Ali J. Sabbah

Optimization study of the femtosecond laser-induced forward-transfer process with thin aluminum films

The parameters for an effective laser-induced forward-transfer (LIFT) process of aluminum thin films using a femtosecond laser are studied. Deposited feature size as a function of laser fluence, donor film thickness, quality of focus, and the pulse duration are varied, providing a metric of the most desirable conditions for femtosecond LIFT with thin aluminum films.

Development of a femtosecond micromachining workstation by use of spectral interferometry

A workstation that permits real-time measurement of ablation depth while micromachining with femtosecond laser pulses is demonstrated. This method incorporates the unamplified pulse train that is available in a chirped-pulse amplification system as the probe in an arrangement that uses spectral interferometry to measure the ablation depth while cutting with the amplified pulse in thin metal films.

Toward silicon-compatible modulation of plasmonic waveguides

We have modeled plasmonic waveguides that support low-loss modes using configurations compatible with metal-oxide-semiconductor devices. Our experimental verification with visible wavelength analogs demonstrates that silicon-based plasmonic modulators are possible.

Silicon-compatible Ultra-long-range Surface Plasmon Modes

Surface plasmon waveguides show promise as sub-wavelength signal elements in integrated optoelectronic devices. Modulation of these signals requires designs that are compatible with existing semiconductor fabrication technologies, such as MOS structures. We show that waveguides of this type are not only practical, but offer distinct advantages. In particular, an asymmetric geometry is shown to support very long-range modes. In preliminary experimental results we report the observation of these modes in an visible wavelength analog structure. Our models predict that these characteristics can be maintained while shifting the operating conditions to the fiber communications band in waveguides constructed in silicon.

Silicon-Compatible Optical Elements

With a model confirmed by visible-wavelength experiments, we show that a MOS capacitor can support surface plasmon modes. We also present finite-element designs for resonant cavity structures that are a step towards Si-based free-carrier modulators.

Femtosecond Pulse Laser Damage in Thin Films

The damage behavior of different dielectric oxide thin films commonly used in optical coatings and the underlying excitation and relaxation mechanisms have been investigated using near-infrared, sub-picosecond laser pulses.

Femtosecond dynamics of highly excited dielectric thin films

Highly excited wide-gap dielectrics are well suited to study the evolution of non-equilibrium conduction band electrons in solids. Compared to semiconductors, the measurable quantities in femtosecond pump-probe experiments – transient reflection (R) and transmission (T) – are less affected by processes such as gap shrinkage and band filling, simplifying the data interpretation. The measurements were performed on three oxides (TiO2, Ta2O5 and HfO2) thin films (~ 500 nm) with different band gaps (3.3 eV, 3.8 eV, 5.1 eV). Pulses at 800 nm and 25 fs duration excited the samples to ~60% of the threshold for dielectric breakdown. Besides their technical importance as optical coatings, we chose thin films to avoid self-focusing. The difficulty when working with thin films is the presence of standing waves (Fabry-Perot effects) of pump and probe, strongly influencing the transmission and reflection behavior after excitation. An algorithm to retrieve the change of the complex dielectric constant, ∆e(t)= ∆eR(t) − i∆e(t), from measured ∆R(t) and ∆T(t) data was developed [1]. This algorithm calculates the optical response of the film due to the periodic excitation by the pump through multi-photon absorption and impact ionization. Detailed studies were performed to characterize and maximize the stability of the algorithm with respect to experimental uncertainties. Figure 1 shows the so obtained time dependent dielectric function for two samples.

Laser breakdown of dielectric oxide films from 20 fs to 1 ps

Dielectric thin films based on oxides are common for optical coatings. We investigated five oxides (TiO2, Ta5O2, HfO2, Al2O3, and SiO2) with different bandgaps ranging from 3.2 eV to 8 eV. The films were deposited by ion-beam sputtering on fused silica substrates with a thickness of 6λ/4n where λ = 790 nm and n is the refractive index of the material. Single-pulse damage studies were performed with pulses from a Ti:sapphire oscillator-amplifier system and pulse durations from 25 fs to 1.2 ps.