Second Harmonic Generation Experiments Research Articles

Near-Unity Polarization of Valley-Dependent Second-Harmonic Generation in Stacked TMDC Layers and Heterostructures at Room Temperature.

With unique valley-dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second-harmonic generation (SHG) in 2D TMDCs monolayers has shown valley-dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion-symmetry-breaking (3R-like phase) TMDCs (WSe2 , WS2 , MoS2 ) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular-polarization-resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.

Symmetry breaking of in-plane anisotropic magnetoresistance with temperature in La-doped Sr2IrO4 films

Electron-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ has been the subject of much recent attention because of its similarities with hole-doped $\mathrm{L}{\mathrm{a}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$. However, direct evidence of its superconductivity has not yet been observed, which has led to various unresolved issues related to this material. In this study, a 14-nm-thick La-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ film was grown on a (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate. We found that the fourfold component of the in-plane anisotropic magnetoresistance disappears and is replaced by a twofold symmetry as the temperature increases. Variable-temperature x-ray diffraction and spatially resolved optical second-harmonic generation experiments eliminate the possibility of crystal structural distortion. The temperature dependence of direct current magnetization measurements indicates that a change in the spin structure may account for the symmetry breaking, similar to the nematic ordering of iron-based and other superconductors. Remarkably, the transition temperature of the symmetry breaking is well consistent with the previously reported onset (50 K) of the low-energy gap in K-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ observed by scanning tunneling spectroscopy. Our results provide a perspective on these materials that helps elucidate the nature of magnetic interactions and explores the expected superconductivity of iridates.

Ultrasensitive Three-Dimensional Orientation Imaging of Single Molecules on Plasmonic Nanohole Arrays Using Second Harmonic Generation.

Recently, fluorescence-based super-resolution techniques such as stimulated emission depletion (STED) and stochastic optical reconstruction microscopy (STORM) have been developed to achieve near molecular-scale resolution. However, such a super-resolution technique for nonlinear label-free microscopy based on second harmonic generation (SHG) is lacking. Since SHG is label-free and does not involve real-energy level transitions, fluorescence-based super-resolution techniques such as STED cannot be applied to improve the resolution. In addition, due to the coherent and non-isotropic emission nature of SHG, single-molecule localization techniques based on isotropic emission of fluorescent molecule such as STORM will not be appropriate. Single molecule SHG microscopy is largely hindered due to the very weak nonlinear optical scattering cross sections of SHG scattering processes. Thus, enhancing SHG using plasmonic nanostructures and nanoantennas has recently gained much attention owing to the potential of various nanoscale geometries to tightly confine electromagnetic fields into small volumes. This confinement provides substantial enhancement of electromagnetic field in nanoscale regions of interest, which can significantly boost the nonlinear signal produced by molecules located in the plasmonic hotspots. However, to date, plasmon-enhanced SHG has been primarily applied for the measurement of bulk properties of the materials/molecules, and single molecule SHG imaging along with its orientation information has not been realized yet. Herein, we achieved simultaneous visualization and three-dimensional (3D) orientation imaging of individual rhodamine 6G (R6G) molecules in the presence of plasmonic silver nanohole arrays. SHG and two-photon fluorescence microscopy experiments together with finite-difference time-domain (FDTD) simulations revealed a ∼106-fold nonlinear enhancement factor at the hot spots on the plasmonic silver nanohole substrate, enabling detection of single molecules using SHG. The position and 3D orientation of R6G molecules were determined using the template matching algorithm by comparing the experimental data with the calculated dipole emission images. These findings could enable SHG-based single molecule detection and orientation imaging of molecules which could lead to a wide range of applications from nanophotonics to super-resolution SHG imaging of biological cells and tissues.

Highly efficient femtosecond second-harmonic generation from Yb:CaF2-regenerative amplifier

We develop a diode-pumped Yb:CaF2-regenerative amplifier with 1 mJ/220 fs centered at 1.04 μm. Applying a KH2PO4 (KDP) crystal, we execute second-harmonic generation (SHG) experiment and a maximum conversion efficiency of 74% is achieved. To the best of our knowledge, this is the highest SHG efficiency, yet achieved with ~ 200 fs fundamental pulses at a wavelength of 1 μm.

Linear and nonlinear absorption, SHG and photobleaching behaviors of Dy doped GaSe single crystal

The linear absorption, nonlinear absorption (NA), second harmonic generation (SHG) and carrier mobilities of undoped and 0.1 at% Dy doped GaSe single crystals grown by modified Bridgman method have been studied by UV–vis, open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments. Both linear absorption and SHG measurements clearly showed that the doping of GaSe crystal with Dy3+ leads to a shift in the linear absorption edge. The mechanisms contributing to the NA behavior were explained and discussed. The photobleaching behaviors of the crystals were investigated by delaying of the probe signal in transmission pump-probe spectroscopy experiments.

