We dispersed crystalline Si into a colloid of ultrasmall nano particles (∼1 nm), and reconstituted it into microcrystallites films on device-quality Si. The film is excited by near-infrared femtosecond two-photon process in the range 765-835 nm, with incident average power in the range 15-70 mW, focused to ∼1 μm. We have observed strong radiation at half the wavelength of the incident beam. The results are analyzed in terms of second-harmonic generation, a process that is not allowed in silicon due to the centrosymmetry. Ionic vibration of or/and excitonic self-trapping on novel radiative Si-Si dimer phase, found only in ultrasmall nanoparticles, are suggested as a basic mechanism for inducing anharmonicity that breaks the centrosymmetry. © 2000 American Institute of Physics

Akcakir, O

Barry, N

Belomoin, G

Gratton, E

Nayfeh, MH

Therrien, J

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UC IrvineUC Irvine Previously Published WorksTitleSecond harmonic generation in microcrystallite films of ultrasmall Si nanoparticlesPermalinkhttps://escholarship.org/uc/item/3bb8n9btJournalApplied Physics Letters, 77(25)ISSN0003-6951AuthorsNayfeh, MHAkcakir, OBelomoin, Get al.Publication Date2000-12-18DOI10.1063/1.1334945Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaSecond harmonic generation in microcrystallite filmsof ultrasmall Si nanoparticlesM. H. Nayfeh,a) O. Akcakir, G. Belomoin, N. Barry, J. Therrien, and E. GrattonDepartment of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana,Illinois 61801Received 14 July 2000; accepted for publication 25 October 2000We dispersed crystalline Si into a colloid of ultrasmall nano particles 1 nm, and reconstituted itinto microcrystallites films on device-quality Si. The film is excited by near-infrared femtosecondtwo-photon process in the range 765–835 nm, with incident average power in the range 15–70 mW,focused to 1 m. We have observed strong radiation at half the wavelength of the incident beam.The results are analyzed in terms of second-harmonic generation, a process that is not allowed insilicon due to the centrosymmetry. Ionic vibration of or/and excitonic self-trapping on novelradiative Si–Si dimer phase, found only in ultrasmall nanoparticles, are suggested as a basicmechanism for inducing anharmonicity that breaks the centrosymmetry. © 2000 AmericanInstitute of Physics. S0003-69510005452-8Recently, there has been interest in the nonlinear opticalresponse and harmonic generation in siliconnanostructures,1–3 stimulated by the discovery of the opticalactivity of porous silicon and the associated nanoscale struc-ture of the material.4 In measurements by Wang et al.1 using50 ps pulsed 1.06 m excitation, it was suggested that a thirdharmonic photon at 355 nm is first generated in the core ofnanostructures. The photoluminescence is then produced bya single photon excitation by the internally generated UVradiation. However, no radiation at the third harmonic wasdirectly detectable in these experiments. In more recent mea-surements by Chin et al.2 using short pulse at 0.870, 1.06,and 1.3 m radiation, there was no evidence for second orthird harmonic generation. The internal generation of secondharmonic photon was ruled out since second harmonic gen-eration is not allowed in bulk silicon due to the centrosym-metry. On the theoretical side, silicon is known to have neg-ligible nonlinearity, being zero at the second order level notallowed, and very small at the third order level.In this letter, we report the observation of second har-monic generation in films of ultrasmall silicon nanoparticles,an effect that is not allowed in bulk silicon due to the cen-trosymmetry. Silicon was dispersed into a suspension nano-particle colloid of 1 nm across.5,6 The particles were thenreconstituted to create large, thick, uniform, and under con-trol layers of microcrystallites on device quality Si sub-strates. The film is excited by near-infrared femtosecond twophoton process at 750–830 nm. Ionic vibration of or/andexcitonic self-trapping on radiative Si–Si dimer phase,7,8found only in ultrasmall nanoparticles, are suggested as abasic mechanism for inducing anharmonicity that breaks thecentrosymmetry which forbids the process in bulk.The substrates were 100 oriented, 1–10  cm resistiv-ity, p-type boron doped silicon, laterally anodized in hydro-gen peroxide and HF acid5,6,9 while advancing the wafer inthe etchant at a reduced speed of 1 mm per hour. Subse-quent immersion in an ultrasound acetone or water bathcrumbles the top film into ultrasmall particles. When it isexcited by a nanosecond radiation at 355 nm, deep blueemission is observed with the naked eye, in room light. Di-rect imaging, using high resolution transmission electron mi-croscopy, of a thin graphite grid which was coated with theparticles shows that the particles are 1 nm in diameter with10% dispersion.10 Electron photospectroscopy and infraredFourier spectroscopy show that the particles are composed ofsilicon, dominated by hydrogen termination with less than10% oxygen. The particles are precipitated from a water sol-vent to reconstitute the material into a thin film a few mi-crons thick on a device quality silicon or glass. Under so-lidification the film shatters. Optical imaging see Fig. 1shows that colloidal crystals of 50–100 m across. We usedmode locked femtosecond Ti:sapphire near-infrared laser of150 fs pulse duration at a repetition rate of 80 MHz. At theaElectronic mail: m-nayfeh@uiuc.eduFIG. 