We introduce a floating-probe force spectroscopy technique with femtonewton resolution and piconewton range, capable of simultaneous measurement of a three-dimensional force field . The technique is uniquely suited to optical force measurements and can be tailored to generate a force map in a plane or a three-dimensional volume.
We apply our technique to the measurement of helicity-dependent spin-momentum forces on a non-chiral microsphere in an evanescent optical field. A direct measurement of such spin-dependent forces has thus-far been elusive, as the widely differing force magnitudes in the three spatial dimensions place stringent demands on a measurement’s sensitivity and range. Our results show quantitative agreement with theory and represent the first direct and simultaneous measurement of all components of this polarization-dependent optical force.
 Liu, Lulu, et al. "Three-Dimensional Measurement of the Helicity-Dependent Forces on a Mie Particle." Physical review letters 120.22 (2018): 223901.
In this work we describe a novel kind of scanning probe microscope carried out through an optical fiber extrinsic microcavity.
The micro-cavity is realized by approaching a single mode fiber to a sample placed on a piezo-scanner. The distal
end of the fiber and the sample realize the optical resonator. The probe is fed by a low-coherence source and the reflected
intensity is acquired by an optical spectrum analyzer. The resonant behavior of the cavity enables to overcome the
conventional Rayleigh limit. For this system the transverse resolution is not defined by the NA of fiber but it is a
function of the transverse electromagnetic field inside the micro-cavity. The lens-free system paves the way towards
quantitative measurements in air and liquid environment.
Noninvasive, lens-free microscopy methods helps biologists to measure quantitative contrast phase imaging without
damaging the cells. An extrinsic scanning micro-cavity in optical fiber is proposed to achieve surface imaging at infrared
wavelengths. The micro-cavity is realized by approaching a single mode fiber with a numerical aperture NA to a sample
and it is fed by a low-coherence source. The measurement of the reflected optical intensity provides a map of the sample
reflectivity, whereas from the analysis of the reflected spectrum in the time/spatial domain, we disentangle the
topography and contrast phase information. The latter describes the contrast variation of the reflected spectrum from the
cavity due to changes in topography and surface refractive index. The interference of diffracted waves defines the
transverse field behavior of the electromagnetic field inside the micro-cavity, affecting in this way the transverse
resolution, that is not defined by the numerical aperture NA of the fiber and consequently by the conventional Rayleigh
limit (about 0.6λ/NA). The resolution in the normal direction is limited mainly by the source bandwidth and
demodulation algorithm. The system shows a compact and simple architecture.
This paper considers the application of the Generalized Telegrapher’s Equations (GTE) to the electromagnetic modelling and designing of integrated electro-optical devices. This approach allows to eliminate the restrictions introduced by others models, as: weakly guiding condition and isotropic unperturbed medium and to value the modulator time response for a generic modulating signal. The presence of small dielectric perturbations and the large difference between the optical and modulating signal frequencies are the hypotheses considered in deriving the model. The analysis has been applied to a GaAs phase modulator in order to validate the equations and to evidence the effects of the induced anisotropy on the time-domain response. The model can be extended to the analysis of multimode dielectric waveguides, such as independent polarization and directional coupler modulator.