1,721,028 research outputs found
Reaction microscope studies of small molecules in strong laser fields
No full textThe dynamics of molecules and molecular reactions that generate chemistry such as bond breaking, bond formation or bond rearrangement occur on the femtosecond [...] time scale. Real-time observations and investigations of these reactions are fundamental to understanding these dynamics. Ultra-fast laser pulses with intensities around 1014 W=cm2, impact on the electronic and nuclear motion on femtosecond timescale. In order to understand the region of transition states between the initial and final state of a reaction, that determines the fate of the products, detailed experimental investigations are necessary.
This work focuses on three separate investigations, one addressing the strong field control of the electronic dynamics in the hydrogen molecule and two addressing the strong field ionization dynamics in small but complex hydrocarbon molecule acetylene (C2H2). The pump-probe approach is applied to observe the real-time femtochemistry. [...]Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Natural SciencesScience, Environment, Engineering and TechnologyFull Tex
Integrated Photonic Multiplexing for Quantum Information
No full textHerewithin we present multiplexed integrated photonics as a realisable route to large-scale quantum technologies. Quantum photonics is one many platforms for experimental demonstrations of quantum information science. The technological advantages afforded by the combination of integrated photonic circuits with multiplexing techniques can enable performance enhancements to make photonics a leading infrastructure for implementation of real-world quantum information systems.
For this purpose, we create integrated circuits to solve technical challenges in the fields of quantum cryptography, discrete variable and continuous variable quantum computation. These circuits were fabricated in a facility developed at Griffith University, Centre for Quantum Dynamics, for the production of annealed and reverse proton exchange waveguides in congruent lithium niobate and have found applications across the fields of quantum optics and cryptography.
We develop and demonstrate the first integrated many-mode active optical demultiplexing of single photons from a solid-state source. This scheme enables the production of a multiphoton Fock state across multiple spatial modes from a single high brightness solid-state source with temporal indistinguishability. This work addressed a major hurdle in the development of photonic quantum computers, namely generation of a large number of indistinguishable single photons on demand. To perform the demultiplexing of single photons we develop two key technologies; a high-speed 1:4 integrated photonic switch and a many channel arbitrary pulse sequence generator.
Cryptography as a field is increasingly reliant on quantum random number generators for added security. With increased demand comes a requirement for higher bitrate random number generators, and as such we demonstrate multiplexing of a random number generation scheme based on measurements of quantum vacuum fluctuations. Furthermore, we show an increased level of security at high bitrates by implementing a new signal processing scheme
We demonstrate integrated generation, manipulation and homodyning of squeezed light on a single chip for the first time. This scheme is the first successful demonstration of full integration of all the major components needed for continuous variable quantum computation in a temporally multiplexed architecture. These results represent contributions to several fields of study, demonstrating the advantages of integrated quantum photonic multiplexing, and are of interest to the quantum computing, information security, integrated optics, and electronic control communities.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Environment and ScScience, Environment, Engineering and TechnologyFull Tex
Probing the Dynamics of Atoms and Molecules by High Harmonic Generation
No full textIn this dissertation, we have observed the phase shift of high harmonic generation
(HHG) from atomic and molecular target gases by using the Gouy phase interferometric
technique. The aim of this thesis is to probe the molecular structure and dynamics
from the phase of emitted HHG radiation. The work mainly divided into three
parts. The first part is on designing and implementing the "Advanced Gouy Phase
Interferometer". It includes the detail about the design and instrumentation that
has been improvised and modifed from the prototype version. The interferometer
consists of two thin gas jets separable in space along the laser propagation direction
in single laser focus.
The intensity of high harmonic generated pulses is modulated when the separation
between two gas jets is increased. This modulation is observed due to the Gouy
phase shift (which depends on the longitudinal position within the Gaussian beam)
of the driving laser beam experienced by the two gas jets. The Gouy phase di erence
imparts a delay in HHG photon emission time. The fringes visibility or temporal
coherence of several consecutive harmonic orders as a function of gas jets separation
is observed with atomic Ar and molecular H2 target gases. The temporal coherence
properties is observed with 9 fs and 50 fs driving laser pulses. This observation of
temporal coherence of the harmonics with multi-cycle driving laser pulses indicates
the broader usefulness of this interferometric technique. The possible applications of
this interferometer such as HHG yield enhancement, selective control of particular or
range of harmonic order from the typical HHG spectrum and extracting the relative
high harmonic phase shift between two gases are discussed.
