Surface Second-Harmonic Generation from Vertical GaP

Reza Sanatinia , Srinivasan Anand , and Marcin Swillo ... Shula Chen , Yuqing Huang , Dennis Visser , Srinivasan Anand , Irina A. Buyanova , Weimin M...
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Letter pubs.acs.org/NanoLett

Surface Second-Harmonic Generation from Vertical GaP Nanopillars Reza Sanatinia,*,† Marcin Swillo,‡ and Srinivasan Anand† †

School of Information and Communication Technology, KTH Royal Institute of Technology, Electrum 229, S-164 40 Kista, Sweden School of Engineering Sciences, KTH Royal Institute of Technology, S-106 91 Stockholm, Sweden

‡

S Supporting Information *

ABSTRACT: We report on the experimental observation and analysis of secondharmonic generation (SHG) from vertical GaP nanopillars. Periodic arrays of GaP nanopillars with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed a strong dependence of the SHG intensity on pillar diameter. Analysis of surface and bulk contributions to SHG from the pillars including the calculations of the electric field profiles and coupling efficiencies is in very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP nanopillars. KEYWORDS: Nanopillars, nanoscopic light source, second-harmonic generation, surface SHG, surface phonons, gallium phosphide

O

ptical properties of semiconductor nanowires/pillars1 and their interaction with light show great promise for applications such as solid−state lighting,2 photovoltaics,3,4 lasing,5,6 nonlinear optics,7 sensing,7,8 and life sciences.9 Nanowires are suitable platforms for growth of cells10 and neural networks11 and can be useful for single cell endoscopy.12 The nanowire (NW) interface can also provide in situ probing of intercellular molecular processes as well as delivering biomolecules into living cells.13 Very recently it has been shown that vertical nanopillars (NP) interface can be used for in vitro single molecule detection and also can function as highly localized light sources inside the cells.14 Nonlinear optical response of nanowires could be of great interest for nonlinear scanning microscopy, novel nanoscopic light sources, and active nonlinear elements in future nanophotonic devices.1,12 Nonlinear scanning microscopy15,16 have certain advantages over standard fluorescence microscopy such as enhanced resolution, removal of the unwanted background laser light and working at wavelengths which would not damage biological tissues. Second-harmonic generation (SHG), where the incident light of the frequency ω is converted to light at frequency of 2ω, can arise both from bulk and surface nonlinearities.17 Surface SHG was theoretically predicted by Bloembergen and Pershan18 and subsequently experimentally demonstrated.19 Both structural and optical field discontinuities contribute to surface SHG.17 At the surface layer of materials, the inversion symmetry is broken (structural discontinuity) and second-order nonlinearity is present in the electric dipole approximation. Electric quadropole contribution to surface SHG can also arise from the electric field discontinuity at the interface of two materials with different refractive indices. © 2012 American Chemical Society

SHG has been experimentally observed in NWs of different semiconductor materials such as Alkaline Niobates,7,20 GaN,21 ZnO,22 ZnSe,23 GaAs,24 and InP.25 However, in most of the reports the observed SHG from NWs have been primarily attributed to the bulk contribution; while a few reports have addressed the surface effects.22 While noncentrosymmetric crystals possess a nonzero second-order susceptibility χ(2) and exhibit bulk SHG,26 surface SHG can also be observed from materials with inversion symmetry.17 In planar (bulk) samples surface SHG has been used as a probe to detect adsorption and orientation of single layers of organic molecules.27 In nanostructured materials, e.g., nanowires/pillars, the large surface to volume ratio can provide added benefit. For NW/ NP geometries the relative contribution of the bulk and surface effect may depend on the size (diameter) of the wire. In addition it may also be beneficial to distinguish the surface and the bulk contribution to SHG to determine optimal nanostructure (NW/NP) geometries for enhancing SHG. Thus coupling efficiency of the pump to the NPs and the intensity of SHG can be tailored by optimization of the diameter of the pillars. Therefore study of SHG in NPs/NWs and analysis of surface and bulk contribution to the generated light is interesting for applications as well as for understanding the light-matter interaction in semiconductor NWs/NPs. In this letter, we report on the experimental observation and analysis of SHG from vertically aligned GaP NPs fabricated via nanosphere lithography (NSL) and dry etching.28,29 We demonstrate a strong dependence of the SHG intensity on Received: November 2, 2011 Revised: December 17, 2011 Published: January 3, 2012 820

