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Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for producing a wide range of van der Waals nanomaterials that can be incorporated into existing laboratory sample and industrial material production. Liquid-phase exfoliated nanomaterials have the potential to produce devices quickly and at low-cost, with colloidal dispersions easily adaptable to existing production methods. Other methods of nanomaterial production, such as chemical vapour deposition (CVD) and mechanical exfoliation can be costly, time-consuming and require complicated equipment to produce comparatively small area devices. Such methods are excellent for laboratory-scale samples to examine physical properties and produce proof of concept devices. However, LPE can and will bridge the gap towards real-world applications that require faster, easier and more cost-effective production methods. In this work, we investigate the saturable absorption and Kerr nonlinearity of graphene fabricated by LPE and CVD. 

Thin films of LPE graphene were produced on BK7 glass substrates, and compared to CVD graphene transferred onto an identical substrate. Through careful consideration of the concentration of graphene dispersion and deposition methods, very thin films of graphene can be prepared. Atomic force microscopy (AFM) measurements showed effective bi-layer graphene thickness for the LPE samples. Z-scan measurements performed with 180 fs pulses at a wavelength of 1030 nm reveal that both LPE and CVD graphene display strong saturable absorption characteristics, with a nonlinear absorption coefficient (*β*) approaching -10&lt;sup&gt;4&lt;/sup&gt; cm/GW and a Kerr nonlinearity (*n2*) of -1 cm&lt;sup&gt;2&lt;/sup&gt;/GW. Such strong saturable absorption is ideal for devices such as mode-lockers for ultrafast pulsed lasers. The magnitude of the nonlinear absorption coefficient of the LPE graphene increases with pulse duration, *τ*, up to 10&lt;sup&gt;5&lt;/sup&gt; cm/GW at around *τ* = 10 ps. These results pave the way for the use of LPE graphene in nonlinear optical applications such as frequency generation and mode locking.

CAP Virtual Congress 2021

Nonlinear optical properties of graphene: liquid-phase exfoliation vs chemical vapour deposition

saturable absorption

nonlinear optics

graphene

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Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for producing a wide range of van der Waals nanomaterials that can be incorporated into existing laboratory sample and industrial material production. Liquid-phase exfoliated nanomaterials have the potential to produce devices quickly and at low-cost, with colloidal dispersions easily adaptable to existing production methods. Other methods of nanomaterial production, such as chemical vapour deposition (CVD) and mechanical exfoliation can be costly, time-consuming and require complicated equipment to produce comparatively small area devices. Such methods are excellent for laboratory-scale samples to examine physical properties and produce proof of concept devices. However, LPE can and will bridge the gap towards real-world applications that require faster, easier and more cost-effective production methods. In this work, we investigate the saturable absorption and Kerr nonlinearity of graphene fabricated by LPE and CVD. 

Thin films of LPE graphene were produced on BK7 glass substrates, and compared to CVD graphene transferred onto an identical substrate. Through careful consideration of the concentration of graphene dispersion and deposition methods, very thin films of graphene can be prepared. Atomic force microscopy (AFM) measurements showed effective bi-layer graphene thickness for the LPE samples. Z-scan measurements performed with 180 fs pulses at a wavelength of 1030 nm reveal that both LPE and CVD graphene display strong saturable absorption characteristics, with a nonlinear absorption coefficient (*β*) approaching -104 cm/GW and a Kerr nonlinearity (*n2*) of -1 cm2/GW. Such strong saturable absorption is ideal for devices such as mode-lockers for ultrafast pulsed lasers. The magnitude of the nonlinear absorption coefficient of the LPE graphene increases with pulse duration, *τ*, up to 105 cm/GW at around *τ* = 10 ps. These results pave the way for the use of LPE graphene in nonlinear optical applications such as frequency generation and mode locking.

technical paper

In this research project, I calculated the high-harmonic spectrum from a 1D periodic potential. I investigated numerical methods for solving the 1D time-dependent Schrodinger equation of a particle in a double-well potential, as well as determining its ground state. I used the Crank Nicolson method [1], which is a finite difference method that can be used for numerically solving second-order partial differential equations. Using this method, I calculated the time evolution of an electronic wave function in a harmonic potential. My code was bench-marked against analytic solutions of the harmonic oscillator wave functions. I extended the use of this code by implementing the imaginary time method [2] to determine the ground state of an electron in a double-well potential. The time-independent Schrodinger equation is solved in the Bloch state basis to calculate the band structure of two different 1D periodic potentials. The calculations of dispersion relation are used to calculate the High Harmonic Spectrum and the final results are compared with [3].

