Sum Frequency Generation Method Research Articles

Probing Molecular Structures of Buried Interfaces in Thick Multilayered Microelectronic Packages

The fast development of flip-chip (FC) devices has helped to keep ICs on pace for Moore's law. FC devices have many buried interfaces, the structures of which all need to be investigated in order to optimize them. It is extremely difficult to examine molecular structures of buried interfaces in samples with multiple layers due to the lack of appropriate analytical tools. In this paper, real FC-on-leadframe devices, manufactured by Texas Instruments Incorporated, were investigated using the intrinsically interface-sensitive technique and sum-frequency generation (SFG) vibrational spectroscopy. By milling down the Si wafer, we can probe the polyimide (PI)/mold compound (MC) interface. By milling down further, we can examine the air/MC and the MC/leadframe (LF) interfaces. Methylene and methyl groups were detected from each of the three interfaces, and the orientation of methyl groups was determined at each interface. It was calculated that at the PI/MC interface, the methyl group tilt angle was 20° (from the surface normal). For the air/MC interface, the tilt angle was 43.5°, and for the LF/MC interface, the tilt angle was measured to be 45°. The deduced different methyl orientations at the two buried solid/solid interfaces can be interpreted by the interfacial interactions. The PI surface is more hydrophobic than the LF surface; therefore, it has more favorable interactions with the hydrophobic methyl groups, leading to a more “standing up” (smaller tilt angle) position for the methyl groups at the PI/MC interface. In this paper, SFG was successfully applied to study the buried interfaces in real devices. The SFG method used in this paper is general and can be applied to other real-world, complicated thick systems.

Temperature-insensitive sum frequency generation of 355-nm ultraviolet laser radiation in LiB3O5 by compensating thermally induced phase mismatch.

A temperature-insensitive sum frequency generation method based on thermally induced phase mismatch compensation is proposed to enhance the thermal stability and scale up the output power of ultraviolet (UV) lasers. In the method, three LiB3O5 crystals are cascaded for sum frequency generation of a 355-nm UV laser, and the two crystals at the ends are employed for frequency conversion; the middle one, which is not required to be phase matched and has an opposite sign of first temperature derivative of phase mismatch, compensates the thermally induced phase mismatch generated in the first crystal. The temperature acceptance bandwidth of frequency conversion in the three cascaded crystals is about double that in a single long crystal with the same interaction length.

Hydrogen bonding and vibrational energy relaxation of interfacial water: A full DFT molecular dynamics simulation.

The air-water interface has been a subject of extensive theoretical and experimental studies due to its ubiquity in nature and its importance as a model system for aqueous hydrophobic interfaces. We report on the structure and vibrational energy transfer dynamics of this interfacial water system studied with equilibrium and non-equilibrium molecular dynamics simulations employing a density functional theory -based description of the system and the kinetic energy spectral density analysis. The interfacial water molecules are found to make fewer and weaker hydrogen (H)-bonds on average compared to those in the bulk. We also find that (i) the H-bonded OH groups conjugate to the free OH exhibit rather low vibrational frequencies (3000-3500 cm-1); (ii) the presence of a significant fraction (>10%) of free and randomly oriented water molecules at the interface ("labile water"), neither of whose OH groups are strong H-bond donors; (iii) the inertial rotation of free OH groups, especially from the labile water, contribute to the population decay of excited free OH groups with comparable rate and magnitude as intramolecular energy transfer between the OH groups. These results suggest that the labile water, which might not be easily detectable by the conventional vibrational sum frequency generation method, plays an important role in the surface water dynamics.

Hydrogen desorption kinetics from H‐Si (111) surfaces studied by optical sum frequency generation and second harmonic generation

We have studied hydrogen desorption from a flat H‐Si (111)1 × 1 surface at 711 K by observing sum frequency generation (SFG) and second harmonic generation (SHG) spectra. Flat H‐Si (111) surfaces were prepared by dosing hydrogen molecules in an ultra‐high vacuum chamber with a base pressure of 〜10−8 Pa. Combining the SFG and SHG methods, the desorption order has been clarified over the whole hydrogen coverage range from 1 monolayer (ML) to 0 ML. The hydrogen desorption was assigned as second order in the high coverage range of 1 ML–0.18 ML by using SFG spectroscopy and as first order in the coverage range of 0.18 ML–0.0 ML by using SHG spectroscopy. Copyright © 2016 John Wiley & Sons, Ltd.

