Nonlinear Optical Techniques Research Articles

Interaction of Cr(III) and Cr(VI) with Hematite Studied by Second Harmonic Generation

The fate of chromium in the environment relies heavily on its redox chemistry and interaction with iron oxide surfaces. Atomic layer deposition was used to deposit a 10 nm film of polycrystalline α-Fe2O3 (hematite) onto a fused silica substrate which was analyzed using second harmonic generation (SHG), a coherent, surface-specific, nonlinear optical technique. Specifically, the χ(3) technique was used to investigate the adsorption of Cr(III) and Cr(VI) to the hematite/water interface under flow conditions at pH 4 with 10 mM NaCl. We observed partially irreversible adsorption of Cr(III), the extent of which was found to be dependent on the concentration of Cr(III) ions in solution. This result was confirmed using X-ray photoelectron spectroscopy. The interaction of Cr(III) with hematite is compared with the adsorption of Cr(III) to the silica/water interface, which is the substrate for the ALD-prepared hematite films, and found to be fully reversible under the same experimental conditions. The observed binding constant for Cr(III) interacting with the silica surface was found to be 4.0(6) × 103 M–1, which corresponds to an adsorption free energy of −30.5(4) kJ/mol when referenced to 55.5 M water. The surface charge density at maximum metal ion surface coverage was found to be 0.005(1) C/m2, which corresponds to 1.0 × 1012 ions/cm2 assuming a +3 charge for chromium. In contrast, the observed binding constant for Cr(III) interacting reversibly with the hematite surface was calculated to be 2(2) × 104 M–1, corresponding to an adsorption free energy of −35(2) kJ/mol when referenced to 55.5 M water. The surface charge density at maximum metal ion surface coverage was found to be 0.004(5) C/m2 for the reversibly bound chromium species, which corresponds to 8.3 × 1011 reversibly bound ions per cm2, again assuming a +3 charge of chromium. The data also allows us to estimate that about 6.7 × 1012 Cr(III) ions are irreversibly bound per cm2 hematite at saturation coverage. The results of this investigation suggest that the use of hematite in permeable reactive barriers, for cost-effective chromium remediation, allows for Cr(III) remediation at very low concentrations through adsorptive and redox processes but quickly renders the barriers ineffective at high chromium concentrations due to surface saturation.

Non-Linear Optical Microscopy Sheds Light on Cardiovascular Disease

Many cardiac diseases have been associated with increased fibrosis and changes in the organization of fibrillar collagen. The degree of fibrosis is routinely analyzed with invasive histological and immunohistochemical methods, giving a limited and qualitative understanding of the tissue's morphological adaptation to disease. Our aim is to quantitatively evaluate the increase in fibrosis by three-dimensional imaging of the collagen network in the myocardium using the non-linear optical microscopy techniques Two-Photon Excitation microscopy (TPE) and Second Harmonic signal Generation (SHG). No sample staining is needed because numerous endogenous fluorophores are excited by a two-photon mechanism and highly non-centrosymmetric structures such as collagen generate strong second harmonic signals. We propose for the first time a 3D quantitative analysis to carefully evaluate the increased fibrosis in tissue from a rat model of heart failure post myocardial infarction. We show how to measure changes in fibrosis from the backward SHG (BSHG) alone, as only backward-propagating SHG is accessible for true in vivo applications. A 5-fold increase in collagen I fibrosis is detected in the remote surviving myocardium measured 20 weeks after infarction. The spatial distribution is also shown to change markedly, providing insight into the morphology of disease progression.

Optical Characterization of Compensating Defects in Cubic SiC

We present investigation of carrier recombination and optical trap recharge in sublimation grown n- and p-type 3C layers by using time-resolved nonlinear optical techniques. Carrier lifetime and recharged trap recovery were measured by differential transmittivity technique. By monitoring nonequilibrium carrier dynamics, we analyzed impact of carrier density and temperature on carrier lifetime and recharged trap recovery rate. Large carrier lifetime and small diffusivity at low injections in highly compensated samples and their dependences on injection were explained by optical recharge of compensating aluminum impurities. The complete recharge of the compensating impurities by optical means allowed us to calculate the compensating aluminum density in n-type samples and compensating nitrogen in p-type ones.

