Second Harmonic Generation Experiments Research Articles

Direct imaging of lattice-strain-induced stripe phases in an optimally doped manganite film

In a ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}\mathrm{Mn}{\mathrm{O}}_{3}∕\mathrm{Mg}\mathrm{O}$ film without quenched disorder, we show insulating and metallic stripes above and below the Curie temperature $({T}_{C})$, respectively, by a high-resolution scanning tunneling microscopy (STM) and/or spectroscopy measurement. Formation of these stripes involves competing charge, orbital, and lattice orders and is an outcome of an overlapping of electron wave functions mediated by the lattice strain. The presence of the quadrupolar contribution in the second-harmonic generation experiments supports the strain mediated charge-orbital stripes observed by STM.

Optical Second-Harmonic Generation Measurements of Hole Carrier Injection and Trapping in Pentacene Field-Effect Transistor

In order to further clarify the carrier injection into pentacene field-effect transistors (FETs), we focused on the hysteresis behavior of the current–voltage (I–V) and capacitance–voltage (C–V) characteristics, and optical second-harmonic generation (SHG) measurement was employed. It was found that in the on state of pentacene FET, the SHG intensity decreases because of hole injection from the source electrode as well as hole trapping in pentacene. The SHG experiment at the off state of the FET shows that electron injection into pentacene from source and drain electrodes is also probable.

Reactive ion etching of high optical quality GaN∕sapphire photonic crystal slab using CH4–H2 chemistry

Reactive ion etching (RIE) using a CH4–H2 plasma is investigated for the fabrication of a GaN one-dimensional (1D) photonic crystal (PhC) slab. The dominant control parameter for the etch rate and the sidewall profile is the dc bias. The influence of operating pressure, CH4∕H2 ratio, and total gas flow rate on the etching characteristics is also presented. An etch rate as high as 85nm∕min and an overcut angle as low as 5° obtained in this work are among the best values reported for conventional RIE technique. The CH4–H2 process is used to etch 1D PhCs with a lattice parameter ranging from 700to350nm and an air filling factor of 0.30 into a 600-nm-thick GaN∕sapphire slab. Sharp peaks corresponding to the resonant modes of the nanopatterned structures are observed in the experimental reflection spectra for all the lattice periods. Furthermore, the good optical quality of the nanostructures is evidenced by a resonantly enhanced second-harmonic generation experiment around 400nm. A second-harmonic generation enhancement factor as high as 105 is obtained, compared with the unpatterned GaN reference slab. These results demonstrate that the CH4–H2 conventional RIE technique is well adapted to the etching of GaN PhC for the fabrication of next generation photonic devices exploiting nonlinear processes.

Structural investigation of CuIn5Se8 single crystals by optical second harmonic generation, ellipsometry, and photoluminescence

Cu In 5 Se 8 bulk single crystals were characterized using ellipsometry, photoluminescence (PL), and optical second harmonic generation (SHG). The refractive n(E) and absorption k(E) indices as functions of photon energy E are determined. The structure of the PL spectrum suggests the presence of the Cu-rich nonstoichiometric phases at the natural surfaces of the CuIn5Se8 sample. The best fit of the experimental SHG data is achieved by assuming that CuIn5Se8 belongs to the 4¯2m symmetry group. This result is in contradiction with x-ray analysis (6¯2m group), which testifies to the difference in surface and bulk structure.

Optical Second-Harmonic Generation Study of Exfoliated MoS2−Malachite Green Inclusion Materials

We have examined the properties of exfoliated and restacked MoS(2)-malachite green (MG) inclusion compounds to provide insight into the MG-MoS(2) interactions that characterize these materials. The results of X-ray diffraction experiments indicate that MG included into the restacked structure adopts a flat orientation approximately parallel to the MoS(2) sheets. Second-harmonic generation experiments conducted on the exfoliated and restacked materials provide information regarding the averaged orientation of the MG. At low MG coverage, our results support the X-ray diffraction findings, and yield large averaged orientation angles, consistent with a flat orientation of MG between the MoS(2) layers. However, as the MG coverage is increased, the SHG results indicate averaged MG orientations that are much more upright, consistent with the expulsion of excess MG from the layers to the outside of the restacked crystallites. Together with X-ray diffraction and adsorption isotherm data, our SHG results provide a model for the exfoliation, adsorption, and subsequent restacking of these MG-based inclusion materials and demonstrate the utility of nonlinear optical techniques as probes of these interesting layered structures.

