We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10 −16 m 2 /V 2 , which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths. By measuring a set of flakes with different numbers of atomic layers, we find that the emission scales with the square of the number of atomic layers, which suggests that the susceptibility of graphene is independent of layer number, at least for a few layers.Graphene is a monolayer of carbon atoms arranged in a hexagonal two-dimensional lattice. It has a linear dispersion relationship between energy, E, and wavenumber, k, E(k) = ±hv F k, where the Fermi velocity v F ≈ 10 6 m/s [see Fig. 1 -3]. Since the material became readily available less than a decade ago [4], it has been the subject of extensive investigations [e.g., see Refs. 5 and 6 and references therein]. Graphene exhibits a number of unusual and remarkable transport properties that make it attractive for nano-electronic applications [7][8][9], including high mobilities [4,5,9, 10] and nearly-ballistic transport at room temperature [5,6]; however, it is the optical properties of graphene that are of primary interest here.The linear absorbance of graphene is flat and approximately 2.3% across the entire visible spectrum [11,12]. Thus, graphene can be considered to be both highly absorbing and/or highly transmitting, depending upon one's point of view or application. Since a negligible fraction (< 0.1%) is reflected [12], 97.7% of the incident light is transmitted. In this sense the sample is certainly transparent. On the other hand, if one were to assign an effective absorption coefficient, α, to a monolayer of thickness 0.33 nm (even though it may be questionable to use macroscopic parameters, such as α, for such thin samples), it would be very large (α ≈ 7 × 10 5 /cm). As suggested by this large effective absorption coefficient, graphene interacts strongly with light. The strong broadband nature of the interaction of light with graphene is consistent with its linear bandstructure, where interband transitions of roughly equal strength are available throughout the visible. The combination of broadband transparency and the high mobilities mentioned earlier makes graphene a promising candidate for use as a transparent conductor [13][14][15][16] in photovoltaic devices [17,18] and touch screens [14]. The high broadband absorption (i.e., optical conductivity) and high carrier mobility suggest that graphene may find applications in a range of optically-controlled transport devices, such as broadband and ultrafast photodetectors [19][20][21][22][23][24][25] The s...

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm. Results agree with recent theoretical predictions.

Using two-color optical coherence control techniques in intrinsic GaAs at 80 K with orthogonally polarized 70 fs, 1430 and 715 nm pulses, we generate a pure spin source current that yields a transverse Hall pure charge current; or alternatively, with parallel polarized pulses, we generate a pure charge source current that yields a pure spin current. By varying the relative phase or polarization of the incident pulses, one can effectively tune the type, magnitude and direction of both the source and transverse currents without application of electric or magnetic fields.

CONTENTS 1. Introduction 2. Theory 3. Experiment and data 3.1. Experiment 3.2. Data 4. Comparison of ß2 values to theory 4.1. Two-photon absorption theory 4.2. Comparison to theory 5. Self-refraction 6. Optical limiter 7. Conclusion 8. Acknowledgments 9. References Abstract. Two-photon absorption coefficients ß2 of ten direct gap semiconductors with band-gap energy Eg varying between 1.4 and 3.7 eV were measured using 1.06 pm and 0.53 pm picosecond pulses. ß2 was found to scale as E93, as predicted by theory for the samples measured. Extension of the empirical relationship between ß2 and Eg to InSb with Eg = 0.2 eV also provides agreement between previously measured values and the predicted ß2. In addition, the absolute values of ß2 are in excellent agreement (the average difference being <26 %) with recent theory, which includes the effects of nonparabolic bands. The nonlinear refraction induced in these materials was monitored and found to agree well with the assumption that the self-refraction originates from the two-photon-generated free carriers. The observed self-defocusing yields an effective nonlinear index as much as two orders of magnitude larger than CS2 for comparable irradiances. This self-defocusing, in conjunction with twophoton absorption, was used to construct a simple, effective optical limiter that has high transmission at low input irradiance and low transmission at high input irradiance. The device is the optical analog of a Zener diode.

Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.