Plasmon enhanced spectroscopy

Surface enhanced spectroscopy encompasses a broad field of linear and nonlinear optical techniques that arose with the discovery of the surface-enhanced Raman scattering (SERS) effect. SERS enabled ultrasensitive and single molecule detection with molecular fingerprint specificity, opening the door for a large variety of chemical sensing applications. Basically, from the beginning it was realized that the necessary condition for SERS to be observed was the presence of a metallic nanostructure, and with this condition, the optical enhancement found a home in the field of plasmonics. Although plasmonic practitioners claim that SERS is "the most spectacular application of plasmonics", perhaps it is more appropriate to say that the spectacular development of plasmonics is due to SERS. Here is a brief recollection from surface enhanced spectroscopy to plasmon enhanced spectroscopy.

Experimental and Theoretical Study on the One- and Two-Photon Absorption Properties of Novel Organic Molecules Based on Phenylacetylene and Azoaromatic Moieties

This Article reports a combined experimental and theoretical analysis on the one and two-photon absorption properties of a novel class of organic molecules with a π-conjugated backbone based on phenylacetylene (JCM874, FD43, and FD48) and azoaromatic (YB3p25) moieties. Linear optical properties show that the phenylacetylene-based compounds exhibit strong molar absorptivity in the UV and high fluorescence quantum yield with lifetimes of approximately 2.0 ns, while the azoaromatic-compound has a strong absorption in the visible region with very low fluorescence quantum yield. The two-photon absorption was investigated employing nonlinear optical techniques and quantum chemical calculations based on the response functions formalism within the density functional theory framework. The experimental data revealed well-defined 2PA spectra with reasonable cross-section values in the visible and IR. Along the nonlinear spectra we observed two 2PA allowed bands, as well as the resonance enhancement effect due to the presence of one intermediate one-photon allowed state. Quantum chemical calculations revealed that the 2PA allowed bands correspond to transitions to states that are also one-photon allowed, indicating the relaxation of the electric-dipole selection rules. Moreover, using the theoretical results, we were able to interpret the experimental trends of the 2PA spectra. Finally, using a few-energy-level diagram, within the sum-over-essential states approach, we observed strong qualitative and quantitative correlation between experimental and theoretical results.

Differential Diagnosis of Lung Carcinoma With Coherent Anti-Stokes Raman Scattering Imaging

Aimed at bridging imaging technology development with cancer diagnosis, this paper first presents the prevailing challenges of lung cancer detection and diagnosis, with an emphasis on imaging techniques. It then elaborates on the working principle of coherent anti-Stokes Raman scattering microscopy, along with a description of pathologic applications to show the effectiveness and potential of this novel technology for lung cancer diagnosis. As a nonlinear optical technique probing intrinsic molecular vibrations, coherent anti-Stokes Raman scattering microscopy offers an unparalleled, label-free strategy for clinical cancer diagnosis and allows differential diagnosis of fresh specimens based on cell morphology information and patterns, without any histology staining. This powerful feature promises a higher biopsy yield for early cancer detection by incorporating a real-time imaging feed with a biopsy needle. In addition, molecularly targeted therapies would also benefit from early access to surgical specimen with high accuracy but minimum tissue consumption, therefore potentially saving specimens for follow-up diagnostic tests. Finally, we also introduce the potential of a coherent anti-Stokes Raman scattering-based endoscopy system to support intraoperative applications at the cellular level.

Optical Biomarkers of Serous and Mucinous Human Ovarian Tumor Assessed with Nonlinear Optics Microscopies

