“Forbidden” Second Harmonic Generation in Bacteriorhodopsin: Steady-State and Time-Resolved Studies

We have investigated the diffusion of the photosensitizer Chlorin-p(6) (Cp(6)) across a egg lecithin lipid bilayer at different pH by the Second Harmonic Generation (SHG) method. Cp(6) has three ionizable carboxylic acid groups, and consequently, neutral and several ionic forms of Cp(6) are expected to be present in the pH range 3-8. The absorption spectra of Cp(6) get considerably modified in the presence of liposomes as the pH is decreased indicating that the drug liposome binding is pH dependent. The first pK(a) of interconversion (D-C) has been identified at pH ~7.0 by fluorescence measurement in an earlier work. In this work, the second pK(a) of interconversion (C-B) has been identified at pH ~4.8 by the hyper-Rayleigh scattering method. At acidic pH (3, 4, and 5), where species A, B, and C are dominant, the addition of liposomes to a Cp(6) solution generates an instantaneous rise (less than 1 s) in the second harmonic (SH) signal followed by decays whose time constants ranged from ten to hundreds of seconds. The instantaneous rise is attributed to the adsorption of Cp(6) to the outer lipid bilayer, and the decay is attributed to the diffusion of the neutral and charged (A and B) species of the drug. The observed fast and slow time constants for diffusion in the pH range 3-5 are attributed to the neutral (A) and ionic form (B) of Cp(6), respectively. At pH 6, the intensity of the generated SH signals on the addition of liposome reduced, and at physiological pH, it was too weak to be detected. These results are consistent with previous studies that show that the interaction between Cp(6) and egg-PC liposomes is pH dependent. At lower pH due to the presence of the hydrophobic species (A and B) of Cp(6), its interaction with liposomes is strong, and at higher pH, the abundance of the negatively charged hydrophilic species (C and D) decreases the interaction with the like charged liposomes. We have also studied the effect of increasing the bilayer rigidity by decreasing the temperature of the medium or by incorporating 50 mol % cholesterol in the lipid bilayer and observed that lowering of temperature has more profound effect on the diffusion rates. The characteristics of the SH signal changed significantly when liposomes incorporating 50 mol % cholesterol were used at a low (3 °C) temperature. Under these conditions, the SH signal consisted of an instantaneous (<1s) followed by a slower rise (10-90s), and then, it decayed on a much longer time scale. This slow rise of the SH signal at pH 3 and 4 may be attributed to the temperature dependent adsorption of the anionic species (B) of Cp(6) with the liposomes. Further investigations are required in order to understand clearly the pH dependent diffusion of this drug across lipid bilayers.

In this paper, we investigated the second harmonic (SH) and the third harmonic (TH) generations in nanorods and nanotubes. Nonlinear free and bound electrons responses of metal combined with the third-order nonlinearity of dielectric core were utilized to study the nonlinear response of nanotubes made of noble metals such as gold. The impacts of the dielectric constant of core medium, thickness of metal shell, and size of nanotube on the SH and TH conversion efficiency are reported. The physical interpretations behind the results are discussed. Finally, we have shown that enhancement of the SH and TH generations are more noticeable and adjustable by varying the core-shell parameters compared to those of nanorods.

The changes of the morphology during heating and the degree of crystallinity of dry and hydrated starch granules are investigated using second harmonic generation (SHG) based Stokes polarimetry. A spatial distribution of various polarization parameters, such as the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP) are extracted and compared with the two dimensional second harmonic (SH) Stokes images of starch granules. The SH signal from hydrated and dry starch on heating differed significantly in DOLP and DOCP values, indicating that hydrated starch has a greater degree of ultrastructural amylopectin disorder. The detail of denaturation and the phase transition of hydrated starch demonstrate the significant influence of thermal processing.

Electric field induced second harmonic measurements applying femtosecond laser pulses (1.59 eV, 80±5 fs, 80 MHz) to substantially boron doped p+-Si/SiO2 interfaces reveal a temporal evolution of the second harmonic (SH) signal, which differs drastically from that of weakly doped samples. A significant initial SH signal is observed in native p+-Si/SiO2 interfaces for boron doping concentrations &amp;gt;7.5×1017 cm−3. This SH signal is attributed to a built-in interfacial electric field E0 generated by the doping induced accumulation of charges at the Si/SiO2 interface following the ionization of interface defect states. A sign reversal is observed in the azimuthal SH anisotropy pattern of the initial SH signal relative to that of the saturated SH signal in p+-Si/SiO2 indicating that the doping related and electron induced interfacial field components oppose each other. Furthermore, the intensity dependence of the initial SH signal in p+-Si/SiO2 is found to be nonquadratic and, in particular, shows a nonmonotonic behavior at incident intensities above ∼60 GW/cm2. These observations are attributed to ultrafast screening of E0 caused by linearly as well as nonlinearly photoexcited electron-hole pairs.

A number of liquid crystalline cyclic pentamethylsiloxane compounds were synthesized containing high concentrations of covalently attached disperse red 1(4-[N-ethyl, N- hydroxyethylamino]-4'-nitroazobenzene) pendant groups. Corona poling (1.0 (mu) A/-5 kV) was used to align these pendant groups in thin films (1 - 2 micrometers ) produced by spin coating onto ITO covered glass substrates. The measured second harmonic (SH) intensities of these films were comparable to 1 mm thick, y-cut quartz. The initial rise in the SH signal at 25 degree(s)C occurred in less than 10 seconds. This was followed by an additional 20 - 50% increase in the signal over the next 1 - 15 minutes to a final equilibrium value. The SH signal of compounds in the smectic or nematic phase increased to a value of 0.7 - 1.0 X the quartz standard upon heating. An upper temperature limit for each material was observed prior to the dielectric breakdown of the film. The SH intensity typically decayed back to its prepoling value within one hour for films that were poled at 25 degree(s)C. Long term SH stability was observed in films that were poled at elevated temperatures. The intensity of the SH signal was found to be a function of the composition, liquid crystalline phase, and temperature.

We theoretically study how the intensity of second harmonic generation (SHG) for a sheet of two-dimensional (2D) material is controlled by an underlying layered structure. By utilizing the transfer matrix method with the inclusion of a nonlinear sheet current to describe the response of the 2D material, an explicit expression for the intensity of upward propagating second harmonic (SH) light is obtained, and the effects of the layered structure can be identified by a structure factor β, defined as the ratio of SH intensity from such a structure to that from a freely suspended 2D material. Our results show that the influence of a layered structure on the SHG intensity arises from interference effects of both the fundamental light and the SH light; the value of the structure factor is 0 ≤ β ≤ 64. Furthermore, when the incident light is pulsed, the interference effects are partially canceled due to the existence of many wave vectors and frequencies, and the cancellation becomes severe for thick films, small beam spots, and short pulses. For a specific structure of 2D material/dielectric film/substrate, the thickness of the dielectric film can effectively tune the value of β in an interval [βmin, βmax], and detailed discussions are performed for the thicknesses when these two extreme values can be obtained. When there is optical loss or the substrate is not perfectly reflective, the extreme value of βmax or βmin cannot reach 64 or 0. A large βmax requires two conditions to be fulfilled: (1) the substrate should be highly reflective, and (2) the refractive indices of the dielectric film at the fundamental and the SH frequencies should differ. Our results indicate how practical substrate structures can be used to achieve high SH signals, and the simple expression we give for the SH enhancement will be useful in characterizing the nonlinear susceptibility of 2D materials.

