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.
We present lasing experiments in index-antiguided (IAG) planar waveguides made of Nd:YAG (refractive index = 1.820)
sandwiched by Terbium Gallium Garnet (refractive index = 1.954). For a core width of 220 μm, we observe fundamental
mode oscillation with maximal 2.62 W output power and 0.109 slope efficiency. An analytical model is developed
taking into account simultaneous index-antiguiding, gain guiding and thermal induced refractive focusing. For a core
width of 400 μm, fundamental mode oscillation is only observed near the lasing threshold, suggesting that the maximally
core width in IAG waveguides for fundamental mode operation depends on the strength of index antiguiding.
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 electronphonon 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.
Gain-guided and index anti-guided waveguides hold great promise to increase laser output power while maintaining single mode operation. Although lasing in GG-IAG fibers was demonstrated, slope efficiency is poor and output power is low. This work presents the first, to the best of our knowledge, lasing characteristics of a diode-pumped GG-IAG planar waveguide in a plano-plano resonator configuration. The laser waveguide is fabricated using 1% doped Nd:YAG core with diffusion bonded TGG claddings. With continuous-wave pumping, 8.5% slope efficiency is demonstrated and an output power of 1.5W achieved, the highest power reported so far from a GG-IAG waveguide laser. Under such pumping condition, however, thermal lensing is shown to override gain guiding and mode narrowing is observed.
Leaky-waveguide laser amplifiers can sometimes have significant advantages for single-transverse-mode
propagation in high-power waveguide laser systems. Most recent studies of such systems have either not included
gain saturation, or they have assumed homogeneously broadened gain media. However, some of the most promising
media for such amplifiers are inhomogeneously broadened. Results reported here include detailed numerical mode
solutions for saturated inhomogeneous broadening, as well as useful analytical methods and approximations for both
homogeneous and inhomogeneous broadening.
We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in
a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less
confined, while at the same time its peak intensity increases more slowly. The modal gain coefficient is shown to scale
linearly with the modal confinement in the cladding. At steady state, both the mode shape and power remain constant.
We show that, at large index antiguiding, the field profile inside the core remains nearly constant during propagation,
which allows an analytical description of the evolution of the optical power inside the core.
In this work, interferometric sensors based on antiresonance reflecting waveguide (ARROW) fibers were developed and
their performances were characterized for measurement of physical parameters, particularly strain and temperature. Two
types of ARROW fibers were considered and signal demodulation was achieved using the white light interferometric
technique. Application issues of sensing heads based on these fibers are also addressed.
We observed the spontaneous formation of periodic nano-structures in both femtosecond laser ablation and deposition.
The former involved 400-nm femtosecond pulses from a 250-KHz regenerated amplified mode-locked Ti:sapphire laser
and periodic nanocracks and the nano-structure are in the form of periodic nanocracks in the substrate, the latter applied
an 80-MHz mode-locked Ti:sapphire oscillator with pulse energy less than half nanojoule in a laser-induced chemical
vapor deposition configuration and tungsten nanogratings grow heterogeneously on top of the substrates. These two
observed periodic nanostructures have opposite orientations respecting to laser polarization: the periodic nanocracks are
perpendicular to, whereas the deposited tungsten nanogratings are parallel to laser polarization direction. By translating
the substrate respecting to the laser focus, both the periodic nanocrack and tungsten nanograting extend to the whole
scanning range. The deposited tungsten nanogratings possess excellent uniformity on both the grating period and tooth
length. Both the attributes can be tuned precisely by controlling the laser power and scanning speed. Furthermore, we
discovered that the teeth of transverse tungsten nanogratings are self aligned along their axial direction during multiple
scanning with appropriate offset between scans. We demonstrate the feasibility of fabricating large-area one-dimensional
grating by exploiting such unique property. These distinct phenomena of nanocracks and tungsten nanogratings indicate
different responsible mechanisms.
We demonstrate deposition of periodic tungsten nanoripple on different substrate using a single 400nm femtosecond
laser beam at room temperature. The laser beam generated by frequency doubling the output from mode-locked 80MHz
Ti: sapphire oscillator was applied in a laser-induced chemical vapor deposition configuration, in which tungsten
hexacarbonyl was used as precursor. The deposition strongly depended on laser polarization. With linearly polarized
light, periodic ripple structure with willow-leaf shape was formed inside the exposure area. The ripple orientation was
found parallel to the laser polarization direction. Affects of laser power and exposure time on ripple formation were
investigated. By translating the substrate with respective to the laser beam, longitudinal or transverse grating structure
was observed. The period of this grating structure is about 150nm on sapphire, and the orientation is parallel to laser
polarization. Simply by programming the translation of the substrate, large area patterns and other patterns such as circle
and characters were formed. Similar ripple and grating structures observed on all the substrates we investigated,
including insulators, semiconductors and metals, implies that ripple formation might be a universal phenomenon.
Considering the simplicity of this process and material flexibility of laser CVD, this technique may provide a novel costeffective
patterning method to produce periodic subwavelength nanostructures of a wide range of materials on many
substrates.
KEYWORDS: Silica, Ultrafast phenomena, Scanning electron microscopy, Absorption, Atomic force microscopy, Electrons, Objectives, Picosecond phenomena, Sapphire, 3D optical data storage
Submicron-diameter structures can be produced inside many transparent materials by tightly focused 100-fs laser pulses. The ultrafast energy deposition creates very high temperature and pressure inside the region, initiating a 'microexplosion'. Material is ejected from the center and forced into the surrounding volume, forming a void surrounded by densified material. Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.
Laser induced formation of CO2 and desorption of O2 are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two-pulse correlation experiments measuring total O2 desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O2 and CO2 are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.
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