Second-harmonic Enhancement Research Articles

Second-Harmonic Enhancement from a Nonlinear Plasmonic Metasurface Coupled to an Optical Waveguide.

Metasurfaces are commonly constructed from two-dimensional arrangements of nanoresonators. Coherent coupling of the nanoresonators through extended photonic modes of the metasurface results in a modified collective optical response, and enhances light–matter interactions. Here we experimentally demonstrate that strong collective resonances can arise also from coupling the metasurface to an optical waveguide. We explore the effect this waveguide-assisted collective interaction has on second-harmonic generation from the hybrid system. Our measurements indicate an enhancement factor of 8 for the transmitted second harmonic in comparison to incoherent collective scattering. In addition, complementary simulations predict about a 100-fold enhancement for the second harmonic that remains confined inside the waveguide. The ability to control the hybrid modes by the waveguide’s design provides broader control over the formation of the collective interaction and new tools to tailor the nonlinear interactions. Our findings pave a promising direction to realize nonlinear photonic circuits with metasurfaces.

Structural second-order nonlinearity in plasmonic metamaterials

Nonlinear processes are at the core of many optical technologies whose further development require optimized materials suitable for nanoscale integration. Here we demonstrate the emergence of a strong bulk second-order nonlinear response in a plasmonic nanorod composite comprised of centrosymmetric materials. We develop an effective-medium description of the underlying physics, compare its predictions to the experimental results, and analyze the limits of its applicability. We demonstrate strong tunable generation of the p-polarized second-harmonic light in response to either s- or p-polarized excitation. High second-harmonic enhancement is observed for fundamental frequencies in the epsilon-near-zero spectral range. The work demonstrates emergence of structurally tunable nonlinear optical response in plasmonic composites and presents a new nonlinear optical platform suitable for integrated nonlinear photonics.

Mixing of spin and orbital angular momenta via second-harmonic generation in plasmonic and dielectric chiral nanostructures

We present a theoretical study of the characteristics of the nonlinear spin-orbital angular momentum coupling induced by second-harmonic generation in plasmonic and dielectric nanostructures made of centrosymmetric materials. In particular, the connection between the phase singularities and polarization helicities in the longitudinal components of the fundamental and second-harmonic optical fields and the scatterer symmetry properties are discussed. By in-depth comparison between the interaction of structured optical beams with plasmonic and dielectric nanostructures, we have found that all-dielectric and plasmonic nanostructures that exhibit magnetic and electric resonances have comparable second-harmonic conversion efficiency. In addition, mechanisms for second-harmonic enhancement for single and chiral clusters of scatterers are unveiled and the relationships between the content of optical angular momentum of the incident optical beams and the enhancement of nonlinear light scattering is discussed. In particular, we formulate a general angular momenta conservation law for the nonlinear spin-orbital angular momentum interaction, which includes the quasi-angular-momentum of chiral structures with different-order rotational symmetry. As a key conclusion of our study relevant to nanophotonics, we argue that all-dielectric nanostructures provide a more suitable platform to investigate experimentally the nonlinear interaction between spin and orbital angular momenta, as compared to plasmonic ones, chiefly due to their narrower resonance peaks, lower intrinsic losses, and higher sustainable optical power.

Second-Harmonic Enhancement with Mie Resonances in Perovskite Nanoparticles

Second-harmonic generation (SHG) in nanostructures gives rise to many applications such as lab-on-a-chip and imaging by frequency doubling. However, the SHG signal decreases with volume, and the conversion efficiency is limited. Thus, means to enhance nonlinear signals at the nanoscale are needed. For instance, while plasmonic nanostructures offer a high enhancement due to the strong confinement of the electromagnetic field, they have high losses and the fabrication methods are difficult. In this work, we propose to enhance the SHG by using the intrinsic scattering properties of an all-dielectric perovskite nanostructure. We demonstrate the Mie scattering resonances of individual barium titanate (BaTiO3) nanoparticles with diameters between 200 and 250 nm. We distinguish contributions of the magnetic dipole and magnetic quadrupole. Then, we use the Mie resonances to achieve an SHG enhancement of 4 orders of magnitude within the same nanoparticle. Our results suggest that a strong increase of the SHG signa...

