Trevor L. Courtney

Bulklike Hot Carrier Dynamics in Lead Sulfide Quantum Dots

Hot electronic dynamics in lead sulfide nanocrystals is interrogated by degenerate pump-probe spectroscopy with 20-25 fs pulses over a broad frequency range around three times the nanocrystal band gap. For each nanocrystal diameter, an initial reduction in absorption is seen only at the peak of the quantum confined E1 transition, while increased absorption is seen at all other wavelengths. The signals from the nanocrystals are approximately 300 times weaker than expected for a two-level system with the same absorbance and molar extinction coefficient and are weaker near time zero. These results appear to be inconsistent with quantum confinement of the initially excited high energy states. Arguments based on carrier scattering length, the wave packet size supported by the band structure, and effective mass are advanced to support the hypothesis that, for many direct-gap semiconductor quantum dots, the carrier dynamics at three times the band gap is localized on the 1-2 nm length scale and essentially bulklike except for frequent collisions with the surface.

Enhanced interferometric detection in two-dimensional spectroscopy with a Sagnac interferometer.

An intrinsically phase-stable Sagnac interferometer is introduced for optimized interferometric detection in partially collinear two-dimensional (2D) spectroscopy. With a pump-pulse pair from an actively stabilized Mach-Zehnder interferometer, the Sagnac scheme is demonstrated in broadband, short-wave IR (1-2 μm), 2D electronic spectroscopy of IR-26 dye.

Absolute measurement of femtosecond pump-probe signal strength.

The absolute femtosecond pump-probe signal strength of deprotonated fluorescein in basic methanol is measured. Calculations of the absolute pump-probe signal based on the steady-state absorption and emission spectrum that use only independently measured experimental parameters are carried out. The calculation of the pump-probe signal strength assumes the pump and probe fields are both weak and includes the following factors: the transverse spatial profile of the laser beams; the pulse spectra; attenuation of the propagating pulses with depth in the sample; the anisotropic transition probability for polarized light; and time-dependent electronic population relaxation. After vibrational and solvent relaxation are complete, the calculation matches the measurement to within 10% error without any adjustable parameters. This demonstrates quantitative measurement of absolute excited state population.

Lightweight hollow rooftop mirrors for stabilized interferometry

Abstract. Hollow rooftop mirrors, also known as dihedral retroreflectors, can simultaneously preserve polarization, minimize chromatic dispersion, and allow beams to be stacked inside an interferometer. Two hollow rooftop mirrors were fabricated and characterized using a Fizeau interferometer and an inexpensive home-built jig instead of a master cube. The mass was 3.3 g for a clear aperture surface area of 110  mm2 with maximum retroreflected beam deviation of 12 arc s. With a hollow rooftop mirror mounted on a piezoelectric transducer in one arm of a Mach-Zehnder interferometer, a displacement stability of ±0.8  nm rms was achieved using active feedback. The rooftop mirrors’ suitability for Fourier transform spectroscopy was demonstrated.

Sagnac Interferometer for Two-Dimensional Spectroscopy in the Pump-Probe Geometry

An intrinsically phase-stable Sagnac interferometer is introduced for enhanced sensitivity detection in partially collinear two-dimensional spectroscopy. The sensitivity and phase accuracy of the apparatus are demonstrated on the dye IR-26 in the short-wave IR.

Hot Carrier Dynamics in Lead Sulfide Nanocrystals

Hot carriers in PbS nanocrystals are directly probed. The data are consistent with bulk-like small, high velocity electron and hole wavepackets that initially feel little or no quantum confinement, except for collisions with the surface.

Femtosecond Spectral Interferometry with Attosecond Accuracy by Correction for Spectrometer Resolution Asymmetry

Asymmetry in the line spread function of the spectrometer causes delay dependent phase shifts. Fourier deconvolution with the complex-valued optical transfer function allows accurate spectral phase recovery.