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In optics, caustics are bright, sharp lines and shapes created by the natural focusing of light. Some examples include rainbows, the wavy lines on the bottom of swimming pools and the patterns produced by gravitational lensing. The intensity at a caustic diverges in the classical ray theory, but can be smoothed by taking into account the wave nature of light. In this work we consider a new type of caustic that occurs in quantum systems due to phase singularities; because phase is such a central concept in wave theory, this heralds the breakdown of the wave description and is an example of a quantum caustic. In particular, we consider analogue black holes which can be formed in a flowing Bose-Einstein condensate (BEC) gas. Waves flowing near the event horizon (describing analogue Hawking radiation) appear to suffer a logarithmic phase divergence, which is known as the trans-Planckian problem for gravitational black holes. We describe the regularization procedure required to cure this quantum caustic in our BEC system, and make connections to catastrophe theory.

CAP Virtual Congress 2021

Hawking radiation as a quantum caustic

cold atoms

hawking radiation

quantum mechanics

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technical paper

Purpose: A quantitative measure of delivered ionizing radiation is recommended for quality assurance and quality control purposes for patients undergoing radiotherapy treatments. Current dosimeters are not well suited for direct measurements due to atomic composition and size limitations. We are developing a fiber optic probe dosimeter based on radiochromic material for in vivo dosimetry. Through measuring the change in optical absorption of the radiochromic sensor, we can quantify the absorbed dose of ionization radiation delivered in real-time. The radiation-sensitive material is composed of lithium-10,12-pentacosa diynoate (LiPCDA), which upon exposure, polymerizes and results in an increased optical density. We observed that monomers of LiPCDA can have two distinct crystalline morphologies, with aspect ratios 10:1 producing hair-like structures and 2:1 resulting in platelets, with polymerized absorbance peaks typically centred at 635 nm and 674 nm, respectively. We aim to characterize and compare the dose-response of the two crystal morphologies achieved through desiccation and Li+ concentration. 

Method: The hair-like LiPCDA in commercial film was desiccated, producing crystals with an absorbance peak at 674 nm. Both materials were exposed to 50-3000 cGy using a clinical linear accelerator with a 6MV X-ray beam; samples of varying Li+ concentration were exposed to 200-400 cGy. Absorbance spectra for all samples were collected and were imaged with a scanning electron microscope to compare their crystal morphology. 

Results: Differences in crystal morphology were not observed when hair-like LiPCDA was desiccated. However, varying the molar ratio of Li+ to PCDA to produce crystals with either 635 nm or 677 nm absorbance peak, differentiable crystal morphologies were observed. The platelet form is ~3x less sensitive to dose but with a more extensive dynamic range relative to hair-like. 

Conclusion: Crystals can be preferentially grown and exhibit differing dose-response. The macrostructure effect on radiation sensitivity in the context of radiotherapy will be explored.

Characterization of modified radiochromic materials for measuring ionizing radiation

In this joint experimental and theoretical work [1], photoelectron emission from excited states of laser-dressed atomic helium is analyzed. We successfully demonstrate a method that is complimentary to transient absorption (e.g. [2]) for the assignment of light-induced states (LIS). The experiment is carried out at DESY in Hamburg and uses the FLASH2 free-electron laser to produce an extreme ultraviolet (XUV) pulse to which the helium atom is subjected along with a temporally overlapping infrared (IR) pulse in the multi-photon ionization regime ($\approx$10$^{12}$ W/cm$^2$). Analysis of the experiment occurs at the reaction microscope (REMI) end station [3] at FLASH2. The XUV pulse is scanned over the energy range 20.4 eV to 24.6 eV, corresponding to excited states of helium. The resonant, electric dipole-allowed $n$P states corresponding to a first step of single XUV photon excitation are shown to lead to ionization, independent of whether or not the lasers temporally overlap. However, dipole-forbidden transitions to $n$S and $n$D states corresponding to multiphoton (XUV $\pm$ $n$IR) excitation are observed during temporal overlap. