ABSTRACT
Dihydroxyphenylalanine (DOPA), its methyl ester (DOPAM) and the N-acetylated derivative of the ester (DOPAMNA) are found to undergo rapid oxidation in air-saturated alkaline solution. Some of the products of oxidation exhibit fluorescent emission in the 300-500 nm spectral range and their excitation-emission spectra have been determined in acidic and alkaline aqueous solutions. The spectral distributions and positions of the maxima depend on the pH of the solution. Excitation-emission maxima associated with the protonated phenolic form of the compounds occur at shorter wavelengths than those of the conjugate base. At some pH values the phenolic forms of these molecules are excited and undergo rapid deprotonation in the excited state; as a consequence, emission is observed from the phenolate anion. The fluorescence excitation-emission spectrum of an authentic sample of 3,4-dihydroxycinnamic (caffeic) acid has also been determined and features of the fluorescence spectra of the principal oxidation products are consistent with the presence of 3,4-hydroxycinnamoyl compounds in solutions of oxidized DOPAM and DOPAMNA.
Subject(s)
Dihydroxyphenylalanine/chemistry , Caffeic Acids/chemistry , Dihydroxyphenylalanine/analogs & derivatives , Dihydroxyphenylalanine/radiation effects , Esters , Hydrogen-Ion Concentration , Oxidation-Reduction , Spectrometry, Fluorescence , SpectrophotometryABSTRACT
A novel adaptive wave-front correction system based on an all-optical feedback interferometer is described. In this system the two-dimensional output fringe intensity from a Mach-Zehnder interferometer with large radial shear is optically fed back to an optically addressed phase-only liquid-crystal spatial light modulator. Consequently, without a separate aberration-free reference wave, the modulator phase approximates the conjugate of the interferometer phase that is directly related to the phase of the input aberrated wave front, so this system is applicable in adaptive optics. We successfully achieved real-time correction of aberrated wave fronts: A diffraction pattern that was seriously distorted because of aberrations was transformed into a diffraction-limited spot immediately after the feedback loop was closed.
ABSTRACT
We present measurements of thermal and optical properties of poly(methyl methacrylate) (PMMA), PMMA modified by the addition of ethanol (MPMMA), and copolymers of methyl methacrylate with 2-hydroxyethyl methacrylate [P(HEMA:MMA)]. Spectral transmission of the polymers is excellent (alpha = 0.5 cm(-1) at 400 nm, decreasing to 0.04 cm(-1) at 633 nm). Measured laser damage thresholds of MPMMA and P(HEMA:MMA) show at least a twofold increase over PMMA. Thermal lensing measured in these hosts doped with Rhodamine 6G is shown to be similar (f = -450 mm for pump power of 200 mW in a 2-mm-diameter spot, scaling with pump intensity). Compared with MPMMA, P(HEMA:MMA) offers an improved surface quality and a more uniform dye distribution.
ABSTRACT
A free-space optical logic technique is presented that utilizes a two-dimensional array of diffractive optical elements. Each optical element focuses light to multiple, separate positions in the output focal plane. The focal spots from different optical elements are allowed to overlap spatially, resulting in interference. By changing the phase shift between the optical elements, one can create different optical logic operations in the focal plane. The technique is demonstrated by the use of two input beams incident onto a multiplexed optical element written onto a programmable spatial light modulator. The optical element simultaneously creates both AND and XOR logic functions in the output plane.