BackgroundNonlinear optical (NLO) microscopy techniques have potential to improve the early detection of epithelial ovarian cancer. In this study we showed that multimodal NLO microscopies, including two-photon excitation fluorescence (TPEF), second-harmonic generation (SHG), third-harmonic generation (THG) and fluorescence lifetime imaging microscopy (FLIM) can detect morphological and metabolic changes associated with ovarian cancer progression.Methodology/Principal FindingsWe obtained strong TPEF + SHG + THG signals from fixed samples stained with Hematoxylin & Eosin (H&E) and robust FLIM signal from fixed unstained samples. Particularly, we imaged 34 ovarian biopsies from different patients (median age, 49 years) including 5 normal ovarian tissue, 18 serous tumors and 11 mucinous tumors with the multimodal NLO platform developed in our laboratory. We have been able to distinguish adenomas, borderline, and adenocarcinomas specimens. Using a complete set of scoring methods we found significant differences in the content, distribution and organization of collagen fibrils in the stroma as well as in the morphology and fluorescence lifetime from epithelial ovarian cells.Conclusions/SignificanceNLO microscopes provide complementary information about tissue microstructure, showing distinctive patterns for serous and mucinous ovarian tumors. The results provide a basis to interpret future NLO images of ovarian tissue and lay the foundation for future in vivo optical evaluation of premature ovarian lesions.

Linear Technique for Discrimination of Optically Coded Waveforms Using Optical Frequency Combs

We have demonstrated a novel coherent optical detection architecture that serves as a matched filter and successfully discriminates between various encoded optical waveforms with high confidence. The waveforms are produced by encoding the spectral phase of an optical frequency comb with codes from a Walsh-Hadamard set. The encoded optical waveforms are decoded with a coherent detection technique implemented using an optical frequency comb as a set of local oscillators, an interferometer, and a pair of differential balanced photodetectors. The use of only linear optical devices in the receiver results in a higher sensitivity compared to the use of nonlinear optical thresholding and gating techniques. Decoding is demonstrated at power levels of ~ 25 μW, which are much lower than previously reported. Possible applications of the receiver architecture include laser radar and optical communications in an optical code-division multiple access network.

Characterization of Nanostructured Plasmonic Surfaces with Second Harmonic Generation

Because of its high surface and interface sensitivity, the nonlinear optical technique of second harmonic generation (SHG) is a designated method for investigating nanostructured metal surfaces. Indeed, the latter present a high surface-to-volume ratio, but even more importantly, they can exhibit strong near-field enhancements or "hot spots". Hot spots often appear as a result of geometric features on the nanoscale or surface plasmon resonances, which are collective electron oscillations on the surface that, on the nanoscale, can readily be excited by light. In the last 10 years, near-field hot spots have been responsible for dramatic developments in the field of nano-optics. In this Feature Article, the influence of hot spots on the SHG response of nanostructured metal surfaces is discussed on both the microscopic and macroscopic levels. On the microscopic level, the nanostructured metal surfaces were characterized by scanning SHG microscopy, complemented by rigorous numerical simulations of the near-field and of the local electric currents at the fundamental frequency. On the macroscopic level, SHG-circular dichroism and magnetization-induced SHG characterization techniques were employed.

Towards automated segmentation of cells and cell nuclei in nonlinear optical microscopy

Nonlinear optical (NLO) imaging techniques based e.g. on coherent anti-Stokes Raman scattering (CARS) or two photon excited fluorescence (TPEF) show great potential for biomedical imaging. In order to facilitate the diagnostic process based on NLO imaging, there is need for an automated calculation of quantitative values such as cell density, nucleus-to-cytoplasm ratio, average nuclear size. Extraction of these parameters is helpful for the histological assessment in general and specifically e.g. for the determination of tumor grades. This requires an accurate image segmentation and detection of locations and boundaries of cells and nuclei. Here we present an image processing approach for the detection of nuclei and cells in co-registered TPEF and CARS images. The algorithm developed utilizes the gray-scale information for the detection of the nuclei locations and the gradient information for the delineation of the nuclear and cellular boundaries. The approach reported is capable for an automated segmentation of cells and nuclei in multimodal TPEF-CARS images of human brain tumor samples. The results are important for the development of NLO microscopy into a clinically relevant diagnostic tool.