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Figure 1. (a−c) Schematic illustration of the GaP nanopillar fabrication process steps. (a) Size reduction (by RIE) of hexagonally close packed silica colloidal particles dispersed on GaP surface. (b) Anisotropic etching to produce GaP nanopillars by Ar/Cl2 CAIBE using silica particles as masks. (c) Etched nanopillar array after removal of the remnant silica mask by wet chemical etching using HF. (d) A representive SEM image (titled view) of a GaP nanopillar array. The etch-process is optimized to obtain cylindrical pillars. (e) Schematic sketch of the setup used for SHG measurements on the GaP nanopillar array showing the backscattering geometry. The wavelength of the pump was 840 nm and the SHG light was collected by a 50× objective.

single photon detector (Si avalanche photodiode). Figure 2a plots the intensity of the generated 420 nm light as the function

pillar diameter. The surface and bulk contributions to SHG from the pillars are analyzed, taking into account the calculated electric field profiles and coupling efficiencies. Complementary measurements of surface optical phonons by Raman spectroscopy were also performed and shown to be in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP NPs. Colloidal silica nanospheres (Sigma Aldrich), 500 nm diameter, were dispersed on double side polished undoped (100) GaP substrates by spin coating to obtain monolayer (ML) coverage. The dispersion of particles by spin coating results in both large area patches (typically a few mm2) with close packed nanoparticle arrays and areas with smaller islands of close packed arrays. In some areas isolated particles are also present. Thus both single NPs and array of pillars can be investigated. After deposition, the silica particles were sizereduced by reactive ion etching (RIE), using fluorine based chemistry. Subsequently, using the nanospheres as etch masks, GaP NPs were fabricated by Ar/Cl2 chemically assisted ion beam etching (CAIBE). The fabricated GaP pillars were typically about 1 μm high. By changing the size of the nanospheres (silica mask), GaP pillars with different diameters ranging from 100 to 250 nm were fabricated (details of the fabrication process can be found in the Supporting Information). Panels a−c of Figure 1 schematically illustrate the NPs fabrication steps. Figure 1d shows a representative SEM image of an array of GaP NPs; the etch process results in nearly cylindrical pillars. In principle, the particle size and etching conditions including chemistry can be optimized to vary size, shape, and geometry of NPs.29 GaP belongs to III−V compound semiconductors, with a wide transparency range (λgap = 548 nm). We performed the SHG measurements at room temperature (RT) in a backscattering geometry (Figure 1e) with Ti:saphire laser operating at 840 nm wavelength with a pulse duration of 100 fs at 82 MHz repletion rate. An objective with 50× magnification and numerical aperture (NA) of 0.5 was used. With this configuration we could obtain a laser spot size with a diameter of ∼2 μm. The generated SH light was collected with a fiber and transferred to a monochromator and subsequently to a

Figure 2. (a) Measured SHG light intensity as a function of the pump power obtained from an array of vertical GaP nanopillars with an average diameter of 150 nm. The determined slope of the line is 2 confirming SHG. The inset shows an optical microscope image of the SHG light (blue light, 420 nm) from the pillar array. (b) The measured intensity of the SHG from different GaP pillar arrays having different average diameters; the pump power was 2.8 mW at 840 nm for all of the measurements.

of the input pump power on an array of vertical GaP NPs with an average diameter of 150 nm. The linear response (log−log scale) with the slope of two confirms the SHG process. We carried out the same set of measurements on arrays of pillars with different diameters. Figure 2b shows the intensity of SHG light, for the average pump power of 2.8 mW, from different arrays of pillars with average diameters ranging from 110 to 250 821

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Figure 3. (a) Shows the calculated profile of the electric field component Ex (x, y), of the 0th order mode in nanopillar waveguide for 840 nm (pump) wavelength and the refractive index profile of an array of GaP nanopillars. The pillar diameter and the spatial distance (period) are 150 and 500 nm, respectively. (b) Plots the calculated coupling efficiency represented by blue triangles (left axis) and the normalized mode size represented by red squares (right axis) for λ = 840 nm (pump wavelength), as a function of pillar diameter. The solid lines are drawn to indicate the trend.

