The ultrafast coherent manipulation of electrons using waveform-controlled laser pulses 1-9 is a key issue in the development of modern electronics 10,11. Developing such an approach for a tunnel junction will provide a new platform for governing ultrafast currents on an ever smaller scale, which will be indispensable for the advancement of next-generation quantum nanocircuits 12-15 and plasmonic devices 16-18. Here, we demonstrate that carrier-envelope phase controlled single-cycle terahertz electric fields can coherently drive electron tunnelling either from a nanotip to a sample or vice versa. Spatially confined electric fields of more than 10 V/nm strongly modulate the potential barrier at a nanogap in a scanning tunnelling microscope (STM) within a sub-picosecond time scale and can steer a huge number of electrons in an extremely nonlinear regime, which is not possible using a conventional STM. Our results are expected to pave the way for the future development of nanoscale science and technologies.

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The ferroelectric soft mode in a SrTiO(3) thin film was impulsively driven to a large amplitude using intense picosecond terahertz pulses. As the terahertz electric field increased, the soft-mode absorption peak exhibited blueshifting and spectral narrowing. A classical anharmonic oscillator model suggests that the induced displacement is comparable to that of the ferroelectric phase transition. The spectral narrowing indicates that the displacement exceeds that induced by any inhomogeneities in the film, demonstrating that the method can be used to explore intrinsic quartic anharmonicity.

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We have developed a femtosecond time-resolved luminescence spectroscopy by the optical Kerr-gate ͑OKG͒ method to investigate ultrafast carrier dynamics and relaxation processes of materials. Solid glasses with a high nonlinear refractive index were used as the Kerr media to obtain a subpicosecond time resolution. When a quartz plate was used as the Kerr medium, the Kerr efficiency and the instrumental response time of our spectroscopic system were 5-10 % and ϳ250 fs, respectively. By employing the OKG method, we revealed the internal conversion from S 2 to S 1 state of ␤-carotene with a low fluorescence quantum yield and an ultrafast fluorescence decay time, and the lifetime of the S 2 state was determined to be 210 fs. An advantage of the OKG method relative to the conventional up-conversion technique is its ability to directly obtain time-resolved luminescence spectra, and thus the OKG method might be superior to the up-conversion technique to investigate ultrafast carrier dynamics and relaxation processes of materials.

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Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology.

We have observed an irreversible ultrafast crystalline-to-amorphous phase transition in Ge2Sb2Te5 chalcogenide alloy thin film using broadband single-shot imaging spectroscopy. The absorbance change that accompanied the ultrafast amorphization was measured via single-shot detection even for laser fluences above the critical value, where a permanent amorphized mark was formed. The observed rise time to reach the amorphization was found to be ∼130–200 fs, which was in good agreement with the half period of the A1 phonon frequency in the octahedral GeTe6 structure. This result strongly suggests that the ultrafast amorphization can be attributed to the rearrangement of Ge atoms from an octahedral structure to a tetrahedral structure. Finally, based on the dependence of the absorbance change on the laser fluence, the stability of the photoinduced amorphous phase is discussed.