Roland Terborg

Steering and guiding light with light in a nanosuspension

We experimentally demonstrate guiding of a low-power probe beam (633 nm wavelength) by means of a light-induced waveguide generated by the self-focusing of a strong pump beam (532 nm wavelength) in an artificial nonlinear medium, constituted by a colloidal suspension of dielectric nanoparticles. We also demonstrate optical steering of the probe beam by controlling the direction of propagation of the pump beam. The distance over which guiding is demonstrated (5 mm) is remarkably long when compared with previous experiments.

Ultrasensitive interferometric on-chip microscopy of transparent objects

An optical reader made of consumer electronics components interferometrically detects ultrathin glass and protein patterns. Light microscopes can detect objects through several physical processes, such as scattering, absorption, and reflection. In transparent objects, these mechanisms are often too weak, and interference effects are more suitable to observe the tiny refractive index variations that produce phase shifts. We propose an on-chip microscope design that exploits birefringence in an unconventional geometry. It makes use of two sheared and quasi-overlapped illuminating beams experiencing relative phase shifts when going through the object, and a complementary metal-oxide-semiconductor image sensor array to record the resulting interference pattern. Unlike conventional microscopes, the beams are unfocused, leading to a very large field of view (20 mm2) and detection volume (more than 0.5 cm3), at the expense of lateral resolution. The high axial sensitivity (<1 nm) achieved using a novel phase-shifting interferometric operation makes the proposed device ideal for examining transparent substrates and reading microarrays of biomarkers. This is demonstrated by detecting nanometer-thick surface modulations on glass and single and double protein layers.

Phase-sensitive plasmonic biosensor using a portable and large field-of-view interferometric microarray imager

Nanophotonics, and more specifically plasmonics, provides a rich toolbox for biomolecular sensing, since the engineered metasurfaces can enhance light–matter interactions to unprecedented levels. So far, biosensing associated with high-quality factor plasmonic resonances has almost exclusively relied on detection of spectral shifts and their associated intensity changes. However, the phase response of the plasmonic resonances have rarely been exploited, mainly because this requires a more sophisticated optical arrangement. Here we present a new phase-sensitive platform for high-throughput and label-free biosensing enhanced by plasmonics. It employs specifically designed Au nanohole arrays and a large field-of-view interferometric lens-free imaging reader operating in a collinear optical path configuration. This unique combination allows the detection of atomically thin (angstrom-level) topographical features over large areas, enabling simultaneous reading of thousands of microarray elements. As the plasmonic chips are fabricated using scalable techniques and the imaging reader is built with low-cost off-the-shelf consumer electronic and optical components, the proposed platform is ideal for point-of-care ultrasensitive biomarker detection from small sample volumes. Our research opens new horizons for on-site disease diagnostics and remote health monitoring.

Quantitative characterization of the energy circulation in helical beams by means of near-field diffraction

We present a method to measure the skew angle of the wave-fronts in an optical vortex, which is directly related with the energy flux. It is based on the analysis of the evolution on propagation of the near-field diffraction pattern generated by a single-slit, consisting of two main lobes that shift in opposite directions depending on the vortex helicity. The transverse displacement of each lobe as a function of the propagation distance allows to quantify the energy circulation. Analytical, numerical and experimental results are compared, showing good agreement. We illustrate the method for the case of Bessel beams, although we also discuss other types of helical beams, such as Laguerre-Gauss and Mathieu beams.

Multispectral interferometrie on-chip microscopy for biosensing

Recent advances towards portable medical devices have been made in the field of lens-free microscopy [1, 2] where computational imaging can replace optical elements such as lenses. This enables increasing the field of view (FOV) while maintaining a spatial resolution of microns. At the same time cost and size of devices can also be reduced.

Steering and switching of soliton-like beams via interaction in a nanocolloid with positive polarizability

We unveil different regimes for the interaction between two orthogonally polarized soliton-like beams in a colloidal suspension of nanoparticles with positive polarizability. The interaction is always attractive. However, it noticeably changes as a function of the angle and the power distribution between the input beams. For small angles, both interacting solitons fuse into a single entity, whose propagation direction can be continuously steered. As the interaction angle increases, the resulting self-collimated beam can be practically switched between two positions when the power imbalance between the beams is changed. For interaction angles larger than ∼10°, the result is no longer a single emerging soliton when the input power is balanced between the two beams.

Waveguides in colloidal nanosuspensions

We present and discuss a set of experiments based on the application of the nonlinear properties of colloidal nanosuspensions to induce waveguides with a high‐power CW laser beam (wavelength 532nm) and its use for controlling an additional probe beam. The probe is a CW laser of a different wavelength (632nm), whose power is well below the critical value to induce nonlinear effects in the colloidal medium. We also discuss a technique for the characterization of the induced waveguides.

Beam-splitting waveguides induced in nanocolloids

It has been shown that a spatial soliton can be created when a CW laser travels through a suspension of dielectric nanoparticles, provided its power is above a critical value [Opt. Lett. Vol. 7: 276 (1982)]. Recently, it was demonstrated that these soliton-like beams can be used as waveguides for controlling an additional low-power laser (probe beam) [Opt. Lett. Vol. 38: 5284 (2013)]. Here we present an experimental study of the interaction between two solitons propagating through a nanocolloid and we analyze their use to create a beam splitter for a probe beam.

Transverse energy flux estimation in optical vortices by single-slit diffraction

We propose a simple technique for the estimation of the local inclination angle of the helical wave fronts, and thus the direction of the transverse energy flux, in beams with embedded optical vortices. It is based on the analysis of the evolution on propagation of the asymmetric diffraction pattern produced by a single-slit aperture.