Temporal Characterization of the Laser Field Using Tunnel Ionization

A way to measure the shape of laser pulses in ambient air has been developed. In contrast to the existing method for studying waveforms, it does not require a vacuum environment or x-ray pulses. The method (dubbed TIPTOE) can be applied to laser beams of different wavelengths, including UV, visible, or longer.

TIPTOE (tunneling ionization with a perturbation for the time-domain observation of an electric field) uses tunnel ionization for the temporal characterization of laser pulse waveforms.

Special mirrors divide a laser beam into strong (E_F) and weak (E_S) laser pulses. When the laser beams reach the air or gas-filled chamber, electrons “escape” from their atoms (tunnel ionization) and are caught by the metal plates. Changes in ionization are used to measure the shape of the laser pulse. Courtesy of the Institute for Basic Science (IBS).

Atoms or molecules exposed to intensive electric fields, like the ones created by strong laser pulses, can lose some of their electrons through tunnel ionization. The TIPTOE method uses two superimposed laser pulses — a strong one and a weak one — and bases its characterization on the electric field and the tunnel ionization of the electrons of the atoms in the air.

Time differences between the strong and the weak superimposed laser pulses will cause the intensity of the electric field to vary. A higher electric field intensity corresponds to higher ionization. Any changes in the electric field will therefore be reflected on the tunnel ionization. These changes in tunnel ionization are used to measure the shape of the laser pulse. Since tunneling ionization lasts only 200 attoseconds, the TIPTOE method can provide enough temporal resolution to measure UV, visible, and longer wavelength pulses.

Scientists at the Institute for Basic Science (IBS) validated TIPTOE by comparing it with the conventional x-ray pulse technique. The two techniques demonstrated the same results. The validity of the method was confirmed by comparing the results obtained with a petahertz optical oscilloscope using few-cycle laser pulses that covered the wavelength range from 500 nm to 1 μm for different dispersion conditions.

The waveforms measured with TIPTOE match the ones obtained with the conventional method. Courtesy of the Institute for Basic Science (IBS).

According to researchers, the TIPTOE approach enables the complete temporal characterization of the laser field, including the carrier-envelope phase of the laser pulse, for a broad spectral range in ambient air, providing a universal tool for the precise measurement of the laser field.

“TIPTOE’s biggest advantage is the universality of this technique at different wavelengths,” said professor Kyung Taec Kim.

The patented technique is currently available for technology transfer and commercialization. The technique could accelerate studies on light-matter interaction.

The research was published in Optica, a publication of OSA, The Optical Society (doi:10.1364/OPTICA.5.000402).