Time-domain ptychography

Through dedicated measurements in the optical regime we demonstrate that ptychography can be applied to reconstruct complex-valued object functions that vary with time from a sequence of spectral measurements. A probe pulse of approximately 1 ps duration, time delayed in increments of 0.25 ps is shown to recover dynamics on a ten times faster time scale with an experimental limit of approximately 5 fs.Comment: 5 pages, 4 figures, new title and minor text change

Electric field induced second harmonic (EFISH) measurements of highly boron doped p-type Si/SiO2

Thesis (PhD (Physics))--Stellenbosch University, 2008.The advent of high intensity short pulse lasers has opened the door to investigating buried solid-solid interfaces through the technique of optical second harmonic generation (SHG). This has led to extensive study of technologically important systems such as the Si/SiO2 interface. In this study, SHG is employed to study the interface between highly boron doped p+-type Si and its native oxide layer (SiO2). Previous studies from this laboratory have extensively investigated the photo-induced charge transfer process across the Si/SiO2 interface in the case of undoped natively oxidized Si by means of SHG, with initial SHG measurements being performed on boron doped p+-type Si. The natively oxidized p+-type Si/SiO2 sample was placed on a computer controlled positioning system which allowed for translation of the sample and rotation around the azimuth. The laser system employed was characterized in terms of spectral composition, pulse duration, pulse repetition rate, spatial pro le and pulse energy in order to ensure quantitative measurements. The SHG signal generated from the sample interface was recorded in re ection. Under the applied irradiation conditions, defects are created at the interface by the near infra red (NIR) femtosecond radiation from the laser. These defects are then populated via multi-photon processes by electrons and to a lesser extent holes. The charge transfer across the interface induces an interfacial electric eld. This photo-induced electric eld is in addition to the built-in interfacial electric eld caused by positive ionization of naturally occurring interfacial defects due to the strong doping of the bulk Si. It is this interfacial electric eld, consisting of the built-in doping induced eld and the photo-induced electron and hole elds, that is probed by SHG. The SHG signal is strongly dependent on the magnitude of this interfacial electric eld as the electric eld induced second harmonic (EFISH) signal dominates all other contributions to the observed SHG signal in the case of the Si/SiO2 system. The temporal evolution of the SHG signal is recorded for di erent intensities from virgin as well as the pre-irradiated samples. This yields information about the time scales on which the charge separation occurs as well as the in- uence of existing photo-induced trap sites on the charge separation process, since the strength of the SHG signal is an indirect measure of the interfacial electric eld strength. The angular dependence of the SHG signal (SH rotational anisotropy measurements) for both the initial signal (when the doping induced electric eld dominates) and the saturated signal (when the electron induced electric eld dominates) is measured. Both these measurements show a four fold symmetry but with a relative 45 phase shift between them. This iii is taken as con rmation of the reversal of the interfacial electric eld direction. The initial SHG signal as a function of intensity is also recorded for di erent incident wavelengths. The variation in the non-quadratic dependence of the initial SHG signal on the incident intensity is attributed to a resonant enhancement of two-photon absorption and subsequent screening of the interfacial electric eld by charge carriers. The measurement performed and the results obtained contribute to the understanding of the photo-induced charge separation process across buried solid-solid interfaces, speci cally as it applies to the important Si/SiO2 interface

Investigation of atmospheric insect wing-beat frequencies and iridescence features using a multispectral kHz remote detection system

Quantitative investigation of insect activity in their natural habitat is a challenging task for entomologists. It is difficult to address questions such as flight direction, predation strength, and overall activities using the current techniques such as traps and sweep nets. A multispectral kHz remote detection system using sunlight as an illumination source is presented. We explore the possibilities of remote optical classification of insects based on their wing-beat frequencies and iridescence features. It is shown that the wing-beat frequency of the fast insect events can be resolved by implementing high-sampling frequency. The iridescence features generated from the change of color in two channels (visible and near-infrared) during wing-beat cycle are presented. We show that the shape of the wing-beat trajectory is different for different insects. The flight direction of an atmospheric insect is also determined using a silicon quadrant detector. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE

Novel time domain ptychography, i2PIE, for improved contrast in nonlinear microscopy

We present a novel nonlinear microscopy modality using a time-domain ptychographic phase measurement, i2PIE, to compress 80 MHz supercontinuum pulses from an ANDi PCF used as excitation source, improving contrast at reduced average power

