Andrea Cattoni

λ3/1000 Plasmonic Nanocavities for Biosensing Fabricated by Soft UV Nanoimprint Lithography

Arrays of plasmonic nanocavities with very low volumes, down to λ(3)/1000, have been fabricated by soft UV nanoimprint lithography. Nearly perfect omnidirectional absorption (3-70°) is demonstrated for the fundamental mode of the cavity (λ ≃ 1.15 μm). The second-order mode exhibits a sharper resonance with strong angular dependence and total optical absorption when the critical coupling condition is fulfilled (45-50°, λ ≃ 750 nm). It leads to high refractive index sensitivity (405 nm/RIU) and figure of merit (∼21) and offers new perspectives for efficient biosensing experiments in ultralow volumes.

Ultrathin GaAs Solar Cells With a Silver Back Mirror

We report on the fabrication and characterization of ultrathin GaAs solar cells with a silver back mirror and absorber thicknesses of only t = 120 nm and t = 220 nm. The silver back mirror is combined with localized ohmic contacts. Without antireflection coating, Fabry-Perot resonances lead to strong enhancement over single-pass absorption (up to 4), and external quantum efficiency reaches 0.8 at resonance wavelengths. An analytical model is used to determine the resonance wavelengths and the absorption maxima. A short-circuit improvement of 27% results from the enhanced absorption induced by the Fabry-Perot resonances. By implementing an additional antireflection coating, short-circuit currents reach 16.3 mA/cm2 fort = 120 nm and 20.7 mA/cm2 for t = 220 nm, corresponding to efficiencies of 8.7 % and 12.9 %, respectively.

Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping

Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm(2) for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.

Single particle demultiplexer based on domain wall conduits

The remote manipulation of micro and nano-sized magnetic particles carrying molecules or biological entities over a chip surface is of paramount importance for future on-chip applications in biology and medicine. In this paper, we present a method for the on-chip demultiplexing of individual magnetic particles using bifurcated magnetic nano-conduits for the propagation of constrained domain walls (DWs). We demonstrate that the controlled injection and propagation of a domain wall in a bifurcation allow capturing, transporting, and sorting a single magnetic particle between two predefined paths. The cascade of n levels of such building blocks allows for the implementation of a variety of complex sorting devices as, e.g., a demultiplexer for the controlled sorting among 2n paths.

Proximity effects induced by a gold layer on La0.67Sr0.33MnO3 thin films

The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4nm thick manganite films reveal a dramatic reduction (≃185K) of the Curie temperature TC and a lower saturation low temperature magnetization M0. A sizable TC reduction (≃60K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4nm thick manganite layer, suggesting that this effect might have an electrostatic origin.

Absorption enhancement through Fabry-Pérot resonant modes in a 430 nm thick InGaAs/GaAsP multiple quantum wells solar cell

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 pairs of InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the external quantum efficiency below GaAs bandgap by a factor of four through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 10% conversion efficiency. We demonstrate numerically that this promising result can be further improved by anti-reflection layers. This study paves the way to very thin MQWs solar cells.

Ultrasensitive Characterization of Mechanical Oscillations and Plasmon Energy Shift in Gold Nanorods

Mechanical vibrational resonances in metal nanoparticles are intensively studied because they provide insight into nanoscale elasticity and for their potential application to ultrasensitive mass detection. In this paper, we use broadband femtosecond pump-probe spectroscopy to study the longitudinal acoustic phonons of arrays of gold nanorods with different aspect ratios, fabricated by electron beam lithography with very high size uniformity. We follow in real time the impulsively excited extensional oscillations of the nanorods by measuring the transient shift of the localized surface plasmon band. Broadband and high-sensitivity detection of the time-dependent extinction spectra enables one to develop a model that quantitatively describes the periodic variation of the plasmon extinction coefficient starting from the steady-state spectrum with only one additional free parameter. This model allows us to retrieve the time-dependent elongation of the nanorods with an ultrahigh sensitivity and to measure oscillation amplitudes of just a few picometers and plasmon energy shifts on the order of 10(-2) meV.

Soft UV Nanoimprint Lithography: A Versatile Tool for Nanostructuration at the 20nm Scale

1.1Why soft UV nanoimprint lithography? Since the pioneering work of Whitesides and coworkers on microContact Printing (mCP) and Soft Lithography (Kumar & Whitesides (1993)) (Xia & Whitsides (1998)), considerable progress has been made in the last years and Soft Lithography is now a well consolidated technology utilized in academic and industrial laboratories (Rogers & Nuzzo (2005)). These printing methods use a flexible elastomer material named PDMS (poly(dimethylsiloxane)) to transfer molecules on a surface thus creating localized chemical patterns (Cerf & Vieu (2010)). The PDMS stamp inked with the desired molecules is placed in contact with the substrate to perform the molecular transfer. mCP has received large attention for biological applications since this soft transfer occurs in a gentlemanner which allows the biomolecules to be transferred without any damage. In addition, this powerful technique is cheap because the soft PDMS stamp can be replicated an indefinite number of times by simply pouring the PDMS prepolymer onto a single expensive siliconmaster mold that containsmicro or nanostructures. Since the flexibility of the elastomeric stamp ensures a perfect conformal adhesion within the substrate, thus allowing replication on large areas up to several tens of cm2, the use of such flexible PDMS stamps was also efficiently applied to another low-cost and high-throughput manufacturing technique called Soft UV Nanoimprinting Lithography (Soft UV-NIL). This technique creates a thickness contrast by embossing thin polymeric films, highlighting the advantages of using a flexible PDMS stamp. Historically, Nanomprint Lithography (NIL) in its original version was proposed by Stephen Chou in 1995 (Chou et al. (1995)) as an alternative technique for the embossing of high resolution patterns in thermoplastic materials. The patterning of features as small as 10 nm has been demonstrated from the beginning (Chou (1997)). This nanoimprint process, usually referred to as thermal-assisted NIL (T-NIL), is based on the use of a hard mold, namely a silicon wafer. As schematically shown in Figure 1, this hard mold containing nanoscale surface-relief features is pressed at high pressure (50-100 bar) onto a thin polymeric resist film. The resist is held some 90-100 C above its glass-transition temperature (Tg) for fewminutes to allow the flowing of the polymer in the mold nanocavities. The thin residual layer of polymer intentionally left to prevent the direct contact between the substrate and the rigid mold is Soft UV Nanoimprint Lithography: A Versatile Tool for Nanostructuration at the 20nm Scale

Broadband light-trapping in ultra-thin nano-structured solar cells

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.

X-ray photoemission study of the Au/La0.67Sr0.33MnO3 interface formation

We report an x-ray photoemission spectroscopy study of the Au∕La0.67Sr0.33MnO3 interface formation, aiming to investigate interface abruptness and possible chemical interdiffusion. Our results indicate that the gold deposition does not affect the chemical properties of manganite film and no interdiffusion or segregation takes place.