same 500 nm period. If substrate would contribute to SHG, the effect would be visible for all investigated pillars. As we can see on Figure 2a the intensity of SHG for pillars with small diameter (110 nm) is below 10% of the one measured for the larger diameter (150 nm). It indicates that the contribution to SHG from the substrate caused by diffraction of the pump beam is negligible. To further confirm that the measured SHG light is originating from the NPs, the SHG measurements were carried out in transmission geometry (from the backside of the double side polished substrate). As the results were similar, we can exclude the effect from the GaP substrate on the measured SHG light. In order to investigate surface and/or bulk contribution to the SHG, we compared the measured results with numerical simulations. For simulations, we used commercial numerical finite element model software (COMSOL 3.5a). We simulated the electric field mode profiles in the pillars for different diameters for both pump (840 nm) and the SHG light (420 nm). The refractive indices for GaP were set to 3.2 and 4 for pump and SH light wavelengths, respectively.33 Figure 3a shows the refractive index and the zeroth order mode profile (for pump 840 nm) for an array of GaP NPs in air; the pillar diameter is 150 nm and the period is 500 nm. Due to the symmetry of the NPs, the guided mode is represented as a superposition of two degenerated eigenmodes. On top of the Figure 3a, the calculated profile of the electric field component Ex in the zeroth order mode (from the first degenerated eigenmode solutions) of the NP waveguide is shown; the wavelength is that of the pump, 840 nm. For the same eigenmode, the electric field Ey is 1 order of magnitude lower compared to Ex. Since the two degenerated eigenmodes are orthogonal and linearly polarized, the guided mode has the same polarization as the pump. As it can be seen, the guided mode is not totally confined in the pillars (higher refractive index area) and has a significant evanescent field in the air. The electric field discontinuity and gradient at the edge of the pillars can also be clearly seen. Both the gradient and the electric field amplitude at the edge of NP depend on the amplitude and the confinement of the guided mode. The coupling efficiency of the pump into the NP waveguide was calculated from overlap integral between the field amplitude profiles of the laser beam and the two degenerated zeroth order eigenmodes. Figure 3b plots the calculated coupling efficiency and normalized size of

nm. The measured SHG intensity shows a strong dependence on the pillar diameter, in the range 120−200 nm. As the diameter increases the SH intensity reaches a maximum for the pillar diameter of ∼150 nm; then shows a slow variation beyond a diameter of ∼200 nm. As noted earlier the fabrication process mostly results in arrays of pillars, however there are areas on the sample with isolated pillars sufficiently well separated to make measurements on single pillars. SHG was also experimentally observed for single GaP pillars with different diameters (see the Supporting Information). GaP has a zincblende structure and belongs to 43̅ m crystal class. It is a noncentrosymmetric crystal (i.e., SHG is allowed for certain directions) and has a significant second order nonlinear susceptibility with the only nonzero components d14 = d25 = d36 = 159 pm/V at λ = 852 nm.30 A simple estimate of the efficiency of the measured SH generated light from array of pillars with the average diameter of 150 nm (about 1 μm tall) is about 2 × 10−7%. We note that this estimate does not take into account the light collection efficiency of the microscope objective. For an array, the efficiency also depends on the spatial density of nanopillars which defines how many of them would contribute to the SHG. For high spatial density of pillars, the spacing also influences the mode coupling between nanopillar waveguides and consequently the efficiency of launching the pump into the guided mode of the array. In this work, we use arrays of pillars with period of 500 nm and the laser beam spot size of ∼2 μm in all experiments; in this case the mode overlap between adjacent nanopillars waveguides can be neglected. GaP as a bulk lacks birefringence and in order to enhance the efficiency of SHG, phase matching condition can be obtained by induced birefringence,31 e.g., as in porous GaP.31,32 Because of the symmetry of the χ(2) tensor we cannot see any SHG from bulk (100) GaP, with the incident light being normal to the (100) surface. Therefore the observed SHG originates from the NPs and the substrate contribution can be neglected. Thus the choice of (100) GaP is advantageous to investigate SHG from the NPs. However it could be possible that in the backscattering geometry, diffraction of light from the NPs could cause the pump to hit the substrate at nonvertical angles and this light can result in SHG from the substrate. Diffraction of light from the nanopillars would occur for all samples with different diameters (110−250 nm). All arrays of pillars have the 822

