Replicating biological patterns is promising for designing materials with multifaceted properties. Twisted cholesteric liquid crystal patterns are found in the iridescent tessellated cuticles of many ...insects and a few fruits. Their accurate replication is extremely difficult since discontinuous patterns and colors must coexist in a single layer without discontinuity of the structures. Here, a solution is demonstrated by addressing striped insect cuticles with a complex twisted organization. Geometric constraints are met by controlling the thermal diffusion in a cholesteric oligomer bilayer subjected to local changes in the molecular anchoring conditions. A multicriterion comparison reveals a very high level of biomimicry. Proof-of-concept prototypes of anti-counterfeiting tags are presented. The present design involves an economy of resources and a high versatility of chiral patterns unreached by the current manufacturing techniques such as metallic layer vacuum deposition, template embossing and various forms of lithography which are limited and often prohibitively expensive.
The ever increasing field of application of nanodielectrics in electrical insulations calls for description of the mechanisms underlying the performance of these systems and for identification of the ...signs exposing their aging under high electric fields. Such approach is of particular interest to electrically insulating polymers because their chemical defects are of deleterious nature for their electrical properties and can largely degrade their performance at high electric fields. Although these defects usually leave spectroscopic signatures in terms of characteristic luminescence peaks, it is nontrivial to assign, in an unambiguous way, the identified peaks to specific chemical groups or defects because of the low intensity of the signal with the main reason being that the insulating polymers are weakly emitting materials under electric field. In this work, we go beyond the conventional electroluminescence technique to record spectroscopic features of insulating polymers. By introducing a single plane of silver nanoparticles (AgNPs) at the near-surface of thin polypropylene films, the electroluminescent signal is strongly enhanced by surface plasmons processes. The presence of AgNPs leads not only to a much higher electroluminescence intensity but also to a strong decrease of the electric field threshold for detection of light emission and to a phase-stabilization of the recorded spectra, thus improving the assignment of the characteristic luminescence peaks. Besides, the performed analyses bring evidence on the capability of AgNPs to trap and eject charges, and on the possibility to adjust the energetics of charge carriers in electrically insulating polymers at the electrode/dielectric contact via AgNPs.
Biological systems inspire the design of multifunctional materials and devices. However, current synthetic replicas rarely capture the range of structural complexity observed in natural materials. ...Prior to the definition of a biomimetic design, a dual investigation with a common set of criteria for comparing the biological material and the replica is required. Here, we deal with this issue by addressing the non-trivial case of insect cuticles tessellated with polygonal microcells with iridescent colours due to the twisted cholesteric organization of chitin fibres. By using hyperspectral imaging within a common methodology, we compare, at several length scales, the textural, structural and spectral properties of the microcells found in the two-band cuticle of the scarab beetle
with those of the polygonal texture formed in flat films of cholesteric liquid crystal oligomers. The hyperspectral imaging technique offers a unique opportunity to reveal the common features and differences in the spectral-spatial signatures of biological and synthetic samples at a 6-nm spectral resolution over 400 nm-1000 nm and a spatial resolution of 150 nm. The biomimetic design of chiral tessellations is relevant to the field of non-specular properties such as deflection and lensing in geometric phase planar optics.
We demonstrate the ability to manipulate ultrashort pulses in cholesteric liquid crystals in the linear regime. We present an extensive analysis of the spectral changes undergone by 20fs pulses when ...propagating through band edges of cholesteric liquid crystals. The accurate quantification of the introduced optical dispersion opens the way to controlled stretching and compression of ultrashort pulses. The behaviors of cholesteric liquid crystal films with different thickness, bandgap and structural parameters (monotonic pitch versus pitch-gradient films) are compared. A statistical approach is disclosed to fidelize and deepen the set of experimental investigations.
Due to the great development of light sources for several applications from displays to lighting, great efforts are devoted to find stable and efficient visible emitting materials. Moreover, the ...requirement of Si compatibility could enlarge the range of applications inside microelectronic chips. In this scenario, we have studied the emission properties of bismuth doped yttrium oxide thin films grown on crystalline silicon. Under optical pumping at room temperature a stable and strong visible luminescence has been observed. In particular, by the involvement of Bi ions in the two available lattice sites, the emission can be tuned from violet to green by changing the excitation wavelength. Moreover, under electron beam at low accelerating voltages (3 keV) a blue emission with high efficiency and excellent stability has been recorded. The color is generated by the involvement of Bi ions in both the lattice sites. These peculiarities make this material interesting as a luminescent medium for applications in light emitting devices and field emission displays by opening new perspectives for the realization of silicon-technology compatible light sources operating at room temperature.
