Results of optical studies of thin solid C60 films are presented which highlight some of the unusual optical phenomena observed in fullerene-derived materials. Results of several experiments to determine quantitatively the optical dielectric function are first presented and discussed. We also present results on two different phototransformations of C60 films from a toluene-soluble to a toluene-insoluble state. These processes may have photolithographic application. The first involves the photopolymerization of C60, and the second is a photo-induced intercalation of di-oxygen into the C60 lattice leading to a film with composition C60(O2).
We report the successful preparation of a solid solution of C60 in a silicon dioxide (SiO2) glass matrix by means of sol-gel chemistry. Raman spectroscopy and x-ray diffraction were used to verify that our synthetic route produced glasses with a homogeneous dispersion of intact fullerenes. The vibrational spectrum of C60 is preserved in the C60/SiO2 gel glass. Raman and X-ray diffraction data confirm that the C60 is microscopically dispersed, and does not form detectable phase-separated, microcrystalline regions. We report preliminary observations of optical limiting in these gels, with intensity and concentration dependence consistent with that observed for C60 in solution.
We present a comprehensive experimental study of the vibrational spectra of nanotubes. There are two main lines observed in the Raman spectrum, one positioned at 1350 cm-1, the D line, and the other at 1580 cm-1, the G line. Both these lines are very similar to those seen with disordered graphite. The disorder induced D line is very weak compared to the G line which is indicative of high crystalline materials. The position and intensity of the D line strongly depends on the energy of the exciting laser. This dispersion effect was also observed for graphitic particles and may be explained by a photoselective resonance process of nanotubes with different sizes. There are two optically active modes in the Infrared spectrum for highly orientated polycrystalline graphite which are the E1u and A2u modes. The E1u mode is positioned at 1587 cm-1 while the A2u mode is positioned at 868 cm-1. The Infrared spectrum of the nanotubes shows both modes although the E1u mode is downshifted to 1575 cm-1.
After the development of a method for macroscopic production of fullerenes, in particular the C60 molecule, a lot of attention has been given to the optical and collective properties of C60 in gas phase, solid and film. The more recent development in fullerene related research is the synthesis of coaxial carbon sheets, usually referred to as carbon tubes or nano tubes. Also spherical concentric graphitic shells called carbon onions have made the field still more exciting and opened the doorway to a new area of materials science. In this work we present how a model based on classical electrodynamics can be used to describe the collective dynamics of electrons in these systems and give insight into the most important physics. We demonstrate the existence of a rich spectrum of collective resonances for both carbon onions and carbon tubes. Evaluated polarizability, oscillator strength distributions, and the EELS loss function are compared to available experimental data and other theoretical studies for especially C60.
Ground state local density calculations have been performed to evaluate the free response of the wavelength dependent microscopic nonlinear polarizabilities (gamma) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ), for the C60 molecule and the macroscopic susceptibilities (chi) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ) i.e. Third Harmonic Generation, for films using a sum-over-states approach. The influence of screening was determined by applying an external static electric field in separate self-consistent calculations to evaluate the induced dipole moment which was used to determine the static linear and nonlinear polarizabilities. The polarizabilities calculated in the static limit were used to determine effective screening parameters which were in turn used together with an RPA approach to calculate screened wave-dispersed, third-order nonlinear optical properties such as (gamma) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ) and (chi) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ). Inclusion of screening results in susceptibilities about two orders of magnitude below the experimental values.
We measure the magnitude and phase of both independent components of the femtosecond (fs) third-order nonlinear optical susceptibility tensor (chi) (3)(-(omega) ;(omega) ,(omega) ,-(omega) ) of a C60 film. We observe only the contribution of mechanisms whose rise and decay times are short compared to our 120 fs microjoule pulses. The value of (chi) 1111 varies between (6.39 +/- 0.35) X 10-13 esu at 745 nm and (19.7 +/- 1.1) X 10-13 esu at 875 nm, relative to fused silica, and rises monotonically with wavelength. Its phase angles lie between 100 and 150 degrees. Our results indicate that neither our, nor any previously published, measurements can be assumed to have approached the low frequency limit in C60.
We present a phenomenological theory of nonlinear optical response of fullerenes. An empirical tight-binding model is used in conjunction with a classical electromagnetic picture for the screening. Since in bulk media such a picture of screening corresponds to the self- consistent field approach, the only additional approximation involved in our approach is the neglect of nonlocality. We obtain reliable estimates for the linear and nonlinear susceptibilities of C60, C70, C76 and other pure carbon fullerenes and also substituted fullerenes. The relatively large values of (beta) that we obtain for C76 and substituted fullerenes appear promising for the development of fullerene-based nonlinear optical materials. Our phenomenological picture of screening provides a good understanding of the linear absorption spectra of higher fullerenes and predicts that a comparison of the one-photon and multi-photon spectra will provide an insight into screening effects in these systems.