Temperature bandwidth of second-harmonic-generation in GdCOB crystal.

The temperature bandwidth of the second harmonic generation (SHG) phase-matching process was investigated for the GdCa4O(BO3)3 (GdCOB) crystal. GdCOB exhibits a much broader temperature bandwidth in comparison with many familiar nonlinear optical (NLO) crystals. For a fundamental wave of 1,064nm, the maximum temperature bandwidth appeared at (θ=135°, ϕ=47.3°), as predicted by the theoretical calculation and demonstrated by the SHG experiments. The GdCOB crystal is a good candidate for NLO frequency conversions under extreme temperatures.

Dynamical Behavior and Second Harmonic Generation Responses in Acido-Triggered Molecular Switches

Second-order nonlinear optical molecular switches are systems displaying marked variations of their second harmonic generation (SHG) responses upon external stimulation. In this article, we combine a multiscale computational method and experimental characterizations to provide a full description of the SHG responses of molecular switches built from the association of the indolino-oxazolidine unit to a bithiophene donor. In chloroform solutions, the addition of trifluoroacetic acid triggers the switching from a neutral closed form to a protonated open form, making an ion pair with the trifluoroacetate counterion and induces a strong enhancement of the SHG responses. The numerical simulations (i) evidence how the large and rapid thermally induced geometrical fluctuations lead to broadening the SHG responses distributions, making even difficult the determination of their sign, (ii) rationalize the variations of these responses as a function of the closure/opening of the oxazolidine ring and of the nature of its chemical substitution, and (iii) call into question common assumptions employed when analyzing the experimental SHG responses.

Nonlinear absorption, SHG behavior and carrier dynamics of Nd and Pr doped GaSe single crystals

Undoped, 0.1 at% Nd and 0.1 at% Pr doped GaSe single crystals were grown by using modified Bridgman method. The nonlinear absorption (NA), second harmonic generation (SHG) and carrier dynamics of undoped, 0.1 at% Nd and 0.1 at% Pr doped GaSe single crystals have been studied by open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments, respectively. All of the studied crystals showed nonlinear absorption under 100 fs pulse duration and 1200 nm wavelength excitations. It was found that the doping of Nd3+ and Pr3+ into GaSe single crystals caused shifting of SHG signal peaks to the shorter wavelengths. However, transmission pumb-probe experiments revealed that all of the crystals showed NA signals with long lifetime under 620 nm pump and 626 nm probe excitations.

A polarizable embedding approach to second harmonic generation (SHG) of molecular systems in aqueous solutions

We present the extension to second harmonic generation (SHG) of the atomistic fully polarizable QM/FQ method that treats the solvent atomistically and embeds each atom in the solvent with a fluctuating charge (FQ) which responds to the solute quantum mechanical (QM) electrostatic potential in a self-consistent manner. The proposed approach is able to achieve an adequate modeling of solvent effects both in the quantum mechanical response equations and on the conformational properties of the system, which is sampled by resorting to molecular dynamics simulations. The application of the model to selected organic acids in aqueous solution, for which the interaction with the surrounding environment is dominated by HB interactions, shows a good agreement in both the modeling of solvent effects and in the reproduction of experimental SHG data extracted from hyper-Raman scattering experiments.

(Invited) High-Index Dielectric Nanoantennas for Light Management, Nonlinear Optics, and Controlled Photoluminescence of Quantum Emitters

