Browsing by Subject "Second harmonic generation"

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Nonlinear and wavelength-tunable plasmonic metasurfaces and devices
(2014-12) Lee, Jongwon; Belkin, Mikhail A.
Wavelength-tunable optical response from solid-state optoelectronic devices is a desired feature for a variety of applications such as spectroscopy, laser emission tuning, and telecommunications. Nonlinear optical response, on the other hand, has an important role in modern photonic functionalities, including efficient frequency conversions, all-optical signal processing, and ultrafast switching. This study presents the development of optical devices with wavelength tunable or nonlinear optical functionality based on plasmonic effects. For the first part of this study, widely wavelength tunable optical bandpass filters based on the unique properties of long-range surface plasmon polaritons (LR SPP) are presented. Planar metal stripe waveguides surrounded by two different cladding layers that have dissimilar refractive index dispersions were used to develop a wide wavelength tuning. The concept was demonstrated using a set of index-matching fluids and over 200nm of wavelength tuning was achieved with only 0.004 of index variation. For practical application of the proposed concept, a thermo-optic polymer was used to develop a widely tunable thermo-optic bandpass filter and over 220 nm of wavelength tuning was achieved with only 8 ºC of temperature variation. Another novel approach to produce a widely wavelength tunable optical response for free-space optical applications involves integrating plasmonic metasurfaces with quantum-electronic engineered semiconductor layers for giant electro-optic effect, which is proposed and experimentally demonstrated in the second part of this study. Coupling of surface plasmon modes formed by plasmonic nanoresonators with Stark tunable intersubband transitions in multi-quantum well structures induced by applying bias voltages through the semiconductor layer was used to develop tunable spectral responses in the mid-infrared range. Experimentally, over 310 nm of spectral peak tuning around 7 μm of wavelength with 10 ns response time was achieved. As the final part of this study, highly nonlinear metasurfaces based on coupling of electromagnetically engineered plasmonic nanoresonators with quantum-engineered intersubband nonlinearities are proposed and experimentally demonstrated. In the proof-of-concept demonstration, an effective nonlinear susceptibility over 50 nm/V was measured and, after further optimization, over 480 nm/V was measured for second harmonic generation under normal incidence. The proposed concept shows that it is possible to engineer virtually any element of the nonlinear susceptibility tensor of the nonlinear metasurface.
Nonlinear optical characterization of Si/high-k dielectric interfaces
(2005) Carriles Jaimes, Ramón; Downer, Michael Coffin
Sum frequency generation between a Ti:Sapphire beam and white light continuum is performed to obtain spectral information from GaAs(001). The experimental difficulties are analyzed and possible solutions and extensions to the technique proposed. The phase of second harmonic generation (SHG) is measured using a frequency domain technique. Phase shifts in GaAs under azimuthal rotation are observed and explained. The phase of the surface second order susceptibility tensor elements of Si is measured. For the first time SHG phase from Si/SiO2 and Si/SiO2/Hf-silicate is measured with a temporal resolution better than one second. Si/SiO2/Hf(1−x)SixO2 stack structures are studied using rotational anisotropic, spectroscopic, and time dependent SHG. It is found that the signal is affected by Si content (x) and by annealing history of the samples. The results show that SHG is sensitive to the phase separation of the silicates bulk due to spinodal decomposition. A heuristic model based on Hf-O-Si polarizable units that explains the observed trends is presented.
Optical second harmonic generation in bitumen films
(2012-12) Roberts, Aaron Joseph; Downer, Michael Coffin
The ability of asphalt binders (bitumen) in road surfaces to self-heal after cracking is important to developing a robust transportation system that can tolerate heavy traffic and varying weather conditions. In order to develop improved binders, there is a need for noninvasive, in-situ, interface-specific methods of monitoring the kinetics, physics and chemistry of self-healing bitumen interfaces. Here the feasibility of using optical second-harmonic generation (SHG) by focused femtosecond laser pulses to monitor bitumen surfaces is demonstrated. SHG signals are observed in transmission through a sample composed of bitumen spin-coated onto a borosilicate microscope coverslip. The SHG signals are absent from uncoated coverslips, demonstrating that they originate from the bitumen layer. Further tests demonstrate that the bitumen-air surface makes the dominant contribution to the SHG signal. The SHG signal is observed to decay on a time scale comparable to typical self-healing times because of sample heating by the incident laser irradiation. Methods to control this effect by translating the sample during data acquisition are developed. Although the present results were obtained with a single incident wavelength (800 nm), they demonstrate the feasibility of probing bitumen interfaces spectroscopically with tunable light sources in order to monitor bond-specific chemical kinetics.
