Cellulose nanocrystals (CNCs) were modified with natural di-and tricarboxylic acids using two concurrent acid-catalyzed reactions including hydrolysis of amorphous cellulose segments and Fischer esterification, resulting in the introduction of free carboxylic acid functionality onto CNC surfaces. CNC esterification was characterized by Fourier Transform Infrared Spectroscopy, 13 C solid state magic-angle spinning (MAS) and conductometric titration experiments. Average degree of substitution values for malonate, malate, and citrate CNCs are 0.16, 0.22 and 0.18, respectively. Despite differences in organic acid pKa, optimal HCl cocatalyst concentrations were similar for malonic, malic and citric acids. After isolation of modified CNCs, residual cellulose co-products were identified that are similar to microcrystalline cellulose based on SEM and XRD analysis. As proof of concept, recycling experiments were carried to increase the yield of citrate CNCs. The by-product was then recycled by subsequent citric acid/HCl treatments that resulted in 55% total yield of citrate CNCs.The crystallinity, morphology, and substitution of citrate CNCs from recycled cellulose coproduct is similar to modified citrate CNCs formed in the first reaction cycle. Thermal stability of all modified CNCs under air and nitrogen resulted in T 10% and T 50% values above 256 °C and 323 °C, respectively. Thus, they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. Nanocomposites of polyvinyl alcohol with modified CNCs (1 wt% malonate-, malate-, citrate and unmodified CNCs) wereprepared. An increase in the thermal decomposition temperature by almost 40 °C was obtained for PVOH-citrate modified CNC nanocomposites.Furthermore, since TGA determined weight loss up to 150°C is attributed to loss of bound water, this provides a tool to determine CNC water affinity. By 150°C, the weight loss of modified and non-modified CNCs is 6% and 2%, respectively. Consequently, modified CNCs with surface carboxylate groups have higher water affinity than non-modified CNCs. Moreover, significant differences in the thermal stability are observed as a function of the di-or triacid used for CNC modification. Based on the peaks of the derivative thermogravimetric curve (DTG, T 50% ), modified CNCs have the following thermal stability: malonate = HCl > malate > citrate CNCs.Corresponding values for T 50% are 366°C, 365°C, 350°C and 345°C, respectively. The amount of residual char at 600°C for HCl, malonate, malate and citrate CNCs is 8, 13, 16 and 20%, respectively. Hence, it follows that increasing the T 50% for di-and tri-acid modified CNCs results in correspondingly lower char formation. Increased char amounts is likely due to CNC functionalizations that lead to relatively larger number of cross-linking events at elevated temperatures.Since the exclusion of air during melt processing is generally not practical, the effect of CNC modification on thermal stability in air was also determined and the correspo...

We investigate whether microscopic cascading of second-order nonlinearities of two molecules in the side-by-side configuration can lead to a third-order molecular nonlinear-optical response that exceeds the fundamental limit. We find that for large values of the second hyperpolarizability, the side-by-side configuration has a cascading contribution that lowers the direct contribution. However, we do find that there is a cascading contribution to the second hyperpolarizability when there is no direct contribution. Thus, while cascading can never lead to a larger nonlinear-optical response than for a single molecule with the same number of electrons, it may provide design flexibility in making large third-order susceptibility materials when the molecular second hyperpolarizability vanishes.

Azo-dye doped liquid crystal elastomers (LCE) are known to show a strong photomechanical response. We report on experiments that suggest that photothermal heating is the underlying mechanism in the 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.

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.PACS numbers: 42.25. Bs, 78.20.Ls, 42.25.Hz Coherent Perfect Absorption (CPA) [1,2] illuminates the role optical coherence plays in the perfect conversion of optical energy into other modes (typically incoherent fluorescence or heat). CPA is a non-conservative linear process, typically modeled using a non-Hermitian Hamiltonian. In its original formulation, this non-Hermitian Hamiltonian included absorption/gain to explicitly break the time reversal invariance of the underlying fundamental processes. This is also the case with the formulation of CPA in PT-invariant theories [3,4], which has led to a fertile way to explore many subtleties in optical processes [5,6].In this paper we develop theory for Coherent Perfect Rotation (CPR), the conservative transfer of any fixed input polarization state of coherent counterpropagating light fields completely into its orthogonal polarization. CPR highlights the necessity of combining T-odd processes (in, for example, magneto-optics) with optical coherence to achieve this perfect conversion. By contrast T-even conservative processes cannot effect such a transformation. CPR denotes a conservative (thus fully Hermitian Hamiltonian) process that first appears at a particular "threshold" value of the parameter scaling the Todd process, and there are many phenomenological correspondences between CPA and CPR, illustrated schematically in Fig. 1. Beyond revealing a connection between T-odd processes, Hermiticity, and CPA, CPR may inform the design of novel magneto-optical sensors and devices.We adopt a 4 × 4 transfer matrix approach to describe linear optical transport of a monochromatic ray moving back and forth along theẑ-axis,where the M 's, B and C are 2 × 2 (in general complex) matrices; here we are working in the basis where the local field (complex) amplitudes are v = (E x , H y , E y , −H x ). Note that for birefringent materials M = M ′ , but since we are interested in systems that transform any input polarization into the orthogonal polarization, we will focritical coupling Λ, α 1. CPA and CPR are distinct from critical coupling and critical rotation [7][8][9]. For a fixed value of Λ, the system's length in terms of the vacuum wavelength, critical coupling and CPA occur at a particular value of the absorption α and index. Critical half-wave rotation and CPR first occur at "threshold" value...

We report on improved gain and spectral control in co-extruded all-polymer multilayer distributed feedback (DFB) lasers achieved by folding and deliberate modification of the center "defect" layer. Because DFB laser gain is greater at spectral defects inside the reflection band than at the band edges, manipulation of structural defects can be used to alter spectral defects and thereby tune the output wavelength and improve laser efficiency. By experimentally terracing the layer that becomes the center of the fold, we tuned the lasing wavelength across the reflection stop-band (∼25 nm) in controllable, discrete steps. The increased density of states associated with the defect resulted in a lower lasing threshold and, typically, a 3- to 6-fold increase in lasing efficiency over non-folded samples.