Surface Second Harmonic Generation Signal Research Articles

Surface second harmonic generation from silicon pillar arrays with strong geometrical dependence

We present experimental demonstration and analysis of enhanced surface second harmonic generation (SHG) from hexagonal arrays of silicon pillars. Three sets of Si pillar samples with truncated cone-shaped pillar arrays having periods of 500, 1000, and 2000 nm, and corresponding average diameters of 200, 585 and 1550 nm, respectively, are fabricated by colloidal lithography and plasma dry etching. We have observed strong dependence of SHG intensity on the pillar geometry. Pillar arrays with a 1000 nm period and a 585 nm average diameter give more than a one order of magnitude higher SHG signal compared to the other two samples. We theoretically verified the dependence of SHG intensity on pillar geometry by finite difference time domain simulations in terms of the surface normal E-field component. The enhanced surface SHG light can be useful for nonlinear silicon photonics, surface/interface characterization, and optical biosensing.

Laterally Emitted Surface Second Harmonic Generation in a Single ZnTe Nanowire

We report a direct observation on the laterally emitted surface second harmonic generation (SHG) in a single ZnTe nanowire. The highly directional surface SHG radiates vertically to the nanowire growth-axis with a high conversion efficiency of 5 × 10(-6) and a low divergence angle of 4°. Polarization state, emission intensity, and direction of the surface SHG are found to be dependent on the polarization of the pumping laser, which is further confirmed by numerical simulations. The laterally emitted surface SHG with a high efficiency and low divergence angle has great potential for short-wavelength nanolasers, nonlinear microscopic imaging, and polarization-dependent photonic integrating.

Increased interfacial thickness of the NaF, NaCl and NaBr salt aqueous solutions probed with non-resonant surface second harmonic generation (SHG)

Specific ion effects on the nonlinear optical response from the water molecules at the air/sodium halide solution interfaces are measured using non-resonant surface second harmonic generation (SHG). Procedures have been developed to monitor and remove the impurities in the salt solution samples to ensure measurement of small changes in the SHG signal. Quantitative polarization analysis of the measured SHG data indicated that the average orientation of the interfacial water molecules changed only slightly around 40 degrees with the increase of the bulk concentration of the three sodium halides, namely NaF, NaCl and NaBr, from that of the neat air/water interface. The observed significant SHG signal increase with the bulk salt concentration is attributed to the overall increase of the thickness of the interfacial water molecular layer, following the order of NaBr > NaCl approximately NaF. The absence of the electric-field-induced SHG (EFISHG) effect indicated that the electric double layer at the salt aqueous solution interface is much weaker than that predicted from the molecular dynamics (MD) simulations. These results provided quantitative data to the specific anion effects on the interfacial water molecules of the electrolyte aqueous solution, not only for the larger and more polarizable Br(-) anion, but also for the smaller and less polarizable F(-) and Cl(-) anions.

Second Harmonic Generation Phase Measurements of Cr(VI) at a Buried Interface

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.

Surface Magnetism during Oxygen-Aided Fe Homoepitaxy

Magnetization-induced optical surface second harmonic generation (SHG) in the longitudinal geometry was used to study magnetism on the p(1 x 1)O/Fe(001) surface during a layer-by-layer Fe growth with surfactant oxygen. Considerable one-monolayer oscillations were observed. Minima of the magnetization-induced contributions to the second order dielectric susceptibility were detected at half-filled monolayers. These contributions were accessed either by measuring a purely magnetic SHG yield, or indirectly from the magnetization dependence of the overall SHG signal, both providing consistent results. The origin of the oscillatory surface magnetism is consistent with the expected oscillating oxygen induced relaxation due to the surface Fe islands.

Chiroptical Effects in the Second Harmonic Signal of Collagens I and IV

We performed polarization-resolved surface second harmonic generation (SHG) experiments on thin films of collagen I and IV molecules, as well as conventional CD measurements. We found that collagen IV presents little CD and no SHG optical activity, whereas collagen I exhibits large chiroptical effects involving both one-electron and excitonic coupling mechanisms. We estimated that these chiral components enhance the SHG signal from fibrillar collagen in biological tissues by typically a factor of 2. By comparing the distinct behaviors of collagens I and IV in SHG microscopy and in surface SHG experiments, we concluded that SHG microscopy is a sensitive probe of the micrometer-scale structural organization of collagen in biological tissues.

Effect of ultrathin Cr layers on surface second-harmonic generation from Cr-SiO2-Si(001) structures

An ultrathin semitransparent Cr coating layer on oxidized Si(001) significantly alters surface second-harmonic generation (SHG) behavior. The peak locations of the rotational-anisotropy SHG (RA-SHG) signals from Cr coated SiO2-Si(001) surfaces are shifted by 45° from that of uncoated surfaces consistently for several polarizations, but only for specific photon energies. Moreover, for appropriate polarizations, a peak-to-valley transition of the RA-SHG signal by varying photon energy is observed for Cr coated samples but not for uncoated samples. The time-dependent SHG signal decreases with continuous laser illumination for Cr coated surfaces, but it increases for uncoated surfaces. Through these comparisons, we find that the Cr coating introduces an additional SHG. Interference between the additional and original SHG contributions modifies both the phase and amplitude of the net SHG, and thus the measured spectroscopic and time dependence of SHG. To determine the source of this additional SHG, we perform further surface SHG measurements on a thick Cr film and the SiO2-Cr interface. We find that it mainly comes from the outermost surface of the Cr coating layer. In addition, we investigate the effect of the thickness of the SiO2 layer on the SHG response in reflection from Cr-SiO2-Si(001) structures.

