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 mu m 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, similar to 100-fs pulses with 1.5 ((h) over bar omega) and 3 eV (2 (h) over bar omega) 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 (h) over bar omega is absorbed via 5-photon absorption, and the light at 2 (h) over bar omega 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 similar to 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.

We report ultrafast photoelectron emission from aligned single-wall carbon nanotubes utilizing strong exciton resonances inherent in this prototypical one-dimensional material. These results establish SWCNT films as novel and promising ultrafast photocathode material. © 2019 The Author(s)

We demonstrate deposition of azimuthally uniform single- or multiple-layer thin films of silicon nitride and silica on fibers using plasma-enhanced chemical vapor deposition by continuously rotating the fibers during growth. Our fibers exhibit distinctive and uniform iridescence that strongly depends on coating configuration. We also report a non-invasive technique to measure refractive index and film thickness of coated fibers simultaneously based on Mie scattering. We found the films grown on fibers have very different characteristics from those grown on flat substrates. We deposit a 1-μm-thick SiNx film on a spheroidal microrod resonator, which is shown numerically to push the guided fundamental mode into the silica core. We demonstrate a Q factor of 2.2 × 106, indicating reasonably good thin film quality that could be further increased with improved process control. Our technique can be applied to coat whispering gallery mode microresonators with engineered (e.g., step, graded, or stratified) refractive index profiles, which are expected to enable many new applications.

We study nonlinear above-threshold photoemission (ATPE) in single-wall carbon nanotubes at two wavelengths. NIR photoemission demonstrates 5-photon ATPE, while UV ATPE is dominantly a 2nd process. Two-pulse correlation exhibits enhanced photoemission with a very short lifetime less than 200 fs.

Mie scattering of Bragg fibers made of periodic layers of SiNx and SiO2 is demonstrated. The scattering pattern is very sensitive to Bragg layers’ configuration, making it attractive to characterize Bragg fibers non-invasively.

We show single-shot nonlinear absorption is enhanced by 5% when pump and probe pulses have parallel polarization. At very large pump energy, we observe a new and fast decay component of self-trapped excitons ~0.5ps.

We investigate femtosecond pulse transmission through BK7 in the presence of its second harmonic as a seeding pulse. Enhanced absorption is observed when the pulses are co-polarized. Such phenomenon can be used to control absorption.

This paper reports the fabrication and characterization of uniform conformal dielectric thin films on optical fibers using plasma-enhanced chemical vapor deposition. We show convincingly that, using both iridescence and scanning electron microscopy, uniform deposition on the fiber can only be obtained if it is rotated at a constant speed during deposition. Our study is important for depositing single or multiple uniform dielectric thin films on objects with cylindrical or spherical symmetry, which could enable new optical functionalities.

This paper reports the fabrication and characterization of uniform conformal dielectric thin films on optical fibers using plasma-enhanced chemical vapor deposition. We show convincingly that, using both iridescence and scanning electron microscopy, uniform deposition on the fiber can only be obtained if it is rotated at a constant speed during deposition. Our study is important for depositing single or multiple uniform dielectric thin films on objects with cylindrical or spherical symmetry, which could enable new optical functionalities.

We have previously reported the observation of self-organized tungsten nanogratings during chemical vapor deposition of tungsten induced by a 400-nm 80-MHz laser oscillator on a wide range of substrates. We show that the growth of nanostructures begins with a thin tungsten film, followed by a rapid formation of periodic texture, when the laser power exceeds a threshold value. The threshold power is found strongly substrate dependent. The ubiquitous presence of thin films prior to nanograting growth suggests adatom diffusion induced by laser heating is vital, as the strong electron-phonon coupling in tungsten is expected to turn absorbed photon energy rapidly into heat. Using a simplified 1D heat diffusion model, we estimate the critical surface temperature on various substrates at the onset of nanograting formation, based on substrate-specific threshold power and material properties. We found interesting correlation of critical temperatures: all the covalent substrates (AlN, Al2O3, quartz, silica, and glass) exhibit a common critical temperature while the ionic substrates (MgO, MgF2, and CaF2) share another yet different critical temperature. The critical temperature of covalent substrates is found higher than that of ionic substrates, indicating the former possesses larger activation energy for adatom diffusion. Based on this model, we can also extract a substrate-independent enthalpy for nanograting formation. Although the present 1D model overestimates the surface temperature, the correlation of critical temperatures among substrates and the presence of a unique enthalpy independent of substrates strongly support the role of laser heating and adatom diffusion in the formation of tungsten nanogratings.