The unique optical properties of fluorophores nanoparticles doped with rare earth elements have attracted a lot of attention in the scientific community due to their potential application from biological imaging to quantum information.
In this work, we compare the photoluminescence of nanoparticles measured by two different means: traditional objective based microscopy and fiber based optical tweezers.
Our doped NaYF4 nanocrystals are prepared through solvothermal synthesis. Ytterbium and erbium codoping provides nanoparticles with luminescence properties. Under IR laser excitation, the nanoparticles present strong and photostable upconversion signals in the visible range. In addition, by changing the gadolinium content of the host matrix, we obtain nanorods with a controlled aspect ratio up to 20 and a well defined crystalline structure.
The high anisotropy of the nanoparticles results in a strong polarisation of the photoluminescence. To investigate this property, we observed our nanoparticles using a confocal microscope and studied the dependency of the polarisation with the length of the particles.
To complete our characterization, we used optical tweezers to trap nanoparticles in water. We first show the possibility to trap these nanoparticles with an original optical tweezers based on two chemically etched fibers. Due to the optical forces applied by the laser beam coupled into the fibers, the nanorods align themselves between the two fibers along their long axis.
Afterwards, the fibers are not only used to trap the particles but also to collect the luminescence emitted only by the trapped nanoparticles. By this mean, we can analyse the emitted light with a spatial resolution. This result will be compare to previous observation done on the same particles with our confocal microscope.
Moreover, an orthogonal third fiber was implemented in the set up. This fiber can move along the particle and collect the light emitted at different point. We present the link between the photoluminescence properties and the emission point by moving this last fiber.
In addition, our optical tweezers are associated to a traditional objective-based optical microscope. We compared the photoluminescence emitted by particles in a homogeneous medium (water) or at an interface when drop casted on a coverslip.
The optical transmission between two metalized optical fiber tips with sub-wavelength open apertures was studied
for tip-to-tip distances down to ten nanometers. Transverse transmission maps with sub-wavelength structures
clearly indicated optical near-field coupling. Depending on light polarization in the emission fiber tips one or
two transmission peaks were observed. All these results were explained by a straightforward analytical model.
We present the stable trapping of luminescent 300-nm cerium-doped YAG particles in aqueous suspension using a dual fiber tip optical tweezers. The particles were elaborated using a specific glycothermal synthesis route together with an original protected annealing step. We obtained harmonic trap potentials in the direction transverse to the optical fiber axes. In the longitudinal direction, the potential shows some sub-structure revealed by two peaks in the distribution statistics with a distance of about half the wavelength of the trapping laser. We calculated intensity normalized trapping stiffness of 36 pN•μm-1W-1. These results are compared to previous work of microparticle trapping and discussed thanks to numerical simulations based on finite element method.
An original optical tweezers using two chemically etched fiber nano-tips is presented. Optical trapping of 1 micrometer polystyrene spheres is demonstrated at optical powers down to 2.6 mW and tip-to-tip distances up to 28 µm. Harmonic trap potentials are found by analyzing the trapped particle position fluctuations. The trap stiffness is deduced using three different models. Consistent values of up to 0.5 fN nm-1 are found. The trap stiffness is linearly decreasing with decreasing light intensity and increasing fiber tip-to-tip distance.