Spectral compression of femtosecond pulses using chirped volume Bragg gratings.

In this Letter, we demonstrate a 360 fold spectral bandwidth reduction of femtosecond laser pulses using the method of sum frequency generation of pulses with opposite chirps. The reduction has been achieved in a compact setup in which a single chirped volume Bragg grating replaces conventional stretcher and compressor units. Starting with 180 fs pulses, we have obtained, with a 30% overall efficiency, pulses longer than 100 ps with the spectral bandwidth of 0.23 cm-1 (7 GHz). We also discuss our method on theoretical grounds.

Retrieval of complex χ((2)) parts for quantitative analysis of sum-frequency generation intensity spectra.

Vibrational sum-frequency generation (SFG) spectroscopy has become an established technique for in situ surface analysis. While spectral recording procedures and hardware have been optimized, unique data analysis routines have yet to be established. The SFG intensity is related to probing geometries and properties of the system under investigation such as the absolute square of the second-order susceptibility χ((2)) (2). A conventional SFG intensity measurement does not grant access to the complex parts of χ((2)) unless further assumptions have been made. It is therefore difficult, sometimes impossible, to establish a unique fitting solution for SFG intensity spectra. Recently, interferometric phase-sensitive SFG or heterodyne detection methods have been introduced to measure real and imaginary parts of χ((2)) experimentally. Here, we demonstrate that iterative phase-matching between complex spectra retrieved from maximum entropy method analysis and fitting of intensity SFG spectra (iMEMfit) leads to a unique solution for the complex parts of χ((2)) and enables quantitative analysis of SFG intensity spectra. A comparison between complex parts retrieved by iMEMfit applied to intensity spectra and phase sensitive experimental data shows excellent agreement between the two methods.

Lipid Compositions Modulate Fluidity and Stability of Bilayers: Characterization by Surface Pressure and Sum Frequency Generation Spectroscopy

Cell membranes are crucial to many biological processes. Because of their complexity, however, lipid bilayers are often used as model systems. Lipid structures influence the physical properties of bilayers, but their interplay, especially in multiple-component lipid bilayers, has not been fully explored. Here, we used the Langmuir-Blodgett method to make mono- and bilayers of 1,2-dihexadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), and 1-hexadecanoyl-2-(9Z-octadecenoyl)-sn-glycero-3-phospho-L-serine (POPS) as well as their 1:1 binary mixtures. We studied the fluidity, stability, and rigidity of these structures using sum frequency generation (SFG) spectroscopy combined with analyses of surface pressure-area isotherms, compression modulus, and stability. Our results show that single-component bilayers, both saturated and unsaturated, may not be ideal membrane mimics because of their low fluidity and/or stability. However, the binary saturated and unsaturated DPPG/POPG and DPPG/POPS systems show not only high stability and fluidity but also high resistance to changes in surface pressure, especially in the range of 25-35 mN/m, the range typical of cell membranes. Because the ratio of saturated to unsaturated lipids is highly regulated in cells, our results underline the possibility of modulating biological properties using lipid compositions. Also, our use of flat optical windows as solid substrates in SFG experiments should make the SFG method more compatible with other techniques, enabling more comprehensive future surface characterizations of bilayers.