Diagnosis of BCC by multiphoton laser tomography

Multiphoton Laser Tomography (MPT) is a non-linear optical technique that gives access to morphology and structure of both cells and extracellular matrix of the skin through the combination of autofluorescence imaging and second harmonic generation (SHG). The aim of this study was to identify MPT descriptors on ex vivo specimens of basal cell carcinoma (BCC) to assess the sensitivity and specificity of these criteria for the diagnosis of BCC and its differentiation from other skin tumours, inflammatory diseases and healthy skin. In the preliminary study, MPT images referring to 24 BCCs and 24 healthy skin samples were simultaneously evaluated by three observers for the identification of features characteristic of BCC. In the main study, the presence/absence of the descriptors identified in the preliminary study was blindly evaluated on a test set, comprising 66 BCCs, 66 healthy skin samples and 66 skin lesions, including 23 nevi, 8 melanomas, 17 skin tumours and other skin lesions by 3 independent observers. In the preliminary study, three epidermal descriptors and six descriptors for BCC were identified. The latter included aligned elongated cells, double alignment of cells, cell nests with palisading and phantom islands. From the test set, 56 BCCs were correctly diagnosed, whereas in 10 cases the diagnosis was 'other lesions'. However, it was always possible to exclude the diagnosis of BCC in healthy skin and other lesion samples. Thus, overall sensitivity of the method was 84.85, whereas a specificity of 100% was observed with respect to both healthy skin and 'other lesions'. This study describes new morphological descriptors of BCC enabling its characterization and its distinction from healthy skin and other skin lesions in ex vivo samples, and demonstrates for the first time that MPT represents a sensitive and specific technique for the diagnosis of BCC.

Pump-probe imaging of historical pigments used in paintings

A recently developed nonlinear optical pump-probe microscopy technique uses modulation transfer to sensitively extract excited-state dynamics of endogenous biological pigments, such as eumelanin and pheomelanin. In this work, we use this method to image and characterize several inorganic and organic pigments used in historical art. We show substantial differences in the near-IR pump-probe signatures from nominally similar pigments and suggest extensions to art restoration.

Optical studies of ballistic currents in semiconductors [Invited

We present a summary of recent studies of ballistic currents using nonlinear optical techniques. Quantum interference between one- and two-photon absorption pathways is used to inject and control ballistic currents in GaAs samples. With this, a pure charge current, pure spin current, or spin-polarized charge current can be injected by changing the polarization configuration of the two pump pulses. Such currents are temporally and spatially resolved using high-resolution pump–probe techniques, including a derivative-detection scheme, which allows detection of the motion of carriers as small as 0.1 nm. Observation of the intrinsic inverse spin Hall effect in the ballistic regime, a study of time-resolved ballistic spin-polarized charge currents, and a study of the efficiency of spin current injection by quantum interference were all achieved using these techniques. Additionally, we discuss demonstrations of second-order nonlinear optical effects induced by charge and spin currents, which allow for the nondestructive, noninvasive, and real-time imaging of currents.

Observation of a blue shift in the optical response at the fundamental band gap in Ga1−xMnxAs

We report the observation of a sharp band-edge response in spectrally resolved differential reflectivity experiments on GaMnAs, in contrast to linear optical experiments in which large band-tail effects are known to dominate. The differential reflectivity response exhibits a blue shift relative to results in GaAs and LT-GaAs, consistent with the valence-band model of ferromagnetism. Our results demonstrate the utility of nonlinear optical techniques for studying the electronic structure of III-Mn-V diluted magnetic semiconductors.

A Powerful Nonlinear Optical Technique to Characterize Surfaces and Interfaces Sum Frequency Generation Vibrational Spectroscopy

In the last two decades, a nonlinear optical technique - sum frequency generation (SFG) vibrational spectroscopy was developed into a powerful characterization method to probe the molecular structures of various surfaces and interfaces in situ. In this paper, we gave a general introduction of the SFG theory and two case studies to demonstrate the power of the SFG spectroscopy.

Nonlinear Optical Techniques for Characterization of Wide Bandgap Semiconductor Electronic Properties: III-nitrides, SiC, and Diamonds

