T. B. Norris

Adaptive correction of depth-induced aberrations in multiphoton scanning microscopy using a deformable mirror

We demonstrate adaptive aberration correction for depth‐induced spherical aberration in a multiphoton scanning microscope with a micromachined deformable mirror. Correction was made using a genetic learning algorithm with two‐photon fluorescence intensity feedback to determine the desired shape for an adaptive mirror. For a 40×/0.6 NA long working distance objective, the axial scanning range was increased from 150 mm to 600 mm.

Smart microscope: an adaptive optics learning system for aberration correction in multiphoton confocal microscopy

Off-axis aberrations in a beam-scanning multiphoton confocal microscope are corrected with a deformable mirror. The optimal mirror shape for each pixel is determined by a genetic learning algorithm, in which the second-harmonic or two-photon fluorescence signal from a reference sample is maximized. The speed of the convergence is improved by use of a Zernike polynomial basis for the deformable mirror shape. This adaptive optical correction scheme is implemented in an all-reflective system by use of extremely short (10-fs) optical pulses, and it is shown that the scanning area of an f:1 off-axis parabola can be increased by nine times with this technique.

Photoluminescence and time-resolved photoluminescence characteristics of InxGa(1−x)As/GaAs self-organized single- and multiple-layer quantum dot laser structures

The characteristics of ground and excited state luminescent transitions in In0.4Ga0.6As/GaAs and In0.35Ga0.65As/GaAs self-organized single- and multiple-layer quantum dots forming the active regions of lasers have been studied as a function of incident excitation intensity, temperature and number of dot layers. The results have been correlated with molecular beam epitaxial growth conditions. The threshold excitation density for the saturation of the ground state increases with the number of dot layers and no saturation is observed in samples with more than six dot layers up to an excitation power density of 2 kW/cm2. The luminescent decay times for the ground and excited states are around 700 and 250 ps, respectively, almost independent of the number of dot layers.

Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots

We measured the transient temperature-dependent carrier population in the confined states of self-assembled In0.4Ga0.6As quantum dots as well as those of the surrounding wetting layer and barrier region using differential transmission spectroscopy. Results show directly that thermal reemission and nonradiative recombination contribute significantly to the dynamics above 100 K. We offer results of an ensemble Monte Carlo simulation to explain the contribution of these thermally activated processes.

Chirped-pulse amplification of 85-fs pulses at 250 kHz with third-order dispersion compensation by use of holographic transmission gratings

We demonstrate pulse stretching and compression in a high-repetition-rate chirped-pulse Ti:sapphire regenerative amplifier, using high-efficiency holographic transmission gratings. A quantitative dispersion measurement technique is developed to characterize dispersion of the system to the third order. After recompression with third-order dispersion compensation, 3.1-microJ 85-fs, nearly transform-limited pulses are obtained.

Time reversal three-dimensional imaging using single-cycle terahertz pulses

We demonstrate three-dimensional imaging using single-cycle terahertz electromagnetic pulses. Reflection-mode imaging is performed with a photoconductive transmitter and receiver and a reconstruction algorithm based on time reversal. A two-dimensional array is synthesized from ten concentric ring annular arrays with numerical apertures ranging from 0.27 to 0.43. The system clearly distinguishes image planes separated by 1.5 mm and achieves a −6 dB lateral resolution of 1.1 mm. In terms of the illuminating terahertz power spectrum, the lateral resolution is 38% and 81% of the peak and mean wavelengths, respectively.

Generation of terahertz pulses with arbitrary elliptical polarization

We employ two different methods to generate controllable elliptical polarization of teraherz (THz) pulses. First, THz pulses are generated via optical rectification in nonlinear crystals using a pair of temporally separated and perpendicularly polarized optical pulses. The THz ellipticity is controlled by adjusting the relative time delay and polarization of the two optical pulses. We generate mixed polarization states of single-cycle THz pulses using ZnTe, and elliptically polarized multicycle THz pulses in periodically poled lithium niobate crystals. Second, we generate elliptically polarized THz pulses by making a THz “wave plate” using a combination of a wire-grid THz polarizer and a mirror to transform linearly polarized multicycle THz pulses into elliptical polarization.

Transient velocity overshoot dynamics in GaAs for electric fields ≤ 200 kV/cm

We have experimentally studied the transient velocity overshoot dynamics of photoexcited carriers in GaAs for electric fields as great as 200 kV/cm. Time domain waveforms proportional to the velocity and the acceleration of carriers have been acquired, respectively, from guided and free‐space radiating signals which contain terahertz frequency components. The measurements demonstrated that the degree of overshoot was maximized for an electric field on the GaAs between 40 and 50 kV/cm when 1.44‐eV photons in an 80‐fs laser pulse excited the sample. For carriers excited with higher initial energy (1.55 eV), the degree of overshoot decreased and the maximum degree of overshoot occurred at a higher electric field.

Chirped-pulse amplification of ultrashort pulses with a multimode Tm:ZBLAN fiber upconversion amplifier

Microjoule pulse energies are achieved from a single-stage upconversion fiber amplifier for the first time, to our knowledge, in this demonstration of chirped-pulse amplification with a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the fiber upconversion amplifier that produces picosecond pulse trains with energies as great as 16 μJ at a repetition rate of 4.4 kHz.