Second-harmonic Generation Radiation Research Articles

Bond model of second harmonic generation in tetragonal and orthorhombic perovskite structures

We apply the simplified-bond-hyperpolarizability model (SBHM) to investigate second harmonic generation (SHG) from tetragonal and orthorhombic methylammonium (MA) lead (Pb) iodide (I) perovskite structures. Assuming that the SHG radiation comes from the anharmonic charges parallel to the Pb-I covalent bond vectors, we show explicitly how the reflective rotational anysotropy SHG (RASHG) intensity profiles arise via dipole superposition from the perovskite surface and that their bulk contribution is zero for both the tetragonal and orthorhombic bulk structures if the contribution from MA molecules can be neglected. In addition, we compare the third order susceptibility tensor obtained from group theory and SBHM and calculate the values for the nonzero components as well as derive the SHG intensity field formulas. We found that the tetragonal and orthorhombic perovskite SHG intensity profiles are different. Furthermore, our results show that even in relative complex systems such as metal halide perovskites (MHPs), RASHG experimental data can be fitted using only one independent fitting parameter, which is the effective Pb-I nonlinear hyperpolarizability. Therefore, the analysis of perovskite using an anharmonic bond model can lead to better efficiencies of photovoltaic devices, while it will also pave a way for the use of SHG in MHP crystals as frequency conversion and optical signal processing.

Polarization Engineered Second Harmonic Generation Enhancement from Amorphous Silicon - Gallium Nitride Heterogeneous Resonant Metasurface

We experimentally demonstrate second harmonic generation (SHG) enhancement from a heterogeneous resonant metasurface consisting of amorphous silicon sub-wavelength gratings supporting resonances within an underlying gallium nitride (GaN) layer. Electromagnetic design studies used to choose the sub-wavelength grating dimensions, corresponding spectral resonances, its angular sensitivity, electric field and non-linear polarization profiles are discussed. The efficient excitation of second-order nonlinearities within the GaN layer requiring longitudinal fundamental field component and efficient extraction of on-axis SHG using in-plane nonlinear polarization components are simultaneously satisfied for fundamental excitation polarized at 45^o, resulting in excitation of TE and TM-like resonances. Experimental studies on the fabricated structures characterizing the linear and nonlinear SHG properties including power, polarization, and spectral dependence are also discussed. The SHG spectral measurements show 1300-times enhancement for 1460 nm fundamental excitation. The SHG enhancement observed for the higher order resonance at 1460 nm is attributed to the trade-off between the increase in the in-plane to out-of-plane nonlinear polarization ratio, resulting in relative increase in on-axis SHG radiation and the decrease in quality factor of the resonances, which lowers the resonance field strength with increasing guided-mode order.

Efficient second-harmonic and terahertz generation from single BiB3O6crystal using nanosecond and femtosecond lasers.

The paper reports the efficient UV and terahertz generation from a 1.29 mm thick and Type I, θ=28.9∘ cut BiB3O6 (bismuth triborate, BIBO) crystal using femtosecond and nanoseconds laser pulses. We have employed 800 nm wavelength pulses of 50 and 140 fs obtained from a Ti:sapphire laser amplifier and oscillators at 1 kHz and 80 MHz repetition rates, respectively. The conversion efficiency of second-harmonic generation (SHG) was ∼50% while that obtained for terahertz (THz) generations was of the order of 1.85×10-5%. In addition, LDS-698 dye laser radiation tunable between 650-700 nm was also used as a source for SHG between the 325-350 nm range. The dye laser was pumped by SHG (532 nm) radiation from an electro-optically Q-switched Nd:YAG laser having a pulse repetition rate of 10 Hz and a pulse width of 10 ns. A conversion efficiency of 4.01% was obtained for generation of UV at 343.5 nm. Finally, we have measured the transmission, refractive index, absorbance, and conductivity properties of BIBO crystal in the THz domain. We also ascertained the coherence length, relative permittivity and reflectivity of the crystal.

Inch-Scale Ball-in-Bowl Plasmonic Nanostructure Arrays for Polarization-Independent Second-Harmonic Generation.

