Cristina Rodriguez

Characterization of laser-induced air plasmas by third harmonic generation.

Third harmonic generation by a femtosecond probe intersecting a laser-induced air plasma is measured and a general model developed to describe such signal. The electron density distribution and nonlinear coefficient of the plasma are determined.

Frequency tripling mirror.

A frequency tripling mirror (FTM) is designed, fabricated and demonstrated. The mirror consists of an aperiodic sequence of metal oxide layers on a fused silica substrate tailored to produce the third harmonic in reflection. An optimized 25-layer structure is predicted to increase the reflected TH by more than five orders of magnitude compared to a single hafnia layer, which is a result of global compensation of the phase mismatch of TH and fundamental, field enhancement and design favoring reflection. Single pulse conversion efficiencies approaching one percent have been observed with the 25-layer stack for fundamental wavelengths in the near infrared and 55 fs pulse duration. The FTM is scalable to higher conversion, larger bandwidths and other wavelength regions making it an attractive novel nonlinear optical component based on optical interference coatings.

Third harmonic microscopy of intrinsic and induced material anisotropy in dielectric thin films

Abstract. Third harmonic (TH) microscopy with circularly polarized illumination directly reveals material anisotropy owing to suppression of background optical signals from isotropic media. Because optical thin films and their substrates are expected to be highly isotropic, TH microscopy presents a path to study induced and intrinsic anisotropy in films, providing both insight into laser-induced material modification that precedes damage and feedback about the deposition process. Because nanoscale defects and material strain influence the damage behavior of films, we examined TH sensitivity to similar sources of contrast. We demonstrate imaging of individual 10 nm colloidal gold nanoparticles and 100 mN nanoindentations in fused silica both with signal-to-noise ratio (SNR)≥100∶1. We present TH images (SNR≥210∶1) of sites exposed to femtosecond laser pulses below damage in 100 nm HfO2 films that are barely visible (SNR≤2.3∶1) with Nomarski and polarization imaging, traditional microscopic techniques known to display contrast for material anisotropy. At our detection limit (320 mW, 50 fs, 790 nm, ≈106 photomultiplier tube gain), we examined root mean square in the TH image of nascent films that correlated to the film’s macrostrain. TH microscopy presents a relatively simple all-optical method to monitor nanoscale anisotropy in thin films during exposure to high-intensity radiation and during deposition.

Characterization and χ(3) measurements of thin films by third-harmonic microscopy.

Third-harmonic (TH) generation from a thin layer on a substrate is analyzed in reflection and transmission geometry taking into account interference effects of fundamental and TH waves in the film, the backward-generated TH, and the pump-beam profile. Conditions are derived for both geometries where the signal from the film dominates, which is important for TH microscopy. The analysis results are applied to retrieve from experiment nonlinear susceptibilities χ(3) of hafnia/silica mixture (Hf(x)Si(1-x)O₂), alumina (Al₂O₃), and scandia (Sc₂O₃) thin films.

Optical coatings excited by femtosecond lasers near the damage threshold: challenges and opportunities

The push for ever more powerful and shorter laser pulse sources demands optical coatings that can handle large bandwidths while operating near the threshold of permanent damage. We review some of the current challenges in the understanding of film behavior under load and opportunities that arise from tailored coating stacks for nonlinear optical harmonic generation.

Femtosecond to nanosecond laser damage in dielectric materials

We present a consistent model supported by experimental data of damage of dielectric films by femtosecond to nanosecond pulses. Special emphasis is given to the role of defects and transient processes in the material. New imaging and diagnostic techniques are discussed to characterize the film performance.

Third-harmonic generation in one-dimensional photonic crystal meta-materials

Stacks of dielectric optical films are tailored to generate the third harmonic of femtosecond laser pulses. The improvement of efficiency compared to bulk samples of the same material is a combination of periodic reset of the phase mismatch and field enhancements due to resonant substructures.

New developments in optical coating characterization and utilization under high power laser irradiation

We present a novel technique to characterize coating imperfections that limit the performance under pulsed laser irradiation. We also discuss the use of 1D photonic crystals based on dielectric metal oxides as nonlinear optical frequency converters.

Modeling third harmonic generation from layered materials using nonlinear optical matrices: erratum.

We present corrected versions of Equations (11), (13), and (14) where typos were made.

Characterization of laser-induced air plasmas and thin films using third harmonic generation microscopy

Third harmonic microscopy is applied to probe electron densities in femtosecond laser-induced air plasmas, determine nonlinear susceptibilities of thin films, and image nascent, laser incubated, and laser damaged dielectric coatings with unprecedented contrast. The relative contribution of signals from the layer of interest and its surroundings is analyzed.