The discovery of the fullerenes in 1985 by Kroto, Heath, O'Brien, Curl and Smalley and the development of a method for production of macroscopic amounts in 1990 by Kraetschmer, Lamb, Fostiropoulos and Huffman opened a new area of carbon research with possible production of new materials with unique properties. The field has developed further later on with discoveries of nanotubes, metal filled nanotubes, carbon onions and more recently metal covered fullerenes. All these new discoveries show how cluster science opens approaches to the area of meososcopic physics. The general trend is here in the direction from small to large contrary to the general trend of modern meososcopic physics or micro-electronics where the movement is from large to small. It is especially fascinating how the whole area of fullerene research was initiated by problems in astrophysics. Originally Kraetschmer and Huffman had the intention to explain an observed strong extinction form interstellar dust and produced in experiments special carbon soot with a characteristics optical absorption known as 'the camel hump smoke'. This paper gives a short overview of some of our more recent theoretical work of the electronic properties of C₆₀, metal covered C₆₀ and nanotubes. In addition some results are also presented of optical properties of metal covered C₆₀ as a function of metal coverage.

Electron spin resonance (ESR) spectroscopy is used in this study in connection with the mass spectrometry one of the most promising method to characterize the state of endohedral fullerenes. The aim of this contribution is to study scandium, yttrium, lanthanum, cerium, praseodymium, samarium, europium, holmium, thulium and lutetium containing fullerenes in dependence on the production and handling conditions with respect to their electronic structure. For the most metallofullerenes both single filled and double filled fullerenes were measured. The mass spectrometric signals of single filled fullerenes are much larger than the signals of double and higher filled metallofullerenes. ESR spectroscopic properties were followed especially in solution of isolated metallofullerenes of high purity characterized by well resolved ESR spectra but also of the solid soot extract resulting in low resolved ESR spectra. In this way new information on the electronic states and the influence of oxygen on endohedral metallofullerene were obtained. The interaction of the lanthanofullerence with oxygen is studied in detail. Different stabilities in different solvents in the presence of oxygen were found for La at C₈₂ molecules. For scandium, yttrium and lanthanum containing fullerenes ESR spectra with hyperfine structure splittings corresponding to the nuclear spin of the metal ion were found. Experiments with lanthanoides results in fullerenes, showing mass spectra with lines equivalent to the corresponding endohedral compounds with single and double filed cages, but no ESR spectra due to the interaction of an unpaired electron with the nuclear spin of the lanthanoide ion could be detected in the magnetic field range of 0 to 800 mT.

The optical excitations in C₇₀ and higher fullerenes, including isomers of C₇₆, C₇₈, and C₈₄, are theoretically investigated. We use a tight binding model with long-range Coulomb interactions, treated by the Hartree-Fock and configuration-interaction methods. We find that the optical excitation sin the energy region smaller than about 4eV have most of their amplitudes at the pentagons. The oscillator strengths of projected absorption almost accord with those of the total absorption. When the projection is performed on each pentagon and pentagon dimers, the resultant spectrum in the low energy region is quite different form that of the total absorption. The spectral shapes of the total absorption turned out to be determined mainly by the geometrical distributions of the pentagons in the fullerene structures.

We show that recent fluorescence and fluorescence-excitation spectra of C₆₀ can be combined with earlier theoretical and experimental results to locate the origins of the tow lowest singlet excited states. At the same time, we assign the prominent features of fluorescence and fluorescence- excitation spectra.

Second and third order nonlinear optical properties of C₆₀ based composites and multilayered structures with TPP and TPN are studied by transverse optical second and third harmonic generation, respectively. The results are compared with those obtained from pristine, photolyzed in vacuum and in oxygen atmosphere C₆₀ thin films. Second harmonic generation is observed from structures containing C₆₀ molecules. An enhancement of the quadratic susceptibility is observed from the multilayered structures. The third harmonic generation experiments show an enhancement of the cubic susceptibility from photolyzed C₆₀ thin films. No increase of (chi) ⁽³) in the multilayered structures or composites is observed. A slightly larger response is obtained with the multilayered thin films as compared to the composite material.

We present an overview of what has been learned form nonlinear optical spectral studies in C₆₀ and C₇₀ by our group and others. These include third harmonic generation (THG), degenerate four-wave mixing (DFWM), and electroabsorption (EA). We present empirical models of interacting states consistent with spectra of EA, THG, and DFWM in C₆₀ and C₇₀. The spectra in the dispersed and solid forms differ significantly. In C₆₀ the lowest- energy resonances in (chi) ^(3_ are dominated by 'forbidden' transitions made allowed by vibronic coupling in the molecule and intermolecular coupling in the solid. These interactions must be included in theoretical models of C₆₀ if comparison is to be made between calculated and experimental nonlinear optical spectra. Major features of (chi) ⁽³) spectra in C₇₀ can be modeled with similar states redshifted from those of C₆₀.

Photophysical properties of fullerene thin films were investigated by carious spectroscopic measurements including absorption, luminescence, luminescence excitation, electric filed induced luminescence quenching, and electroabsorption. These results were compared with photoconduction spectra, giving a picture of solid fullerene's excited states characterized by substantial charge transfer (CT) nature, that is, excited states delocalized over neighboring molecules in the solid lattice. For C60 films, the CT exciton emerges around 540nm, while for C70 films, it was located at 690 and 535nm. Electroabsorption spectra of the two showed strong modulation at these same energies, and their spectral shapes were well fitted by the second derivative of the corresponding absorption spectra, another evidence for CT excitons. In a different context of work, an attempt was made to develop a new photonic material by the alternate deposition of C60/Si composite layers. We have found that, for appropriate Si content, the composite films emit strong white light under Ar ion laser excitation. The emission spectrum, after correction for the detecting system, was found to be very broad with no specific structures and increased monotonously form visible to 1.5 micrometers , the limit of our measurement apparatus. Moreover, the spectral shape was well fitted by a Planck distribution, giving a surprisingly high black-body temperature of 1400- 1600K. Also discussed are similarities and differences as compared with the previously reported white luminescence from C60 crystallites or powder and their implications.

We discuss the photophysical properties of the supramolecular composites of two (pi) electron semiconductors; e.g. conjugated oligothiophenes as electron donors and Buckminsterfullerene, C60, as electron acceptor in different solvents as well as in solid composite films. Thus, the supramolecular solid state composites of these two conjugated materials exhibit an ultrafast, reversible, metastable photoinduced electron transfer and long lived charge separation. Upon changing the polarity of the surrounding media, however, we observe a profound effect on the photophysical properties of these composites in solution. Polar solvents induce electron transfer upon photoexcitation. In non-polar solvents the energy transfer is dominant. To investigate the effects of molecular ordering we embedded conjugated oligomers into nematic liquid crystals and achieved a high degree of orientation. Furthermore, in these spontaneously oriented media, the orientation of the liquid crystal host alongside with the conjugated oligomer guest can be switched upon applying an electric field. This results in dynamic switching of the polarized photoluminescence of oligomers.

We have studied photoexcitation dynamics in pristine and C60-doped PPV films under bias illumination by photoinduced absorption (PA), optically-detected magnetic resonance (ODMR), and electromodulated PA (EPA). Intrachain polaron pair species are the dominant excitations under these conditions. The polaron pairs on the polymer chains are identified in ODMR by the existence of an anisotropic spin- exchange interaction; their characteristics absorption spectrum is blueshifted with respect to that of isolated polarons by approximately 0.4 eV. We also identified the PPV⁺/C60 complexes by EPA spectroscopy.

We have succeeded in adsorbing individual C60 molecules onto the tunneling region of an STM tip. The individual tip- adsorbed molecules are imaged by scanning the fullerene- adsorbed tip over a defect covered graphite surface. The nanometer-size defects serve as a surface tip array which 'inverse images' the molecules adsorbed to the tip when the surface is scanned. These tips were subsequently used to observe threefold symmetric electron scattering from point defects on a graphite surface, an effect that could not be observed using bare metal tips. Functionalizing an STM tip with an appropriate molecule adsorbate alters the density of states near the Fermi level of the tip and changes its imaging characteristics.

