Laser-assisted field emission in single-walled carbon nanotubes

Carbon nanotubes (CNTs) have many uses in energy storage, electron emission, molecular electronics, and optoelectronics. Understanding their light-matter interactions is crucial to their development. Here, we study a film of single-walled CNTs with a thickness of 1.67 μm and a 2D orientational order parameter of 0.51, measured by polarized Raman spectroscopy. The film is expected to have a work function of about 5.1 eV. In this study, ~100-fs pulses with 1.5 (ℏω) and 3 eV (2ℏω) photon energy are used to pump the CNT film while observing its electron emission in vacuum. Ultrafast pulses produce nonlinear phenomena in enhanced field emission, as the CNTs absorb strongly enough that thermally excited carriers can tunnel through the potential barrier. Through curve fitting of the power dependence for each pump energy, we find that the light at ℏω is absorbed via 5-photon absorption, and the light at 2ℏω is absorbed via a combination of 2- and 3-photon absorption. Further study reveals a space-charge limited regime with low applied bias, a photoemission regime with moderate bias, and a laser-assisted field emission regime when the bias is high enough that the photon pump is no longer important. Cross-correlation pumping with the two colors simultaneously shows 4x enhancement of the emission, with a FWHM that suggests a lifetime of ~190 fs, similar to the dephasing time of electrons in CNTs. These studies help illuminate the properties of CNTs as a nonlinear optical material and go towards a more thorough understanding of their optoelectronic properties.

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