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the mode excited by the focused pump light (with λ = 840 nm and spot size of 2 μm, which corresponds to the waist of the Gaussian beam at the focal point of 50× microscope objective) as a function of the pillar diameter. The left vertical axis shows the coupling efficiency of the pump to the NP (blue triangles). The coupling efficiency was obtained after extracting out the evanescent field outside the pillar, as it does not contribute to SHG. As seen from the figure the coupling efficiency is close to zero (4 × 10−4%) for pillars with diameters of 50 nm and it increases as the pillar diameter increases. The coupling efficiency reaches to more than 6.4% for the pillars with a diameter of 400 nm. For the pulsed laser with average pump power of 3 mW, the values of the coupling efficiency for the 50 and 400 nm diameter pillars correspond to optical peak powers (in the pillars) of 1.4 mW and 23.4 W, respectively. On the right vertical axis the normalized mode size is plotted as a function of the pillar diameter. The plot shows the full width at half-maximum (fwhm) of the mode size normalized to the pillar diameter. We can see from the plot that for pillars with diameters larger than 250 nm the normalized mode size is almost constant and approximately 0.5, which means the fwhm corresponds to half of the pillar diameter. This indicates that the mode is almost fully confined in the pillar and the intensity of the electric filed at the edges (boundary between pillar and air) is significantly reduced. On the other hand, for pillars with diameters smaller than 250 nm the mode is less confined inside the pillars, thus stronger electric field exists at the edges. For pillars with diameters less than 100 nm the fwhm is larger than pillar diameter (indicated as a dashed line in Figure 3b). Overall, we can conclude that more light can couple into pillars with larger diameters while the normalized mode size is larger in pillars with smaller diameters. This indicates that the criterion for larger electric field at the surface is basically a trade-off between light coupling into the pillars and the normalized mode size inside the pillars. In order to experimentally study the interaction of light with the surface of NPs, we performed a set of μ-Raman spectroscopy measurements at RT. We used HORIBA JobinYvon LabRam micro-Raman system to collect the Raman spectra. We used a 50× objective of NA = 0.45 and the optical excitation with the 514 nm line of an Ar+ laser. The incident laser on the sample was around 1 mW in power and had the spot size of 2−3 μm. We used low laser powers to minimize thermal effects on the Raman spectra. Figure 4a plots the result of the μ-Raman measurements on GaP (1 0 0) bulk crystal and on two GaP NP samples (pillar diameter: 170 and 210 nm). It can be seen from the figure that bulk GaP (red dashed line), has the longitudinal optical (LO) phonon peak at 403 cm−1. The LO peak has a Lorentzian line shape (see the Supporting Information). In the current set of measurements we use backscattering configuration with the excitation light being perpendicular to the {100} plane. In this geometry, transversal optical phonons (TO) mode is forbidden and therefore is absent in (100) GaP bulk (see the Supporting Information). Interestingly for GaP NPs (for both 170 and 210 nm diameter pillars) the TO peak is present and is located at 366 cm−1 which is in good agreement with the values for bulk crystalline GaP.34 The peak for the LO peaks position for the NPs and the bulk GaP appear at the same position indicating the good quality of the fabricated NPs. Besides the LO and TO peaks, the Raman spectra for the NPs (Figure 4a) shows a third peak lying in between LO and TO, on the lower frequency shoulder of the LO peak, which is

Figure 4. (a) Raman spectra from bulk GaP (red dashed line) and from GaP nanopillars with two different diameters 170 nm (blue solid line) and 210 nm (black dotted line). Longitudinal, transverse, and surface optical phonons are indicated by arrows (LO, TO, and SO, respectively). The measured Raman spectra from nanopillars are normalized in reference to the TO modes. (b) Plots the dependence of the measured SO phonon peak intensity (black triangles) and the calculated intensity of the electric field at the edge (sidewall) of the pillars (red circles) on the pillar diameter; the excitation wavelength is 514 nm. The solid line is drawn to indicate the trend.

known to be related to the surface optical (SO) phonons.35,36 The peak position of the SO modes shifts slightly to shorter frequencies for pillars with smaller diameters, although this shift is not much for the dimensions of the GaP pillars studied here. The other interesting phenomenon we observe is that the intensity of the SO phonons peak increases as the pillar diameter reduces. This difference can be clearly seen in Figure 4a: The SO phonon intensity increases by nearly a factor of 2 as the pillar diameter reduces from 210 to 170 nm. In Raman measurements intensity of the Raman scattered light is proportional to the intensity of the electric field of the incident excitation light in the sample. Therefore the intensity of SO phonons can be expected to be proportional to the intensity of the electric field of the excitation light at the surface. The intensity of both surface phonons and surface SHG depends on the intensity of the electric field of the excitation light at the surface. In the case of surface phonons, the intensity of SO phonons is proportional to the electric filed intensity at the edge of the pillars while the intensity of surface SHG light is proportional to the square of electric field intensity at the edges. Figure 4b shows the influence of NP diameter on electric field intensity (excitation pump in Raman measurements is 514 nm) at the NP surface (electric field normal to NP surface). The electric field intensity was numerically calculated by integration of the intensity of mode profile over a 10 nm thick inner ring at the edge of NPs. The 10 nm thickness of the ring 823

