Juefei Zhou

Pushing the hyperpolarizability to the limit

We use numerical optimization to find a one-dimensional potential energy function that yields the largest hyperpolarizability, which we find is within 30% of the fundamental limit. Our results reveal insights into the character of the potential energy functions and wave functions that lead to the largest hyperpolarizability. We suggest that donor-acceptor molecules with a conjugated bridge with many sites of reduced conjugation to impart conjugation modulation may be the best paradigm for making materials with huge hyperpolarizabilities that approach the fundamental limit.

Studies on optimizing potential energy functions for maximal intrinsic hyperpolarizability

We use numerical optimization to study the properties of (1) the class of one-dimensional potential energy functions and (2) systems of point nuclei in two dimensions that yield the largest intrinsic hyperpolarizabilities, which we find to be within 30% of the fundamental limit. In all cases, we use a one-electron model. It is found that a broad range of optimized potentials, each of very different character, yield the same intrinsic hyperpolarizability ceiling of 0.709. Furthermore, all optimized potential energy functions share common features such as (1) the value of the normalized transition dipole moment to the dominant state, which forces the hyperpolarizability to be dominated by only two excited states and (2) the energy ratio between the two dominant states. All optimized potentials are found to obey the three-level ansatz to within about 1%. Many of these potential energy functions may be implementable in multiple quantum well structures. The subset of potentials with undulations reaffirm that modulation of conjugation may be an approach for making better organic molecules, though there appear to be many others. Additionally, our results suggest that one-dimensional molecules may have larger diagonal intrinsic hyperpolarizability {beta}{sub xxx}{sup int} than higher-dimensional systems.

Two-photon fluorescence measurements of reversible photodegradation in a dye-doped polymer

We report on the dynamics of photodegradation and subsequent recovery of two-photon fluorescence in a dye-doped polymer. The energy dependence suggests that photodegradation is a linear process, while recovery is entropic. Such recovery could be useful to high-intensity devices such as two-photon absorbers, which can be used in many applications.

Intrinsic Hyperpolarizabilities as a Figure of Merit for Electro-optic Molecules†

We propose the scale-invariant intrinsic hyperpolarizability as a measure of the figure of merit for electrooptic molecules. By applying our analysis to the best second-order nonlinear-optical molecules that are made using the present paradigms, we conclude that it should be possible to make dye-doped polymers with electrooptic coefficients of several thousand picometers per volt.

Experimental verification of a self-consistent theory of the first-, second-, and third-order (non)linear optical response

We show that a combination of linear absorption spectroscopy, hyper-Rayleigh scattering, and a theoretical analysis using sum rules to reduce the size of the parameter space leads to a prediction of the imaginary part of the second hyperpolarizability of the dye AF-455 that agrees with the experimental data gathered through two-photon absorption spectroscopy. Our procedure, which demands self-consistency between several measurement techniques and does not use adjustable parameters, provides a means for determining transition moments between the dominant excited states based strictly on experimental characterization. This is made possible by our new approach that uses sum rules and molecular symmetry to rigorously reduce the number of required physical quantities.

Hardening of polymer optical materials with laser cycling and gamma-rays

When certain molecules are doped into polymers, they are photo-chemically more stable to photodegradation than the same molecule in liquid solution or crystalline form. Furthermore, such composite materials are also found to self heal after photo-damage when they are stored in the dark. Repeated laser cycling of these dye-doped polymers make them more immune to future photodegradation. In parallel, it has been observed that electrooptic devices made with poled dye-doped polymers become more robust upon irradiation by gamma-rays. In the present work, we combine both gamma radiation and laser cycling and find that the synergism between the two processes may result in more robust materials.

Reply to "Comment on pushing the hyperpolarizability to the limit"

Experimental results and a new new-sum-over-states calculation of the hyperpolarizability show that conjugation modulation is a viable approach to enhancing the nonlinear-optical response. Thus, the conclusion is consistent with our previous results.

Self-healing of polymer lasers and two-photon absorbers

This paper reports the photodegradation and recovery of two-photon fluorescence (TPF) in the chromophore AF455 doped in poly(methyl methacrylate) (PMMA) polymer, known to have a large two-photon absorption cross-section and a promising optical material for many applications. The self-healing of amplified spontaneous emission (ASE) in the DO11 system is also analysed.

Using Numerical Optimization Techniques and Conjugation Modulation to Design the Ultimate Nonlinear-Optical Molecule

Guided by numerical calculations and sum rules, we made and characterized molecules with varying degrees of modulated conjugation between donor and acceptor ends. Hyper-Rayleigh scattering measurements of the best molecule show a record intrinsic hyperpolarizability.

Efforts toward fabricating micro-structured fiber with enhanced optical nonlinearity

We report on a procedure to fabricate a specific micro-structured fiber with metal wires array running down its length. The goal is to develop an unique photonic bandgap lattices with enhanced optical nonlinearity. Mechanism of enhanced nonlinearity is briefly discussed. FDTD simulation on single-wire fiber is presented.