beam axis to an annular region around it, whose radius is almost independent from the beam intensity. Only above a certain intensity value (i.e., about 4.0 and 2.0mW/μm2 at 514 and 1064 nm, respectively) the circular orbit becomes unstable and the AuNMPs are pushed out from the illuminated region. Below these values, due to the circular polarization of the light, the particles constrained in the annular region simultaneously spin and orbit around the beam axis. Such behavior has been observed in the intensity ranges 1.5–4.0 and 1.0–2.0mW/μm2 for λ=514 and 1064 nm, respectively, while no rotation has been detected for lower intensity values. In Fig. 4(a) the frames sequence from video 2 (see Visualization 2) shows the rotational motion, with a period of about 1.7 s, of an AuNMP with a mean radius 2.8±0.2μm illuminated by the 1064 nm laser source at 1.7mW/μm2 intensity. The white dashed lines cross at the position of the beam axis (i.e., the peak intensity position of the Gaussian beam). A peculiar feature of the AuNMPs’ rotational motion emerges: at each orbital position, the particle optical axis keeps the same angle with the radial direction (i.e., the line connecting the particle center with the beam center), indicative of the synchronous spinning and orbital motion performed by the AuNMP. The graph in Fig. 4(b) shows the orbits described by the AuNMP center of mass in 9 s (about five full turns) superimposed to the light beam spatial distribution in the trapping plane. The orbits show an almost elliptical shape with major and minor semiaxis 5.5±0.5μm and 4.5±0.5μm, respectively. In Fig. 4(c), the rotational frequencies measured as a function of the light beam intensity for both wavelengths are reported.

Comparing these measurements with the ones of the bare NMPs proves that the presence of a shell of AuNPs around the polymeric NMP boosts the rotational dynamics at both beam trap wavelengths. The rotational frequency enhancement is even more distinct for the 514 nm beam; the fitting line slope [Fig. 4(c)] is here 0.44μm2(mWs)−1, even slightly higher than the one at 1064 nm, the latter being 0.32μm2(mWs)−1. Depending on the above-mentioned threshold behavior, the fitting line does not cross the origin of the graph.

Starting from the above considerations, we discuss the observed dynamics and relate it to the optical and geometrical features of the NMPs, as well as to the trapping configuration.

The bare NMPs are confined in a plane near the bottom glass substrate. For this trapping geometry, the longitudinal scattering force is reduced, while the transverse gradient force exerted by the laser beam confines the NMPs on its axis, where they spin due to the SAM transfer of the circularly polarized light. Preventing 3D trapping that would align the ellipsoidal particle along the beam axis [40], the NMPs orient with the major axis parallel to the glass substrate (exhibiting a larger birefringence) and rotate around a minor axis parallel to the beam’s one.

On the other hand, we observe that the AuNPs’ shell significantly alters the entrapment position and the motion of the AuNMPs. For the 1064 nm laser source wavelength, due to the AuNMPs’ increased reflectance, the scattering force contribution is expected to increase in the propagation direction (wave vector k, Fig. 2). Indeed, due to the trap geometry, its transverse component, pointing radially out of the trap, is balanced by the gradient force, pointing toward the beam axis, at the highest light intensity value. This condition determines a new equilibrium position of the particle, out of the beam axis and with the particle major axis oriented radially [41]. The equilibrium distance from the beam axis remains almost unchanged even increasing the light intensity within the above-mentioned range, as it mainly depends on the particle’s reflectance. Optical forces and trap stiffness as a function of the MPs’ reflectance have been previously investigated and reported in Ref. [30]. The numerical simulations assess the stability condition of the optical trap as a function of the reflectance, considering a spherical particle of the same polymer and a radius of 2.5 μm. The simulations revealed that in the reflectance range 0.2–0.3, there is a threshold that separates two opposite dynamics, passing from attractive to repulsive forces. The behavior of the transverse gradient force for a reflectance below this threshold is such that its slope calculated at the equilibrium position is in fact always negative. As the reflectance increases, the strength of the trap reduces, and an inversion of the slope, which corresponds to an instability condition of the trap, is expected for higher values. In a narrow region of the reflectance (around 0.23), the trap varies from a single stable equilibrium position centered at the beam axis, into two symmetric stable equilibrium positions, at the half-maximum of the Gaussian beam intensity. Due to the rotational symmetry of the gradient force in the transverse plane (x,y), those equilibrium positions form an annular region of stable trapping. In the present case, a value of the reflectance giving off axis trapping, is plausible considering the value of the Au refractive index at 1064 nm (n=0.71) and the type of coverage of the NMP (a thin layer of polymer embedded AuNPs).