[1] Wachter, C. (2017). Numerical Solution of the Time-Dependent 1D-Schrodinger Equation using Absorbing Boundary Conditions (Bachelor Thesis, University of Graz, Austria). Retrieved from https://physik.uni-graz.at/~pep/Theses/BachelorThesis_Wachter_2017.pdf

[2] Williamson, A. (1996). Quantum Monte Carlo Calculations of Electronic Excitations.
Retrieved from http://www.tcm.phy.cam.ac.uk/~ajw29/thesis/node27.html

[3] Wu, M. (2015). Attosecond Transient Absorption in Gases and High Harmonic Generation in Solids (Doctoral dissertation, Louisana State University, USA). Retrieved from https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?article=4320&context=gradschool_dissertations

Calculation of High Harmonic Spectrum from a 1D periodic potential

Blood banks all around the world store blood for several weeks ensuring the availability of blood for transfusion medicine. Although the storage conditions have been optimized for decades it has become evident that red blood cells (RBC) undergo numerous changes when being stored.
We investigated the effect of storage on the nanoscopic bending rigidity of RBC membranes with a combination of Molecular Dynamics simulations, inelastic neutron scattering and diffuse X-ray scattering [1]. Coarse grained (CG) models of RBC membranes were created by matching experimental lipidomic analysis. It was found experimentally that the concentration of fatty acids and cholesterol changes during storage and aged membranes were mimicked by adjusting the lipid composition accordingly. 
Solid supported membrane stacks of fresh and stored RBC samples were prepared. X-ray diffraction experiments were then conducted at high relative humidity allowing to reconstruct the membrane surface fluctuations from diffuse scattering signals using a GPU accelerated workstation. These experiments were complemented by Neutron Spin Echo measurements on RBC vesicles which probe the membrane fluctuations directly. Bending moduli of 1.8 kBT and 15.4 kBT were measured in excellent agreement with the simulation data.
[1] Himbert et al., “The Nanoscopic Bending Rigidity of Red Blood Cell Membranes”, in preparation

THE NANOSCOPIC BENDING RIGIDITY OF RED BLOOD CELL MEMBRANES

Accelerator Science is both a discipline in its own right within modern physics and provides highly powerful tools for discovery and innovation in many other fields of scientific research. Accelerators do support different disciplines of subatomic physics, material sciences, life sciences and applications in research and industry. Accelerator Science Community does perform R&D to improve operational facilities and prepare technologies for new facilities unveiling new opportunities and unprecedented performance parameters. Having delivered nearly five decades of discovery, TRIUMF has a vibrant reputation globally as Canada’s particle accelerator laboratory and a hub for particle accelerator physics and technology with a wide network of international connections. In collaboration with CLS, the Fedoruk Centre and Canadian Universities, we want to grow the impact of Accelerator Science in Canada on the society and the potential to address key issues of the society of today.

Accelerator Science in Canada

Magnetic Resonance Imaging (MRI) is a powerful imaging modality with excellent soft tissue contrast. Contrast agent such as iron oxide nanoparticles can be used to “tag” individual cells, distorting the magnetic field around them and allowing the imaging of single cells. Time-lapse MRI can be used to track the motion of tagged cells, providing insights in the studies of inflammatory diseases and metastasis of cancer. Current methods have a very limited temporal resolution resulting in a detection limit of 1µm/s . In addition, the manual cell counting is time consuming and difficult. 
In this work, a dictionary learning based technique has been developed to accelerate the MRI data acquisition and aid in the task of locating cells. Dictionary learning is a machine learning technique, in which features of an image can be ‘learned’ as atoms. Images can be represented using a sparse combination of atoms. The sparsity property can be used as a constraint in non-linear image reconstruction with data sampled below the Nyquist criteria. The undersampling improved the temporal resolution of the in-vivo measurements by approximately an order of magnitude. The dictionary atom coefficients provided information on the cell locations for feature detection.

Magnetic Resonance Imaging Data Acquisition Acceleration and Feature Detection with Dictionary Learning

The versatility of gamma-ray spectroscopy has given rise to its many applications, from quantification of trace elements in a sample to maintaining nuclear material safeguards. Depending on the application and gamma-ray detector, there is often a compromise made between detection efficiency and energy resolution. While characterizing or quantifying trace radionuclide concentrations in an unknown sample, energy resolution is often the more important property. In these situations, high-purity germanium (HPGe) detectors are the detector of choice as they have a superior energy resolution which significantly reduce measurement uncertainties and improve minimum detection limits.