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Figure 5. (a) Plots the dependence of the measured (blue dots) and calculated SH light intensity (surface, red squares and bulk, black triangles) on the GaP nanopillar diameter; the measurements were carried out in backscattering geometry and the pump wavelength was 840 nm. The green lines are drawn to highlight the trend in the calculated data. The purple line shows the best fitting of the simulation data (superposition of bulk and surface effects) to the measured values. (b) Distinguishing surface and bulk SHG by polarization selection. The SH intensity generated from the nanopillars was measured in transmission geometry and was detected after a rotatable polarizer. The normalized polar plot shows the result for two different pillar diameters: 150 nm (blue) and 250 nm (red); for these two pillar diameters either the surface or bulk dominated SHG is present (data of panel a). The electric field polarization for the pump is indicated by an arrow.

Analyzing the contribution of structural discontinuity and/or electric field discontinuity to the surface contribution to the generated surface SH is not trivial and requires further investigation. For the bulk contribution to SHG, the second order nonlinear polarization can be written as

was chosen arbitrarily, based on the mesh size used in mode solver and it only influences the absolute value of the calculated intensity. The calculated field intensity at the surface together with the measured (normalized) surface phonons peak intensity from the NPs as a function of diameter are plotted in Figure 4b. The observed trend of SO phonons intensity with pillar diameter is in very good agreement with the simulation results. Since SO phonons originate from broken symmetry at the surface due to material discontinuity, it suggests that in our NPs the surface contribution to SHG should also be considered. As was mentioned before, GaP NPs should be analyzed both with respect to the surface and the bulk contribution to SHG. The surface contribution to SHG nonlinearity is a result of material discontinuity at the interface NP/air. Structural deviation at the interface is responsible for local second order nonlinear susceptibility, whereas mismatch of dielectric constants gives the electric quadrupole and magnetic dipole contribution to the surface nonlinear polarization.17 In both cases, the optimum diameter of NP for surface contribution to SHG corresponds to the highest intensity at the surface for the pump with electric filed polarization component normal to the interface NP/air. Therefore, the strongest component of second order nonlinear polarization P(2) is also normal to the interface,37 which leads to the approximation: P(2) ≈ P⊥(2) · e⊥ where

P⊥(2) ∝ [E0(x , y) · e⊥]2 e2i(ωt −βωz)

Px(2) = 2d36[E0,2(x , y) · ey]·[E0,2(x , y) · ez] e2i(ωt −βωz)

(2a)

P(2) y = 2d36[E 0,1(x , y) · ex] · [E 0,1(x , y) · ez] e2i(ωt −βωz)

(2b)

where ex, ey, and ez are the primitive vectors along x, y, and z direction, respectively, and E01 and E02 are the first and second degenerated egienmodes, respectively. SHG (bulk contribution) was calculated by applying nondepleted pump approximation over the coherence length for the bulk SHG process (∼100 nm). As one can see from eqs 2a and 2b the intensity of SHG does not depend on the polarization of the pump. The dependence of the bulk contribution to SHG on the pillar diameter is plotted in Figure 5a. It can be seen that the highest SH intensity is obtained for a NP diameter of 170 nm. For this optimum diameter, the guided mode for the pump wavelength is maximally confined, which is an indication that the ratio between longitudinal and transverse component of the zeroth order guided mode reaches the maximum value. In case of larger pillar diameters, this ratio decreases due to a weaker mode confinement while the coupling efficiency to the zeroth order mode increases (Figure 3b). Consequently, the intensity of the bulk contribution is still significant even for larger diameters. It should be mentioned that, in the zeroth order guided mode for the investigated diameters of NPs, the longitudinal and transverse component of the electric field have comparable amplitudes. Therefore this case is far from plane wave approximation, where the longitudinal component of the electric field does not exist and the bulk contribution to SHG vanishes (for [100] orientation). The simulation results show that the strongest surface contribution to SHG is for pillar diameters of 150−160 nm. This diameter is about 10 nm smaller compared to the optimum one for the bulk effect, where it corresponds to the maximum confinement of the mode. In simulations, we have not taken into account the above