Optical quality organic blend films were developed with high photosensitivity. These materials are composed with conjugated polymers, P3OT, or poly(2-methoxy-5-(2-ethyl- hexyloxy)-1,4-phenylene vinylene), MEH-PPV) and fullerene molecules. These blends are soluble to common organic solvents, and are processable at room temperature. Thin film devices in sandwich configurations were fabricated with high quantum efficiency. The carrier collection efficiency and energy conversion efficiency in the MEH-PPV:C60 photovoltaic cells are approximately 29 percent electrons/photons and 2.9 percent respectively, better by more than two orders of magnitude than devices made with MEH-PPV alone. The photosensitivity are even higher with reverse bias, approximately 0.2A/W-0.3A/W in visible region at a few volts. These numbers are comparable to photodiodes made with inorganic semiconductors.

Some recent results of a research work aimed at exploiting the potentialities of fullerene derivatives to obtain sol- gel glassy materials for use as optical limiters are reported. The solubility of fullerene derivatives in organic solvents compatible with the sol-gel process has been increased by more than two orders of magnitude by suitable functionalization of C60. Covalent linking to the glassy matrix is favored by introducing silicon alkoxide groups in the fullerenes. The optical limiting properties of these derivatives in toluene solution are comparable to those of the parent compound and exhibit enhancement in the red spectral range. Optical-quality films of various thickness are obtained by incorporating the C60 derivatives at high concentration in organically modified sol-gel matrices. These materials exhibit high resistance to laser damage and can be used to build multilayer structures optimized for optical limiting.

We compare detailed dynamics of the excited-state absorption for C60 in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C60 molecule plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of the C60-glass composite occur on long timescales, comparable to that in solution; thin film samples, by contrast, show rapid decay. These results demonstrate that the C60-sol-gel glass composites contain C60 in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450-950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range form 620-810 nm.

The results of a recent study of a nonlinear electro-optical effect in dye and fullerene C60-doped nematic liquid crystal films are presented. In particular, photo-induced conduction, self diffraction effects, and optical phase conjugation in the films form near-UV to near-IR wavelengths are explained.

We report two separate studies of the nonlinear optical properties of unique fullerene materials. Degenerate four- wave mixing measurements of the endohedral metallofullerene Er₂ at C₈₂ at 1064 nm show that the third order susceptibility is increased by more than two orders of magnitude relative to the empty cage fullerenes as a result of the metal-to-cage charge transfer. Optical limiting studies of C60 derivatives and higher fullerenes in the 680 to 880 nm spectral region demonstrate that these materials are strong reverse saturable absorbers at wavelengths where C60 itself is transparent. The tremendous variety of available fullerene-derived structures provides important opportunities for optimization of nonlinear optical responses.

Optical limiting behavior, which is generally observed with high peak power, short laser pulses in Fullerene solutions, can also be observed at low intensity with continuous wave irradiation by a He-Ne or Ar laser. The origin of the effect lies, like in the case of pulsed laser excitation, in a strong excited triplet state absorption. We use this effect to demonstrate various types of all-optical switching and optical bistability.

The optical limiting of Q-switched Nd:YAG radiation oat 532 nm is performed using Fullerenes-toluene solution contained in a cell with a random rough surface. One of the inside surface of K9 glass was roughened. At low incident fluences the random rough surface windows index matches with fullerenes solution, so optical limiting behavior in fullerenes-toluene solution is based on excited state absorption. At high incident fluences thermally induced index change causes a sufficient index mismatch at the random rough surface and leads to significant scattering. Additional limiting due to scattering occurs. The limiter of excited state absorption/random surface scattering is superior to the limiter of purely excited state absorption.