ABSTRACTCombining interdisciplinary fields of nonlinear optics, dynamic holography, and photoelectrical phenomena, we developed the optical measurement technologies for monitoring the spatial and temporal non-equilibrium carrier dynamics in wide bandgap semiconductors at wide range of excitations (1015 to 1020 cm-3) and temperatures (10 to 800 K).We explored advantages of non-resonant optical nonlinearities, based on a short laser pulse induced refractive or absorption index modulation (Δn and Δk) by free excess carriers. This approach, based on a direct correlation between the electrical and optical processes, opened a possibility to analyze dynamics of electrical phenomena in “all-optical” way, i.e. without electrical contacts.Carrier diffusion and recombination processes have been investigated in various wide band gap materials - differently grown GaN, SiC, and diamonds - and their key electrical parameters determined, as carrier lifetime, diffusion coefficient, diffusion length and their dependences on temperature and injected carrier density. The studies provided deeper insight into nonradiative and radiative recombination processes in GaN crystals, revealed diffusion-driven long nonradiative carrier lifetimes in bulk GaN and SiC, disclosed impact of delocalization in InGaN layers, and suggested a trap-assisted Auger recombination in highly-excited InN. Injection and temperature dependent diffusivity revealed a strong contribution of carrier-carrier scattering in diamond and bandgap renormalization in SiC.

Development of high-repetition-rate laser pump/x-ray probe methodologies for synchrotron facilities

We describe our implementation of a high repetition rate (54 kHz-6.5 MHz), high power (>10 W), laser system at the 7ID beamline at the Advanced Photon Source for laser pump/x-ray probe studies of optically driven molecular processes. Laser pulses at 1.06 μm wavelength and variable duration (10 or 130 ps) are synchronized to the storage ring rf signal to a precision of ~250 fs rms. Frequency doubling and tripling of the laser radiation using nonlinear optical techniques have been applied to generate 532 and 355 nm light. We demonstrate that by combining a microfocused x-ray probe with focused optical laser radiation the requisite fluence (with <10 μJ/pulse) for efficient optical excitation can be readily achieved with a compact and commercial laser system at megahertz repetition rates. We present results showing the time-evolution of near-edge x-ray spectra of a well-studied, laser-excited metalloporphyrin, Ni(II)-tetramesitylporphyrin. The use of high repetition rate, short pulse lasers as pump sources will dramatically enhance the duty cycle and efficiency in data acquisition and hence capabilities for laser-pump/x-ray probe studies of ultrafast structural dynamics at synchrotron sources.

Molecular Engineering of Nonplanar Porphyrin and Carbon Nanotube Assemblies: A Linear and Nonlinear Spectroscopic and Modeling Study

The importance of molecular conformation to the nature and strength of noncovalent interactions existing between a series of increasingly nonplanar tetraphenylporphyrin (TPP) derivatives and carbon nanotubes was systematically investigated experimentally in solution using a range of linear and nonlinear optical techniques. Additional complementary molecular dynamics studies were found to support the experimental observations. Convincing evidence of binding between single walled nanotubes (SWNTs) and some of these porphyrins was discovered, and a nonplanar macrocycle conformation was found to increase the likelihood of noncovalent binding onto nanotubes. Nonlinear optical studies showed that the optical limiting behavior of the TPP derivatives deteriorated with increasing porphyrin nonplanarity, but that formation of nanotube composites dramatically improved the optical limiting properties of all molecules studied. It was also found that the significant photoluminescence quenching behavior reported in the literature for such porphyrin/SWNT composites is at least partly caused by photoluminescence and excitation self-absorption and is, therefore, an artifact of the system.

Plasmons Reveal the Direction of Magnetization in Nickel Nanostructures

We have applied the surface-sensitive nonlinear optical technique of magnetization-induced second harmonic generation (MSHG) to plasmonic, magnetic nanostructures made of Ni. We show that surface plasmon contributions to the MSHG signal can reveal the direction of the magnetization. Both the plasmonic and the magnetic nonlinear optical responses can be tuned; our results indicate novel ways to combine nanophotonics, nanoelectronics, and nanomagnetics and suggest the possibility for large magneto-chiral effects in metamaterials.

Probing polymer surfaces and interfaces using sum frequency generation vibrational spectroscopy - a powerful nonlinear optical technique

Sum frequency generation (SFG) vibrational spectroscopy has been proved to be a powerful technique which substantially impacts on many research areas in surface and interfacial sciences. This paper reviews the recent progress of applying this nonlinear optical technique in the studies of polymer surfaces and interfaces. The theoretical background of SFG is introduced first. Current applications of SFG in polymer science are then described in more detail to demonstrate the significance of this technique. Finally, a short summary is presented on this relatively new but widely applicable spectroscopic technique.