Second harmonic spectroscopy of silicon nanocrystals
(2007) Figliozzi, Peter Christopher, 1972-; Downer, Michael Coffin
Using a novel two-beam technique developed to greatly enhance quadrupolar contributions to the second-order nonlinear polarization, we performed a nonlinear spectroscopic study of silicon nanocrystals implanted in an SiO₂ matrix.
Second-harmonic generation and reflecance-anisotropy spectroscopy of vicinal Si(001)
(2006) Kwon, Jinhee; Downer, Michael Coffin
Spectroscopic second-harmonic generation (SHG) and reflectance-anisotropy spectroscopy (RAS) of native-oxidized vicinal Si(001) with off-cut angles ζ = 0o , 4o , 6o , 8o and 10o from (001) toward [110] are performed. The SHG spectra are first decomposed using the macroscopic Fourier analysis. As a common microscopic analysis for SHG and RAS, we use a simplified bond hyperpolarizability model. The description of the expanded bond model and the result of the calculation are presented. The derived complex hyperpolarizability spectra are compared with the corresponding inverted real and imaginary parts using nonlinear Kramers-Kronig relations, thus confirming the consistency of the developed bond model. RAS and SHG spectra from clean and selectively H2 adsorbed reconstructed vicinal Si(001):6o surfaces are obtained and analyzed in a similar way.
Study of applications of second harmonic generation
(2011-05) Prem, Adrienne Marie; Downer, Michael Coffin; Sitz, Greg O.
Two applications of second harmonic generation (SHG), a nonlinear optical technique, are studied. First, Fresnel factors are used with a bond model to describe SHG from vicinal silicon at five incidence angles: 7.5°, 22°, 30°, 45°, and 52°. Second, a prototype apparatus for applying SHG to enhance imaging capabilities of optical coherence tomography, a microscopy technique used in many biological fields, is briefly described.
Two-beam SHG from centrosymmetric media
(2006) Sun, Liangfeng; Downer, Michael Coffin
We report a comprehensive study of quadrupolar second-harmonic generation (SHG) from centrosymmetric materials. Because of symmetry, quadrupolar SHG from centrosymmetric materials is forbidden in the forward direction, and is extremely weak when it is obtained using a traditional single-beam SHG geometry. A two-beam geometry is found to enhance quadrupolar SHG greatly compared to single-beam SHG. Two orthogonally polarized laser beams create wavelength-scale, forward-radiating gradients in the second-harmonic polarization that enhance SHG. The radiation pattern of two-beam quadrupolar SHG is observed to have a TEM00 mode, different from single-beam quadrupolar SHG which has a TEM01 mode. The quantitative study of quadrupolar SHG is done with a two-beam geometry on an isotropic glass slide. The polarization of the two-beam quadrupolar SHG is found to be normal to the plane determined by the two crossed fundamental beams, and the SHG intensity depends on the orthogonally polarized fundamental electric fields. Quadrupolar SHG from a glass slide with two tightly focused laser beams appears surface-like – SHG peaks sharply when the glass surfaces are centered on the two-beam overlap region, but vanishes when the overlap is totally immersed in glass bulk. Quasi-phase-matched quadrupolar SHG from two glass slides (and two pellicle films) is demonstrated. Enhanced and phase-matched two-beam SHG from a centrosymmetric and birefringent crystal – calcite – is also realized. Preliminary studies on the application of two-beam SHG in microscopy are done with a scratched glass slide. We find that dipolar SHG and quadrupolar SHG can be selectively imaged by choosing different polarization combinations of the fundamental laser beams. Macroscopically centrosymmetric silicon nanocrystal composites embedded in glass are studied by single-beam and enhanced two-beam SHG. Both techniques are sensitive to the nano-interface chemistry, but two-beam SHG gives rise to a signal that is several orders stronger. Multiple signal discrimination techniques are developed to separate the nanocrystal SHG from the glass SHG which is also enhanced by the two-beam geometry. The enhanced two-beam SHG enables spectroscopy and time resolved studies of silicon nano-interfaces.