We study the antiferromagnetic phase of the C60-polymer doped with one electron per one C60 by using a tight-binding model with long-range Coulomb interactions. The model is solved by the unrestricted Hartree-Fock approximation, and a phase diagram is obtained by changing Coulomb interaction strengths and a conjugation parameter between C60 molecules. We find that the anti-ferromagnetism is well described by the present model. The comparison with the photoemission studies shows that the new band around the Fermi energy of the A₁C60 phase can be explained by the extremely large intrusion of an energy level is to the gap of the neutral system. This indicates that the interaction effects among electrons are important in doped C60-polymers.

Optical power limiting of Fullerenes/PMMA has been studied based on a time-resolved single-beam technique. A nanosecond/picosecond Nd:YAG laser system with pulse width of 8-ns, repetition frequency of 10 Hz and wavelength of 532-nm was employed. Temporal profile of pulse beam is Gaussian profile. When the peak power density of incident laser pulse P_in is 180 WM/cm², the incident and the transmitted laser pulses versus time are measured using a 4402 Boxcar of 0.4 ns time resolution. The experimental results have shown that, at high irradiance the transient transmitted power density increases nonlinearly and limits to less than 30 MW/cm². In our experiment, the switching speed of the optical power limiter is less than 5 ns. The measured results can be fitted on the excited state absorption theory. Theoretical results are shown that the excited state absorption saturation occurred at higher peak power.

Time-resolved and steady-state techniques are employed to explore the photophysical properties of C60C(COOEt)₂, equatorial-C60[C(COOEt)₂]₂, trans₃- C60[C(COOEt)₂]₂, trans₂-C60[C(COOEt)₂]₂, and equatorial-C60[C(COOEt)₂]₃. Picosecond-resolved energy transfer to the fullerene core results in the formation of the excited singlet state with remarkably blue-shifted singlet-singlet transitions in going from C60 to 3. Rapid formation of the triplet-triplet absorption as a consequence of intersystem crossing to the energetically lower lying excited triplet state suffers a deceleration with increasing number of functionalizing addends. The corresponding triplet-triplet absorption energies also show a significant dependence on the degree and site of functionalization, spreading over a range of 100 nm. Energy transfer from radiolytically excited biphenyl to the fullerene's ground state, corroborates the photolytic data. 0->0 transitions from the lowest level of the excited singlet state and excited triplet state are mirror- images to the reversed 0->0 absorption transitions. Red- shifts of these emission, relative to pristine C60, again sensitively reflect the degree and site of functionalization. Cyclic voltammetry and reductive quenching of triplet excited fullerenes demonstrate that functionalization of the fullerene's (pi) -system obstructs the ease of reduction in the ground and excited triplet state. An increasing number of bis(ethoxycarbonyl)methylene groups shifts the redox potential of the ground state from -0.54 to -0.86 V versus SCE and of the excited triplet state from +1.01 V versus SCE to +0.64 V versus SCE for C60 and 3, respectively.

We discuss the production of carbon nanotubes by the electric arc-discharge method. Co-evaporation of metals, Yttrium, Lutetium, Nickel and Boron have been used with the following stoichiometry: YNi₂B₂C or LuNi₂B₂C. We have obtained singlewalled and multiwalled nanotubes and also crystalline nanoparticles embedded in glassy carbon as evidenced by HRTEM and x-ray diffraction observations. The SQUID measurements showed a superconducting transition which would mean that crystalline particles present the superconducting phases of YNi₂B₂C, YNi₄B₄C or LuNi₂B₂C. HRTEM investigations are corroborated by Raman spectra which exhibit the features characteristic of singlewalled nanotubes by the presence of three main peaks at approximately 1580 cm^-1, approximately 1350 cm^-1 and approximately 180 cm^-1. Other samples present on the E_2g2 mode and those assigned to glassy carbon at approximately 1350 cm^-1 and approximately 1600 cm^-1.