Quantitative measurement and interpretation of optical second harmonic generation from molecular interfaces

Second harmonic generation (SHG) has been proven a uniquely effective technique in the investigation of molecular structure and conformations, as well as dynamics of molecular interfaces. The ability to apply SHG to molecular interface studies depends on the ability to abstract quantitative information from the measurable quantities in the actual SHG experiments. In this review, we try to assess recent developments in the SHG experimental methodologies towards quantitative analysis of the nonlinear optical properties of the achiral molecular interfaces with rotational isotropy along the interface normal. These developments include the methodology for orientational analysis of the SHG experimental data, the experimental approaches for more accurate SHG measurements, and a novel treatment of the symmetry properties of the molecular polarizability tensors in association with the experimentally measurable quantities. In the end, the recent developments on the problem of surface versus bulk contribution in SHG surface studies is discussed. These developments can put SHG on a more solid foundation for molecular interface studies, and to pave the way for better understanding and application of SHG surface studies in general.

Real-time study of HWCVD a-Si:H film growth using optical second harmonic generation spectroscopy

Hydrogenated amorphous silicon (a-Si:H) thin films deposited by HWCVD have been investigated by the surface/interface-sensitive nonlinear technique of optical second harmonic generation (SHG). In situ spectral SHG scans have revealed two resonance peaks at ∼ 1.2 eV and ∼ 1.45 eV corresponding with electronic states (e.g., Si dangling bonds, Si–Si strained bonds) at the surface/interface of the a-Si:H with an isotropic distribution. The data are compared to SHG results obtained ex situ for a-Si:H films deposited by an rf plasma. The first real-time SHG experiments during HWCVD a-Si:H film growth are presented.

Grid-based dynamic electronic publication: a case study using combined experiment and simulation studies of crown ethers at the air/water interface

Conventional paper-based scientific publications are limited in the amount of data and interaction they can provide. Simply placing an electronic copy of such a publication on the web makes for easier distribution, but more can be achieved by making use of the technology to navigate through the paper, to show the links between calculated parameters and the data, and provide access to the chain of calculated results all the way back to the raw data and ultimately the laboratory notebook. While the paper version of this publication is in a relatively conventional form, an interactive demonstrator version (available at http://epaper.combe.chem.soton.ac.uk and as an installable package) illustrates the concepts of publication@source, whereby all the figures and data presented in the paper are linked back to the original raw data together with a description of the processes by which the raw data were analysed. This level of interactivity is achieved using semantic technologies, which have the additional advantage of making the final document subsequently available and navigable by automated techniques. We present the combined information from experimental studies of surface tension and second harmonic generation (SHG) on the behaviour of benzo-15-crown-5 at the solution/air interface, together with a molecular dynamics computer simulation, to demonstrate how the simulation aids the interpretation of the SHG experiment. The adsorption isotherm was determined using SHG and fitted to a Langmuir isotherm giving Delta ads G0=26 kJ mol-1.

Polar Order in Spin‐Coated Films of a Regioregular Chiral Poly[(S)‐3‐(3,7‐dimethyloctyl)thiophene

Films of regioregular chiral poly[(S)-3-(3,7-dimethyloctyl)thiophene] (see Figure) prepared via spin-coating and characterized by polarized UV-vis spectroscopy show that order along the film normal is present (which is proportional to the spinning rate). Phase-sensitive second-harmonic generation experiments demonstrate this order to be polar.

Electrochemical stability of self-assembled monolayers of biphenyl based thiols studied by cyclic voltammetry and second harmonic generation

The reductive desorption of self-assembled monolayers (SAMs) of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH 3–C 6H 4–C 6H 4–(CH 2) n –SH, BP n) on Au(1 1 1) on mica was studied in 0.5 M KOH solution as a function of the length of the aliphatic spacer chain ( n = 1–6 and 12) and for two different preparations temperatures (295 K and 343 K). Second harmonic generation (SHG) was applied in situ parallel to cyclic voltammetry (CV). Odd–even differences in the structure of the BP n monolayers are clearly reflected in the electrochemical stability, as well as by the charge and shape of the desorption peak. For n = 1–5 a single desorption peak is detected whereas multiple peaks occur for BP6 similar to hexadecane thiol which was also studied for comparison. An increased preparation temperature affects the shape and width of the desorption peak but not the position. BP1 exhibits a temperature dependence different from the other homologues. The relationship between coverage monitored by SHG and desorption charge determined from the CVs is found to be linear and surprisingly independent from the details of the SAMs. The combined SHG and CV experiments suggest that capacitive and faradaic current are always closely coupled even for BP6 and hexadecane thiol which exhibit multiple desorption peaks.

