We elucidate the crystalline nature and the three-dimensional orientation of isolated organic nanocrystals embedded in a sol-gel matrix, using a polarized nonlinear microscopy technique that combines two-photon fluorescence and second harmonic generation. This technique allows the distinction between mono-crystalline structures and nano-scale poly-crystalline aggregates responsible for incoherent second harmonic signals.PACS numbers: 78.67. Bf, 61.82.Rx The optical properties of nanoparticles have recently attracted much attention. In addition to metallic and semiconductor nanoparticles which are now used as biomarkers and as the building blocks of nanostructured materials [1], their organic counterparts constitute an interesting alternative. Advances in molecular engineering have enabled the design of molecular structures of various resonances and symmetries with optimized one-and two-photon absorption cross sections [2], or combining different optical properties such as luminescence and second harmonic generation (SHG) [3]. In addition, macroscopic molecular arrangements have been optimized using the tensorial oriented gas model [4], which predicts that an enhancement of the SHG efficiency is expected from the non-centrosymmetric crystalline arrangement of efficient nonlinear molecules. Molecular nanocrystals can be therefore envisioned as a new class of multi-functional nano-scale materials. In the case of organic nanocrystals however, the traditional crystalline characterization techniques have raised many practical barriers due to their low concentration and fragility. Consequently, the elucidation of their crystalline nature has been so far indirect or averaged over a large number of nanocrystals [5].In this letter, we show that two-photon nonlinear microscopy permits in-situ characterization of isolated organic nanocrystals grown in an amorphous sol-gel matrix. The diagnostic is based on polarization resolved two-photon excited fluorescence (TPF) and SHG. TPF is an incoherent process allowed in centrosymmetric media, which exhibits a specific anisotropy depending on the medium symmetry. On the other hand, SHG is the signature of a crystalline non-centrosymmetric phase in the sample, with a sensitivity down to the nanometric scale [6,7]. We show that the polarization analysis of both TPF and SHG from nanocrystals allows the unambiguous discrimination between isolated mono-crystalline and poly-crystalline systems. Moreover, once a nanocrystal has been identified as mono-crystalline, a detailed model for both TPF and SHG polarization responses accounting for the unit-cell symmetry allows the determination of its three-dimensional orientation within the host matrix.The organic nanocrystals that we investigate are based upon the α-((4'-methoxyphenyl)methylene)-4-nitro-benzene-acetonitrile) molecule (CMONS), which exhibits efficient luminescence and quadratic nonlinearity under two-photon excitation [8,9,10]. The bulk crystalline phases of such crystals have three possible polymorphic forms, two being noncen...

We studied intensity fluctuations of a single photon source relying on the pulsed excitation of the fluorescence of a single molecule at room temperature. We directly measured the Mandel parameter Q(T ) over 4 orders of magnitude of observation timescale T , by recording every photocount. On timescale of a few excitation periods, subpoissonian statistics is clearly observed and the probablility of two-photons events is 10 times smaller than Poissonian pulses. On longer times, blinking in the fluorescence, due to the molecular triplet state, produces an excess of noise.PACS numbers: 42.50. Dv, 03.67.Dd, Over the past few years, there has been a growing interest for generating a regular stream of single photons on demand. This was mainly motivated by applications in the field of quantum cryptography [1]. An ideal single photon source (SPS) should produce light pulses containing exactly one photon per pulse, triggered with a repetition period τ rep , and delivered at the place of interest with 100% efficiency. For any given measurement time T , this source would emit exactly N = T /τ rep photons, so that the standard deviation ∆N ≡ N 2 T − N 2 T = 0 ( T has to be understood as a mean value over a set of measurements lasting T ). Such a source would then be virtually free of intensity fluctuations, therefore corresponding to perfect intensity squeezing [2].A first category of SPSs already realized consists of sources operating at cryogenic temperature. They rely on optically [3,4,5,6] or electrically [7] pumped semiconductor nanostructures or on the fluorescence of a two level system coherently prepared in its excited state [8]. A one-atom micromaser has also been used to prepare arbitrary photon number states on demand [9]. However the collection efficiency of photons is barely higher than a few 10 −3 in these experiments. Due to this very strong attenuation, the intensity statistics are very close to a Poisson law at the place where the stream of photons is available. Another route is to realize SPSs at room temperature. In this case higher collection efficiency (around 5%) is achieved. The existing room-temperature SPSs rely on the pulse saturated emission of a single 4-levels emitter [10,11].When the pulse duration τ p is much shorter than the dipole radiative lifetime τ rad , such a single emitter can only emit one photon per pulse. This temporal control of the dipole excitation allows therefore to easily produce individual photons on demand [12,13]. However, in previous SPS realizations, little attention has been paid to analyse their intensity fluctuations. To address this problem we realized a room-temperature SPS relying on the pulsed saturation of a single molecule embedded in a thin polymer film [14].The samples are made of cyanine dye DiIC 18 (3) molecules dispersed at a concentration of about one molecule per 10 µm 2 into a 30 nm thick PMMA layer, spincoated over a microscope coverplate. The fluorescence from the single molecule is excited and collected by the standard technique of scanning confocal opt...