(1)

e is a primitive vector, ⊥ represents the component normal to the interface NP/air, E0(x,y) is the electric field of the zeroth order mode (superposition of the two degenerated eigenmodes) for the pump frequency ω, and βω is the propagation constant of the zeroth order mode. Intensity of the electric field at the surface for SHG was numerically simulated by applying nondepleted pump approximation and integrating the intensity over a 10 nm thick inner ring at the edge of NP. The thickness of the ring is chosen arbitrary based on the mesh size used in simulation in order to ensure a stable result for numerical computation. However, we note that the interaction length in surface SHG process depends on the skin depth of surface nonlinearity.37 Figure 5a shows the influence of the pillar diameter on the surface contribution to SHG obtained by calculations based in Figure 3. The results predict that in GaP NPs, 160 nm is the optimum diameter for the surface contribution to SHG for pump wavelength of 840 nm. 824

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band edge absorption of GaP (∼200 nm penetration length at 420 nm wavelength) for second-harmonic light considering the coherence length of SHG process. Intensity of the measured SHG is also plotted in Figure 5. The (purple) solid line shows the best fit of the simulation data to the experimental results. The simulation data obtained for purely bulk contribution to SHG was fitted to the measured values for pillars with large diameters (larger than 200 nm) and the superposition of the simulated bulk and surface contribution to SHG was fitted to the maximum values of the measured SHG intensity. The simulation results are in very good agreement with the measured intensity of SHG from NPs and clearly demonstrate the importance of surface contribution to the SHG in GaP NPs. As one can see from the presented model, the intensity of SHG light from GaP NPs does not depend on polarization of the pump in neither surface nor bulk contribution. However, the surface and bulk contribution can be distinguished by polarization measurements of SHG light for pump polarization along x or y axes of GaP crystal. In the case of linearly polarized pump in either x or y direction, SHG from the bulk would have an orthogonal polarization with respect to the pump, whereas the SHG from surface would have the same polarization as the pump. From the data of Figure 5a, one can see that for the pillar diameters of 150 and 250 nm the surface and bulk contribution to SHG dominate, respectively. This was experimentally confirmed by applying the pump laser with linear polarization along the x or y axes of GaP crystal and detecting second-harmonic light intensity after a rotatable polarizer in transmission geometry. We used the transmission geometry since in the backscattering geometry the polarization of SHG light can change due to multiple reflections. The results are presented in Figure 5b, where the pump polarization is indicated at 0°. In the case of pillars with 250 nm diameter the polar plot shows dominant polarization orthogonal to the pump; which corresponds to bulk SHG. For pillars with 150 nm diameter the polar plot shows stronger polarization along the pump direction which is clearly indicative of surface contribution to the generated light. In conclusion, we experimentally observed and analyzed SHG from vertical GaP NPs. GaP NPs with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed strong dependence of the SHG intensity on pillar diameter. The surface and bulk contributions to SHG from the pillars were analyzed, including the calculations of the electric field profiles and coupling efficiencies and the results showed very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. We also performed a set of experiment where the polarization of the measured SHG light from GaP NPs was used to distinguish between the bulk and surface contribution to SHG. Since the intensity of bulk SHG in GaP NPs is limited due to the phase mismatch between the propagation constants of SH light and the pump, surface SHG can be used to enhance the SHG intensity. The results presented in this work are applicable to SHG in other NW materials and can be used to optimize pillar/wire diameter depending on the material and wavelength of interest. The generated light from NPs can be used as an ultrafast nanoscopic light source and may have potential applications in sensing, bio imaging and single cell and molecule imaging.

Letter

ASSOCIATED CONTENT

S Supporting Information *

Details of GaP nanopillars fabrication, SHG measurements on single pillars, Raman measurements, as well as additional materials on COMSOL simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

The work was performed within the Linné Center for Advanced Optics and Photonics funded by the Swedish Research Council. Partial supports from the network of excellence, Nanophotonics for Energy Efficiency and from Nanordsun funded by Nordic Innovation centre are also acknowledged.

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