Unveiling low THz Dynamics of Liquid Crystals: Identification of Intermolecular Interaction among Intramolecular Modes

Liquid crystal based technologies have found considerably diversified uses and areas of application over the last few decades, proving to be excellent materials for tunable optical elements from visible to near-infrared frequencies. Currently, much effort is devoted to demonstrating their applicability in the far-infrared or THz spectral frequency (1 - 10 THz), where tremendous advances have been achieved in terms of broadband and intense sources. Yet a detailed understanding of the dynamics triggered by THz light in liquid crystals is far from complete. In this work, we perform broadband THz Time Domain Spectroscopy on the low-frequency modes of 4-cyano-4'-alkylbiphenyl (nCB) and 5-phenylcyclohexanes (PCH5) across different mesophases. DFT calculations on isolated molecules capture the majority of the measured response above 3 THz. In particular, the pronounced modes around 4.5 and 5.5 THz mainly originate from in-plane and out-of-plane bending of the cyano group. In contrast, the broad response below 3 THz, linked to modes of the alkyl chain, disagrees with the single molecule calculation. Here, we identify a clear intermolecular character of the response, supported by dimer and trimer calculations.Comment: 14 pages, 4 figure

All optical sub diffraction multilevel data encoding onto azo polymeric thin films

By exploiting photo-induced reorientation in azo-polymer thin films, we demonstrate all-optical polarization-encoded information storage with a scanning near-field optical microscope. In the writing routine, 5-level bits are created by associating different bit values to different birefringence directions, induced in the polymer after illumination with linearly polarized light. The reading routine is then performed by implementing polarization-modulation techniques on the same near-field microscope, in order to measure the encoded birefringence direction.Comment: 13 pages, 5 figure

Apparatus for vectorial Kerr confocal microscopy

Apparatus for vectorial Kerr confocal microscop

Near-field circular polarization probed by chiral polyfluorene

We demonstrate that a high degree of circular polarization can be delivered to the near field (NF) of an aperture at the apex of hollow-pyramid probes for scanning optical microscopy. This result is achieved by analyzing the dichroic properties of an annealed thin polymer film containing a chiral polyfluorene derivative, placed in close proximity to the optical probe. We also prove that the degree of circular polarization in the probe NF does not depend in a significant way on the shape of the aperture, at variance with the far-field behavior. These results demonstrate the feasibility of nano-optics applications exploiting circularly polarized NFs

Impact ionization in low-band-gap semiconductors driven by ultrafast terahertz excitation: Beyond the ballistic regime

Using two-dimensional THz spectroscopy in combination with numerical models, we investigate the dynamics linked to carrier multiplication caused by high-field THz excitation of the low-gap semiconductor InSb. In addition to previously reported dynamics connected with quasiballistic carrier dynamics, we observe other spectral and temporal features that we attribute to impact ionization for peak fields above 60 kV/cm, which continue up to the maximum investigated peak field of 430 kV/cm. At the highest fields we estimate a carrier multiplication factor greater than 10 due to impact ionization, which is well-reproduced by a numerical simulation of the impact ionization process which we have developed.ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-3795ISSN:2469-995