Janelle Shane

Control of Molecular Fragmentation Using Shaped Femtosecond Pulses

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 1013 to 1016 W/cm2. Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (ISHG), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (IMS) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with IMS and/or ISHG. When the extensive changes in fragmentation yields as a function of IMS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for IMS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[eta5-1-methyl-2,4-cyclopentadien-1-yl]-manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.

Effect of pulse temporal shape on optical trapping and impulse transfer using ultrashort pulsed lasers

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscopes temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

Spectral phase optimization of femtosecond laser pulses for narrow-band, low-background nonlinear spectroscopy

We use experimental search space mapping to examine the problem of selective nonlinear excitation with binary phase shaped femtosecond laser pulses. The search space maps represent a graphical view of all the possible solutions to the selective nonlinear excitation problem along with their experimental degrees of success. Using the information learned from these maps, we generate narrow lines with low background in second harmonic generation and stimulated Raman scattering spectra.

Amorphous Al 2 O 3 Shield for Thermal Management in Electrically Pumped Metallo-Dielectric Nanolasers

We analyze amorphous Al2O3 (α-Al2O3) for use as a thick thermally conductive shield in metallo-dielectric semiconductor nanolasers, and show that the use of α-Al2O3 allows a laser to efficiently dissipate heat through its shield. This new mechanism for thermal management leads to a significantly lower operating temperature within the laser, compared with lasers with less thermally conductive shields, such as SiO2. We implement the shield in a continuous wave electrically pumped cavity, and analyze its experimental performance by jointly investigating its optical, electrical, thermal, and material gain properties. Our analysis shows that the primary obstacle to room temperature lasing was the devices high threshold gain. At the high pump levels required to achieve the gain threshold, particularly at room temperature, the gain spectrum broadened and shifted, leading to detrimental mode competition. Further simulations predict that an increase in the pedestal undercut depth should enable room temperature lasing in a device with the same footprint and gain volume. Through the integrated treatment of various physical effects, this analysis shows the promise of α-Al2O3 for nanolaser thermal management, and enables better understanding of nanolaser behavior, as well as more informed design of reliable nanolasers.

Wafer bonded distributed feedback laser with sidewall modulated Bragg gratings

We demonstrate an optically pumped hybrid III-V/Si distributed feedback laser with a small footprint, using sidewall modulated Bragg gratings for optical feedback. Our approach provides high overlap between lasing mode and gain medium and may enable hybrid lasers with improved efficiency, reduced threshold, and minimal size. We fabricate the structure using plasma-assisted wafer bonding, followed by self-aligned lithography and etching. The latter allows us to avoid alignment errors. This approach is a promising avenue toward ultracompact, energy efficient, and scalable monolithically integrated photonic circuits.

Temperature effects in metal-clad semiconductor nanolasers

Abstract As the field of semiconductor nanolasers becomes mature in terms of both the miniaturization to the true sub-wavelength scale, and the realization of room temperature devices, the integrated treatment of multiple design aspects beyond pure electromagnetic consideration becomes necessary to further advance the field. In this review, we focus on one such design aspect: temperature effects in nanolasers. We summarize recent efforts in understanding the interplay of various temperature-dependent parameters, and study their effects on optical mode and emission characteristics. Building on this knowledge, nanolasers with improved thermal performance can be designed, and their performance evaluated. Although this review focuses on metal-clad semiconductor lasers because of their suitability for dense chip-scale integration, these thermal considerations also apply to the broader field of nanolasers.

Thermal considerations in electrically-pumped metallo-dielectric nanolasers

Metal nanocavity-based lasers show promise for dense integration in nanophotonic devices, thanks to their compact size and lack of crosstalk. Thermal considerations in these devices have been largely overlooked in design, despite the importance of self-heating and heat dissipation to device performance. We discuss the sources of self-heating in electrically-pumped wavelength-scale nanolasers, and the incorporation of these heat sources into a heat dissipation model to calculate laser operating temperature. We apply this thermal model to an example electrically-pumped nanolaser operating at room temperature.

Optical trapping using ultrashort 12.9fs pulses

We demonstrate stable three-dimensional optical trapping of 780nm silica particles using a dispersion-compensated 12.9fs infrared pulsed laser and a trapping microscope system with 1.40NA objective. To achieve these pulse durations we use the Multiphoton Intrapulse Inteference Phase Scan (MIIPS) method to compensate for the significant temporal dispersion introduced by the trapping system. We demonstrate orders of magnitude reduction in pulse duration at the sample, and a dramatic increase in the efficiency of multiphoton excitation at the sample. The use of dispersion-compensated ultrashort pulses will therefore be a valuable tool for enhancing non-linear processes in optically trapped particles. In addition, ultrashort pulses can allow the use of pulse shaping to control these nonlinear processes, yielding the possibility of advanced applications using coherent control of trapped particles.

Effect of Undercut Etch on Performance and Fabrication Robustness of Metal-Clad Semiconductor Nanolasers

We use optical, thermal, and electrical simulation to evaluate the effects of using varying amounts of undercut etch on wavelength-scale and subwavelength metal-clad semiconductor nanolasers (MCSELs). We find that as MCSEL diameter decreases, the optical performance becomes more sensitive to slight amounts of sidewall tilt. A modest amount of undercut (25%) dramatically improves the optical performance, reducing modal threshold gain to 100 cm-1 or less for lasers with core radius of 225, 550, or 775 nm, even in the presence of significant sidewall tilt (20° gain sidewall or ±8° pedestal sidewall tilt). Finally, we examine the effects of the increased undercut on nanolaser thermal performance and find that the increased resistive heating is insignificant near threshold, even for subwavelength nanolasers.

Automated phase characterization and adaptive pulse compression using multiphoton intrapulse interference phase scan in air.

We introduce a non-interferometric single beam method for automated spectral phase characterization and adaptive pulse compression of amplified ultrashort femtosecond pulses taking advantage of third order harmonic generation in air. This new method, air-MIIPS, compensates high-order phase distortions based on multiphoton intrapulse interference phase scan (MIIPS).