Over the last twenty years, nanoscale optics was mainly driven by plasmonics since noble metal nanostructures, thanks to localized surface plasmon resonances (LSPR), can be used to manipulate, concentrate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics. Recently, an alternative to plasmonics emerged with high refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same range of applications as plasmonics by manipulating Mie optical resonances instead of LSPRs. The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures. Furthermore, high index dielectric nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap (near UV to near IR for Si), presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for nanostructure fabrication (essential for easier transfer towards applications). We present three examples describing the interest of using silicon nanoantennas: enhancement and directionality control of light scattering, enhancement and polarization control of second harmonic generation (SHG), and emission rate modification of quantum emitters placed in the silicon nanoantenna near-field. We focus also on the important role of both electric and magnetic components of light. First, we show that silicon nanospheres and nanowires are able to scatter and/or absorb very efficiently visible light. Furthermore, due to a kind of Fano process involving the interference between a broad resonance and a sharp higher mode simultaneously excited, the direction of light scattering can be very different (either dominating forward or backward scattering). Second, we investigate the nonlinear emission of silicon nanowires (Si-NWs). We show that SHG is strongly increased if a Mie optical mode corresponding to the fundamental (laser) wavelength is supported by the nanowire. According to nonlinear scattering theory and the so-called Miller's rule, the highest SHG is predicted for an optimum overlap of resonant modes and linear susceptibilities, at both the fundamental and harmonic wavelengths, respectively. Contrary, no SHG can be detected in absence of any resonance. This behavior, and experimental SHG mappings of individual Si-NWs, are well reproduced by numerical simulations (Green Dyadic Method - GDM). We also show that, depending on the Si-NW diameter, different surface of bulk-like contributions to the SHG are addressed. This also results in different polarizations of SHG as function of both the Si-NW diameter and incident light polarization. Third, we show the effect of a high index dielectric nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of molecules or quantum dots placed around simple Si nanoantennas (nanowires, nanodisk dimers). We also present preliminary results of the photoluminescence of Eu3+-doped clusters deposited on top of Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the farfield.

Nature of the tensor order in Cd2Re2O7

The pyrochlore metal Cd2Re2O7 has been recently investigated by second-harmonic generation (SHG) reflectivity. In this paper, we develop a general formalism that allows for the identification of the relevant tensor components of the SHG from azimuthal scans. We demonstrate that the secondary order parameter identified by SHG at the structural phase transition is the x2-y2 component of the axial toroidal quadrupole. This differs from the 3z2-r2 symmetry of the atomic displacements associated with the I-4m2 crystal structure that was previously thought to be its origin. Within the same formalism, we suggest that the primary order parameter detected in the SHG experiment is the 3z2-r2 component of the magnetic quadrupole. We discuss the general mechanism driving the phase transition in our proposed framework, and suggest experiments, particularly resonant X-ray scattering ones, that could clarify this issue.

Time-reversal symmetry breaking hidden order in Sr2(Ir,Rh)O4

Layered 5d transition iridium oxides, Sr2(Ir,Rh)O4, are described as unconventional Mott insulators with strong spin-orbit coupling. The undoped compound, Sr2IrO4, is a nearly ideal two-dimensional pseudospin-1/2 Heisenberg antiferromagnet, similarly to the insulating parent compound of high-temperature superconducting copper oxides. Using polarized neutron diffraction, we here report a hidden magnetic order in pure and doped Sr2(Ir,Rh)O4, distinct from the usual antiferromagnetic pseudospin ordering. We find that time-reversal symmetry is broken while the lattice translation invariance is preserved in the hidden order phase. The onset temperature matches that of the odd-parity hidden order recently highlighted using optical second-harmonic generation experiments. The novel magnetic order and broken symmetries can be explained by the loop-current model, previously predicted for the copper oxide superconductors.

Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation

The orientation angle is an important parameter that reflects the structure of molecules at interfaces. In order to obtain this parameter, second order nonlinear spectroscopic techniques including second harmonic generation (SHG) and sum frequency generation-vibrational spectroscopy (SFG-VS) have been successfully applied through analysis of the nonlinear signal from various polarizations. In some SHG and SFG-VS experiments, total internal reflection (TIR) configuration has been adopted to get enhanced signals. However, the reports on the detailed procedure of the polarization analysis and the calculation of the orientation angle of interfacial molecules under TIR configuration are still very few. In this paper, we measured the orientation angles of two molecules at the hexadecane-water interface under TIR and Non-TIR experimental configurations. The results measured from polarization analysis in TIR configuration consist with those obtained from Non-TIR configuration. This work demonstrates the feasibility and accuracy of polarization analysis in the determination of the orientation angle of molecules at the interfaces under TIR-SHG configuration.

Wave-Mixing Origin and Optimization in Single and Compact Aluminum Nanoantennas

The outstanding optical properties of plasmon resonances in noble metal nanoparticles enable the observation of nonlinear optical processes such as second-harmonic generation (SHG) at the nanoscale. Here, we investigate the SHG process in single rectangular aluminum nanoantennas and demonstrate that (i) a doubly resonant regime can be achieved in very compact nanostructures, yielding a 7.5 enhancement compared to singly resonant structures, and (ii) the χ⊥⊥⊥ local surface and γbulk nonlocal bulk contributions can be separated while imaging resonant nanostructures excited by a tightly focused beam, provided the χ⊥∥∥ local surface is assumed to be zero, as is the case in all existing models for metals. Thanks to the quantitative agreement between experimental and simulated far-field SHG maps, taking into account the real experimental configuration (focusing and substrate), we identify the physical origin of the SHG in aluminum nanoantennas as arising mainly from χ⊥⊥⊥ local surface sources.

