Andrea V. Bragas

High-Efficiency Second Harmonic Generation from a Single Hybrid ZnO Nanowire/Au Plasmonic Nano-Oligomer

We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits a second harmonic (SH) conversion efficiency of ∼3 × 10(-5)%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.

Coverage and Aggregation of Gold Nanoparticles on Silanized Glasses

We study the dynamics of the coverage and aggregation of gold nanoparticles over organosilanized glass substrates for different sizes of nanoparticles. We present measurements of extinction spectra and nanoparticle counting statistics and demonstrate that both methods are equivalent describing those processes. We introduce models that describe the mentioned dynamics, which are characterized by an exponential-like function with two relevant parameters: a saturation value and a characteristic time. The electrostatic repulsion plays a significant role in both processes. The aggregation is dominated by the mobility of the isolated nanoparticles, which first join in dimers and, further in time, in clusters of higher number of nanoparticles.

Field-enhanced scanning optical microscope.

We present experimental results of an imaging technique that uses as a local probe the optical field enhanced at the junction of a scanning tunneling microscope illuminated by a p-polarized laser beam. Images of highly oriented pyrolithic graphite, recorded at a constant height mode, show a lateral optical resolution of as much as 10 nm. Approach curves exhibit sensitivity on a subnanometer scale of the optical signal to the tip-sample distance, yielding the ultrahigh vertical resolution reached in the images.

Femtosecond soliton source with fast and broad spectral tunability

We present a complete set of measurements and numerical simulations of a femtosecond soliton source with fast and broad spectral tunability and nearly constant pulse width and average power. Solitons generated in a photonic crystal fiber, at the low-power coupling regime, can be tuned in a broad range of wavelengths, from 850 to 1200 nm using the input power as the control parameter. These solitons keep almost constant time duration (approximately 40 fs) and spectral widths (approximately 20 nm) over the entire measured spectra regardless of input power. Our numerical simulations agree well with measurements and predict a wide working wavelength range and robustness to input parameters.

Hierarchical ZnO nanostructures: Growth mechanisms and surface correlated photoluminescence

ZnO nanowires were grown by vapor-transport and deposition on Au nanocluster covered fused and thermal silica and c-Si. The nanowire size and density depended strongly on the substrate type. By decreasing the O2 to local Zn partial pressure ratio, the growth pattern changed to nanocombs and nanosheets. ZnO nanohedgehogs were found on bare c-Si. We observe a remarkable correlation between the defect to exciton photoluminescence intensity ratio and the nanostructures specific surface areas. These results indicate that changes in strain and O deficiency defects at surfaces are behind the observed morphology changes, one to two-dimensional growth transition, and corresponding luminescence.

Metal Nanoparticle Ensembles: Tunable Laser Pulses Distinguish Monomer from Dimer Vibrations

Resonant interaction of laser pulses with plasmons is used to identify vibrations associated with isolated spheres and pairs of contacting spheres in a system of gold nanoparticles. The optical pulses generate coherent mechanical oscillations of both monomers and dimers in the 5-150 GHz range, the amplitudes of which exhibit a strong enhancement when the laser central wavelength is tuned to resonate with the corresponding plasmon. Because of the resonant selection in the excitation process, the widths of the acoustic modes are significantly smaller than broadening caused by the spread in radii in the ensemble.

Spectroscopic response of photoinduced currents in a laser-assisted scanning tunneling microscope

Thermal expansion of the sample or tip in a laser-assisted scanning tunneling microscope (STM) junction can obstruct the study of other phenomena related with STM–light interactions, when photoinduced currents are analyzed. In this article, we show that the thermal contribution is proportional to the average tunneling current and that this can be used to distinguish it from other contributions. Simultaneous tunneling current–voltage and photoinduced current–voltage curves are recorded for highly oriented pyrolitic graphite and gold samples with a Pt tip. We have done the measurements for two different polarizations of the incident beam. We show that the I–V curves can be used to discriminate between different mechanisms that appear, producing photoinduced currents.Thermal expansion of the sample or tip in a laser-assisted scanning tunneling microscope (STM) junction can obstruct the study of other phenomena related with STM–light interactions, when photoinduced currents are analyzed. In this article, we show that the thermal contribution is proportional to the average tunneling current and that this can be used to distinguish it from other contributions. Simultaneous tunneling current–voltage and photoinduced current–voltage curves are recorded for highly oriented pyrolitic graphite and gold samples with a Pt tip. We have done the measurements for two different polarizations of the incident beam. We show that the I–V curves can be used to discriminate between different mechanisms that appear, producing photoinduced currents.

Photothermal measurement of absorption and scattering losses in thin films excited by surface plasmons

We present a novel noncontact, photothermal technique, based on the focus error signal of a commercial CD pickup head that allows direct determination of absorption in thin films. Combined with extinction methods, this technique yields the scattering contribution to the losses. Surface plasmon polaritons are excited using the Kretschmann configuration in thin Au films of varying thickness. By measuring the extinction and absorption simultaneously, it is shown that dielectric constants and thickness retrieval leads to inconsistencies if the model does not account for scattering.

Photon counting statistics using a digital oscilloscope

We present a photon counting experiment designed for an undergraduate physics laboratory. The statistics of the number of photons of a pseudo-thermal light source is studied in two limiting cases: much longer and much shorter than the coherence time, giving Poisson and Bose-Einstein distributions, respectively. The experiment can be done in a reasonable time using a digital oscilloscope without the need of counting boards. The use of the oscilloscope has the advantage of allowing the storage of the data for further processing. The stochastic nature of the detection phenomena adds additional value because students are forced to do data processing and analysis.

Tailored Hypersound Generation in Single Plasmonic Nanoantennas

Ultrashort laser pulses impinging on a plasmonic nanostructure trigger a highly dynamic scenario in the interplay of electronic relaxation with lattice vibrations, which can be experimentally probed via the generation of coherent phonons. In this Letter, we present studies of hypersound generation in the range of a few to tens of gigahertz on single gold plasmonic nanoantennas, which have additionally been subjected to predesigned mechanical constraints via silica bridges. Using these hybrid gold/silica nanoantennas, we demonstrate experimentally and via numerical simulations how mechanical constraints allow control over their vibrational mode spectrum. Degenerate pump-probe techniques with double modulation are performed in order to detect the small changes produced in the probe transmission by the mechanical oscillations of these single nanoantennas.