Femtosecond spectral phase shaping for CARS spectroscopy and imaging

Coherent Anti-Stokes Raman Scattering (CARS) is a third-order non-linear optical process that provides label-free, chemically selective microscopy by probing the internal vibrational structure of molecules. Due to the resonant enhancement of the CARS process, faster imaging is possible compared to Raman microscopy. CARS is unaffected by background fluorescence, but the inherent non-resonant background signal can overwhelm the resonant signal. We demonstrate how simple phase shapes on the pump (and probe) beam reduce the background signal and enhance the resonant signal. We demonstrate chemically selective microscopy using these shaped pulses on plastic beads

Spiral amplifiers in a-Al₂O₃:Er on a silicon chip with 20 dB internal net gain

Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of 1-µm-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are overgrown by a SiO2 cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium concentrations between 0.5-3.0×10^20 cm^-3 are experimentally characterized. A maximum internal net gain of 20 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2×10^20 cm^-3 and 1×10^20 cm^-3, respectively. The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain regime up to ~100 µW of signal power, but extension to the mW regime seems feasible. The experimental results are compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU) and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8 dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the amplifier performance of the Al2O3:Er3+ waveguide amplifiers. This effect is accentuated for concentrations higher than 2×10^20 cm^-3

Direct confocal lifetime measurements on rare-earth-doped media exhibiting radiation trapping

Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 – 5 × 1021 cm−3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid

Temperature-dependent absorption and emission of potassium double tungstates with high ytterbium content

We study the spectroscopic properties of thin films of potassium ytterbium gadolinium double tungstates, KYb0.57Gd0.43(WO4)2, and potassium ytterbium lutetium double tungstates, KYb0.76Lu0.24(WO4)2, specifically at the central absorption line near 981 nm wavelength, which is important for amplifiers and lasers. The absorption cross-section of both thin films is found to be similar to those of bulk potassium rare-earth double tungstates, suggesting that the crystalline layers retain their spectroscopic properties albeit having >50 at.% Yb3+ concentration. The influence of sample temperature is investigated and found to substantially affect the measured absorption cross-section. Since amplifiers and lasers typically operate above room temperature due to pump-induced heating, the temperature dependence of the peak-absorption cross-section of the KYb0.57Gd0.43(WO4)2 is evaluated for the sample being heated from 20 °C to 170 °C, resulting in a measured reduction of peak-absorption cross-section at the transitions near 933 nm and 981 nm by ~40% and ~52%, respectively. It is shown that two effects, the change of Stark-level population and linewidth broadening due to intra-manifold relaxation induced by temperature-dependent electron-phonon interaction, contribute to the observed behavior. The effective emission cross-sections versus temperature have been calculated. Luminescence-decay measurements show no significant dependence of the luminescence lifetime on temperature

Robust orthogonal parameterization of evolution strategy for adaptive laser pulse shaping

Many spectroscopic applications of femtosecond laser pulses require properly-shaped spectral phase profiles. The optimal phase profile can be programmed on the pulse by adaptive pulse shaping. A promising optimization algorithm for such adaptive experiments is evolution strategy (ES). Here, we report a four fold increase in the rate of convergence and ten percent increase in the final yield of the optimization, compared to the direct parameterization approach, by using a new version of ES in combination with Legendre polynomials and frequency-resolved detection. Such a fast learning rate is of paramount importance in spectroscopy for reducing the artifacts of laser drift, optical degradation, and precipitation

Femtochemistry and reactive intermediates : application to atmospheric and organic chemistry

The logical sequence of primary steps in a reaction mechanism is governed by the reactive intermediates which are postulated to ensure smooth transformations between stable species. Historically, such constructs were merely hypothetical as their (assumed) extreme reactivity and/or unstable nature precluded their observation. The introduction of femtosecond lasers to chemistry has allowed insight and understanding to many fundamental processes in reaction dynamics. This thesis presents applications of femtosecond spectroscopy to atmospheric and organic chemistry, specifically probing the reactive intermediates mediating various chemical transformations. Direct observation of these fleeting species can validate previously proposed mechanisms as well as offer new perspectives to their reaction dynamics. Studies of diradicals, the molecular species hypothesized to be archetypal of chemical bond transformations in many reactions in organic chemistry, have been made using femtosecond laser techniques combined with mass spectrometry in a molecular beam. Evolution of the diradical intermediate is monitored in real time throughout the course of the chemical reaction. These studies offer the first direct evidence of singlet 1,3- and 1,4-diradicals. Hydrogen-atom transfer processes are fundamental to many reactions in organic chemistry. The dynamics of the motion may be localized, with little nuclear motion aside from the H-atom transfer between two "anchors," or may be global, involving a multidimensional potential. Using methyl salicylate as a prototype system, direct studies of the nuclear motion in this important transformation are made with femtosecond resolution. Many reactive intermediates have been linked to stratospheric ozone depletion in recent years. In particular, catalytic cycles involving halogens are considered critical to establishing the overall mechanism for ozone depletion. Direct studies of the photochemically-activated dissociation of key species in atmospheric chemistry can offer insight to the global dynamics by providing important rate constants and branching ratios. Femtosecond reaction dynamics of OClO in a supersonic molecular beam are reported. The observations reveal the nuclear motions and couplings between potential energy surfaces relevant to the dissociation process. Comparisons with results of ab initio calculations are made