Coherent control is an aspiration of the physical and life sciences, with the potential to revolutionize these disciplines by providing a diverse optical tool kit. So far, control has been shown over chemical reactions, ionization pathways and biomolecular interactions, on the one hand, and injection of conventional and spin currents in gases and solids, on the other. This ability derives from the coherence properties of the light field of pulsed lasers, which can be shaped using interferometers and spatial light modulators, to give complete control of the radiation in space, time and frequency. Since the amplitude and phase information can be transferred into matter, manipulating the light field can be mapped onto the atomic, molecular or electronic interactions too. This knowledge has been one of the driving forces behind coherent control for several decades. Nevertheless, to date, few applications have been perfected or commercialized – perhaps due to the inherent complexities.
One of the simplest ways to provide a light field for coherent control is to superpose laser pulses that are harmonically related, e.g. adding femtosecond pulses centered at 1550 nm and 775 nm. Using this combination of pulses in metals and semiconductor nanostructures produces a photocurrent that is dependent on the relative phase of the carrier frequency, due to quantum interference in the medium. The best way to confirm the phase dependence is to use a two-color interferometer, scan the time delay to vary the phase between the two colors and watch the photocurrent reverse sign.
Recently, the Betz group began to use this photocurrent as a detection scheme for optically induced phase changes in a medium that has been placed in one arm of the interferometer. In this case, varying the relative phase leads to results that differ from the ones obtained by having empty interferometer arms; one obtains a complex signature related to the medium under examination. In essence, this time-domain field correlation can be Fourier transformed to provide the transmission spectrum and spectral phase. Hence, this technique has similarities to orthodox spectral interferometry, which uses a reference pulse to heterodyne an optical signal in a multichannel imaging spectrograph. One convenience in the case of phase-resolved photocurrent injection is that the detector is significantly more compact.
In this paper by Ruppert et al., the group has now demonstrated that the phase-resolved photocurrent can measure dynamic nonlinear-optical effects by incorporating it as the detection scheme in a pump-probe experiment. Samples are probed in the short wavelength arm of the two-color interferometer and the photocurrent is measured. Now, a narrow band pump, below the optical band gap of the sample, is used to produce a time-varying nonlinear optical response. The photocurrent is Fourier-transformed as before to provide the complex spectrum for various pump delays. The examples presented in the paper include: 1) a two-photon-induced exciton bleaching in a thin films CdTe cooled to liquid-helium temperatures at positive time delays, with previously unseen two-photon-induced spectral oscillation at negative time delays; and 2) isolation of cross-phase modulation and two-photon absorption in the real and imaginary contributions from bulk ZnTe at room temperature.
In both examples presented, the power of the technique is shown to be the phase-resolved spectrum. This optical pump and photocurrent probe experiment show promise for in situ measurement of dynamic nonlinear optical processes that occur during or impede the operation of optoelectronic heterostructure devices. Moreover, with the design of a simplified package this detection scheme may one day be used in an ultrafast electric-field-resolved oscilloscope. This letter demonstrates that the light-field should not be understated in the generation of some photocurrents.
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