Detection of organic colorants in historical painting layers using UV laser ablation surface-enhanced Raman microspectroscopy.

Surface-enhanced Raman spectroscopy (SERS) has been increasingly used in the study of works of art to identify organic pigments and dyes in paintings, which (depending on the material) are difficult or not possible to detect by other current methods. The application of SERS to the study of paintings has been limited, however, by the lack of a sampling approach with sufficient sensitivity and spatial resolution. We show that ultraviolet laser ablation (LA) sampling coupled with SERS detection can be successfully used to study paint layers. LA-SERS permitted the isolation of signals from colorants in individual thin paint layers in sample cross-sections, avoiding contamination from adjacent layers. These results expand the range of analytical applications of SERS demonstrating how the technique can be used to sensitively detect minor organic components in complex matrices. While this is fundamental for the study of cultural heritage, it is also relevant in other fields such as forensic analysis, food science, and pharmacology.

Laser ablation surface-enhanced Raman microspectroscopy.

Improved identification of trace organic compounds in complex matrixes is critical for a variety of fields such as material science, heritage science, and forensics. Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique that can attain single-molecule sensitivity and has been shown to complement mass spectrometry, but lacks widespread application without a robust method that utilizes the effect. We demonstrate a new, highly sensitive, and widely applicable approach to SERS analysis based on laser ablation in the presence of a tailored plasmonic substrate. We analyze several challenging compounds, including non-water-soluble pigments and dyed leather from an ancient Egyptian chariot, achieving sensitivity as high as 120 amol for a 1:1 signal-to-noise ratio and 5 µm spatial resolution. This represents orders of magnitude improvement in spatial resolution and sensitivity compared to those of other SERS approaches intended for widespread application, greatly increasing the applicability of SERS.

Deploying RNA and DNA with Functionalized Carbon Nanotubes.

Carbon nanotubes internalize into cells and are potential molecular platforms for siRNA and DNA delivery. A comprehensive understanding of the identity and stability of ammoniumfunctionalized carbon nanotube (f-CNT)-based nucleic acid constructs is critical to deploying them in vivo as gene delivery vehicles. This work explored the capability of f-CNT to bind single- and double-strand oligonucleotides by determining the thermodynamics and kinetics of assembly and the stoichiometric composition in aqueous solution. Surprisingly, the binding affinity of f-CNT and short oligonucleotide sequences was in the nanomolar range, kinetics of complexation were extremely rapid, and from one to five sequences were loaded per nanotube platform. Mechanistic evidence for an assembly process that involved electrostatic, hydrogen-bonding and π-stacking bonding interactions was obtained by varying nanotube functionalities, oligonucleotides, and reaction conditions. 31P-NMR and spectrophotometric fluorescence emission data described the conditions required to assemble and stably bind a DNA or RNA cargo for delivery in vivo and the amount of oligonucleotide that could be transported. The soluble oligonucleic acid-f-CNT supramolecular assemblies were suitable for use in vivo. Importantly, key evidence in support of an elegant mechanism by which the bound nucleic acid material can be 'off-loaded' from the f-CNT was discovered.

Few-photon all-optical modulation in a photonic band-gap fiber.

We demonstrate 25% all-optical modulation with <20 photons, i.e., a few attojoules of energy, using nondegenerate two-photon absorption in rubidium atoms confined to a hollow-core photonic band-gap fiber. An attenuation of up to 3 dB is induced on an optical field with a switching energy density of less than one photon per (λ(2)/2π). We show that the temporal response of the system is determined by the 5-ns transit time of the atoms across the optical mode of the fiber, which results in a modulation bandwidth up to 50 MHz.

Spectral amplitude and phase measurement of ultrafast pulses using all-optical differential tomography.

We demonstrate a simple, all-optical, fiber-based method for characterizing the spectral amplitude and phase of ultrafast pulses using a differential tomographic measurement realized via four-wave mixing. The technique is applied to subpicosecond pulses in the C-band of the telecommunication spectrum. Characterization of amplified pulses and propagation through dispersive media is demonstrated and compared with autocorrelation measurements and calculated predictions. We show how our approach can be extended to larger bandwidths in similar systems, extending tomographic reconstruction of coherent fields to nearly an octave of bandwidth while maintaining a robust, waveguide-based geometry.

All-optical modulation of four-wave mixing in an Rb-filled photonic bandgap fiber.

We demonstrate efficient all-optical modulation using Rb vapor confined to a hollow-core photonic bandgap fiber. The intensity of a signal field participating in the four-wave-mixing process is modulated using a weak switching field. We observe 3 dB of attenuation in the signal field with only 3600 photons of switching energy, corresponding to 23 photons per atomic cross section lambda(2)/(2pi). Modulation bandwidths as high as 300 MHz are observed.

Ultralow-power four-wave mixing with Rb in a hollow-core photonic band-gap fiber.

We demonstrate extremely efficient four-wave mixing with gains greater than 100 at microwatt pump powers and signal-to-idler conversion of 50% in Rb vapor confined to a hollow-core photonic band-gap fiber. We present a theoretical model that demonstrates such efficiency is consistent with the dimensions of the fiber and the optical depths attained. This is, to our knowledge, the largest four-wave mixing gain observed at such low total pump powers and the first demonstrated example of four-wave mixing in an alkali-metal vapor system with a large (approximately 30 MHz) ground state decoherence rate.

Coherent control of decoherence.

Manipulation of quantum interference requires that the system under control remains coherent, avoiding (or at least postponing) the phase randomization that can ensue from coupling to an uncontrolled environment. We show that closed-loop coherent control can be used to mitigate the rate of quantum dephasing in a gas-phase ensemble of potassium dimers (K2), which acts as a model system for testing the general concepts of controlling decoherence. Specifically, we adaptively shaped the light pulse used to prepare a vibrational wave packet in electronically excited K2, with the amplitude of quantum beats in the fluorescence signal used as an easily measured surrogate for the purpose of optimizing coherence. The optimal pulse increased the beat amplitude from below the noise level to well above it, and thereby increased the coherence life time as compared with the beats produced by a transform-limited pulse. Closed-loop methods can thus effectively identify states that are robust against dephasing without any previous information about the system-environment interaction.

Generation of large alkali vapor densities inside bare hollow-core photonic band-gap fibers.

We demonstrate the ability to generate extremely large rubidium densities in uncoated hollow-core photonic band-gap fibers using light-induced atomic desorption. Once the fiber is exposed to Rb vapor for 1-2 weeks, and this atomic source is removed, the fiber yields large desorbable densities for an extended period of time. We show that optical depths greater than e(-1200) can be created within seconds. Our observed Rb densities are several orders of magnitude larger than any previously reported to be generated optically, and allow for the demonstration of a relatively easy-to-use fiber-based vapor cell capable of producing large optical depths without the need for thermal tuning.