1. 275 m300 m optical image, showing two microcrystallite frag-ments from a film reconstituted from the silicon nanoparticles.APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 25 18 DECEMBER 200040860003-6951/2000/77(25)/4086/3/$17.00 © 2000 American Institute of PhysicsP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.200.102.124 On: Fri, 222:41:17target, the average power, 5–300 mW, is focused to diffrac-tion limited spots 1 m diameter, giving an average inten-sity of 2105 to 4107 W/cm2 peak intensity of21010– 41012 W/cm2). The configuration of the excita-tion and detection optics is such that the incident beamstrikes the sample in near normal incidence, and the emissionis detected in the backscattering direction 180° relative tothe incident beam. The silicon substrate provides a reflec-tivity of 0.5 at 400 nm, and 0.4 at 780 nm. Thus the signalis partly due to direct backscattering and partly due to re-flected forward scattering. The beam–sample interaction isviewed via an optical microscope 10 and the excitationlight was raster scanned. Emission is detected by a photo-multiplier and stored in a two-dimensional array. The spec-tral response is analyzed with a resolution of 4 Å by a grat-ing dispersive element and a charge sensitive detector cooledto 0° to reduce the dark current count.Figure 2 gives the emission spectra for the three excita-tion wavelengths 780, 800, and 832 nm of different incidentintensities. Each shows a double peak spectrum. The ratio ofthe peak is 0.60, essentially flat with incident intensity.Those and other measurements show that the emitted wave-length tracks the incident wavelength. The peaks at 390, 400,and 416 nm, respectively, are at half the wavelengths of theincident beam second harmonic. The peaks at 380, 390,and 406 nm are blueshifted by 10 nm from the second har-monic peak. We find no measurable harmonic generationfrom control Si substrates with no nanoparticles deposited,and that the incident beam does not have a double peakprofile. We examined the power dependence of the emission.The spectra of increasing strength shown in Fig. 3 wererecorded using radiation at 832 nm wavelength at, respec-tively, 15, 29, and 59 mW incident intensity, shows a qua-dratic response with incident intensity. However, in somemeasurements, the response was found slightly subquadraticwhich we may attribute to sample degradation or damage.A plausible mechanism for the second harmonic genera-tion may be derived from a recent model.7,8 Allan et al.7theoretically discovered, using first principle calculations,the formation, in ultrasmall nanocrystallites 1.75 nm, ofnew stable configuration or phase, distinct from but inter-connected to diamond-like structure by a potential barrier. Itis based on pairing of surface atoms to form intrinsic Si–Sidimer bonds that act as self-trapped excitons on a singlebond. Figure 4 gives a schematic of the interatomic potentialof the bond7 and the various pathways for absorption andemission in a 1.03 nm diameter particle.8 To estimate theanharmonicity, we expand the interatomic potential as afunction of the bond length Fig. 4 about its minimum.V(r)ar2Dr3 where r is the displacement from the po-tential minimum, and the second term is the first anharmonicterm. The fit gives mD5.11013 V/m3, where m is themass of the electron. For centrosymmetric material, the co-efficient D vanishes. From D we can determine the frequencyindependent nonlinear optical coefficient 	mD/2e3N2FIG. 2. Emission spectra for the three excitation wavelengths 780 solid,800 dot, and 832 dense dot nm at different incident intensities. Eachshows a peak with a shoulder on the red wing. The shoulders are set at 390,400, and 416 nm, respectively.FIG. 3. Emission spectra of increasing strength recorded at 832 nm for 15,29, and 59 mW incident power, showing a quadratic dependence.FIG. 4. Schematic of the interatomic potential of the Si–Si bond and thevarious pathways for absorption and emission in a 1.03 nm diameter par-ticle.4087Appl. Phys. Lett., Vol. 77, No. 25, 18 December 2000 Nayfeh et al.P Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.200.102.124 On: Fri, 222:41:17where N is the density of electrons that contribute to thepolarization.11 This gives 	71013 MKS for N61028/m3. This is a large coefficient in view of the fact thatthe mean value of the parameter 	 for 25 noncentrosymmet-ric crystals known for second harmonic generation is 2109, insuring a large