[en] 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

[en] We dispersed electrochemical etched Si into a colloid of ultrabright blue luminescent nanoparticles (1 nm in diameter) and reconstituted it into films or microcrystallites. When the film is excited by a near-infrared two-photon process at 780 nm, the emission exhibits a sharp threshold near 10⁶W/cm², rising by many orders of magnitude, beyond which a low power dependence sets in. Under some conditions, spontaneous recrystallization forms crystals of smooth shape from which we observe collimated beam emission, pointing to very large gain coefficients. The results are discussed in terms of population inversion, produced by quantum tunneling or/and thermal activation, and stimulated emission in the quantum confinement-engineered Si--Si phase found only on ultrasmall Si nanoparticles. The Si--Si phase model provides gain coefficients as large as 10³--10⁵cm^-1

[en] Ultrabright ultrasmall (∼1 nm) blue luminescent Si₂₉ nanoparticles are chlorinated by reaction with Cl₂ gas. A Si - N linkage is formed by the reaction of the chlorinated particles with the functional amine group in butylamine. Fourier transform infrared spectroscopy and x-ray photospectroscopy measurements confirm the N linkage and the presence of the butyl group, while emission, excitation, and autocorrelation femtosecond optical spectroscopy show that, after the linkage formation, the particles with the ultrabright blue luminescent remain, but with a redshift of 40 nm. [copyright] 2001 American Institute of Physics

[en] Detection of UV photons is becoming increasingly necessary with the use of noble gases and liquids in elementary particle experiments. Cerenkov light in crystals and glasses, scintillation light in neutrino, dark matter, and rare decay experiments all require sensitivity to UV photons. New sensor materials are needed that can directly detect UV photons and/or absorb UV photons and re-emit light in the visible range measurable by existing photosensors. It has been shown that silicon nanoparticles are sensitive to UV light in a wavelength range around ∼ 200 nm. UV light is absorbed and re-emitted at wavelengths in the visible range depending on the size of the nanoparticles. Initial tests of the wavelength-shifting properties of silicon nanoparticles are presented here that indicate by placing a film of nanoparticles in front of a standard visible-wavelength detecting photosensor, the response of the sensor is significantly enhanced at wavelengths < 320 nm

[en] We examine the photostability of silicon nanoparticles when they are dispersed in liquid or immobilized in gels or on surfaces. We show that the photoluminescence in static solution develops, under UV irradiation, a long-term stability at the 50% level. Under the same conditions, common dye molecules such as coumarin and stilbene quench with time at rates 8 and 50 fold faster, and exhibit no long-term stability. For the case of immobilized particles in agarose gel as well as on a quartz substrate we used two-photon near infrared femtosecond excitation at 780 nm to induce the blue luminescence. ''Parking'' the excitation beam, focused on such stationery particles shows that they, unlike similarly immobilized dye molecules, are highly photostable at more than 80%-90% level and do not bleach. The photostability is discussed in terms of excited state interactions and structuring of the silicon outer shell.

[en] We dispersed electrochemical etched Si into a colloid of ultrasmall blue luminescent nanoparticles, observable with the naked eye, in room light. We use two-photon near-infrared femtosecond excitation at 780 nm to record the fluctuating time series of the luminescence, and determine the number density, brightness, and size of diffusing fluorescent particles. The luminescence efficiency of particles is high enough such that we are able to detect a single particle, in a focal volume, of 1 pcm3. The measurements yield a particle size of 1 nm, consistent with direct imaging by transmission electron microscopy. They also yield an excitation efficiency under two-photon excitation two to threefold larger than that of fluorescein. Detection of single particles paves the way for their use as labels in biosensing applications. (c) 2000 American Institute of Physics

[en] Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ∼10⁶ W/cm² (8x10¹⁰ W/cm² peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si-Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption. (c) 1999 American Institute of Physics