Berna Yalizay

Optical element for generation of accelerating Airy beams.

We demonstrate an optical element for generation of accelerating Airy beams. The element is conveniently constructed by combination of positive and negative cylindrical lenses of matching radii of curvature. With proper choice of lens curvatures, the resulting surface profile closely follows a cubic polynomial. Passing a gaussian beam through this element and performing optical Fourier transform yields beam profiles close to the Airy function. Our experiments demonstrate parabolic propagation, or acceleration, of the resulting focal spots.

Fabrication of nanometer-size structures in metal thin films using femtosecond laser Bessel beams

We report nanometer-scale fabrication on metal thin films using ablation by femtosecond laser pulses, with Bessel beam profiles. Choosing the laser fluence around ablation threshold allows control of the structure size below the diffraction limit. We show that using Bessel beams has several advantages. Bessel beams have focal spot sizes insensitive to longitudinal position, which significantly relaxes alignment constraints. Tighter foci are easier to generate, less costly, and less prone to aberrations. Scaling the method to shorter wavelengths, and hence increasing the resolution is also straightforward. By using the proposed method, we generate structures with resolution below 200 nm.

Free-standing optofluidic waveguides formed on patterned superhydrophobic surfaces

We report on the preparation and characterization of free-standing optofluidic waveguides created on solid superhydrophobic (SH) substrates with patterned wetting properties. In order to locally modify the liquid-solid contact angle, we employed selective laser ablation of SH layers deposited on magnesium-fluoride substrates with low refractive index. Upon ablation, surfaces with hydrophilic channels surrounded by SH areas were obtained. Subsequently, we created liquid optical waveguides based on total internal reflection using ethylene glycol, a polar liquid with high refractive index spreading spontaneously along the hydrophilic surface channels. We evaluated the light guiding performance and losses of these optofluidic waveguides.

Propagation characteristics of Bessel beams generated by continuous, incoherent light sources.

We investigate the propagation behavior of Bessel beams generated by incoherent, continuous light sources. We perform experiments with narrowband and broadband light emitting diodes, and, for comparison, with a laser diode. We observe that the formation of Bessel beams is affected minimally by temporal coherence, while spatial coherence determines the longitudinal evolution of the beam profile. With spatially incoherent beams, the fringe contrast is comparable to the coherent case at the beginning of the Bessel zone, while it completely fades away by propagation, turning into a cylindrical light pipe. Our results show that beam shaping methods can be extended to cases of limited spatial coherence, paving the way for potential new uses and applications of such sources.

Optofluidic waveguides written in hydrophobic silica aerogels with a femtosecond laser

We present a new method to form liquid-core optofluidic waveguides inside hydrophobic silica aerogels. Due to their unique material properties, aerogels are very attractive for a wide variety of applications; however, it is very challenging to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore, there is a need to develop alternative processes for formation of complex structures within the aerogels without damaging the material. In our study, we used focused femtosecond laser pulses for high-precision ablation of hydrophobic silica aerogels. During the ablation, we directed the laser beam with a galvo-mirror system and, subsequently, focused the beam through a scanning lens on the surface of bulk aerogel which was placed on a three-axis translation stage. We succeeded in obtaining high-quality linear microchannels inside aerogel monoliths by synchronizing the motion of the galvo-mirror scanner and the translation stage. Upon ablation, we created multimode liquid-core optical waveguides by filling the empty channels inside low-refractive index aerogel blocks with highrefractive index ethylene glycol. In order to demonstrate light guiding and measure optical attenuation of these waveguides, we coupled light into the waveguides with an optical fiber and measured the intensity of transmitted light as a function of the propagation distance inside the channel. The measured propagation losses of 9.9 dB/cm demonstrate the potential of aerogel-based waveguides for efficient routing of light in optofluidic lightwave circuits.

Enhancement of polycrystalline silicon solar cells efficiency using indium nitride particles

In this work, we present a hybrid indium nitride particle/polycrystalline silicon solar cell based on 230 nm size indium nitride particles (InN-Ps) obtained through laser ablation. The solar cell performance measurements indicate that there is an absolute 1.5% increase (Δη) in the overall solar cell efficiency due to the presence of InN-Ps. Within the spectral range 300–1100 nm, improvements of up to 8.26% are observed in the external quantum efficiency (EQE) and increases of up to 8.75% are observed in the internal quantum efficiency (IQE) values of the corresponding solar cell. The enhancement in power performance is due to the down-shifting properties of the InN-Ps. The electrical measurements are supplemented by TEM, Raman, UV/ VIS and PL spectroscopy of the InN-Ps.

PROPAGATION OF DIFFRACTION-FREE AND ACCELERATING LASER BEAMS IN TURBID MEDIA

We experimentally investigate propagation of laser beams with Gaussian, Bessel and Airy transverse profiles in turbid media. We evaluate and compare the self- healing properties of these beams.

Investigation of shaped laser beam propagation in turbid media

Shaping laser beams has been a fascinating topic in optic studies lately, because they exhibit intriguing characteristics and have potential applications. One of the most commonly used profiles is the Bessel beams. These beams, which are first shown by Durnin and others, are called also diffraction-free beams [1]. They have much long length of focus and their profiles do not change in free space propagation. Bessel beams also indicate self-healing property [2]. When the beams encounter an obstacle, they can reconstruct their initially intensity profiles after short propagation. This property enables imaging deeper into highly scattering media like biological tissues [3]. Recently, Siviloglou and others have demonstrated accelerating Airy beams [4]. Airy beams are also diffraction-free beams like Bessel beams. Furthermore, they expose acceleration. Focus of an Airy beam moves along a parabola. Airy beams also exhibit self-healing. However, the self-healing behaviour of Airy beams in turbid media has not yet been explored comprehensively.