In the last decades, in the field of silicon microphotonics many efforts have been made to find strategies in order to obtain efficient luminescence from silicon or from Si compatible materials that ...could be totally integrated on Si microchips. Among the several approaches, mixed rare earth silicates and oxides have received great attention as active media, owing to their compatibility with the standard Si technology and the possibility to dissolve inside high concentrations of luminescent centers. In this paper we will review the last achievements on the investigation of the structural and optical properties of erbium doped yttrium silicates and oxides. We will further show the advantages of using bismuth as Er co-dopant, as strategy to improve optical efficiency at 1540 nm, wavelength of interest for telecommunications. The stabilization and the role of the Bi3+ oxidation state will be evidenced along the paper by comparing the results of the Bi influence on the Er emission in the two hosts. Photoluminescence in excitation measurements will permit to demonstrate that Bi ions act as efficient sensitizers to transfer energy to Er ions. Hence, through mediated excitation the Er excitation cross section is increased up to two or three orders of magnitude in silicates and oxide, respectively, if compared to the Er resonant condition at 488 nm. In the second part of the paper we will focus on the investigation of Bi doped yttrium oxide as efficient visible emitter. In particular, the peculiar dependence of the Bi emission on the excitation wavelength, which makes the Bi-doped oxide a tunable light source, together with the estimation of its optical efficiency, permits to clearly evidence its potentiality for application in visible light emitting devices.
Description of the relationship between protein structure and function remains a primary focus in molecular biology, biochemistry, protein engineering and bioelectronics. Moreover, the investigation ...of the protein conformational changes after adhesion and dehydration is of importance to tackle problems related to the interaction of proteins with solid surfaces. In this paper the conformational changes of wild-type Discosoma recombinant red fluorescent proteins (DsRed) adhered on silver nanoparticles (AgNPs)-based nanocomposites are explored via surface-enhanced Raman scattering (SERS). Originality in the present approach is to work on dehydrated DsRed thin protein layers in link with natural conditions during drying. To enable the SERS effect, plasmonic substrates consisting of a single layer of AgNPs encapsulated by an ultra-thin silica cover layer were elaborated by plasma process. The achieved enhancement of the electromagnetic field in the vicinity of the AgNPs is as high as 10
. This very strong enhancement factor allowed detecting Raman signals from discontinuous layers of DsRed issued from solution with protein concentration of only 80 nM. Three different conformations of the DsRed proteins after adhesion and dehydration on the plasmonic substrates were identified. It was found that the DsRed chromophore structure of the adsorbed proteins undergoes optically assisted chemical transformations when interacting with the optical beam, which leads to reversible transitions between the three different conformations. The proposed time-evolution scenario endorses the dynamical character of the relationship between protein structure and function. It also confirms that the conformational changes of proteins with strong internal coherence, like DsRed proteins, are reversible.
Although in the last decades rare earths (REs)-containing materials have been proposed as active media in Si compatible light sources, the reached optical efficiency is still limited by the low REs ...excitation cross section. In this work, the introduction of Bi as an Er sensitizer in a Si-compatible yttrium oxide thin film has been proposed in order to overcome this limit. The stabilization of optically active Bi3+ ions in place of Y3+ in the two reticular sites, S6 and C2, of Y2O3 was demonstrated. Photoluminescence (PL) and photoluminescence excitation measurements have revealed the existence of two different intense emission bands in the visible range ascribed to Bi3+ in the two reticular sites. They can be selectively excited by properly changing the excitation wavelength. Moreover, the Bi3+→Er3+ energy transfer processes in the (Bi+Er)-codoped samples were demonstrated for both Bi3+ sites by the spectral overlap between the Bi emission bands and Er excitation peaks. We estimated an Er emission enhancement by a factor of about 80 for the C2 site and 16 for the S6 site with respect to the Er direct excitation. By the recorded reduction of the Bi PL intensities and the shortening of the related lifetimes in presence of Er ions, the energy transfer efficiencies were evaluated to be about 70% for the Bi (C2) and 35% for the Bi (S6). These interesting findings suggest (Bi+Er)-codoped yttrium oxide as an efficient Si-compatible material for photonic applications.
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