Such AuNMP trapping is observed until the longitudinal component of the force becomes relevant, and after that the particle is expelled from the trap; this effect occurs for light intensities above 2mW/μm2.

The shell reflectance also affects the rotational motion. Due to the circular polarization of the light beam, the particle rotates around its minor axis and contextually moves on a circular trajectory, displaying an orbital motion that cannot be accounted for by the OAM of the light beam. A plausible explanation could be put forward considering the optical forces in the transverse plane and the SAM transfer. When the particle starts to rotate, it also moves along the orbit, following its equilibrium position in the annular trap. Based on this feature, at any orbit position the particle has the same orientation with respect to the beam center [see Fig. 4(a)], giving evidence of a synchronous rotation. By consequence, the orbital motion is triggered by the ellipsoidal AuNMPs’ spinning within the specific trapping potential field.

In Fig. 5(a), the kinetic energy as a function of the light intensity, at 1064 nm wavelength, is reported for a bare NMP and an AuNMP with the same radius (2.8 μm). Although the trend is linear for both NMPs, the slope is steeper for the AuNMPs, suggesting the occurrence of an additional contribution to the optical torque in Eq. (2). Since no transfer of angular momentum is expected due to the reflection at the water/AuNMP interface, we can assume that the increased reflectance at the interface between the dielectric core and the AuNPs’ shell favors a greater circulation of light within the AuNMPs, which is responsible of the increased optical torque, and consequently of the higher rotational frequency with respect to the bare NMPs. Such interpretation is supported by the rotational frequency measurements of the NMPs and AuNMPs as a function of the particle radius, at 1064 nm laser wavelength and 1.7mW/μm2 intensity [see Figs. 5(b) and 5(c)]. As expected from Eq. (2), for both particles, the rotational frequency is inversely proportional to the NMPs’ cubic radius, and its value is higher for the NMPs covered by the AuNPs. The solid lines in Figs. 5(b) and 5(c) represent the theoretical fits of the experimental data and have been carried out taking into account the rigid body approximation, suitable for polymerized NMPs, and two contributions to the optical torque [42], including an effective absorption parameter αeff involving the whole particle, in addition to the birefringence. According to Ref. [41], the rotational frequency takes the following expression: ν=ν0σin(1−e−2αeffRcosΔϕ)2,(3)where ν0=P/2πKω and Δϕ=2kRΔn is the total birefringence retardation phase, where k=2π/λ0, and Δn is the polymer birefringence.

The fitting curves in Figs. 5(b) and 5(c) account for the enhanced optical torque on the AuNMPs associated with their doubled effective absorption (αeff=230m−1) with respect to the NMPs one (αeff=100m−1). This parameter can be rationalized by a stronger light trapping inside the nematic core, i.e., the circulating light enhancement effect inside a cavity, which results in an increased transfer of SAM.

A greater contribution to the rotational frequency of the AuNMPs and, consequently, to the kinetic energy, is obtained for the 514 nm laser beam. In Fig. 5(d), the acquired rotational kinetic energy as a function of the light intensity is reported for a bare NMP and an AuNMP of comparable size. A more efficient angular momentum transfer from the absorption-mediated process enables one to increase the rotational kinetic energy of the core-shell AuNMPs by two orders of magnitude with respect to the bare NMPs. Looking at the spectra in Fig. 2, the optical properties at 514 nm suggest the optical torque enhancement as the only responsible of the rotational frequency increase due to the photons absorbed by the plasmonic nanoshell; see Figs. 3(c) and 4(c). Moreover, compared to 1064 nm wavelength, plasmonic resonance and lower reflectance at 514 nm are clearly observed (Fig. 2).