Magnetic ground state ofSr2IrO4and implications for second-harmonic generation

The currently accepted magnetic ground state of Sr2IrO4 (the -++- state) preserves inversion symmetry. This is at odds, though, with recent experiments that indicate a magnetoelectric ground state, leading to the speculation that orbital currents or more exotic magnetic multipoles might exist in this material. Here, we analyze various magnetic configurations and demonstrate that two of them, the magnetoelectric -+-+ state and the non-magnetoelectric ++++ state, can explain these recent second-harmonic generation (SHG) experiments, obviating the need to invoke orbital currents. The SHG-probed magnetic order parameter has the symmetry of a parity-breaking multipole in the -+-+ state and of a parity-preserving multipole in the ++++ state. We speculate that either might have been created by the laser pump used in the experiments. An alternative is that the observed magnetic SHG signal is a surface effect. We suggest experiments that could be performed to test these various possibilities, and also address the important issue of the suppression of the RXS intensity at the L2 edge.

Probing the 3D structure of cornea-like collagen liquid crystals with polarization-resolved SHG microscopy.

This work aims at characterizing the three-dimensional organization of liquid crystals composed of collagen, in order to determine the physico-chemical conditions leading to highly organized structures found in biological tissues such as cornea. To that end, we use second-harmonic generation (SHG) microscopy, since aligned collagen structures have been shown to exhibit intrinsic SHG signals. We combine polarization-resolved SHG experiments (P-SHG) with the theoretical derivation of the SHG signal of collagen molecules tilted with respect to the focal plane. Our P-SHG images exhibit striated patterns with variable contrast, as expected from our analytical and numerical calculations for plywood-like nematic structures similar to the ones found in the cornea. This study demonstrates the benefits of P-SHG microscopy for in situ characterization of highly organized biopolymers at micrometer scale, and the unique sensitivity of this nonlinear optical technique to the orientation of collagen molecules.

Tracking quintuple layer oxidation on cleavedBi2Se3by optical second-harmonic anisotropy

We find that the major and minor intensity lobes of a measured rotational-anisotropy pattern of optical second-harmonic generation (SHG) from a cleaved $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ (111) surface can gradually change with time and eventually switch their relative amplitudes. This switching provides a means for tracking the progress of surface oxidation inside a quintuple layer of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. At different stages of oxidation, we perform pump-probe SHG experiments to study charge dynamics at the oxide/$\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ interface and to detect spin polarization of photoexcited surface states in the $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ topological insulator.

(Invited) Linear and Nonlinear Optical Properties of Individual Silicon Nanowires