Quenching of optical second harmonic generation at the Si(001) surface by hydrogen adsorption

A detailed study of the surface second harmonic generation (SHG) efficiency at $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ is presented. We have measured the SHG efficiency as a function of hydrogen coverage, relative to that of the clean surface, at a fundamental wavelength of 1064 nm, for substrate temperatures between 200 and 600 K, and for different polarization directions of both the fundamental and second harmonic light beams. The dependence of hydrogen-induced SHG quenching on H coverage is explained with a statistical mechanical model that has previously been invoked to describe the kinetics and dynamics of ${\mathrm{H}}_{2}$ adsorption/desorption. The model predicts an increase in the SHG intensity upon rearranging randomly adsorbed H atoms by thermal annealing, which was confirmed experimentally. An empirical parametrization of the SHG efficiency curves is presented which may be used as a practical calibration to determine hydrogen coverage on Si(001) as a function of SHG signal and temperature.

Transient Second Harmonic Generation from Oxazine Dyes at the Air/Water Interface

Ultrafast surface second harmonic generation (SHG) studies are reported for several oxazine dyes at the air/water interface. As we demonstrated previously [J. Phys. Chem. B 2001, 105, 3062] in our steady-state SHG studies of aqueous oxazine dye solutions, the SHG signals are almost entirely due to dimers. The transient SHG recovery is biexponential with time constants of 4−8 ps and 22−44 ps depending on the dye and the solution composition. The SHG recovery is one-photon resonant and is attributed to two parallel ground state recovery mechanisms. Transient absorption signals were also measured for bulk dye solutions of water and methanol, and the results indicate a fast (2 ps) solvation dynamics component as well as a longer component due to the excited state lifetime (>400 ps) of the dye monomer. Therefore, both decay times observed for the dyes at the surface are different than what is seen for bulk solution, and they resemble what has been previously reported for dye aggregates.

The temperature dependence of surface second-harmonic generation from the air-water interface

An investigation of the polarization and temperature dependence of surface second-harmonic generation (SHG) at the air-water interface shows that the intensity of the SHG signal decreases over the range 20–70°C. Accounting for the changes in the SHG polarization behaviour with temperature to extract the relative temperature dependence of Xzzz shows that it is similar to that obtained from simulation studies. The role of the interfacial refractive index in the interpretation of the SHG signals is addressed.

Ultrafast dynamics of electron thermalization in gold.

Time-resolved surface second-harmonic generation (SHG) is used to probe electron relaxation processes in gold following intense laser excitation at 1.55 eV. For the first time, an electron temperature ( T(e)) dependent enhancement in the SHG signal is clearly observed at T(e) above 0.7 eV, which is shown to relate to the thermalization of nonequilibrium hot electrons. Therefore, the relaxation dynamics of the transient nonequilibrium electrons in the high T(e) regime is directly resolved by monitoring the time evolution of the SHG signal.

Surface second harmonic generation studies of the dodecane/water interface: the equilibrium and kinetic behaviour of p-nitrophenol and tri-n-butyl phosphate

The behaviour of p-nitrophenol (PNP) absorbed at the dodecane/water interface and the interaction with tri-n-butyl phosphate (TBP) were studied by surface second harmonic generation (SHG). The polarisation dependence of the PNP SHG signal, which is the same for all concentrations of PNP and TBP, is best described in terms of a limiting weak orientational order. In the presence of TBP, the SHG signal initially increases with PNP concentration, passes through a maximum and then it decreases. At low PNP concentrations, increasing the TBP concentration leads to an increase in the SHG intensity, but for higher PNP concentrations the intensity decreases. The free energies of adsorption for PNP and TBP to the dodecane/water interface were determined using Frumkin and Langmuir isotherms. The kinetics of the interaction between TBP and PNP was observed by monitoring the SHG signal along a liquid–liquid flow cell and found to be first-order at high interfacial coverage with rate constant, k, of 0.1±0.01 s−1.

On the Factor in Gibbs Equation for Ionic Surfactants

The adsorption layer of soluble surfactants can be described by the well-known Gibbs equation. It states that the surface excess Γ is given by the slope of the isotherm according to Γ = −1/(mRT)∂σe/∂ ln a. The rigorous thermodynamic treatment requires a factor m = 2 for 1:1 electrolyte in the case of the absence of indifferent electrolyte. This aspect is addressed in the following contribution. For this purpose, a soluble C12 and an insoluble cationic amphiphile C20 was designed and synthesized. Both contain the same head group which possess a sufficiently high hyperpolarizability for surface second-harmonic generation (SHG) measurements. SHG is highly surface specific, and polarization dependent measurements allow a determination of the orientational order within the adsorption layer. The intensity of the SHG signal is proportional to the square of the number density N, and with a calibration based on the SHG signal of the Langmuir monolayer of the insoluble compound the absolute number density can be de...

