Richard J. Saykally

Controlled Growth of ZnO Nanowires and Their Optical Properties

This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010 cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.

Water clusters: Untangling the mysteries of the liquid, one molecule at a time

Extensive terahertz laser vibration-rotation-tunneling spectra and mid-IR laser spectra have been compiled for several isotopomers of small (dimer through hexamer) water clusters. These data, in conjunction with new theoretical advances, quantify the structures, force fields, dipole moments, and hydrogen bond rearrangement dynamics in these clusters. This new information permits us to systematically untangle the intricacies associated with cooperative hydrogen bonding and promises to lead to a more complete molecular description of the liquid and solid phases of water, including an accurate universal force field.

Tunable nanowire nonlinear optical probe

One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion, structured illumination, and photoactivated localization microscopy, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.

Quantifying Hydrogen Bond Cooperativity in Water: VRT Spectroscopy of the Water Tetramer

Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures.

Far infrared laser magnetic resonance of singlet methylene: Singlet–triplet perturbations, singlet–triplet transitions, and the singlet–triplet splittinga)

We have observed and assigned a number of far infrared laser magnetic resonance spectra of CH2 arising from rotational transitions within the lowest vibrational state of the a 1A1 electronic excited state and from transitions between such singlet levels and vibrationally excited levels of the X 3B1 electronic ground state. The singlet–singlet transitions are magnetically active, and the singlet–triplet transitions have electric dipole intensity because of the spin‐orbit mixing of singlet levels with vibrationally excited levels of the triplet state. By identifying four pairs of singlet and triplet levels that perturb each other we can accurately position the singlet and triplet state relative to each other and determine the single–triplet energy splitting. We determine that T0(a 1A1)=3165±20 cm−1 (9.05±0.06 kcal/mol; 0.392±0.003 eV), and Te(a 1A1)=2994±30 cm−1 (8.56±0.09 kcal/mol; 0.371±0.004 eV). A new ab initio calculation of the spin‐orbit matrix element between these two states has been of assista...

An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+ – e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

Vibration-Rotation Tunneling Spectra of the Water Pentamer: Structure and Dynamics

Far-infrared laser vibration-rotation tunneling spectroscopy was used to measure an intermolecular vibration (81.19198 wave numbers) of the isolated water (D2O) pentamer. Rotational analysis supports the chiral, slightly puckered ring structure predicted by theory. The experimentally deduced interoxygen separations for the water clusters up to the pentamer showed exponential convergence toward the corresponding distance in bulk phase water.

Optical routing and sensing with nanowire assemblies

The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

Determination of an improved intermolecular global potential energy surface for Ar–H2O from vibration–rotation–tunneling spectroscopy

A new highly accurate and detailed intermolecular potential surface for Ar–H2O is derived by a direct nonlinear least squares fit to 37 far infrared, infrared, and microwave spectroscopic measurements. The new potential (denoted AW2) gives a much better description of the strong radial dependence of the anisotropic forces and of the binding energy than its predecessor, the AW1 surface [Cohen and Saykally, J. Phys. Chem. 94, 7991 (1990)]. The global minimum on the AW2 potential (De=142.98 cm−1) occurs at the position R=3.636 A, θ=74.3°, and φ=0°. At these coordinates the argon is located in the monomer plane between the perpendicular to the C2 axis (θ=90°) and the hydrogen bonded geometry (θ=55°). This orientation of the minimum is opposite of that found in recent ab initio calculations of Bulski et al. [J. Chem. Phys. 94, 8097 (1991)] and Chalisinski et al. [J. Chem. Phys. 94, 2807 (1991)]. Both sets of authors find a minimum at an antihydrogen bonded geometry corresponding to an orientation Ar–OH (θ=125°).