We present high resolution two-photon excitation microscopy studies combining two-photon fluorescence (TPF) and second harmonic generation (SHG) in order to probe orientational distributions of molecular ensembles at room temperature. A detailed polarization analysis of TPF and SHG signals is used in order to unravel the parameters of the molecular orientational statistical distribution, using a technique which can be extended and generalized to a broad variety of molecular arrangements. A polymer film containing molecules active for TPF and/or SHG emission is studied as a model system. Polarized TPF is shown to provide information on specific properties pertaining to incoherent emission in molecular media, such as excitation transfer. SHG, being highly sensitive to a slight departure from centrosymmetry such as induced by an external electric field in the medium, complements TPF. The response of each signal to a variable excitation polarization allows investigation of molecular behavior in complex environments which affect their orientations and interactions.

using a commercially available capacitance meter operating at 800 Hz (Iso-Tech 9023). The capacitors were connected to the capacitance meter via Karl Süss MicroTec PH100 miniature probe heads. The measurement was repeated on a minimum of six spots located on the whole length of anodized part of the foil. The leakage measurements were performed directly after capacitance measurements using the same connections. The bottom (Ti) electrode was biased either positively (as it was during anodization) or negatively (as it would be in a p-type transistor). Subsequently, organic field-effect transistors were fabricated by evaporating pentacene (97 %, Aldrich, used as received) under a high vacuum < 5 10 ±7 torr with a rate of 0.3 nm s ±1 at room temperature. The pentacene film thickness measured with a Dektak profilometer was~50 nm. The transistors were completed by evaporation of gold source and drain contacts through a shadow mask on the top of the pentacene layer. The channel width and length was 2 mm and 25 lm, respectively. Characterization was performed at ambient conditions without any special precautions. Two Keithley 2400 source/measure units were used to control source±drain (V SD ) and gate (V G ) voltages. The study of octupolar molecules [1,2] has opened up new research directions in the design and synthesis of new families of molecules that have large nonlinear optical susceptibilities (hyperpolarizabilities), b, with possible applications in optical and opto-electronic devices. Among the advantages of octupolar structures is that they have a broader range of nonlinear tensorial coefficients b ijk (where i, j, k are the molecular-frame indices) compared to earlier 1D schemes, [3] which provides an optimum nonlinear efficiency with a polarization-independent second-harmonic response to incident light. Optimization at the molecular level has made great progress, as exemplified by the established structure±property relationship of two-dimensional octupoles and by the development of highly efficient molecules.[4±7] Different strategies have been proposed to obtain a significant bulk second-order optical nonlinearity, v (2) , by the non-centrosymmetric alignment of octupoles such as optical poling in polymers, [8] supramolecular arrangement, [9±11] and crystal engineering. [12] Although the crystalline structures would be more advantageous in terms of stability, achieving three-dimensional octupolar geometry at the mac-

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