Although tightly focused input beams for efficient second harmonic (SH) generation (SHG) are usually avoided because of material damage, this is exactly the condition needed for the generation of spatial solitons (QSS) based on quadratic (χ(2)) nonlinearities.[1,2] These QSS’es are mutually self-trapped fundamental (FW) and harmonic (SH) optical beams in which the mutual beam narrowing due to photon exchange incipient in up and down conversion balances diffraction. The generation of quadratic solitons significantly affects the SHG process, for example by placing an upper limit on the amount of fundamental that can be converted to SH, by increasing the bandwidth of the process etc.[3] Of particular interest are periodically poled ferroelectric media which are an important limit of non-critical phase-matching. In fact, periodically poled LiNbO3 (PPLN) has been shown to have the lowest soliton threshold of 1.5 GW/cm2 reported to date [4]. That pioneering work established the existence of quadratic solitons in periodically poled media but neither this work, or any other work on QSSes has addressed the impact of QSS on the SHG tuning curves.

We theoretically study the gain-assisted double plasmonic resonances to enhance second harmonic generation (SHG) in a centrosymmetric multilayered silver-dielectric-gold-dielectric (SDGD) nanostructure. Introducing gain media into the dielectric layers can not only compensate the dissipation and lead to giant amplification of surface plasmons (SPs), but also excite local quadrupolar plasmon which can boost SHG by mode matching. Specifically, as the quadrupolar mode dominates SHG in our nanostructure, under the mode matching condition, the intensity of second harmonic near-field can be enhanced by 4.43 × 102 and 1.21 × 105 times when the super-resonance is matched only at the second harmonic (SH) frequency or fundamental frequency, respectively. Moreover, the intensity of SHG near-field is enhanced by as high as 6.55 × 107 times when the nanostructure is tuned to double super-resonances at both fundamental and SH frequencies. The findings in this work have potential applications in the design of nanosensors and nanolasers.

Depending on the chosen fabrication process, nonlinear waveguides realized in periodically oriented material such as periodically poled lithium niobate (PPLN) can present different fabrication errors. In this paper, we present a detailed numerical study of the impact on the nonlinear performances of one or several stitching errors occurring during the realization of the periodic domains by e-beam writing. This study shows that contrary to what was expected, a single finite stitching error does not simply decrease the nonlinear efficiency but splits the second harmonic (SH) signal into a double peak spectrum, where the position of the peaks and their width at half-maximum depend not only on the poling period, the total length of the grating, and the waveguide parameters, but also on the amplitude and the position of the defect. The numerical results are confirmed by second harmonic generation (SHG) experiments performed in PPLN waveguides obtained by Soft Proton Exchange and where the nonlinear grating was composed of one to four e-beam-written 1.5-mm-long PPLN sections between which important stitching errors can occur. In this case the spectrum can be quite complicated. It is worth noting that similar errors (domains merging or missing domains) can occur also in the e-field poling process but they are randomly distributed along the propagation path. Therefore, this study is a first attempt to take them into account to explain experimental results and indicate the steps that have to be taken to improve the quality of the components.

We have found that the resonance energy of optical second-harmonic (SH) generation from Au films on NaCl(100) depends strongly on the film thickness. The resonance peak of the SH intensity was found at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ for the island film of mean thickness 0.4 nm, while it appeared at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the continuous film of thickness 40 nm. We suggest that the SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ originates from a resonant transition from occupied to unoccupied electronic levels of the (100) surface on top of the Au islands. The SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the film thickness of 40 nm originates from the roughness-induced linear dielectric property of the film.