The discovery in 1985 by Kroto, Heath, O'Brien, Curl and Smalley of the existence of a new form of carbon known as Buckminsterfullerene or C60 initiated a new field of carbon research. The development of the field was however rather limited and it was not until Kratschmer, Lamb, Fostiropoulos and Huffman developed a technique for production of macroscopic amounts that a number of new applications became possible. Originally Kratschmer and Huffman had the intention to explain an observed strong extinction from interstellar dust and produced a special carbon soot with a characteristic optical absorption known as `the camel hump smoke'. The observed absorption was in rather good agreement with excitation energies and oscillator strengths for C60 evaluated by Larsson, Volosov and Rosen (one of the authors) in 1987, using the semi-empirical CNDO/S-CI method. This good agreement seems to have encouraged Kratschmer and Huffman to continue with the development of the technique. This paper gives a historical overview and a presentation of recent calculations of optical spectra for C60 in the gas phase and the dielectric constants for films of C60 with a comparison with experimental data.
We present here the results for frequency-dependent linear polarizability, (alpha) , and second hyperpolarizability, (gamma) , coefficients of the icosahedral C60 molecule calculated from minimal basis ab initio and semiempirical methods in the framework of the time-dependent purturbation theory. The ab initio results of (alpha) are too small by a factor of two compared to their experimental counterpart. The semiempirical methods predict yet smaller values of (alpha) . The (gamma) coefficients obtained by the ab initio calculations are two orders of magnitude smaller than those derived from experiments. The corresponding values obtained by semiempirical calculations show improved agreement with the experiment. The ratio xxxx/xyyx for (gamma) (-3(omega) ;(omega) ,(omega) ,(omega) ) is calculated to be 1/3 up to an optical wavelength of 1.37 micrometers . The calculated values of (gamma) for the various third order effects follow the order: (gamma) (-3(omega) ;(omega) ,(omega) ,(omega) ) > (gamma) (-2(omega) ;0,(omega) ,(omega) ) > (gamma) (-(omega) ;(omega) ,(omega) ,-(omega) ) > (gamma) (-(omega) ;0,0,(omega) ) > (gamma) (0;0;0;0).
Two-photon excitation measurements of C60 single crystal at 4 K have been performed. The TPE spectrum shows a sharp band at 1.846 eV which is assigned to the C60 lowest forbidden Frenkel singlet exciton of T1g symmetry. This assignment is supported by the analysis of Herzberg-Teller induced photoluminescence.
The resonant nonlinear optical (NLO) properties and excited state dynamics of pristine, chemically and photochemically modified C60 are studied at 590.5 nm by time-resolved degenerate four-wave mixing and nonlinear transmission. The chemically modified fullerenes consist of (a) composite films of C60 with meso-tetraphenyl porphine (TPP) prepared by vacuum vapor deposition, and (b) spin-coated films of solutions of C60 mixed with the conjugated polymer, poly-[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The photochemically modified fullerene, poly-C60, is a distribution of oligomers of single C60 molecules proposed to be linked together through four-membered rings. Photopolymerization of C60 leads to an enhancement of its physical properties without substantial alteration of its NLO properties. C60 and TPP form a ground state charge transfer complex with an enhanced NLO response. C60 and MEH-PPV form a charge transfer complex in the excited state which gives rise to more than an order of magnitude increase in the NLO response and the figure of merit. The dynamics of the NLO response for C60, poly-C60, C60:TPP, and C60:MEH-PPV films show strong fluence dependence. Bimolecular decay via exciton-exciton annihilation dominates the temporal response at high-fluence.
We describe the results of the investigations of optical nonlinearities in fullerenes in various forms, ranging from solutions to composites devised in device-oriented forms. Two types of optically-induced nonlinearities are of great interest here: third-order nonlinearity and photorefractivity. The third-order nonlinearity has been investigated by a combination of photoinduced transient absorption and optical Kerr gate, using femtosecond pulses at 615 nm and 796 nm. Specially useful here is the new technique of femtosecond phase-tuned optically heterodyned Kerr gate by which we have measured both the signs and the magnitudes of the real and the imaginary components of (chi) (3). This information is very useful in understanding the roles of one- and two-photon resonances. Another application of fullerenes is in polymeric photorefractivity which is derived from a combination of optically induced space-charge field and the electro-optic effect. Exceptionally rapid progress has been recently witnessed in the development of polymeric photorefractive materials which now exhibit parameters of merit comparable or surpassing best inorganic photorefractives. We have devised polymer composite with C60 as the photosensitizer to produce highly efficient photorefractive materials. The dynamic and the kinetic aspects of the photorefractive process in the polymer composite have been investigated.
Picosecond time resolved photoluminescence and photoconductivity measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerene crystals. A highly nonlinear dependence of both the photoluminescence characteristics and the photoconductive response of the fullerenes is seen and temperature dependent measurements indicate that the nonlinear processes are associated with an insulator- metal phase transition in the material, and thus that the electronic properties of the excited state are dramatically altered at high excited state densities. Application of a simple phenomenological model to calculate the contribution of exchange and correlation energies supports the feasibility of such an interpretation. A further manifestation of this behavior is the emergence of a broadband electroluminescent emission above a critical injection current density.
