We investigated the thermal hysteresis of a pelleted sample of the spin-crossover compound ͓Fe͑NH 2 -trz͒ 3 ͔Br 2 by means of spectroscopic ellipsometry, in the temperature range 264-358 K. The ellipsometric parameters ͑ , ⌬͒ have been recorded in the optical range 240-1000 nm. The corresponding absorption and dispersion spectra show temperature-invariant isobestic points located at 240 and 291 nm, respectively. We found that the high-spin-fraction data, derived from the integrated absorption curves, are in excellent agreement with the magnetic data recorded on the same sample. We also investigated the consistency of the optical data by application of the Kramers-Kronig relations, which are well obeyed above ϳ450 nm. All these results demonstrate that the spectroscopic ellipsometry is well adapted to characterize the spin-crossover transition. We also expect that this nondestructive technique will be highly relevant to investigate the physical properties of thin films of switchable molecular solids, involving a change in the electronic properties upon a thermally or photoinduced phase transition.
This paper compares anisotropic linear optical properties (linear birefringence, linear dichroism, degree of polarization) and performances (absorption coefficient, thermal stability) of two types of birefringent waveplates fabricated in silica glass by femtosecond laser direct writing. The first type of waveplate is based on birefringence induced by self-organized nanogratings imprinted in the glass. One the other hand, the second design is based on birefringence originating from the stress-field formed around the aforementioned nanogratings. In addition to the provided comparison, the manufacturing of stress-engineered half waveplates in the UV-Visible range, and with mm-size clear aperture and negligible excess losses, is reported. Such results contrast with waveplates made of nanogratings, as the later exhibit significantly higher scattering losses and depolarization effects in the UV-Visible range.
Silicon carbon nitride thin films are deposited by sputtering a silicon carbide target under mixed Ar–N2 atmosphere. The flow ratio (RF = [N2]/[N2] + [Ar]) of these two gazes is set between 0 and 1. The deposits were analyzed by Rutherford backscattering (RBS), infrared, Raman, and electron spin resonance (ESR) spectroscopies. Nitrogen content in thin film was found to be sensitive to the sputtering atmosphere until RF = 0.3 and reached a plateau between 0.3 and 1. From a structural point of view, the analysis revealed that the sp2 carbon character and the C≡N or CN bonds increased with the nitrogen percentage in the gas mixture. These effects induced an increase of the structural disorder and the amount of dangling bonds in the layer bulk. The optical properties in terms of optical gap (Eg) and Urbach parameter (Eu) were also determined. Eg was found to increase with nitrogen percentage in the sputtering mixture, whereas Eu exhibited an opposite behavior. We also show that the refractive index and the extinction coefficient determined by ellipsometric spectroscopy are sensitive to the structural modification.
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