This dissertation presents detailed experimental and theoretical investigations of nonlinear and nonreciprocal effects in magnetic garnet films. The dissertation thus comprises two major sections. The first section concentrates on the study of a new class of nonlinear magneto-optic thin film materials possessing strong higher order magnetic susceptibility for nonlinear optical applications. The focus was on enlarging the nonlinear performance of ferrite garnet films by strain generation and compositional gradients in the sputter-deposition growth of these films. Under this project several bismuth-substituted yttrium iron garnet (Bi,Y) 3 (Fe,Ga)5 O12(acronym as Bi:YIG) films have been sputter-deposited over gadolinium gallium garnet (Gd 3 Ga5 O12 ) substrates and characterized for their nonlinear optical response. One of the important findings of this work is that lattice mismatch strain drives the second harmonic (SH) signal in the Bi:YIG films, in agreement with theoretical predictions; whereas micro-strain was found not to correlate significantly with SH signal at the micro-strain levels present in these films. This study also elaborates on the role of the film's constitutive elements and their concentration gradients in nonlinear response of the films. Ultrahigh sensitivity delivered by second harmonic generation provides a new exciting tool for studying magnetized surfaces and buried interfaces, making this work important from both a fundamental and application point of view. The second part of the dissertation addresses an important technological need; namely the development of an on-chip optical isolator for use in photonic integrated circuits. It is based on two related novel effects, nonreciprocal and unidirectional optical Bloch oscillations (BOs), recently proposed and developed by Professor Miguel Levy and myself. This dissertation work has established a comprehensive theoretical background for the implementation of these effects in magneto-optic waveguide arrays. The model systems we developed consist of photonic lattices in the form of one-dimensional waveguide arrays where an optical force is introduced into the array through geometrical design turning the beam sideways. Laterally displaced photons are periodically returned to a central guide by photonic crystal action. The effect leads to a novel oscillatory optical phenomenon that can be magnetically controlled and rendered unidirectional. An on-chip optical isolator was designed based on the unidirectionality of the magneto-opticBloch oscillatory motion. The proposed device delivers an isolation ratio as high as 36 dB that remains above 30 dB in a 0.7 nm wavelength bandwidth, at the telecommunication wavelength 1.55 μm. Slight modifications in isolator design allow one to achieve an even more

The visibility of inverted domain structures using the second harmonic generation (SHG) microscope is discussed based upon the three-wave coupling equations. Without reference second harmonic (SH) waves (non-interference case), the SHG intensity at a domain boundary decreases steeply along the perpendicular direction to the domain wall, which is observed as a dark line. Thus the non-interference SHG microscope reveals the location of domain boundaries but not the domain polarity which the SHG interference microscope can do. In the case of periodically poled domain structures, the clear observation criterion is expressed with the wave-number offset between the fundamental and SH waves.

Based on vector diffraction theory, an analytical integral representation to calculate the field gradient of focused electromagnetic fields is derived by using the differential recursion formula between the Bessel functions with different order. Within the phenomenological model for second harmonic generation (SHG) of centrosymmetric material, the second harmonic (SH) response of a single centrosymmetric spherical particle excited by a focused beam under different values of the NA is investigated theoretically. The results show that, with increasing NA, the SH radiation pattern of the surface response hardly changes. For the larger value of the NA, because the elements of the field gradient are almost the same order of magnitude and the relative magnitudes of Ey and Ez increase, the bulk response related to the parameters δ′ and ζ become remarkable.

Second-harmonic (SH) generation has been applied to study the interfaces of short-period (GenSim)p strained layer superlattices. From the SH signal and its symmetry, we have concluded that (i) over an area of optical wavelength size, the superlattices were grown with nonuniform layer thickness in each period; (ii) along the growth direction, the atomic structure of Si-on-Ge and Ge-on-Si interfaces are not symmetric; and (iii) a structural change of the interfaces, possibly from the increase of misfit dislocation or other defect density, occurs when the thickness of the strained Ge layer in each period exceeds about six layers. The SH spectroscopic results showed defects contributed dominantly for (Ge7Si8)4, but substantial contribution from Si–Ge bonds for (Ge4Si4)3 and (Ge5Si5)3 still exists.

It has been found that a high electric field is created at the interface between copper-tetratert-butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) film/metal due to the exchanged charges. The symmetry of the CuttbPc molecule is broken by this space charge induced electric field (SCIEF), and optical second harmonic (SH) signal is generated from the CuttbPc LB film/metal interface. A theoretical expression of the reflected SH signal intensity is derived using a four-layer model with the SCIEF being treated as a mean field. The origins of the SHG are considered as the quadrupole effect and the electric field induced SHG (EFISHG) due to the SCIEF. On the basis of experimental results and the theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the wavelength of incident fundamental light around 1100 nm the electric quadrupole effect makes a main contribution to the SHG from the interface, while at the wavelength around 1200 nm it is attributed to the EFISHG. It is expected that the SCIEF could be determined by SHG.