Second harmonic generation by an isolated spin

Jian Shu

Loc Le Xuan, Sophie Brasselet, François Treussart +6 more

The authors demonstrate the association of two-photon nonlinear microscopy with balanced homodyne detection for investigating second-harmonic radiation properties at nanoscale dimensions. Variation of the relative phase between second-harmonic and fundamental beams is retrieved, as a function of the absolute orientation of the nonlinear emitters. Sensitivity down to a few photons per second in the spatiotemporal mode of the local oscillator is obtained. This value is high enough to efficiently detect the coherent second-harmonic emission from a single KTiOPO4 crystal of subwavelength size.

Daniel F. Walls, C. T. Tindle

G. S. Agarwal

Alexander I. Lvovsky, Sven R. Hartmann

Time-delayed second harmonic generation has recently been reported in an atomic vapor of Cs at a temperature near 180 degrees Celsius and situated in a magnetic field of the order of a killogauss. Short 10 ps excitation laser pulses at 885 nm were used to excite a macroscopic electric quadrupole density which was then observed to radiate at 442 nm after a delay of a few hundred picoseconds. The experiment is complicated by the presence of several superfluorescence transitions some of which radiate close to the 442 nm wavelength. These emissions are also delayed and our strategy is to set the experimental conditions so that they only become appreciable after the second harmonic emission is complete. In this paper we show how the second harmonic develops by considering the evolution of the radiation pattern of an isolated atomic radiator (not forbidden to radiate at the second harmonic even in the absence of a magnetic field). We estimate the intensity of the superradiant second harmonic emission and obtain agreement with experiment. Excellent agreement is also obtained between measured and calculated delay.

Alexander I. Lvovsky, Sven R. Hartmann

Second harmonic generation has hitherto been thought of as simultaneous with the excitation pulse which generates it. Recent experiments in Cs vapor demonstrate otherwise. A short two-photon 6D - 6S resonant excitation pulse in Cs vapor generates a superposition state which would radiate immediately were it not for destructive interference arising from the sample being large compared to the radiation wavelength. The radiation pattern of an isolated radiator (atom) starts out with a null in the plane transverse to the polarization (linear) direction but then rotates about the applied transverse magnetic field developing a radiation component along the phase matched direction which we detect. Experimental results are presented in excellent agreement with theory. Previous delay discrepancies have been resolved by using a nonmetallic sample cell. Cascade and yoked superfluorescence take place but not significantly until after the second harmonic emissions. Quantitative measurements of signal intensity are also presented.