Second-harmonic generation (SHG) in plasmonic nanostructures has been investigated for decades due to their wide applications in photonic circuit, quantum optics and biosensing. Development of large-scale, uniform, and efficient plasmonic nanostructure system with tunable modes is desirable for their feasible utilizations. Herein, we design an efficient inch-scale SHG source by a solution-processed method instead of traditional high-cost processes. By assembling the gold nanoparticles with the porous anodic alumina templates, multiresonance in both visible and near-infrared regions can be achieved in hexagonal plasmonic nanostructure arrays, which provide strong electric field enhancement at the gap region. Polarization-independence SHG radiation has been realized owing to the in-plane isotropic characteristic of assembled unit. The tilt-angle dependent and angle-resolved measurement showed that wide-angle nonlinear response is achieved in our device because of the gap geometry of ball-in-bowl nanostructure with nonlinear emission electric dipoles distributed on the concave surface, which makes it competitive in practical applications. Our progress not only makes it possible to produce uniform inch-scale nonlinear arrays through low-cost solution process; and also advances the understanding of the SHG radiation in plasmonic nanostructures.

Multipolar analysis of second-harmonic generation in (111) Gallium Arsenide nanoparticles

We perform multipolar analysis of second-harmonic generation (SHG) from (111)-grown gallium arsenide (GaAs) nanoantennas and discuss its specifics. It was experimentally demonstrated that the conversion efficiency in axially-symmetric (111) GaAs nanoparticles remains constant under the polarization rotation of normally incident radiation in a wide range of particle sizes, while the SHG radiation pattern changes. We apply the analytical method based on the Lorentz lemma to explain this behaviour. The induced nonlinear current is decomposed into two rotating contributions, which are shown to generate multipoles of different parities. Thus, the total SHG intensity in the far-field is proved to be independent of the in-plane rotation of the pump polarization. Nevertheless, due to the threefold symmetry of the crystal with regard to the (111) direction, the SHG radiation pattern rotates around the polar axis repeating its shape every 60°.

Modes interplay and dynamics in the second harmonic generation of plasmonic nanostructures.

The full wave surface integral equation computation of the second harmonic generation (SHG) dynamics for metal spheres and nanorods - presented as multimedia files - is performed to reveal the dynamics of the modes supported by the nanostructure. We demonstrate that the interplay between different modes controls the nonlinear response and that the size-induced redshift of the eigenmodes can be manipulated by adjusting the nanostructure geometry, so that the SHG signal can be boosted at specified frequencies. We show that the SHG radiation is not necessarily quadrupolar in spherical nanoparticles, as it is often assumed. Finally, we introduce an efficient way to reduce the SHG calculation time.

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Bond model of second-harmonic generation in wurtzite ZnO(0002) structures with twin boundaries

We describe second-harmonic generation (SHG) in ZnO(0002) wurtzite structures with [11¯00]/(1102) twin boundaries using the simplified bond hyperpolarizability model (SBHM). We show explicitly how the reflective second-harmonic generation (RSHG) intensity profile for the s-incoming fundamental and s-outgoing SHG polarized light arise via superposition of the SHG dipole fields. The nonlinear fields originate from anharmonic motions of electric charges along each bond. The total dipole fields from within the ZnO bulk sum up to zero but produce a constructive SHG radiation from the unreduced surface and twin boundary. In addition, we compare the third-order susceptibility tensor obtained from group theory and the SBHM and calculate the values for the nonzero components. We found that the off-resonance RSHG intensity data in diatomic wurtzite structures even with the presence of twin boundaries can be modeled using only three independent fitting parameters, namely, the effective bulk, reduced surface, and twin boundary SHG hyperpolarizability. The results show that the SBHM can be used to investigate impurities and surface defects in ZnO as well as their contribution to nonlinear radiation with potential application to frequency conversion in nanoscale optical circuitry.

Thermal dephasing in an enhanced cavity based high-power second harmonic generation.