Structure of water at zwitterionic copolymer film–liquid water interfaces as examined by the sum frequency generation method

A copolymer film composed of zwitterionic carboxymethylbetaine (CMB) and n-butyl methacrylate (BMA), Poly(CMB-r-BMA), was cast on a flat plane of an octadecyltrichlorosilane (ODS)-modified fused quartz prism with a semi-cylindrical shape. CH stretching of the polymer film and OH stretching of water at the surface of the film were examined using the sum frequency generation (SFG) technique. The CH stretching band of the cast film, indicating a gauche defect of the film, was affected by the contact medium including dry nitrogen, water vapor-saturated nitrogen and liquid water. In contrast, the CH stretching of an octadecyl group introduced onto the quartz prism for stable attachment of the cast film was not significantly changed by the contact medium. The OH stretching band indicated that water molecules at the surface of the Poly(CMB-r-BMA) film in contact with liquid water were not greatly oriented in comparison with those at the surfaces of a bare prism, an ODS SAM-modified prism, and a prism covered with a PolyBMA film or a copolymer film of BMA and methacrylic acid or 2-(dimethylamino)ethyl methacrylate. A similar small perturbation of the structure of water was previously observed in the vicinity of water-soluble zwitterionic polymers and zwitterionic copolymer films using Raman and attenuated total reflection infrared spectroscopies, respectively. A distinct effect of charge neutralization to diminish the perturbation of the structure of interfacial water around polymer materials was suggested.

Structure of water in the vicinity of a zwitterionic polymer brush as examined by sum frequency generation method

A zwitterionic poly(carboxymethylbetaine) (PCMB) brush was prepared on a fused quartz prism by the surface-initiated atom transfer radical polymerization (SI-ATRP) of CMB monomer. The conformation of PCMB brush and the state of water at the surface of the brush were examined using sum frequency generation (SFG) technique. The CH stretching band of the brush, indicating the gauche defect of the brush, was affected by the contact medium such as dry nitrogen, water vapor-saturated nitrogen and liquid water. The water molecules at the PCMB–water interfaces were not largely oriented in comparison with the interfacial water of both bare and the ATRP-initiator-modified quartz prisms. The similar tendency was previously observed for water in the vicinity of water-soluble zwitterionic polymers and polymer thin films using Raman and attenuated total reflection (ATR) infrared spectroscopies, respectively. The electrical neutralization between neighboring positive and negative charges might diminish the electrostatic adsorption of water molecules to the vicinity of zwitterionic polymer brushes.

Stick–Slip Phenomenon in Measurements of Dynamic Contact Angles and Surface Viscoelasticity of Poly(styrene-b-isoprene-b-styrene) Triblock Copolymers

In this paper, a series of poly(styrene-b-isoprene-b-styrene) triblock copolymers (SIS), with different chemical components, was synthesized by anionic polymerization. The relationships between surface structures of these block copolymers and their stick-slip phenomena were investigated. There is a transition from stick-slip to a closely smooth motion for the SIS films with increasing PS content; the patterns almost vanish and the three-phase line appears to move overall smoothly on the film surface. The results show that the observed stick-slip pattern is strongly dependent on surface viscoelasticity. The jumping angle Δθ, which is defined as θ(1) - θ(2) (when a higher limit to θ(1) is obtained, the triple line "jumps" from θ(1) to θ(2) with increases in drop volume), was employed to scale the stick-slip behavior on various SIS film surfaces. Scanning force microscopy/atomic force microscopy (AFM) and sum frequency generation methods were used to investigate the surface structures of the films and the contributions of various possible factors to the observed stick-slip behavior. It was found that there is a linear relationship between jumping angle Δθ and the slope of the approach curve obtained from AFM force measurement. This means that the stick-slip behavior may be attributed mainly to surface viscoelasticity for SIS block copolymers. The measurement of jumping angle Δθ may be a valuable method for studying surface structure relaxation of polymer films.

Sum frequency generation study on the structure of water in the vicinity of an amphoteric polymer brush

A polyampholyte brush was prepared on the surface of a quartz prism by surface-initiated reversible addition-fragmentation chain-transfer polymerization of methacrylic acid (MA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) at a monomer ratio of 1:1. The sum frequency generation method indicated that water molecules at the amphoteric polymer brush–water interfaces were not greatly oriented in comparison with those at the surfaces of bare quartz prisms, and negatively charged PolyMA and positively charged PolyDMAEMA brushes. The small perturbation effect of an amphoteric polymer brush on the structure of vicinal water was in accordance with the tendency of aqueous solution and thin films of amphoteric polymers observed by using Raman and infrared spectroscopies, respectively.

