We report a study of the determination of polymer cross-linking, namely the degree of conversion and refractive index of the microstructures created by two-photon polymerization (TPP). The influence of TPP processing parameters such as laser intensity and scanning velocity is investigated. The degree of conversion is analyzed via Raman microspectroscopy and the refractive index is measured with the interferometric technique employing a Michelson interferometer. Moreover, the relationship between these two properties is revealed and details are discussed. The largest refractive index change that we have obtained is of the order of 10 −2 . Finally, we propose and demonstrate experimentally the realization of the gradientindex (GRIN) structure, resulting from a laser-induced local refractive index modification due to monomer cross-linking, i.e. degree of conversion. This work implies that the TPP technique is a valuable tool for the fabrication of GRIN microoptics for (in)homogeneous molding of light flow at the micrometer scale.

Bifunctional self-assembled monolayers (SAMs) are widely used for construction of surfaces with desirable properties. To predict and control the function of SAMs, molecular level understanding of monolayer architecture is required. In this work, structure and bonding of positive charge bearing SAM of N-(6-mercapto)hexylpyridinium (MHP) on Au and Ag electrodes was probed by SERS, firstprinciples calculations, isotopic substitution, and reductive desorption voltammetry. Based on analysis of immersion timeand temperature-dependent SERS spectra as well as DFT calculations, the marker bands for MHP structure, bonding, and orientation have been established. The presence of a band around 1083 cm −1 was found to be a reliable SERS marker for the all-trans conformation of hydrocarbon chain. Soft C−H stretching mode near 2832 cm −1 was recognized as a marker for the probing of direct interaction of alkyl chain with the metal. Based on solvent-dependent Raman spectra the pyridinium ring C− H band (ν 2 ) was proposed to be a marker for environment dielectric constant. The metal−adsorbate bonding marker bands, ν(Au−S) and ν(Ag−S), were observed at 260 and 236 cm −1 , respectively. Identified in this work SERS marker bands provide novel spectra−structure correlations applicable to molecular level control of structure, bonding, and stability of bifunctional SAMs.

A femto- and picosecond laser assisted periodic nanostructuring of hydrogenated amorphous silicon (a-Si:H) is demonstrated. The grating structure with the subwavelength modulation of refractive index shows form birefringence (Δn ≈ −0.6) which is two orders of magnitude higher than commonly observed in uniaxial crystals and femtosecond laser nanostructured silica glass. The laser-induced giant birefringence and dichroism in a-Si:H film introduce extra dimensions to the polarization sensitive laser writing with applications that include data storage, security marking, and flat optics.

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Methylammonium lead iodide perovskite( MAPI) is ap romising materialf or highly efficient photovoltaic devices.H owever,i t suffers from photooxidation,whichimposes strict requirements for its protection from oxygen during processing and operation. Ah indered amine light stabilizer( HALS) has been found to exert as tabilization effect on methylammonium iodide (MAI) and MAPI against photooxidation. The HALS prevents the degradation of MAI by inhibiting the oxidation of iodide to iodine. Chemical modification of HALS allowsi ts incorporation in MAPI films, whiche xtendst he resistivity of MAPI against photodegradation in ambient air from ac ouple of hours to severald ays,w hile causing no significant changes in key properties, such as optical absorption and charge transport. These results represent an important advance in the stabilizationo f MAPI against decomposition and demonstrate for the first time that antioxidants improve the stability of MAPI.Methylammonium lead iodide perovskite (MAPI) solar cells constitute an emerging photovoltaict echnology that in recent years has shown an outstanding improvement in power conversion efficiency, surpassing 21 %.[1-6] However,d espite this significant progress, poor stabilityo fp erovskite devices remains am ajor issue. Factors such as light, oxygen, humidity, and temperature cause rapid degradation of perovskite solar cells (PSC). [7,8] This drawback hampers potential large-scale application of the devices, which must be able to operate for a sufficient length of time under real environmental conditions. Because PSC is ac omplexd evice, composed of MAPI absorber, charge extractionl ayers, and electrodes, multiple degradation phenomenao ccur in it. Most of the studies, relate the degradation of PSC with MAPI decomposition, yet degradation is also attributed to the role of the hole-transporting material, its additives,t he electron-extracting TiO 2 ,t he electrodes, or even the architecture of the device. [7][8][9][10] Recent studies have demonstrated that photooxidation (POX) of MAPI inducedb yl ight and oxygen is ad ominant factor limiting the lifetimeo fP SC under ambient conditions. [11] Photoexcited MAPI reacts with oxygen, resulting in the formation of superoxide anion,w hich in turn attacks the methylammoniumc ation thus initiating degradation of MAPI. Importantly,t he degradation rate may be reduced when effective electron extraction from the excited perovskite is ensured and the electron transfer from MAPI to oxygen is minimized. However, despite reduced photodegradationw hen TiO 2 is used as an electron acceptor,t he process cannot be completely suppressed, [12] since areas with poor electron extraction are present in ap olycrystalline MAPI layer.[13] These recent findings indicate unequivocally that MAPI is inherently unstable in the presence of oxygen andl ight. Furthermore,M API degradation in PSCs also occurs in the dark at forward bias when electrically injected electrons react with oxygen.[12] Photodegradation of one of the MAPI precursors, namely methyla...