polarization even for smaller suscep-tibilities. The polarization at the second harmonic is propor-tional to 	 and to the susceptibilities at the frequencies and 2: P (2)1/2d (2)E02 cos 2 , where d (2)	( ())2 (2), and  () is the linear susceptibility.To explore the origin of the blueshifted peak, we checkthe characteristic vibration frequency of the system. We pre-viously calculated the vibration structure of the dimers,8yielding 42 meV for the vibrational spacing. For incidentwavelength of 780 nm, this vibration energy produces ananti-Stokes line at 759.4 nm, with a shift of 20.6 nm. Har-monic generation of the Raman shifted radiation gives a lineat 380 nm, with a shift of 10.3 nm. Thus it is possible that theblueshifted peak is a higher order nonlinear coherent processsuch as stimulated anti-Stokes scattering.11,12 AlthoughStokes lines are expected to be more efficient than anti-Stokes lines, there are no resolved Stokes peaks.In cataloging the response from different parts of theshattered film, we noticed some relationship between the sec-ond harmonic and the luminescence. However, we noted nonoticeable differences in the optical image of the film thatmight be correlated with the spectral width or distribution ofthe emission spectrum. The spectra in Figs. 2 and 3 showstrong harmonic generation. Spectra in general, have a har-monic peak, a blue band 400–500 nm, and a weaker green/yellow band 510–623 nm. Figure 5 shows several spectralfrom different parts of the film; they show the harmonic peakanticorrelates with the blue band but correlates with thegreen/yellow band. Is the disappearance of the harmonicgeneration due to internal reabsorption to produce the lumi-nescence, or is it inhibited dynamically in favor of the lumi-nescence channels? or due to phase matching conditions?Those require information on the fundamental structure ofthe nanoparticles as well as the electromagnetic part of theproblem, including issues of phase matching, in addition tothe feedback by the silicon substrate.In conclusion, we dispersed Si into a colloid of ultras-mall nanoparticles 1 nm, and reconstituted into microc-rystallites on device-quality Si. The film is excited by near-infrared femtosecond two-photon process at 750–830 nmwith the radiation focused to 1 m. We have observednarrow radiation at half the incident wavelength. The resultsare discussed in terms of second-harmonic generation. Ionicvibration of or/and excitonic trapping on radiative Si–Sidimer phase, found only in ultrasmall nanoparticles, are sug-gested as a basic mechanism for inducing anharmonicity thatbreaks the centrosymmetry.The authors acknowledge State of Illinois Grant IDCCANo. 00-49106, the U. S. Department of Energy Grant No.DEFG02-91-ER45439, and the National Institute of HealthRR03155 and the University of Illinois at Urbana-Champaign.1 J. Wang, H.-B. Jiang, W.-C. Wang, J.-B. Zheng, F.-L. Zhang, P.-H. Hao,X.-Y. Hou, and X. Wang, Phys. Rev. Lett. 69, 3252 1992.2 R. P. Chin, Y. R. Shen, and V. Petrova-Koch, Science 270, 776 1995.3 M. Nayfeh, O. Akcakir, J. Therrien, Z. Yamani, N. Barry, W. Yu, and E.Gratton, Appl. Phys. Lett. 75, 4112 1999.4 L. T. Canham, Appl. Phys. Lett. 57, 1046 1990.5 O. Akcakir, J. Therrien, G. Belomoin, N. Barry, E. Gratton, and M. Nay-feh, Appl. Phys. Lett. 76, 1857 2000.6 G. Belomoin, J. Therrien, and M. H. Nayfeh, Appl. Phys. Lett. 77, 7792000.7 G. Allan, C. Delerue, and M. Lannoo, Phys. Rev. Lett. 76, 2961 1996.8 M. Nayfeh, N. Rigakis, and Z. Yamani, Phys. Rev. B 56, 2079 1997;Mater. Res. Soc. Symp. Proc. 486, 243 1998.9 Z. Yamani, H. Thompson, L. AbuHassan, and M. H. Nayfeh, Appl. Phys.Lett. 70, 3404 1997; D. Andsager, J. Hilliard, J. M. Hetrick, L. H.AbuHassan, M. Plisch, and M. H. Nayfeh, J. Appl. Phys. 74, 4783 1993;Z. Yamani, S. Ashhab, A. Nayfeh, and M. N. Nayfeh, ibid. 83, 39291998.10 Z. Yamani, A. Alaql, J. Therrien, O. Nayfeh, and M. H. Nayfeh, Appl.Phys. Lett. 74, 3483 1999.11 A. Yariv, Quantum Electronics Wiley, New York, 1975.12 Y. R. Shen and N. Bloembergen, Phys. Rev. 137, A1787 1965; J. A.Armstrong, N. Bloembergen, J. Dunning, and P. S. Pershan, ibid. 127,1918 1962.FIG. 5. Emission spectra at the same intensity and wavelength from severalspots or/and microcrystallite fragments. The harmonic peak anticorrelateswith a blue band at 400–500 nm but correlates with the green/yellow bandat 510–625 nm. No obvious differences in the optical images of the frag-ments that may explain the differences in the spectral distributions.4088 Appl. Phys. Lett., Vol. 77, No. 25, 18 December 2000 Nayfeh et al.P Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.200.102.124 On: Fri, 222:41:17

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Second harmonic generation in microcrystallite films of ultrasmall Si nanoparticles

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Second harmonic generation in microcrystallite films of ultrasmall Si nanoparticles

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