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All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30  20 nm 2 ). Thanks to extreme light confinement, we have significantly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitudes smaller than that in previous graphene-based devices and is the smallest value ever reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional Si waveguides and employed in Si photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing.

[1] Penetration of the magnetospheric electric field to the equatorial ionosphere was examined for the geomagnetic storm on 6 November 2001, by analyzing the difference in magnitude of the geomagnetic storm recorded at the dayside geomagnetic equator, Yap (À0.3°GML) and low latitude, Okinawa (14.47°GML). The penetrated electric field caused the DP2 currents at the equator, i.e., eastward currents during the main phase of the storm, while the overshielding currents, i.e., westward currents dominated during the recovery phase. It is shown that the ring current started to develop simultaneously with the onset of the equatorial DP2 within the temporal resolution of a few minutes. These results imply prompt transmission of the dawn-to-dusk convection electric field to the inner magnetosphere as well as to the equatorial ionosphere. It is found that the equatorial DP2 started to decrease one hour after the onset of the ring current development, indicating shielding effects becoming effective at the equator during the latter half of the storm main phase. The DP2 was then overwhelmed by the overshielding, which resulted in the counter electrojet (CEJ) in the beginning of the storm recovery phase. The IMAGE magnetometer chain data indicate that the westward auroral electrojet (AEJ) in the dawn sector was driven over midlatitude centered at 57°corrected geomagnetic latitude (CGML) during the main phase, while the AEJ shifted rapidly poleward to the auroral latitude centered at 67°CGML in the beginning of the recovery phase. The overshielding must be caused by the abrupt poleward shift of the R1 FACs as inferred from the poleward shift of the AEJ, in addition to the decrease in their magnitude due to the decrease in magnitude of the southward IMF. The geomagnetic storm at the dayside geomagnetic equator was enhanced in amplitude with the ratio of 2.7 as compared with the geomagnetic storm at low latitude. This amplification is a result of both effects of the DP2 currents and the CEJ associated with the main and recovery phases, respectively. It is suggested that the electric field associated with the DP2 currents contributed to the development of the ring current during the main phase, while the overshielding electric field may contribute to cease developing the ring current during the recovery phase.

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Photonic crystal slab enables us to form an ultrasmall laser cavity with a modal volume close to the diffraction limit of light. However, the thermal resistance of such nanolasers, as high as 10(6) K/W, has prevented continuous-wave operation at room temperature. The present paper reports on the first successful continuous-wave operation at room temperature for the smallest nanolaser reported to date, achieved through fabrication of a laser with a low threshold of 1.2 muW. Near-thresholdless lasing and spontaneous emission enhancement due to the Purcell effect are also demonstrated in a moderately low Q nanolaser, both of which are well explained by a detailed rate equation analysis.

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Abstract. The geomagnetic sudden commencement (SC) on February 18, 1999, was preceded by a preliminary positive impulse (PPI) at noon (1146 LT) mid-latitudes (34.9 ø and 26.9 ø geomagnetic latitude (GML)), and by a preliminary reverse impulse (PRI) near the dip equator (-0.3 ø and 4.9 ø GML) in the same local-time sector. By assuming that the step-like SC at a lower latitude (14.5 ø GML) was entirely caused by the Chapman-Ferraro currents, we subtracted this magnetic field from the SC at midlatitudes and equatorial latitudes, to identify the magnetic fields caused by the field-aligned currents (FACs) and ionospheric currents. We found that the PPI was composed of a positive impulse (true-PPI) with a timescale of less than 1 min and a succeeding negative impulse (several minutes), with their amplitudes decreasing with decreasing latitudes. The true-PPI occurred simultaneously with the equatorial PRI, and the succeeding negative impulse occurred with the DP 2-type ionospheric current component of the main impulse (MI) of the SC (DP (MI)). Analysis of 46 well-defined PPI events showed that the afternoon PPIs occurred exclusively in winter, while there was no significant seasonal dependence in the morning PPIs. None of the afternoon PPIs could be explained by the conventional SC model based on the Chapman-Ferraro currents and the DP 2-type ionospheric currents. We apply the Biot-Savart law to a three-dimensional current circuit including FACs to interpret the afternoon PPIs. Model calculations assuming a seasonal asymmetry in the ionospheric conductivity indicate that the FACs played a predominant role at midlatitudes in the winter hemisphere, while the ionospheric currents played a predominant role in the summer hemisphere. It is concluded that the true-PPIs and succeeding negative impulses were dominated by the magnetic effects of the FACs that carry the electric fields responsible for the PRIs and DP (MIs), respectively.