Influence of Monomer Optical Purity on the Conformation and Properties of Chiral, Donor-Embedded Polybinaphthalenes for Nonlinear Optical Purposes

A series of chiral, chromophore-functionalized donor-embedded polybinaphthalenes were prepared by a Stille coupling reaction between a diiodo-functionalized chromophore and a bis(trimethyltin)binaphthalene derivative. The optical purity of the chiral binaphthalene monomer was varied, and its influence on the conformation of the polymers was investigated. Differential scanning calorimetry and second-harmonic generation experiments revealed that the mobility of the chromophore in the polymers, prepared from optically impure monomers, is much higher than that in polymers obtained by polymerization of optically pure monomers. Hyper-Rayleigh scattering measurements demonstrated that this difference in mobility is due to a different macromolecular structure.

Surface-induced orientational phase transition in a lyotropic liquid crystal observed by nonlinear optical techniques.

We have observed a phase transition from a uniaxial to a biaxial nematic phase in a lyotropic liquid crystal as a function of decreasing film thickness. The results, obtained by optical second-harmonic generation experiments in a wedged cell geometry, are supported by additional Z -scan measurements and can be interpreted by wall-induced ordering effects.

Linear and nonlinear optical responses influenced by broken symmetry in an array of gold nanoparticles

An array of low-symmetry, L-shaped gold nanoparticles is shown to exhibit high sensitivity to the state of incident polarization. Small imperfections in the shape of the actual particles, including asymmetric arm lengths and edge distortions, break the symmetry attributed to an ideal particle. This broken symmetry leads to a large angular displacement of the extinction axes from their expected locations. More significantly, second-harmonic generation experiments reveal significant second-order susceptibility tensor components forbidden to the ideal symmetry.

Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate-titanate thin films: “Polaron” artifacts

We report infrared absorption spectroscopy and second-harmonic generation data for barium strontium titanate (BST) and lead zirconate-titanate (PZT) ceramic films in the 3000−5500 cm−1 region. Second-harmonic generation experiments give temperature dependences in accord with oxygen vacancy cluster theory [S. A. Prosandeev, Sov. Phys. JETP 83, 747 (1996); S. A. Prosandeev, V. S. Vikhnin, and S. Kapphan, Integr. Ferroelectr. 32, 1047 (2001); J. Phys. Condens. Matter 14, 4407 (2002)]. A percolation model of vacancy ordering is discussed. The present work shows that earlier data interpreted as polaron spectra in these films were actually artifacts due to interference in the optical apparatus used [B. Guettler, U. Bismayer, P. Groves, and E. Salje, Semicond. Sci. Technol. 10, 245 (1950]; more recent “polaron” spectra in WO3 films may also be artifacts [L. Berggren, A. Azens, and G. A. Niklassson, J. Appl. Phys. 90, 1860 (2001)]. Numerical estimates of the polaron mass m**=16 me in SrTiO3 and BST help prove that dielectric data in strontium titanate interpreted as bipolarons [A. Levstik et al., Appl. Phys. Lett. 81, 4046 (2002)] are also artifacts.

Second-harmonic generation with monolithic walk-off-compensating periodic structures II Experiments

The theory of periodic walk-off compensation in monolithic structures is tested with type II second-harmonic generation experiments in the YZ plane of KTiOPO4 by use of 2N optically contacted, walk-off-compensating (OCWOC) structures with numbers of plates 2N=10 and 2N=4. The results confirm the theoretical prediction that such structures behave as harmonic birefringence filters whose N-dependent transfer functions select ranges of wavelengths for maximum conversion at normal incidence and extinguish others within the tuning bandwidth curve of nominal birefringence phase matching. The residual plate orientation mismatches that alter the periodicity of the phase-mismatch gratings were found to be responsible both for the reduced second-harmonic enhancement compared with that of a reference bulk crystal of the same total length and for broadening of the tuning bandwidth. The shapes of the tuning curves depend critically on the pump wavelength, displaying a variety of modulated patterns that were previously attributed to plate orientation mismatches. The tuning filter response was found to change with periodicity defects. An enhancement factor of 15 (scalable to 22) compared with a reference bulk crystal was measured with a 10-OCWOC structure of length LC=10 mm.

Second-harmonic generation in GaAs: experiment versus theoretical predictions of chi(2)xyz.