In Figs. 5(e) and 5(f) the frequency is reported as a function of the particle’s radius. While for bare NMPs, the trend is the one described by Eqs. (1) and (2), the AuNMPs exhibit a different behavior, being stably trapped only if their radius is larger than 3 μm. Here, the optical force is expected to have a further contribution due to the transferred linear momentum from the absorbed light, which is opposite to the one due to the refracted rays [1,34] and reduces the trap stability. Nevertheless, due to the core-shell structure, the absorption process does not involve the whole MP, but only the thin AuNPs shell. Hence, for large values of the surface-to-volume ratio (i.e., for smaller particles) at fixed surface density of the AuNPs, this contribution is expected to prevail, pushing the AuNMPs out of the irradiated region. With the increase of the particle radius, this effect reduces, thus enabling stable trapping within an annular region, at even higher light intensity values with respect to the 1064 nm beam; see Fig. 4(c).

The absorbed photons, different from the reflected one, may transfer SAM, giving a further contribution to the optical torque applied to the particle [18]. Such a contribution compensates for the lower values of the rotational frequency at 514 nm, as expected from Eqs. (2) and (3). The graph in Fig. 4(c) shows that, for particles with 3.2±0.2μm radius, the optical torque caused by the light absorption at the plasmon resonance gives a contribution about six times larger than the one due to the sole birefringence [bare NMP, Fig. 3(c)]. In addition, the slope of the fitting line is now even steeper than the one at 1064 nm. However, since a very small surface layer of the AuNMPs is involved in the process, a larger effect is expected for particles with a higher surface-to-volume ratio. As a consequence, particles with a radius just above 3 μm achieve a larger increase of frequency.

Beside the ones investigated in the present work, further advantages of the core-shell structure could be linked to the heating phenomena caused by light absorption. Such side effects must be usually avoided, since the structural modifications that the temperature increase may induce in both the environment and/or the MP itself limit the power value that can be used and the rotation rates that can be achieved [43]. Here, the resonant absorption involving the AuNPs in the nanometric shell is able to significantly enhance the optical torque on the MPs, whereas the core-shell structure reduces the temperature rise of the whole particle and improves the heat dissipation processes.

We demonstrate that the dielectric-metallic core-shell structure of birefringent MPs enhances the rotational rates and enables further rotational modes in a basic Gaussian beam optomechanical setup. The comparative investigation between bare NMPs and AuNMPs, at different beam wavelengths, rationalizes the rotational dynamical features of the latter in term of the optical properties of the AuNPs shell.

The increased reflectance expressed in the near-IR region (at 1064 nm) modifies the rotational motion: bare NMPs spin, while the AuNMPs show a synchronous spinning and orbital motion. The efficient absorption expressed in the visible region at the plasmonic resonance of the AuNPs (at 514 nm) also enhances the rotational rates that can be achieved exploiting the same light intensity: the core-shell AuNMPs exhibit a rotational kinetic energy that is two orders of magnitude larger than the bare NMPs one. Finally, the core-shell structure also could represent a useful approach to limit the heating side effects generally linked to the light absorption. Therefore, the reported results reveal a novel strategy to improve the rotational control of MPs and can have direct implications in applications involving light-guided rotating micro-objects, like micromechanics (micromotors and microrotors) and microrheology.

Acknowledgment. The authors would like to thank Dr. Ida Perrotta (Department of Biology, Ecology and Earth Sciences, University of Calabria) for the TEM measurements, Prof. Nicola Scaramuzza (Department of Physics, University of Calabria) for the access to the spectrophotometer, and Dr. Giovanni Desiderio (Institute of Nanotechnologies, CNR) for useful discussions.