Ultrafast Nonlinear Coherent Vibrational Sum-Frequency Spectroscopy Methods To Study Thermal Conductance of Molecules at Interfaces

It is difficult to study molecules at surfaces or interfaces because the total number of molecules is small, and this is especially problematic in studies of interfacial molecular dynamics with high time resolution. Vibrational sum-frequency generation (SFG) spectroscopy, where infrared (IR) and visible pulses are combined at an interface, has emerged as a powerful method to probe interfacial molecular dynamics. The nonlinear coherent nature of SFG helps overcome the sensitivity issues, especially when femtosecond IR pulses are used. With femtosecond pulses, a range of vibrational transitions can be probed simultaneously and high time resolution can be achieved. Ultrafast SFG experiments use three pulses, a pump pulse to generate nonequilibrium conditions with a pair of probe pulses, and two time delay parameters. Mapping SFG intensity as a function of the two time delays creates a two-dimensional surface, where one axis (t(1)) provides information about molecular dynamics driven by the pump pulses, and the other axis (t(2)) about the dynamics of the SFG probing process. We present examples of ultrafast SFG measurements drawn from our studies of heat transport through interfacial molecules that are models for molecular wires in electronic circuits. In these flash-heating experiments, a self-assembled monolayer (SAM) of long-chain molecules adsorbed on a metal surface is subjected to a large amplitude (up to 800 K) temperature jump. Specific vibrational reporter groups on the SAM molecules probed by SFG serve as tiny ultrafast thermometers approximately 1.5 A thick with a approximately 1 ps response time. These SFG thermometers can monitor ultrafast heat transport through the SAM molecules. By varying the lengths of the molecular wires we can tell if the heat is propagating ballistically along the chains, at constant speed, or diffusively. In our analysis of 2D SFG methods, we first describe a simpler situation where the visible probe pulse is effectively infinite in duration. This is the usual way time-resolved SFG measurements are made, and the SFG experiment then becomes a function of a single time delay, the pump-IR probe delay t(1). Unfortunately, in this case the SFG signals have a large contribution from the nonresonant (NR) background generated by the metal surface, which adds a great deal of noise to the data, and the time resolution is limited by the molecule's vibrational dephasing time constant T(2), which is often 1 ps or more. We have recently shown that the NR background can be suppressed using a time delay t(2) between IR and visible probe pulses. In this now 2D SFG method, one would expect that information about the molecular response to the pump pulses would be contained in slices along the t(1) axis, but by simulating the experiment we show that the t(1) and t(2) parameters interact. Changing t(2) to suppress the NR background causes t(1) slices to shift in time. We also show how to improve the time resolution of ultrafast SFG experiments while maintaining NR suppression using femtosecond visible pulses at appropriate t(2) delay values.

New adjustment technique for time coincidence of femtosecond laser pulses using third harmonic generation in air and its application to holograph encoding system

The third harmonic generation of light (266 nm) is enhanced, sensitively depending on the time delay between a pair of pulses split from a single 800 nm femtosecond laser pulse, when they are focused and collided in air. This finding offers a convenient and widely applicable technique to detect temporal and spatial overlapping of two femtosecond pulses. This technique has several advantages over the conventional sum frequency generation method using nonlinear optical crystals, since it obviates the need for expensive crystals, free from phase matching, and elimination of temporal walk off. By applying it to “a holographic encoding system using an interference femtosecond laser pulse,” a periodic fringe spacing is minimized to ∼430 nm by extending the colliding angle between two-pulse beams up to ∼160 °C.

Efficient conversion of visible radiation from a tunable dye laser to the ultraviolet range

An experimental investigation was made of the conversion of the second harmonic of the radiation of a dye laser of wavelengths 545—760 nm into ultraviolet radiation of wavelengths 217–280 nm by the method of sumfrequency generation. The use of a KDP crystal with phase matching by angular adjustment ensured that the conversion efficiency was 35%.