The linear absorption and electroabsorption spectra of C60 crystals are discussed theoretically. Excited states of a model cluster consisting of a few C60 molecules are calculated within the extended Pariser-Parr-Pople model and the single-excitation configuration-interaction method. Electroabsorption spectra are numerically calculated by use of the standard formula for the third-order nonlinear optical susceptibility. We assume moderately strong electron-electron interactions and weak interchain transfer energies. The mixing of intermolecular charge transfer components with intramolecular Frenkel exciton states gives rise to weak humps in the tail parts of the main absorption peak. In the electroabsorption spectrum, the charge transfer states have larger contributions than intramolecular excitons. The results are qualitatively in good agreement with experiments.

We have carried out measurements of real and imaginary parts of second order hyperpolarizability (gamma) of C60, C70, (Ph₃P)₂PtC60, ((C₅H₅)₂Fe)₂C60 and (C₅H₅)₂CoC60 solutions and C60 films using the Z-scan technique with picosecond excitation at 527.5 and 1055 nm wavelengths. It was found that addition of ferrocene or cobaltocene to C60 molecule slightly influences its nonlinearity while Pt-group attachment increases total nonlinearity by an order of magnitude. Strong dependence of (gamma) on wavelength and state of radiation polarization has been observed. Two-photon absorption as a dominant nonlinear process in investigated fullerenes under picosecond excitation at 527.5 nm wavelength is discussed.

We report measurements on a multilayer of (alpha) - sexithienyl (T6) and C60 grown in ultra-high vacuum. The photoluminence spectrum of the multilayer is made up to T6 emission at high energies, and C60 emission at low energies. The T6 emission has more structure than that of bulk T6, even after taking into account reabsorption by the C60. However, the spectrum can be simulated quite well by assuming that there is both emission from bulk T6 and isolated molecules of T6, based on the vibronic structure that we would expect from isolated T6 molecules. Molecules of T6 may conceivably become isolated at the C60 interface, if they migrate into gaps between C60 grains, for example. Thus we cannot conclude that any of the new peaks we see are due to charge transfer between the C60 and T6. The low energy fluorescence is that of C60, but its efficiency is orders of magnitude larger than is seen in films of C60. We cannot explain this yet.

We have extended the wavelength range of our previous study of the third order nonlinear optical susceptibility tensor (chi) ⁽³)(-(omega) , (omega) , (omega) , -(omega) ) of a thin C60 film to 1.7 micrometers . We use time-resolved degenerate four-wave-mixing with femtosecond pulses to measure both the phase and magnitude of (chi) ₁₁₁₁. Our data are well defined in terms of a single two-photon resonance at a fundamental wavelength of 930 nm. From our fit parameters, we predict for (chi) ₁₁₁₁ in the zero-frequency limit a value of (9 +/- 3) X 10^-13 esu and at the resonance maximum a value of i(3.9 +/- 0.6) X 10^-12 esu.

We report photoinduced absorption and photoinduced reflectance spectra of poly(3-octyl thiophene)- methanofullerene films. The data allow direct evaluation of the corresponding changes in the complex refractive index, (Delta) N equals (Delta) n((omega) )+i(Delta) (kappa) ((omega) ), of the films. As a results of the efficient photoinduced intermolecular charge transfer, the magnitudes of (Delta) n((omega) ) and (Delta) (kappa) ((omega) ) are significantly enhanced over those in either of the component materials, with (Delta) n and (Delta) (kappa) approximately equals 10^-2 in the infrared at laser pump intensity of only 50mW/cm². Moreover, the photoinduced absorption shows new features at 1.2 eV and 1.6 eV, both of which are associated with excited state absorptions of the methanofullerene anion. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials, indicating that conducting polymer/methanofullerene films are promising as high- performance nonlinear optical materials.

Holographic recording has been demonstrated in conducting polymer/C60 blends. These materials allow ultrafast holographic recording and the ability to tailor the decay dynamics of the recorded hologram. Diffraction efficiencies up to 1.6 percent have been measured in these materials for gratings recorded with individual laser pulses. Results are presented that demonstrate that an improved signal-to-noise ratio is obtained when holographic detection is used to observe the dynamics of photo-induced absorption.