Determination of Molecular Orientational Distribution Using Surface Second-Harmonic Generation Analyzed by the Five-Layer Model and the Modified Maximum-Entropy Method

We have determined the molecular orientational distribution functions (ODFs) at air and substrate interfaces for unrubbed and rubbed films of polymer possessing second-harmonic generation (SHG) active side chains. Using a thick polymer film and four different optical geometries, the surface SHG signals from the two interfaces could be successfully separated. To theoretically analyze the surface SHG results, we used the five-layer model, in which a five-layered system consisting of air, an unrubbed or rubbed upper SHG active layer, a SHG inactive polymer bulk layer, a lower SHG active layer and a substrate was considered. The local field factors for both active layers were deduced from the boundary conditions among these five layers. In order to obtain ODFs of the side chains at two SHG active layers, the modified maximum-entropy method was used. After achieving the best fit of the theoretical SHG intensities to the experimental results, we unambiguously determined the unbiased ODFs together with information on the in-plane axial and the polar order parameters for the side chains at the air-polymer and substrate-polymer interfaces of unrubbed and rubbed polymer films.

Photochemistry of O2 on Ag(110) using picosecond ultraviolet laser pulses

Surface second-harmonic generation (SHG) was used to study the photochemical reaction of O2 with Ag(110) using ∼6 ps laser pulses. A clean well-ordered Ag(110) surface produces a strong SHG signal at 580 nm. The SHG signal measured in real time during O2 exposure is sensitive to submonolayer coverages of O2 and to the subsequent exposure of this system to picosecond ultraviolet laser pulses. Upon O2 adsorption at 130 and 200 K, the signal decreases by more than a factor of 2 at saturation coverage. Langmuir adsorption kinetics are used to fit the O2 dosage curves relating the decrease in SHG intensity to the oxygen coverage. Subsequent exposure of the surface dosed at 130 K to 290 nm light of the same pulse width also produces a change in the second-harmonic response. Combined with surface SHG temperature programmed desorption measurements, these results are correlated to the photochemical reaction taking place on the surface in which chemisorbed O2 photodissociates and photodesorbs. The results of the surface photochemical reaction initiated by picosecond UV pulses are compared to those previously studied using continuous wave radiation and are found to have similar overall behavior while possibly showing some new features.

Surface temperature of pulsed-laser-irradiated Si(111)-7 × 7 measured by second-harmonic generation

Pulsed laser heating effect on the Si(111)-7 × 7 structure was investigated through surface second-harmonic generation (SHG) of a delayed probe Q-switched YAG laser pulse during a transient temperature jump induced by a pump laser pulse. The time evolution of the surface temperature of the Si(111)-7 × 7 structure induced by a pulsed laser was determined empirically using the time evolution of the SH intensity under a condition for which no structural changes were induced by the laser pulse. The SH intensity from the 7 × 7 structure laser-heated above ∼ 860°C, the transition temperature of the 7 × 7 → “1 × 1”, was also decreased. The results are consistent with a simple extrapolation of the SHG signal from the equilibrium 7 × 7 phase to temperatures above the equilibrium transition over the laser heated period of 0.1 ms.

Second harmonic generation from the air/water interface of an aqueous solution of the dipeptide Boc-Trp-Trp

The air/water interface of aqueous (t-butyloxycarbonyl) -tryptophan-tryptophan (Boc-Trp-Trp) has been studied by surface second harmonic generation (SHG). The SHG signals obtained with a linearly polarised fundamental indicate a Langmuir-like adsorption isotherm. Wavelength-dependent circularly differential harmonic generation (SHG-CD) was observed together with SHG optical rotation. The SHG signals from the LL and DD enantiomer show equal but opposite dependence on the handedness of the fundamental radiation and the DL diastereoisomer gave rise to a different spectrum.

Consequences of chirality in second-order non-linear spectroscopy at surfaces.

Surface second-harmonic generation (SHG) is recognized as a second-order non-linear laser-based spectroscopy offering good surface sensitivity and selectivity. With the goal of developing optical methods to study chiral surfaces, a way to detect dichroic behaviour in SHG in analogy to circular dichroism (CD) spectroscopy has been elucidated. The SHG efficiency from a monolayer of oriented chiral molecules [(R)- or (S)-2,2'-dihydroxy-1,1'-binaphthyl (BN)] has a strong dependence upon whether the excitation beam has left- or right-handed circular polarization. The relative differential SHG signal is much larger than in ordinary CD spectroscopy. The large chiroptical effects in SHG are attributed not to R, the rotational strength responsible for traditional CD (R = Im mu.m), but to the electric dipole-allowed second-order non-linear tensor, chi (2). There is a chiral element of chi (2), chi (xyz), that must be included in descriptions of the SHG response from chiral surfaces. The ability to measure chiral properties of an adsorbed molecule separately, with no background from its achiral properties, provides another form of chiral spectroscopy, different from CD and having no analogue in linear spectroscopy, but related in a well understood manner.