High-refractive index semiconductor nanostructures provide original optical properties thanks to the occurrence of size- and shape-dependent optical resonances, which can be used to enhance and engineer light-matter interaction. In particular scattering and absorption efficiencies or local electromagnetic field intensity can be amplified and spectrally tuned, opening a multitude of possible applications in photovoltaics or field-enhanced spectroscopies. High-refractive index semiconductor nanostructures have thus attracted increasing interest as low-loss alternatives to plasmonic particles. In photonics, nonlinear optical effects also play an important role, providing several functionalities such as coherent up-conversion of light, generation of short pulses, or all-optical signal processing. A hindrance is that nonlinear effects are intrinsically weak. Plasmonic nanostructures, acting as nanoantennas able to strongly localize far-field radiations, are very promising candidates to improve nonlinear optical effects, which is one of the reasons of the increasing interest for nonlinear plasmonics. However, as mentioned above, plasmonic nanoparticles usually suffer from high losses. As an alternative, enhanced nonlinear properties can be obtained in semiconductor nanostructures by properly using resonant optical modes. For example, intense second harmonic generation (SHG) was achieved in nanowires of III-V compounds, or enhanced third harmonic generation (THG) could be produced in silicon nanostructures. SHG (THG) corresponds to the coherent conversion of two (three) low-energy photons into a single photon at double (triple) energy. SHG from bulk centrosymmetric materials including silicon is impossible in the dipole approximation, since they have vanishing second order susceptibility (the inversion symmetry forbidding even order terms in the electric polarization expansion). Second order processes can therefore occur only when symmetry is broken, for instance in the presence of interfaces or field gradients. For very small systems, the high surface to volume ratio allows that rather strong second order effects may arise, possibly further supported by local field enhancement due to resonant modes. This can be interesting for silicon-based photonics compatible with CMOS technology. We present here linear and nonlinear properties of individual strain-free silicon nanowires (Si-NWs). First, we show experimentally and theoretically the optical resonance modes (Mie resonances) supported by the NW as function of diameter and polarization of the excitation, and how these modes may be used to modify the photoluminescence of Si nanocrystals placed in the NW near-field. Second, we investigate the nonlinear emission composed of a broad nonlinear PL band and the SH peak. We show that SHG from the Si-NWs is strongly increased if a Mie optical mode corresponding to the fundamental (laser) wavelength is supported by the nanowire. According to nonlinear scattering theory and the so-called Miller's rule, the highest SHG is predicted for an optimum overlap of resonant modes and linear susceptibilities, at both the fundamental and harmonic wavelengths, respectively. Contrary, no SHG can be detected in absence of any resonance. This behavior as well as experimental SHG mappings of individual Si-NWs, are well reproduced by numerical simulations (Green Dyadic Method - GDM). In first approximation, second order effects in centrosymmetric nanostructures are modeled assuming that the c2 ^^^ surface contribution from field components normal to the surface are the most significant. Other possible sources of SHG are neglected in a first step as commonly done. However, this approach fails explaining the polarization behavior of the SHG from Si-NWs with diameters up to about 150 nm. Using the c2 ^^^ surface contribution leads to SHG polarization perpendicular to the Si-NW axis, independently of the polarization of the incident light parallel of perpendicular to the nanowire axis. This is in contradiction with the experimental findings where, for small diameters below 150 nm, the polarization of both fundamental and harmonic radiations are always observed parallel. Thus, the SH polarization is found parallel to the Si-NW axis for TM excitation (incident electric field along the axis). Using GDM, we explain this behavior with SHG contributions arising from tangential fields at the surface as well as from strong field gradients in the bulk, proving that there is a changeover of the main SHG contribution as function of NW diameter. In summary, Si-NWs provide useful research tools for the investigation of SHG from centrosymmetric materials, with the possibility to address different nonlinear susceptibility contributions. They could also be interesting for nonlinear silicon-based photonic applications if enhanced SHG and polarization control could be achieved. We finally show a few examples of Si nanostructures obtained by e-beam lithography in Silicon On Insulator substrates in order to explicitly tailor the SH response.

Phase Transitions in the Ruddlesden-Popper Phase Li2CaTa2O7: X-ray and Neutron Powder Thermodiffraction, TEM, Raman, and SHG Experiments.

The structure of the Ruddlesden-Popper layered perovskite Li2CaTa2O7, known for its high photocatalytic water activity since its discovery in 2008, is reinvestigated. This oxide has been characterized by powder X-ray and neutron thermodiffraction, TEM, second harmonic generation (SHG), and Raman experiments on powders and single crystals. It is shown that it undergoes two structural phase transitions (i) around 220 °C, mainly characterized by the progressive emergence of SHG signal at low temperatures, and (ii) at 660 °C, mainly characterized by changes of the temperature behavior of lattice parameters and by the emergence of Raman signals that linearly increase on decreasing temperature. It is shown by powder neutron diffraction profile refinements at RT, 400, and 800 °C that the space groups of the successive phases of Li2CaTa2O7 are the acentric Pna21 (RT ≤ T ≤ 220 °C), Pnma (220 °C ≤ T ≤ 660 °C), and Cmcm (T ≥ 660 °C). A soft mode associated with the transition to the highest symmetry for this structural arrangement (I4/mmm) is also found in the Raman spectra. All these transitions appear continuous: the high temperature ones can be attributed to progressive vanishings of the octahedra tiltings (displacives) while the transition in the vicinity of 220 °C from Pna21 to Pnma exhibits order-disorder character.

Focused ion beam milling of gallium phosphide nanostructures for photonic applications

We report on the fabrication of gallium phosphide (GaP) nanowaveguides of controlled dimensions, as small as 0.03 μm and aspect ratio in excess of 20, using focused ion beam (FIB) milling. A known limitation of this fabrication process for photonic applications is the formation of gallium droplets on the surface. We demonstrate a post-fabrication step using a pulsed laser to locally oxidize the excess surface gallium on the FIB milled nanostructures. The process significantly reduces the waveguide losses. The surface optical quality of the fabricated GaP nanowaveguides has been evaluated by second-harmonic generation experiments. Surface and bulk contributions to second-order optical nonlinearities have been identified by polarization measurements. The presented method can potentially be applied to other III-V nanostructures to reduce optical losses.