In the second part, we report on extracting the relative phase shift of HHG
emissions by using the advanced Gouy phase interferometric technique. As a part
of this, first we measured the relative phase di erence from the simplest molecular isotopes in nature i.e., hydrogen (H2) and deuterium (D2). The experimental results
are presented with 9 fs and 50 fs laser pulses. The results show that the relative
phase shift between these isotopes increases monotonically with the harmonic order.
We have observed about 180 +/- 8 mrad phase shift for harmonic order 23 (H23) and
it increases to 230 +/- 26 mrad for H35 with 9 fs pulses. The experimental results
agree well with the simulation results obtained by solving the non-Born-Oppenheimer
time dependent Schrodinger equation. Detailed analysis of the theoretical modelling
uncover the underlying mechanisms of electrons and nuclei responsible for the
measured phase di erence. The results indicate that the electron-ion Coulomb
interaction during the electron acceleration in the continuum as well as the bound
electron interaction with the ionized electron play the role on the observed phase shift.
The phase shift measured with 50 fs pulses is 225 +/- 10 mrad for H23 and 340 +/- 38
mrad for H35. We anticipated that the contribution of high harmonic generated
by the electron return to the higher excited 2p u state of H+2/D+2 molecular ion
along with the ground 1s g state may lead to the larger phase shift with 50 fs pulses
compared to the 9 fs pulses.
The final experimental result presents the HHG phase shift measurement from
methane isotopes i.e., methane (CH4) and deuterated methane (CD4). The observed
phase shift as a function of harmonic order increases at a greater rate for methane
isotopes compared to the hydrogen isotopes. The measured phase shift is 95 6
mrad for H23 and increases to 460 83 mrad for H31. Theoretical analysis predicted
that this phase shift comes from the di erence in field free nuclear dynamics of CH+4 and CD+4 ions.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Environment and ScScience, Environment, Engineering and TechnologyFull Tex
Femtosecond Laser-Induced Structural Transformation and Micromachining of Diamond
No full textAn effective structural transformation from an sp3-hybridised diamond into an sp2-hybridised graphite as well as precisely micromachining the diamond crystal with an acceptable precision and accuracy at the micro- and/or nano- scale can only be achieved with a careful application of well-tuned ultrashort femtosecond (fs)-laser pulses, thus avoiding the generation of thermal stresses and the resultant thermal damage of the surrounding material, which is the main problem encountered in the conventional laser processing. An ultrashort pulsed laser with a pulse duration of 30 fs has been used for the first time in this research project to minimize the heat diffusion into the lattice as the material processing with sub-50 fs pulses is driven predominantly by the photoionization of electrons without the heat transfer to the surrounding material. This thesis aims to nonthermally transform the surface of diamond into an aromatic sp2 graphitic phase with high crystallinity as well as to micromachine the surface of diamond with high machined surface quality and high volumetric processing efficiency. [...]Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Eng & Built EnvScience, Environment, Engineering and TechnologyFull Tex
`Attoclock' experiments on atomic and molecular hydrogen
No full textThe current thesis aims at benchmarking strong- eld physics with the help of precision
measurements performed on the simplest atomic (H) and molecular (H2) systems.
The importance of H in validating strong- eld models is demonstrated through
the rst set of experimental data. It aims at calibrating the absolute Carrier-envelope
phase (CEP) of few-cycle laser pulses using H against complete ab initio solution of
the three dimensional time-dependent Schr odinger equation (3D-TDSE). Subsequent
set of measurements with noble gases against widely used strong- eld models based
on single-active electron (SAE) approximation, is shown to reveal a systematic o set
of 0:25 radians in tagging CEP, questioning the validity of such models.
The second experimental study forms the main result of this thesis, that attempts
to resolve the ongoing debate on tunnelling times (tunnelling delays in the context of
strong- eld physics). We address this by employing the `attoclock' technique with 6
fs pulses on H and validating the results against full numerical solutions of ab initio
3D-TDSE. The validated numerical codes are then used to arti cially screen the
parent ion-electron interaction, concluding that the tunnelling time 1.8 as.
The nal experimental results presented in this dissertation are the alignmentdependent
attoclock measurements using both few-cycle (7 fs) and multi-cycle (28 fs)
pulses on H2. The measured attoclock observable for various molecular orientations
(in laser polarisation frame) shows a strong modulation with a periodicity of .
