Among all known materials, we found that a forest of vertically aligned single-walled carbon nanotubes behaves most similarly to a black body, a theoretical material that absorbs all incident light. A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition. We found a material that can absorb light almost perfectly across a very wide spectral range (0.2-200 m). We attribute this black body behavior to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.absorbance ͉ emissivity ͉ reflectance A black body is a theoretical object that absorbs all light that falls on it, because no light is transmitted or reflected (1). As a result, it appears perfectly black at room temperature and is the most efficient thermal absorber and emitter because any object at thermal equilibrium will emit the same amount of light as it absorbs at every wavelength. The radiation spectrum of a black body is determined solely by the temperature and not by the material, properties, and structure. These features, as an ideal source to emit or absorb radiation, make the black body valuable for many applications. For example, because the black body efficiently converts light to heat, it has great importance to solar energy collectors (2-5) and infrared thermal detectors, such as pyroelectric sensors (6-8). As a perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater and would be valuable for heat liberation (9), particularly in space or in a vacuum where convective cooling is negligible.A requirement for an object to behave as a black body is that it perfectly absorbs light of all wavelengths; yet, in reality, black bodies do not exist. Emissivity is a measure of how similar an object is to a black body and is defined as the ratio of the energy radiated by that object and by a black body. Therefore, a black body would possess emissivity of unity for all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition, and thus, the emissivity of any real object is less than unity and is wavelength dependent.A good approximation of a black body is a cavity; however, this structure limits its utility. A material exhibiting black body behavior would solve this structural limitation and increase its practical usefulness. Hence, various processes and materials have been developed to blacken the surface by chemical treatment (10, 11), plating (4-6), and painting (8). Despite these efforts, emissivities for black coatings (Astro Black), chemically treated black surfaces (Hino Black), and microscale needle-like structure of nickel-phosphorus alloy (Anritsu Black) can be as high as 0.96 at 5-9 m but decreases notably at Ͼ9 m (Fig...

The realization of all-optical switching, modulating and computing devices is an important goal in modern optical technology. Nonlinear optical materials with large third-order nonlinear susceptibilities (chi(3)) are indispensable for such devices, because the magnitude of this quantity dominates the device performance. A key strategy in the development of new materials with large nonlinear susceptibilities is the exploration of quasi-one-dimensional systems, or 'quantum wires'--the quantum confinement of electron-hole motion in one-dimensional space can enhance chi(3). Two types of chemically synthesized quantum wires have been extensively studied: the band insulators of silicon polymers, and Peierls insulators of pi-conjugated polymers and platinum halides. In these systems, chi(3) values of 10(-12) to 10(-7) e.s.u. (electrostatic system of units) have been reported. Here we demonstrate an anomalous enhancement of the third-order nonlinear susceptibility in a different category of quantum wires: one-dimensional Mott insulators of 3d transition-metal oxides and halides. By analysing the electroreflectance spectra of these compounds, we measure chi(3) values in the range 10(-8) to 10(-5) e.s.u. The anomalous enhancement results from a large dipole moment between the lowest two excited states of these systems.

We demonstrate the ultrafast photoinduced Mott transition from a charge transfer insulator to a metal in a halogen-bridged Ni-chain compound by pump-probe reflection spectroscopy. Upon the irradiation of a 130-femtosecond laser pulse, the spectral weight of the gap transition is transferred to the inner-gap region. When the photoexcitation density exceeds 0.1/Ni site, the Drude-like high-reflection band appears in the infrared region, signaling the formation of a metallic state. The photogeneration of the metallic state and the subsequent recovery to the original gapped state occur within a few picoseconds.

Photoinduced transitions from ionic ͑I͒ to neutral ͑N͒ and neutral ͑N͒ to ionic ͑I͒ phases in an organic charge transfer (CT) complex, tetrathiafulvalene-p-chloranil (TTF-CA), were investigated by femtosecond pumpprobe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced N ͑I͒ states in the I ͑N͒ phase. The results reveal that the I to N ͑IN͒ transition induced by the resonant excitation of the CT band at 4 K is composed of three processes; (1) formation of a confined one-dimensional (1D) N domain, that is, a sequence of D 0 A 0 pairs, just after the photoexcitation, (2) multiplication of the 1D N domains to the semimacroscopic N states up to 20 ps within the absorption depth of the excitation light, and (3) proceeding of the IN transition along the direction normal to the sample surface. At 77 K near the NI transition temperature ͑T c =81 K͒, the size of the 1D N domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D N domain to the positively and negatively charged N states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring I state strongly unstable. The dynamics of the photoinduced N to I ͑NI͒ transition has also been investigated. The 1D I domains are initially produced by lights, however, they decay within 20 ps even if the density of the I domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced IN and NI transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of ϳ0.6, ϳ50, and ϳ85 ps have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the NI domain boundary. On the basis of the results, dynamical aspects of the photoinduced IN and NI transitions have been discussed in detail.

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Ultrafast optical switching from an ionic ( I) to a neutral ( N) state in TTF-CA was observed in femtosecond reflection spectroscopy. Charge transfer excitation in the I phase produces N donor ( D0) acceptor ( A0) strings within 2 ps. These initial N states decay with a lifetime of 300 ps at 4 K, but rather multiply leading to macroscopic I-N conversion within 20 ps at 77 K just below the N-I transition temperature T(NI). Near T(NI), we also found the evidence for the coherent motion of the macroscopic N-I domain boundary with a period of 85 ps.

Molecules with many spin-crossover sites could be used as multistepped molecular switches. The CNbridged molecular square [Fe II 4 (m-CN) 4 (bpy) 4 (tpa) 4 ](PF 6 ) 4 (bpy = 2,2'-bipyridine; tpa = tris(2-pyridylmethyl)amine), a Prussian blue analogue, undergoes thermally induced, two-step spin conversion. For more information, see the Communication by H. Oshio and co-workers on the following pages.

Spectra of the third-order nonlinear susceptibility chi((3)) have been investigated for one-dimensional Mott insulators, Sr(2)CuO(3) and Ca(2)CuO(3), by applying the third-harmonic generation (THG) spectroscopy on their single-crystalline thin films. The three-photon resonance to the lowest charge-transfer (CT) state with odd parity strongly enhances chi((3)), which is of the order of 10(-9) esu. The two-photon resonant structure unravels the even-CT state, located close to the odd-CT state. Two types of chi((3)) spectra obtained from THG and the electroreflectance measurements are explained based on the concept of spin-charge separation.