We report our observation of the excited-state enhancement of the nonlinear optical response in C60 toluene solutions by simultaneous resonant pumping. The dynamics of this enhancement was studied and two different relaxation process were obtained, which can be attributed to the singlet and triplet excited states of C60 molecule. The (gamma) xxxx value of the singlet and triplet excited states were calculated to be enhanced by over three and two orders of magnitude respectively, compared with that of the ground state, while no enhancement was observed in the (gamma) xyyx component.
Using a picosecond excitation source and a time-resolved streak camera detection system, we have observed the time-resolved photoluminescence spectra of C60 from toluene solution at room temperature and from thin film at both room temperature and 77 K. For C60 solution, a photoluminescence peak at about 730 nm was detected at room temperature and its decay time was determined to be 1.1 ns. For C60 film, the photoluminescence peak was at about 730 nm at 77 K and 740 nm at room temperature, respectively. Its time decay behavior changed from a slow process with decay time of 0.9 ns to a fast process which was fitted well to a double exponential of decay times of (tau) 1 equals 0.087 ns, (tau) 2 equals 0.68 ns. Some relaxation mechanisms are suggested in explaining this phenomenon.
The observation of a broadband electroluminescent emission from fullerene crystals, with a spectral distribution comparable to that of the photoluminescence at high excitation densities is described. The emission intensity is highly nonlinearly dependent on the current. The response of the crystal to the application of an alternating current is investigated to determine the dynamic parameters of the emission process. In particular the frequency dependence of the emission intensity is described. Since the observed high frequency cut-off behavior cannot be mimicked by a simple equivalent circuit, a rate equation model is used to describe the state of the system. Fits of the model to the observed behavior provide rate constants which compare favorably to those reported for excited state decay in fullerenes. Observation of electroluminescence requires crystals of comparatively high conductivity and it is noted that the conductivities of different crystals from the same batch can vary by many orders of magnitude. Furthermore, when driven by high currents, the electroluminescent crystals undergo an irreversible increase in this conductance. In the region of the irreversibility, the electroluminescence output becomes unstable, and the current threshold for the onset of emission increases in a sample which has been driven to progressively higher currents increases. The process is discussed in terms of a current driven increase in the active volume of highly conducting crystalline pathways in the crystal.
Evidence for ultrafast photoinduced electron transfer from semiconducting polymers to C60 is presented. Upon photoexcitation across the (pi) -(pi) * gap of the conjugated polymer, an electron transfer from the polymer to C60 is initiated. We find that the forward charge transfer occurs in the subpicosecond time domain, and that the charge transferred state is metastable. The ultrafast electron transfer implies that the intrinsic quantum efficiency for photogenerating charged species is close to unity. Hence C60 can serve as an ideal photosensitizing agent for conjugated polymers, as confirmed by picosecond photoconductivity measurements. Steady state photoinduced absorption shows features of the absorption of the C60 anion and the polymer cation, confirming the mechanism of charge transfer from the polymer host to C60.
We examine optical limiting with C60-toluene for nanosecond optical pulses at 532 nm. When the input fluence is less than 50 J/cm2, optical limiting is due to a combination of reverse saturable absorption (excited state absorption) and self-defocusing. Nonlinear scattering was not observed. For a peak input fluence greater than 50 J/cm2, an acoustic report and broad-band emission indicate that optical breakdown occurs. We find C60- tetrahydronaphthalene to be a better optical limiter, at 532 nm, than tetrahydronaphthalene solutions of C76, C78, or C84. C84-tetrahydronaphthalene is shown to be an optical limiter at 1.064 micrometers .
We present experimental results on optical limiting in C60 and C70 solutions in various organic solvents using 30 ns pulses at 527 nm. Angular and temporally resolved measurements of transmitted light and the solvent dependence of optical limiting suggest that in C60 solutions the optical limiting is dominantly of thermal origin. Results of z-scan are presented which show contribution to limiting action from nonlinear refraction as well as scattering. The relevance of these findings to DFWM experiments with visible laser pulses is also discussed.
The X-band photoluminescence detected magnetic resonance (PLDMR) of C60 and C70 films and molecules isolated in toluene/polystyrene glasses is described. Both fullerene glasses have a PLDMR due to a triplet exciton delocalized over the entire molecule. However, the PLDMR of 3C70 strikingly differs from that of 3C60 in both lineshape and dynamics. These differences are due to the much longer lifetime of 3C70 which is consequently phosphorescent. The PLDMR of films indicates that the delocalized triplet is distorted by intermolecular interactions and reveals a half-field resonance consistent with triplet-triplet fusion. In addition, a broad approximately 600 G wide triplet powder pattern at g approximately equals 2, its half-field resonance at g approximately equals 4, and a narrow PL-enhancing resonance at g equals 2.0017 and 2.0029 for C60 and C70, resp., are observed. This second triplet is believed to be localized on a pentagon or hexagon face adjacent to a neighboring molecule. A narrow (approximately 3.3 G wide) PL-enhancing resonance is attributed to recombination of long-lived charged polarons. The PLDMR of photooxidized and photopolymerized films of C60 and C70 is also described. Both processes quench the localized triplet, but PLDMR spectrum of photopolymerized C60 films reveal the presence of a third triplet exciton delocalized across several molecules.