We report on the performance of an enhanced-cavity (EC) designed for obtaining high-power and efficiency second-harmonic generation (SHG). This is performed by numerical simulation of SHG coupled equations in the presence of a thermal dephasing effect that is effectively intensified through embedding the periodically poled LiNbO3 crystal by an oven-surrounded scheme. It is found that by setting the PPLN temperature at an optimum value, adjusting the mirror reflectively, and pumping power at certain values, gaining SHG efficiency of more than 90% is possible. We further realized that by an ECSHG device SHG efficiency can be improved by about 15%-50%. Moreover, compared to a single-pass SHG scheme, the EC-based SHG device is shown to be a very promising candidate to reduce and suppress the effect of thermal dephasing on the stability and efficiency of SHG radiation.

Enhanced second harmonic generation from a plasmonic Fano structure subjected to an azimuthally polarized light beam

We show that an azimuthally polarized beam (APB) excitation of a plasmonic Fano structure made by coupling a split-ring resonator (SRR) to a nanoarc can enhance second harmonic generation (SHG). Strikingly, an almost 30 times enhancement in SHG peak intensity can be achieved when the excitation is switched from a linearly polarized beam (LPB) to an APB. We attribute this significant enhancement of SHG to the corresponding increase in the local field intensity at the fundamental frequency of SHG, resulting from the improved conversion efficiency between the APB excitation and the plasmonic modes of the Fano structure. We also show that unlike LPB, APB excitation creates a symmetric SHG radiation pattern. This effect can be understood by considering an interference model in which the APB can change the total SHG far-field radiation by modifying the amplitudes and phases of two waves originating from the individual SRR and nanoarc of the Fano structure.

Snapshot second-harmonic generation polarimeter based on spectral analysis.

A new high-speed second-harmonic generation (SHG) polarimetric method is reported. It is based on the spectral analysis of the SHG radiation emitted by a nonlinear medium excited with circularly polarized femtosecond laser pulses. The setup uses only passive components for polarization encoding and a fast spectrometer for spectral analysis. The method is validated on a z-cut quartz plate, then on a collagen-rich biological tissue with a view to single image polarization-resolved SHG microscopy.

Polarization-resolved second harmonic generation microscopy of chiral G-shaped metamaterials

Chiral planar metamaterials are known for their possibility to show strong nonlinear optical effects such as second harmonic generation (SHG) circular dichroism or asymmetric SHG. The underlying mechanisms are commonly discussed in terms of local field effects and formation of localized SHG sources (so called ``hotspots'') that are sensitive to the shape and size of meta-atoms. Nevertheless, a full characterization of the polarization state of the nonlinear optical radiation from the hotspots has not been performed until now. Here we present the results of the polarization-resolved second harmonic generation microscopy studies of planar chiral G-shaped metamaterials. We demonstrate that the SHG radiation coming from the hotspots that are localized within a single meta-atom is partially polarized; moreover, the SHG polarization state reveals the chirality of the structure. The observed effects are attributed to the induced plasmonic current oscillations at the fundamental frequency along with the local field distribution.

The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas.

Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

Direct observation of bulk second-harmonic generation inside a glass slide with tightly focused optical fields

Bulk second-harmonic generation (SHG) inside glass slides is directly detected unambiguously without interference from surface contributions. This is enabled by tightly focused and highly localized ultrashort laser pulses. The theoretical calculations based on vector diffraction theory and the phenomenological model of SHG inside centrosymmetric materials agree well with the measured far-field SHG radiation patterns for different polarization states of the fundamental beam. The results indicate that the observed bulk SHG is predominantly related to the bulk parameter ${\ensuremath{\delta}}^{\ensuremath{'}}$ and originates from the three-dimensional field gradient in the focal region.

Optimization of nonlinear crystal location for intracavity second-harmonic generation

We discuss optimization of locations for the nonlinear crystal in intracavity generation of second-harmonic radiation and show that optimizing the distance between the nonlinear crystal and the two-wave end mirror in the laser cavity may support nearly a 100% increase in the maximum emission power of the second harmonic. The optimum distance depends on the type of end mirror used. We describe a variety of effects that mask the advantages conferred by the optimum location of the nonlinear crystal when performing experiments on intracavity generation of strong second-harmonic laser radiation. We describe various approaches for experimental determination of the optimal location of the nonlinear crystal in the cavity.