Generalized spectral phase-only time-domain ptychographic phase reconstruction applied in nonlinear microscopy

Nonlinear microscopy has evolved over the last few decades to become a powerful tool for imaging and spectroscopic applications in biological sciences. In this study, I$^2$PIE, a novel spectral phase control technique, was implemented in order to compress broad-bandwidth supercontinuum light pulses generated in an all-normal-dispersion (ANDi) photonic crystal fiber (PCF). The technique, based on time-domain ptychography, is demonstrated here in a nonlinear microscopy application for the first time, to the best of our knowledge. The first real-world application of this technique for second-harmonic generation and two-photon excitation fluorescence microscopies in biological samples is presented. We further show that in our implementation, I$^2$PIE leads to improved contrast and signal-to-noise ratios in the generated images, compared to conventional compression techniques used in nonlinear microscopy.Comment: Copyright 2020 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibite

Novel time-resolved CARS implementation for application in microscopy

Vibrational dephasing times for benzene and carbon disulfide are measured using a custom single-beam Coherent Anti-Stokes Raman Spectroscopy (CARS) setup. A femtosecond oscillator is used to pump a polarization maintaining all normal dispersion photonic crystal fibre (PM-ANDi-PCF) to generate a broad band supercontinuum, covering a spectral region from 680 to 900 nm. The dispersion properties of the PM-ANDi-PCF ensures the supercontinuum is stable and there exists a fixed phase relationship between the spectral components of the supercontinuum. This enables its temporal compression using i2PIE, implemented using a liquid crystal spatial light modulator (SLM) in a 4f geometry. This SLM is also used to shape the pulse spectrally and temporally. With this setup we could demonstrate time-resolved CARS, measuring the vibrational relaxation times of a carbon disulfide (CS2)/benzene mixture, and eliminate the non-resonant background completely. The main advantage of this setup is the fact that it is a single beam technique, eliminating the requirement for aligning the overlap of the pump and probe, both spatially and temporally, in the focal plane of the microscope. The strengths and limitations of the technique are highlighted and the route to time-resolved/background free vibrational microscopy is proposed

Determining non-linear optical properties using the Z-scan technique

Thesis (MSc (Physics))--University of Stellenbosch, 2005.The extremely high light intensities produced by lasers and the increasing use of lasers highlights the need for measures to prevent damage to materials due to exposure to high intensity laser light. In particular it necessitates the development of systems to protect optical sensors, including the human eye. In this work optical limiters were investigated as a system for protecting sensors. An optical limiter transmits ambient light, but absorbs high intensity light. This makes it ideal for protecting sensors from laser radiation, since it allows the sensor to operate unhindered at design intensities while protecting it from harmful high intensity radiation. There are various mechanisms used for optical limiting, and in this work the nonlinear absorption and the nonlinear index of refraction changes of materials were investigated. A facility was established to measure the nonlinear optical properties of a variety of materials, in order to classify them as possible optical limiters. This entailed creating a so called Zscan setup, which enabled us to measure the nonlinear absorption coefficient and the nonlinear index of refraction of a material. The theory and the design of the setup are discussed and experimental results obtained using this setup are presented. A wide variety of material types were investigated to show the versatility of the experimental setup. These included C60, which was analyzed in solution; ZnO which is a crystal; CdS quantum dots in solution; and poly(dioctyl-fluorene), which is a large polymer molecule, in solution. The materials investigated in this work were chosen based on their known strong nonlinear optical properties. Emphasis was placed on measuring the nonlinear absorption coefficients since it was the dominant optical limiting effect of the materials under investigation. The results obtained displayed the same trends as published results and it shows that the established facility was capable of measuring the nonlinear properties of these samples. The experimental limitations of the setup were determined, and critical experimental parameters were identified for measurements of this nature. Improvements to the experimental facility are suggested to improve the accuracy of future measurements

Targeted single-beam CARS using phase-and-polarization shaping

I2PIE compressed supercontinuum pulses from a femtosecond oscillator pumped ANDi-PCF are phase shaped, using an SLM in a 4f-shaper geometry, with quadratic phase functions. Specific Raman transitions in single-beam CARS measurements are successfully targeted

Applied kHz optical remote sensing for determination of insect flight direction and relative size

An applied kHz optical remote sensing system is implemented to determine the flight direction of an insect with respect to wind direction, and its relative size using near- and shortwave infrared light in situ