Applications using HPGe detectors are limited to counting rates on the order of a few tens of thousands of counts per second (cps) before the performance of the detector is severely diminished. The limiting factor for high counting rate measurements comes from the need to shape signals with a relatively long shaping time, on the order of 1-6 μs, to maintain good energy resolution. At higher counting rates however, if a signal is received while a previous signal is still being shaped, pulse-pileup occurs. Pulse-pileup distorts the energy measurement of the previous signal and entirely drops the measurement of the second signal. Traditionally, this situation is usually handled with a pileup rejection method. At ultra high counting rates, on the order of a million cps, the percentage of the pileup rejection is so high that the spectroscopy system suffers from extremely high deadtimes.

In order to solve this critical problem, the present study aims to develop a data acquisition system capable of deconvoluting pileup signals into two or more recovered signals in real-time. This technique has been applied for NaI(Tl) and silicon drift detectors, which showed very promising results. However, applications of this technique for HPGe detectors have been unsuccessful as it relies on a fixed signal rising edge shape, a feature which cannot applied for HPGe detectors due to the variation in the rising shape. Our study has been focused on developing a deconvolution algorithm to identify and recover piled-up signals using a planar HPGe. During this development stage, signal waveforms are analyzed offline, optimizing the algorithm over a range of counting rates while building a library of the rising edge shapes. Once completed, the deconvolution algorithm will be benchmarked for various high-rate measurements. The preliminary result of the pile-up deconvolution performance will be presented.

Development of a digital data acquisition system capable of pulse-pileup recovery for HPGe detectors

Polycrystalline monolayer graphene films grown by chemical vapor deposition were exposed to a low-pressure inductively-coupled plasma operated in a gaseous mixture of argon and diborane. Optical emission spectroscopy and plasma sampling mass spectrometry reveal high B2H6 fragmentation leading to significant populations of both boron and hydrogen species in the gas phase. X-ray photoelectron spectroscopy indicates the formation of a boron-containing layer at the surface and provides evidence of a substitutional incorporation of boron atoms within the graphene lattice. To probe plasma’s influence on graphene structure, Hyperspectral Raman Imaging (RIMA for Raman Imaging) is used to obtain qualitative as well as quantitative data on a macroscopic scale. Graphene domains doping by graphitic boration is then confirmed by hyperspectral Raman Imaging of graphene domains. These results demonstrate that diborane-containing low-pressure plasmas are an efficient mean for boron substitutional incorporation in graphene with minimal domain hydrogenation and defect generation.

Boron substitutional doping of graphene by low- pressure diborane-argon plasma

We seek to design experimentally feasible broadband multiplexed optical quantum memory with near-term applications to telecom bands. Specifically, we devise dispersion compensation for an impedance-matched narrow-band quantum memory by exploiting Raman processes over two three-level atomic subensembles, one for memory and the other for dispersion compensation. Our proposed broadband quantum memory employs three-level atoms with atomic density, cavity quality, and Raman-laser power and detuning chosen such that inverse cavity lifetime equal so ptical depth, the delay-bandwidth product exceeds $10^6$, power efficiency exceeding 90% and at least one second of storage time, thereby leading to $10^6$ modes for multiplexing. Our design will lead to significant multiplexing enhancement for quantum repeaters to be used for telecom quantum networks.

Broadband quantum memory in a cavity via zero spectral dispersion

The radioactive decay of radon in the home is the leading cause of lung cancer in non-smoking Canadians (REF 1,2). Radon produced by the decay of uranium and thorium minerals entering the home may accumulate in concentrations that exceed the national maximum guideline for indoor air of 200 Bq/m3. There is a critical need to develop a practical tool to assess an individual’s exposure to radon and eventually one’s lung cancer risk. An important opportunity is to use keratinizing tissues in the body (hair, nails) as archives of radon exposure. The lead is sequestered from the environment in toenails, including the relatively long-lived (22 yrs) 210Pb isotope, which comes from 222Rn decay. 

In this project, we are using isotope ratio mass spectrometry to quantify the amount of 210Pb in a known amount of sample. This method has the advantage of providing a direct and relatively rapid count of the numbers of 210Pb atoms. The challenge is that the actual number of 210Pb atoms is very low and achieving reliable results requires high sensitivity methods specifically designed for the extraction of lead from the biological matrix. In the first stages of the project, we are using isotope dilution methods coupled with multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Initial results demonstrate that femtogram quantities of lead can be measured.