Initial ab initio simulations for few-cycle pulses under the frozen-nuclei and SAE
approximations, fail to explain these observations. Further experimental studies with
H2/D2 (50:50 mixed gases) show no signi cant relative di erences among the attoclock
observables, suggesting a prominent role of the electron-electron correlations at play.
The ongoing study is believed to have far reaching implications in applications such
as studying molecular dissociation processes and tomography.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Environment and ScScience, Environment, Engineering and TechnologyFull Tex
Ionisation of Metastable Neon with Few Cycle Laser Pulses
No full textThis thesis describes two separate investigations into the ionisation of the 3P2 metastable state of neon (Ne) by strong-eld light pulses. The first is an investigation into the ionisation yield of Ne and the second experiment examines the electron momentum distribution of the ionised electrons.
Experimental ionisation results were compared to ionisation models via a scaling fit to ion yield and laser intensity. The best laser intensity fit was provided byTDSE based theory with n = 0:82. The effect of the spin state on ionisation yield was also examined, and it was determined that there is an 15:34:6% difference in ion yield between atoms pumped into the stretched mj = 2 states compared to an atomic ensemble with a more even state population distribution. This was an unexpected result that remains unexplained. Experimental measurements of the transverse electron momentum distribution (TEMD) for Ne that is ionised well in the over-the-barrier regime are compared to the TEMD for Ar, which is ionised well in the tunnelling regime. It was observed that as the polarisation of the ionising light becomes circular, the distribution for Ne remains cusp-like, whereas the distribution for Ar broadens and becomes Gaussian. These results are predicted by a theoretical treatment of the interaction with a TDSE model and quantitatively indicate that the free electron is influenced in a non-negligible way by the ionic core in the over-the-barrier ionisation regime. This has implications for the validity of the strong field approximation in this ionisation regime.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Natural SciencesScience, Environment, Engineering and TechnologyFull Tex
Laser produced plasmas for the generation of X-rays
No full textLaser Produced Plasmas (LPP) are a better source to increase the energy cut-off of the harmonics generated due to their larger ionisation potential than the conventional neutral gas media, if optimised properly. The experiments detailed in the thesis aim towards the use of LPP as a medium for the generation of bright coherent EUV/XUV table-top sources. A detailed spatio-temporal characterisation of LPP's generated from Al and Cr targets via non-invasive optical emission spectroscopy (OES) and time-resolved plume imaging are performed to investigate plasma features and optimise them. While ns LPP's are spatio-temporally optimised for Al IV/Al III with the irradiation energy as the control parameter, the ps LPP's are optimised for Al III using the single pulse (SP) and double pulse (DP) schemes. Experiments on Cr plasmas are also performed and the changes in plume morphology upon implementing SP and DP schemes are investigated via plume imaging and the
abundance of various species in the plume were investigated via time-resolved OES. The DP schemes are investigated in detail to find out the influence of parameters such as the polarisation, delay between the two pulses, energy in the first pulse and the ambient pressure on the plume morphology. Building and extending the experimental setup to conduct the proposed experiments aiming towards increasing the energy cut-off of the harmonics using LPP as the nonlinear medium comprises
the second part of the thesis. A High-order Harmonic Generation (HHG) set up is built and tested using harmonics generated in Ar gas. The HHG system developed here would further be modified to take care of the plasma fluorescence as
well as to accommodate higher order harmonics. These spatio-temporally optimised LPP's together with the set-up are anticipated to produce HHG sources with higher brilliance than the commercial tabletop sources, adding energy tunability to these
sources. Additional experiments on the angle resolved X-ray emissions in the range of 30 keV to 150 keV from LPP from metals (Al and Cr) and their alloys (Al-Cr) are carried out and detailed here; which can also be used as a source of X-ray radiation.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Environment and ScScience, Environment, Engineering and TechnologyFull Tex
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Strong-field excitation of argon
No full textThe advancement in laser technology in the past few decades have enabled consistent generation of pulses in the femtosecond (fs, x10^-15 s) timescale. The strong electric fields produced by such pulses are comparable to those experienced by bound electrons within atoms and molecules, leading to highly non-linear interactions. One of the most probable such interaction is that of strong-field excitation where the target is left in an excited state. These excited states have been shown to influence other strong-field phenomena and exhibit unique properties that are useful for further applications such as, generation of coherent extreme-ultraviolet radiation, and lasercooling of noble gases. Therefore, a comprehensive understanding of the fundamental excitation process and how excitation rates are affected is necessary in order to tailor conditions for a desired outcome. So far, there have been only a few experimental studies on excitation yields due to the unique experimental arrangements required for observation of these states. In contrast, there have been more theoretical studies which have yet to be experimentally confirmed. The aforementioned experiments have been undertaken with laser pulses with a duration of 30 fs or more centred at a wavelength of 800 nm which contain many optical cycles. But, numerical calculations predict that excitation yields scale differently as the pulse duration reduces such that it contains only a few optical cycles. This has yet to be experimentally confirmed since there has not been any experimental studies on excitation yields from few-cycle pulses. Furthermore, the use of few-cycle pulses enables precise control over the electric field experienced by the atom which may influence the excitation process.
