Determination of the optical second harmonic response of pharmaceutical solid–solid mixtures

We report on a new method for characterising crystallinity and polymorphism of pharmaceutical drugs and excipients in the solid state. The optical nonlinearity of certain crystalline structures allows under certain circumstances the quantitative determination of polymorphism or crystallinity. An apparatus was set up to measure the second harmonic response of powdered samples when irradiated with a pulsed laser source. Here we present experimental results from various pharmaceutically interesting materials. This rapid, novel technique has potential for industrial monitoring of pharmaceutical manufacturing processes.     

High-accuracy optical clock via three-level coherence in neutral bosonic 88Sr

An optical atomic clock scheme is proposed that utilizes two lasers to establish coherent coupling between the 5s2 1S0 ground state of 88Sr and the first excited state, 5s5p 3P0. The coupling is mediated by the broad 5s5p 1P1 state, exploiting the phenomenon of electromagnetically induced transparency. The effective linewidth of the clock transition can be chosen at will by adjusting the laser intensity. By trapping the 88Sr atoms in an optical lattice, long interaction times with the two lasers are ensured; Doppler and recoil effects are eliminated. Based on a careful analysis of systematic errors, a clock accuracy of better than 2 x 10(-17) is expected.     

Xenon clusters in intense VUV laser fields

A simple model is developed that quantitatively describes intense interactions of a vacuum ultraviolet (VUV) laser pulse with a xenon cluster. We find good agreement with a recent experiment [Nature (London) 420, 482 (2002)]]. In particular, the large number of VUV photons absorbed per atom, at intensities significantly below 10(16) W/cm(2), is now understood.     

Enhanced Ratiometric pH Sensing Using SNAFL-2 on Silver Island Films: Metal-enhanced Fluorescence Sensing

We have explored the opportunities for enhanced ratiometric pH sensing using the well-known carboxy seminaphthofluorescein (SNAFL-2) and silver island films (SiFs). Our results show that the metallic surfaces can provide up to a 40-fold increase in probe fluorescence intensity as compared to nonmetallic surfaces with the same probe coverage. However, while the S/N is significantly better for pH sensing, the emission wavelength ratiometric values are similar to that obtained in solution, due to the fact that the emission of both the acidic and basic forms of the probe are enhanced to similar extents. To the best of our knowledge this is the first report of enhanced ratiometric fluorescence sensing on metallic surfaces.     

Synthesis and reconstruction of computer generated holograms by a double pass technique on a twisted nematic-based liquid crystal spatial light modulator

Achieving phase only modulation from a spatial light modulator (SLM) is useful for many optical processing tasks. In this paper, we demonstrate a simple method of decoupling phase and amplitude modulation in a twisted nematic liquid crystal (TNLC) SLM using a double pass technique. A Jones calculus model is developed which matches our experimental data.     

Multiwavelength pulse generator using time-lens compression

We demonstrate a novel method of generating a multiwavelength pulse train by use of time-lens compression. In addition to pulse compression, this time lens simultaneously displaces the pulses according to their center wavelengths, resulting in a temporally evenly spaced multiwavelength pulse train. We further demonstrate a new aberration-correction technique based on the temporal analog of a spatial correction lens to improve the quality of the compressed pulses. Through the use of cw distributed-feedback lasers and electro-optic phase modulators, the all-fiber system allows complete tunability of temporal spacing, spectral profile, and repetition rate.