Nonlinear conversion in optical waveguide filled with NaNO2

This paper contains the first experimental results of nonlinear conversion in optical waveguide filled with solid nonlinear media. The experimental sample was made from SiO2 fiber of the 1.6 cm length. During the pumping of the sample by the pulse laser, the generation of second harmonic radiation was observed at the whole length of the sample filled with NaNO2. Despite the visibility of this effect, the detected signal was rather weak due to the scattering inside the fiber. Nevertheless, the successful experiment presented in this work may lead to the future development of new nonlinear devices using photonic crystal fiber filled with solid nonlinear medium.

Selectively addressing optically nonlinear nanocrystals by polarization-shaped ultrafast laser pulses

A noncentrosymmetric nanocrystal can produce second-harmonic generation (SHG) and sum-frequency generation (SFG) upon interaction with a laser field. The SHG or SFG radiation depends on the orientation of the nanocrystal as well as the field polarization, which allows for modulating the second-order emission of an arbitrarily oriented nanocrystal by specially tailoring the field polarization. We theoretically study SHG and SFG signals produced by nanocrystals driven with broad-bandwidth laser pulses. Several simulations explore the influence of the field polarization and temporal pulse profile. The latter two factors are decoupled in their influence upon the SHG and SFG signals, and thus polarization and temporal shaping can be independently performed to modulate a nanocrystals’ second-order emission. We consider the possibility of enhancing (suppressing) the signal from one nanocrystal among others by choosing the appropriate polarization, thereby opening up the prospect of selectively addressing optically nonlinear nanocrystals.

Simultaneous determination of the second-harmonic generation emission directionality and reduced scattering coefficient from three-dimensional imaging of thick tissues

Second-harmonic generation (SHG) microscopy has intrinsic contrast for imaging fibrillar collagen and has shown great promise for disease characterization and diagnostics. In addition to morphology, additional information is achievable as the initially emitted SHG radiation directionality is related to subresolution fibril size and distribution. We show that by two parameter fittings, both the emission pattern (FSHG/BSHG)creation and the reduced scattering coefficient μs', can be obtained from the best fits between three-dimensional experimental data and Monte Carlo simulations. The improved simulation framework accounts for collection apertures for the detected forward and backward components. We apply the new simulation framework to mouse tail tendon for validation and show that the spectral slope of μs' obtained is similar to that from bulk optical measurements and that the (FSHG/BSHG)creation values are also similar to previous results. Additionally, we find that the SHG emission becomes increasingly forward directed at longer wavelengths, which is consistent with decreased dispersion in refractive index between the laser and SHG wavelengths. As both the spectral slope of μs' and (FSHG/BSHG)creation have been linked to the underlying tissue structure, simultaneously obtaining these parameters on a microscope platform from the same tissue provides a powerful method for tissue characterization.

Generation of second harmonic radiation from sub-stoichiometric silicon nitride thin films

Enhancing second-order optical processes in Si-compatible materials is important for the demonstration of innovative functionalities and nonlinear optical devices integrated on a chip. Here, we demonstrate significantly enhanced Second-Harmonic Generation (SHG) by silicon-rich silicon nitride materials over a broad spectral range, and show a maximum conversion efficiency of 4.5 × 10−6 for sub-stoichiometric samples with 46 at. % silicon. The SHG process in silicon nitride thin films is systematically investigated over a range of material stoichiometry and thermal annealing conditions. These findings can enable the engineering of innovative Si-based devices for nonlinear signal processing and sensing applications on a Si platform.

Label-free second-harmonic phase imaging of biological specimen by digital holographic microscopy

We have previously developed a new way for nonscanning second-harmonic generation (SHG) microscopy [Opt. Lett. 34, 2450 (2009)]. Based on digital holography, this technique captures, in single-shot hologram acquisition, both the amplitude and the phase of a coherent SHG radiation, which makes possible second harmonic phase microscopy. In this work, we present holographic SHG phase microscopy of a label-free biological tissue and discuss its added value to SHG microscopy.