Nathan J. Dawson

Experimental studies of the mechanisms of photomechanical effects in a nematic liquid crystal elastomer

Azo-dye-doped liquid crystal elastomers (LCEs) are known to show a strong photomechanical response. We report on experiments that suggest that photothermal heating is the underlying mechanism in surface-constrained geometry. In particular, we use optical interferometry to probe the length change of the material and direct temperature measurements to determine heating. LCEs with various dopants and optical density were used to study the individual mechanisms. In the high dye-doped limit, most of the light is absorbed near the entry surface, which causes a local strain from photothermal heating and a nonlocal strain from thermal diffusion. The results of our research on the microscopic mechanisms of the photomechanical response can be applied to designing photomechanical materials for actuating/sensing devices, the potential basis of smart structures.

Cascading of liquid crystal elastomer photomechanical optical devices

Abstract Photomechanical actuation is demonstrated in two coupled liquid crystal elastomer photomechanical optical devices (PODs) acting in series. The response function of an individual POD is characterized and used to predict the temporal response of the coupled system. The predicted coupled-system response agrees with the experiment for several waveforms and frequencies, suggesting that large-scale integration of photomechanical devices is possible.

Coherent Perfect Rotation

Two classes of conservative, linear, optical rotary effects (optical activity and Faraday rotation) are distinguished by their behavior under time reversal. In analogy with coherent perfect absorption, where counterpropagating light fields are controllably converted into other degrees of freedom, we show that only time-odd (Faraday) rotation is capable of coherent perfect rotation in a linear and conservative medium, by which we mean the complete transfer of counterpropagating coherent light fields into their orthogonal polarization. This highlights the necessity of time reversal odd processes (not just absorption) and coherence in perfect mode conversion and may inform device design.

Testing the diffusion hypothesis as a mechanism of self-healing in Disperse Orange 11 doped in poly(methyl methacrylate)

In this work, we show that reversible photodegradation of Disperse Orange 11 doped in poly(methyl methacrylate) is not due to dye diffusion—a common phenomenon observed in many dye-doped polymers. The change in linear absorbance due to photodegradation of the material shows an isobestic point, which is consistent with the formation of a quasi-stable damaged species. Spatially resolved amplified spontaneous emission and fluorescence, both related to the population density, are measured by scanning the pump beam over a burn mark. A numerical model of the time evolution of the population density due to diffusion is inconsistent with the experimental data, suggesting that diffusion is not responsible.

Thermo-spectral study of all-polymer multilayer lasers

We investigate the temperature dependence of the emission wavelength and reflection band of polymer Distributed Bragg Reflector (DBR) and defect Distributed FeedBack (DFB) lasers fabricated using a coextrusion melt-process. We show the measured spectral shifts are a direct consequence of the optical path modifications associated with layer expansion and thermo-optic coefficients. By varying the choice of polymer bilayers and sandwiching the DBR laser films between glass coverslips, we fabricated DBR lasers that are either readily tunable up to 0.36nm/°C or made thermally stable at 0.035nm/°C.

Quantum mechanical model of the upper bounds of the cascading contribution to the second hyperpolarizability

Microscopic cascading of second-order nonlinearities between two molecules has been proposed to yield an enhanced third-order molecular nonlinear-optical response. In this contribution, we investigate the two-molecule cascaded second hyperpolarizability and show that it will never exceed the fundamental limit of a single molecule with the same number of electrons as the two-molecule system. We show the apparent divergence behavior of the cascading contribution to the second hyperpolarizability vanishes when properly taking into account the intermolecular interactions. Although cascading can never lead to a larger nonlinear-optical response than a single molecule, it provides alternative molecular design configurations for creating materials with large third-order susceptibilities that may be difficult to design into a single molecule.

Lowest-order relativistic corrections to the fundamental limits of nonlinear-optical coefficients

The effects of small relativistic corrections to the off-resonant polarizability, hyperpolarizability, and second hyperpolarizability are investigated. Corrections to linear and nonlinear optical coefficients are demonstrated in the three-level ansatz, which includes corrections to the Kuzyk limits when scaled to semi-relativistic energies. It is also shown that the maximum value of the hyperpolarizability is more sensitive than the maximum polarizability or second hyperpolarizability to lowest-order relativistic corrections. These corrections illustrate how the intrinsic nonlinear-optical response is affected at semi-relativistic energies.

The local-field factor and microscopic cascading: a self-consistent method applied to confined systems of molecules

We use a simplified self-consistent method to address nonlinear-optical cascading phenomena, which shows added microscopic cascading contributions in high-ordered nonlinear susceptibilities through fifth order. These cascading terms in the microscopic regime encompass all possible scalar cascading configurations. The imposition of geometric constraints further influences the predicted cascading contributions and opens up additional design parameters for nonlinear-optical materials. These results are used in approximating the effective fifth-order susceptibility in thin films of C60 monomers of varying thickness and concentration.

Optical properties of cellulose nanocrystals decorated with silver nanospheres

We discuss some optical properties of cellulose nanocrystals decorated with silver nanospheres. We give a short description of the discrete dipole interactions, and the broadening effects observed in the extinction spectrum. We also discuss some preliminary results for their use in organic photovoltaic devices.

Interfacial trapping in an aged discotic liquid crystal semiconductor

This study reports on time-of-flight (TOF) hole mobility measurements in aged 2,3,6,7,10,11-Hexakis(pentyloxy)triphenylene columnar liquid crystals. In contrast to the original samples reported in 2006, homeotropically aligned samples yielded TOF transients with an extended non-exponential rise. The experimental data were fit to a simple model that accurately reproduces the TOF transients assuming delayed charge release from traps near the optically excited electrode. While interfacial trapping appears only in the aged materials, the bulk mobility is similar to the pristine material. The model addresses dispersive transport in quasi-one-dimensional materials, determines the charge carrier mobility in systems with interfacial traps, and provides a method for characterizing the traps.