In this dissertation, we experimentally investigate excitation yields of argon interacting with multi- and few-cycle pulses centred at 800 nm and compare them to solutions of the time-dependent Schrodinger equation (TDSE). The first investigation explores the effect of changing the intensity spanning between 50-300 TW/cm2. By directly detecting excited states surviving the flight time to the particle detector, we show that excitation rates exhibit a step-wise increase within the intensity range which correspond to the absorption of 13 and 14 photons with linearly polarized multi-cycle pulses. These were predicted theoretically but were thought to be washed out due to volume-averaging inevitable in the experiment. Analysis of the numerical predictions reveal that these enhancements are mainly due to excitation into low-lying states, specifically the 5g and 6h states for 13- and 14-photon absorption, respectively. These increases are not observed with few-cycle pulses where the offset between the peak of the pulse envelope and the peak of the central electric field cycle, known as the carrier-envelope phase (CEP), was not locked. This is in excellent agreement with TDSE predictions. Population of low-lying states are largely preferred with few-cycle pulses and these enhancements are less pronounced, to the point where they do not persist after volume-averaging.
The second investigation explores excitation with elliptically polarized laser fields of varying ellipticities at select intensities with both multi- and few-cycle pulses. In all cases, excitation rates decrease quicker with increasing ellipticity than that of Ar+ but slower than predicted with the strong-field approximation as well as Ar2+. This indicates a different mechanism than the tunneling-plus-rescattering model proposed for the formation of Ar2+ through non-sequential double ionization. No anomalous peaks at non-zero ellipticity are observed in the experiment for 30 fs and 6 fs pulses at an intensity of 270 TW/cm2 and 200 TW/cm2, respectively, nor were they predicted by TDSE results. At a lower intensity, where previously published results from semi-classical modeling predict anomalous distributions, no obvious deviations from a normal distribution is observed. However, low statistics at this intensity limits any confident conclusions for a peak at very small, non-zero ellipticity values. Lastly, analysis of TDSE results reveals an anomalous distribution for excitation out of the pm= +-1 initial ground state orbitals. Further experiments are required for solid conclusions as well as good agreement between TDSE results and experiments.
The last investigation explores the role of the CEP of a few-cycle pulse. For the first time, we show that excitation rates are highly dependent on both the peak intensity and CEP of the pulse. At a single intensity, TDSE calculations predict up to a 55% variation in excitation rates. Furthermore, the CEP dependent trends can vary significantly with small changes in the intensity, leading to a significant variation in the optimum CEP for maximum excitation yields. In the experiment, volume averaging reduces the maximum observable variation in the CEP dependent yields to 7%. Furthermore, they are still highly dependent on the exact in situ peak intensity of the experimental pulse with many peak intensities resulting in a variation below 5%. This places tight restrictions on conditions which allow successful observation of the variation in yields with varying CEP. Despite the inability to precisely determine the in situ experimental intensity, the agreement with the numerical predictions is very good which serves to validate the theoretical predictions.
The results from these studies reveal that the population of excited states are dependent on the intensity, polarization, and, in the case of few-cycle pulses, the CEP. If the intensity can be precisely controlled, selective excitation to the 5g and 6h states can be achieved with up to a 60% likelihood with the use of multi-cycle pulses. This is reduced with volume-averaging but these states still remain the most populated states. Knowing this, excitation to the metastable state can be increased through direct stimulation via additional radiation. Further studies to determine the precise efficiency of the process is required in order to evaluate it as a suitable replacement for current metastable generation techniques.Thesis (PhD Doctorate)Doctor of Philosophy (PhD)School of Environment and ScScience, Environment, Engineering and TechnologyFull Tex
- …