High-efficiency intracavity second-harmonic enhancement for a few-cycle laser pulse train

This paper presents an intracavity second-harmonic (SH) enhancement technology without the need of input impedance-matching for optimal coupling between the cavity and its input frequency comb. More than 10% SH energy conversion efficiency is available, thus the power of the SH frequency comb can be enhanced beyond 100 relative to single-pass SH generation. Compared with a conventional passive enhancing cavity, for the purpose of high power enhancement, our scheme can operate at much lower finesse and thus broader bandwidth so that it can support several-optical-cycle pulses more easily. If they have the same finesse, both methods perform with similar operating stability. The results show that our novel design is suitable for some applications which need a short wavelength, high intensity, and ultra-broad bandwidth pulse train.

Microscopy of localized second-harmonic enhancement in random metal nanostructures

Using second-harmonic (SH) scanning optical microscopy in reflection we study the effect of localized SH enhancement in random metal nanostructures. With a tightly focused tunable (750--830 nm) laser beam, we obtain SH images of a 55-nm-thick gold film surface covered with randomly distributed 70-nm-high gold bumps showing small $(\ensuremath{\sim}0.7\ensuremath{\mu}\mathrm{m})$ and very bright $(\ensuremath{\sim}{10}^{3}$ times the background) spots. Wavelength and polarization dependencies of both positions and intensities of these SH bright spots as well as their statistics are investigated and compared for two areas having different nominal densities of scatterers, i.e., 25 and $50\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}.$ For relatively large signals, it is found that, the probability density function of the SH signal follows the power-law dependence with the index being in the range of 2.5--3 for both values of the scattering density. We observe that, for incident laser powers in the range 3--40 mW, the SH bright spots exhibit, as expected, quadratic dependencies of the maximum signal on the incident power. However, for higher power levels, the interrogating laser beam causes nonlocal modifications of SH images, i.e., some SH bright spots disappear and others emerge even outside of the area that was exposed to a high power radiation. We relate the observed feature to surface plasmon polariton contribution in the process of multiple scattering, resulting in the formation of resonant eigenmodes (at fundamental and SH frequency) whose excitation in turn gives rise to the SH bright spots.

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.

Direct observation of localized second-harmonic enhancement in random metal nanostructures.

Second harmonic (SH) scanning optical microscopy in reflection is used to image the gold film surface covered with randomly placed scatterers. SH images obtained with a tightly focused tunable (750-830 nm) laser beam show small (approximately 0.7 microm) and very bright (approximately 10(3) times the background) spots, whose locations depend on the wavelength and polarization of light. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to the overlap of FH and SH eigenmodes. The probability density function of the SH signal is found to follow the power-law dependence.

Second-harmonic generation from ellipsoidal silver nanoparticles embedded in silica glass.

Second-harmonic generation of uniformly oriented, ellipsoidal silver nanoparticles in a glass matrix was observed and investigated as a function of incidence angle, light polarization, and spatial arrangement of the particles. The results can be explained by the symmetry of the spatial nanoparticle arrangement and by resonance enhancement that is due to the localized surface plasmons of the particles. Second-harmonic enhancement is observed only in sufficiently thin layers (deltakl < pi); on a sample with two separate layers, strong modulation owing to quasi-phase matching is obtained.

Second-harmonic enhancement using a self-locked 0.63-µ He-Ne laser

Relative second-harmonic power generated in a non-linear crystal has been measured for a multimode He-Ne 0.63-μ laser source operating unlocked and in two different types of spontaneous locking. For the same average laser power the harmonic power was 2.15 times larger for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/2L</tex> pulsing than for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/L</tex> pulsing, in good agreement with a predicted value of 2.18. The harmonic enhancement for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/2L</tex> pulsing compared with unlocked operation was 3.31, considerably lower than the analytic estimate of 4.2, indicating that significant interaction was present between the modes of the unlocked laser.

Internal modulation of ruby lasers and second-harmonic enhancement

Internal modulation of ruby lasers and second-harmonic enhancement