For GaAs we have determined |chi((2))(xyz)(-2omega;omega,omega)| in second-harmonic generation experiments using two-photon energies between 2 and 5 eV. In addition to the E1, E1+delta(1), E'0, and E2 critical-point bulk transitions of GaAs, a surprisingly strong surface transition at 3.35 eV was observed for natively oxidized GaAs(001) samples. A detailed comparison with theoretical predictions reveals that calculations that include many-particle effects at the level of the "scissors" approximation can describe the overall frequency dependence of the second-harmonic susceptibility reasonably well.

Thin Film Cavity Ringdown Spectroscopy and Second Harmonic Generation on Thin a-Si:H Films

AbstractA set of 8 rf deposited a-Si:H thin films of various thickness (4-1031nm) have been used to explore the applicability of two optical techniques, thin film cavity ringdown spectroscopy (tfCRDS) and second harmonic generation (SHG), for the measurement of small defect-related absorptions. In this paper we will give a first overview of the different aspects of these techniques, which are novel in the field of amorphous silicon materials. It is shown that tf-CRDS is capable of measuring defect-related absorptions (associated with dangling bonds) as small as 10-7 for a single measurement, without the need for elaborate calibration procedures. The results are compared with photothermal deflection spectroscopy (PDS) for a broad spectral range (0.7 – 1.7 eV) and show good agreement. Furthermore the existence of a defect-rich surface layer with a defect density of 1.1×1012 cm-2 has been proven. The absorption spectrum of a 4 nm thin film has revealed a different spectral signature than a bulk dominated (1031 nm) film. The SHG experiments on a-Si:H films have shown that the second harmonic signal arises from the surface states and polarization dependent studies have revealed that the surface states probed have an ∞m-symmetry. From this it can be deduced that the absorbing surface states are isotropically distributed. A spectral scan suggests that the second harmonic signal, whose origin has not been unrevealed yet, has a resonance at an incident photon energy of 1.22 eV.

Ab initio calculation of surface nonlinear optical response

We review recent advances in ab initio calculations of surface optical second harmonic (SH) responses of several systems: Si(001)-adsorbate surfaces, Si(111) surface, GaN(0001) surface, and Ni/Cu(001) surface. Optical second harmonic generation (SHG) has recently become a tool for studying surface electronic states. However, the interpretation of experimental SHG data has been rather difficult due to the lack of guiding theoretical predictions. For well understanding of the results of the measured SH intensity, a number of ab initio theoretical calculations have been competitively developed in these several years, and they are reviewed in this contribution. The results indicate that for simple covalent systems such as the Si surface, quantitative agreement between experiment and calculation has been obtained. [DOI: 10.1380/ejssnt.2003.57]

Characterisation of Au films on Si( [formula omitted]) [formula omitted]-Au by photoemission and optical second-harmonic generation

Synchrotron radiation photoemission in combination with optical second-harmonic generation (SHG) is used to study the growth of thin Au films on the 3 × 3 -Au-reconstructed Si(1 1 1) surface. The Si2p and Au4f core level spectra show the formation of a surface compound that has stabilized to a constant level at 5-monolayer coverage. Valence band spectra show thickness-dependent structures interpreted as quantization effects in the direction perpendicular to the surface. This is supported by optical SHG experiments showing oscillations in the signal with film thickness due to the shift of the quantized levels. The oscillating second-harmonic signal depends on the azimuthal sample orientation and thus originates from a film with crystalline order. From the thickness dependence of the second-harmonic signal in combination with the photoemission results it is concluded that the Au film is well-ordered below a thin reacted surface layer.

Enhanced second-order nonlinear optical response of LiNbO3 films upon Er doping

Erbium-doped LiNbO3 films have been produced in a single-step process by alternate pulsed laser deposition. The dopant is incorporated in submonolayers whose nominal indepth separation is varied in the range 1.7–4.0 nm to lead respectively to Er concentrations in the range 3.4–0.6×1020 atoms/cm−3. All the films exhibit the characteristic Er3+ photoluminescence at 1.54 μm with lifetime values as high as 3 ms. The d33 nonlinear coefficients determined from second-harmonic generation experiments are in the range 22–28 pm/V, the films having nominal Er submonolayer indepth separation of 3–4 nm exhibiting d33 values slightly above that of the bulk material. The comparison of the d33 values obtained in the Er-doped films to those reported earlier for similar undoped films shows clearly that both the structural quality and the second-harmonic performance of the films can be enhanced by Er doping.