The next stage of the project involves the design and construction of a laser ablation ion source coupled to the Multiple Reflection Time of Flight (MR-TOF MS) at the TITAN instrument at TRIUMF. The laser ion source in combination with the MR-TOF MS offers high sensitivity and the ability to separate isobars of 210Pb. The laser beam, after passing through optical telescope system and polarizers for pulse energy modulation, is focused on a small point on the sample surface located in a high-vacuum chamber. Thus, the laser source enables spatial mapping of 210Pb isotopic composition and allow one to map the accumulation of the radon daughter products over the growth of tissue. Ultimately, an accurate measurement of the number of accumulated atoms in an individual’s biological tissue may be a personalized biodosimeter for radon.

1. Gogna, Priyanka, et al. "Estimates of the current and future burden of lung cancer attributable to residential radon exposure in Canada." Preventive medicine 122 (2019): 100-108
2. Stanley, F. K., Zarezadeh, S., Dumais, C. D., Dumais, K., MacQueen, R., Clement, F., & Goodarzi, A. A. (2017). Comprehensive survey of household radon gas levels and risk factors in southern Alberta. CMAJ open, 5(1), E255–E264. https://doi.org/10.9778/cmajo.20160142
3. Stanley, Fintan KT, et al. "Radon exposure is rising steadily within the modern North American residential environment and is increasingly uniform across seasons." Scientific reports 9.1 (2019): 1-17.

Development of an isotopic biodosimeter to assess radon gas exposure

We model a number of both closed-packed and non-closed-packed crystals inside a cholesteric liquid crystal (LC) with different pitch values and nematic LC through the Landau–de Gennes free-energy method[1]. We investigated the anisotropic interactions between particles with heterogeneous boundary conditions inside both nematic and cholesteric liquid crystals[2]. The results show that it is energetically favorable for the particles to remain in a plane parallel to the far-field director in a nematic liquid crystal, while for particles immersed in a cholesteric there are multiple energy minima not all located in the same plane. Therefore, We became interested in investigating the stability of binary crystals [3]. The results indicate that body-centered-cubic (BCC) crystals have a lower-energy lattice defect structure than the diamond crystal. Furthermore, it is shown that a pair of binary colloids can be self-assemble into a stable face-centered- cubic lattice structure inside a nematic LC, as it has the lowest energy comparing to diamond and BCC crystals.

1-P.G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford University Press, Oxford, 1993)
2-Phys. Rev. E 95, 052703 – Published 30 May 2017
3-Phys. Rev. E 99, 052701 – Published 13 May 2019

Stability Of Binary Colloidal Crystals Immersed In a Cholesteric Liquid Crystal

High-power lasers are rapidly becoming standard tools in advanced manufacturing, mainly in the form of laser welding, laser cutting, and laser additive manufacturing. Of these applications, laser welding in the electric mobility sector---particularly in the manufacturing of battery packs---presents unique challenges. Weld depth needs to be precisely controlled, not only to ensure joint strength, but also to ensure the weld does not puncture into the lithium ion cell. In addition, these processes often involve highly reflective metals (such as copper), which have material properties that lead to unstable welds; this requires an unprecedented level of control to ensure weld quality and depth. To better monitor and control weld quality, we need fully in-line monitoring during the process. Inline Coherent Imaging (ICI) is a process monitoring technique that has been demonstrated to measure keyhole depth (down to 15 um axial resolution) at high imaging rates (~200 kHz), even in high aspect ratio features. Due to its interferometric nature, coherent imaging has unparalleled sensitivity and dynamic range. However, it suffers from speckle noise, which degrades high-speed measurements by orders of magnitude, and false interfaces that arise from unwanted interferences (“autocorrelation” peaks). These pose a significant challenge to quality assurance and closed-loop control, particularly in highly dynamic laser processing applications, such as copper welding. To mitigate these problems, we have integrated a second, automatically synchronized, imaging channel into a standard ICI system, by exploiting a previously unused part of the imaging window. This “witness” image allows us to identify real signatures based on correlation and filter out the uncorrelated noise. Using this system, we have demonstrated the complete removal of autocorrelation artifacts, and have increased signal to noise by a factor of two, with no loss of imaging rate or spatial resolution compared to standard ICI. When applied to imaging laser keyhole welding, the false interface detection rate is reduced from 10% to 0.15%, yielding improved tracking of the true morphology.

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