Alexander Q. Wu

Ultrasensitive mass sensing using mode localization in coupled microcantilevers

We use Anderson or vibration localization in coupled microcantilevers as an extremely sensitive method to detect the added mass of a target analyte. We focus on the resonance frequencies and eigenstates of two nearly identical coupled gold-foil microcantilevers. Theoretical and experimental results indicate that the relative changes in the eigenstates due to the added mass can be orders of magnitude greater than the relative changes in resonance frequencies. Moreover this sensing paradigm possesses intrinsic common mode rejection characteristics thus providing an alternate way to achieve ultrasensitive mass detection under ambient conditions.

Coupling of ultrafast laser energy to coherent phonons in bismuth

Energy coupling to coherent phonons in Bi during femtosecond laser–bismuth interaction is investigated using a double-pulse femtosecond pulse train generated from a temporal pulse shaper. It is found that the increase of bismuth temperature is dependent on the separation time between the two laser pulses. Using a numerical fitting, which considers the effect of convolution between the incident pulses and the material response, the measured temperature increases using different pulse-to-pulse separations allow quantitative determination of the amount of laser energy coupled from excited electrons to coherent phonon vibration.

Ultrafast dynamics of photoexcited coherent phonon in Bi2Te3 thin films

Nonequilibrium A1g longitudinal optical phonon with a frequency of 1.84THz in bismuth telluride (Bi2Te3) is coherently excited by ultrafast pulses. Time-resolved reflectivity measurements show a distinct second harmonic vibration around 3.68THz at room temperature caused by the nonlinearity of coherent phonon potentially related to the favorable crystal structure of Bi2Te3. The scattering rate between A1g coherent phonon and room temperature incoherent phonon is derived from the pump-fluence-dependent scattering rate of A1g coherent phonon. It is also observed that energy coupling from photoexcited carriers to lattice through coherent phonon vibration is more efficient and faster at higher pump fluence.

Plasma formation in fused silica induced by loosely focused femtosecond laser pulse

The focusing position inside fused silica irradiated by a loosely focused high power femtosecond laser pulse is studied both experimentally and numerically. The experimental measurement of plasma radiation shows that the laser pulse is focused behind the focal plane, which is also found in the numerical calculation and is attributed to a complex interplay between self-focusing due to the Kerr effect and defocusing because of the free electron plasma. Also, when more than one pulse is incident at the same spot in the sample, plasma radiation is observed at more than one spot along the laser propagation direction.

Molecular dynamics simulation of ultrafast laser ablation of fused silica

Ultrafast laser ablation of fused silica is studied using molecular dynamics simulations. Ionization and generation of free electrons, absorption of the laser energy by free electrons and energy coupling between free electrons and ions are considered. The BKS potential is used to describe molecular interactions and is modified to include the effect of free electrons. Temperature, density, and material removal are computed, and the thermal and non-thermal mechanisms of ultrafast laser ablation of fused silica are discussed.

Biochemical Mass Detection Using Mode Localization in Microcantilever Arrays

Microcantilever based biochemical sensors rely on accurately detecting the selective binding of small amounts of the target analyte to a functionalized microcantilever. Commonly, the added mass of the bound analyte is detected by measuring accompanying shift in cantilever resonant frequency. In this paper we explore the possibility of using Anderson or mode localization in coupled oscillator arrays as a potentially more sensitive method for detecting the added mass. According to this method, the eigenvectors of an array of coupled, nominally identical cantilevers are expected to be extremely sensitive to small disorder such as that introduced by the added mass of the target biochemical analyte. A simple lumped parameter model of the cantilever array is used to establish theoretical feasibility of the method. The effects on the eigenvector sensitivity of initial statistical variability introduced due to inherent manufacturing tolerances are investigated. Experimental results are presented for a two cantilever array laser fabricated out of gold foil, and the results compared to theoretical expectations. Both the theoretical and experimental results indicate that added mass induced shifts in eigenvectors of such arrays can be orders of magnitude more sensitive than the resonance frequency shifts.Copyright

Coherent Phonon Excitation by Ultrafast Pulse Trains

Time resolved reflectivity of bismuth thin film evaporated on a silicon substrate is measured by an 80 femtosecond (fs) laser at a center wavelength of 800 nm. The reflectivity data reveal that coherent optical phonons (A1g ) near 2.9 THz (1 THz = 1012 Hz) are excited by the 80 fs laser pulses. Analyses of the reflectivity data reveal key parameters related to electron and phonon dynamics, including phonon excitation and de-phasing and electron-phonon energy coupling. It is also found that the phonon frequency peaks are red-shifted and broadened at higher laser fluences. We also show that the phonon oscillation can be manipulated by specifically designed ultrafast pulse trains with different pulse separation times.Copyright

Ultrafast Diagnostics of Coherent Phonon Excitation and Energy Transfer

Time resolved reflectivity of bismuth thin film evaporated on a silicon substrate is measured by an 80 femtosecond (fs) laser at a center wavelength of 800 nm. The reflectivity data reveal that coherent optical phonons (A1g ) near 2.9 THz (1 THz = 1012 Hz) are excited by the 80 fs laser pulses. Analyses of the reflectivity data reveal key parameters related to electron and phonon dynamics, including phonon excitation and de-phasing and electron-phonon energy coupling. It is also found that the phonon frequency peaks are red-shifted and broadened at higher laser fluences.Copyright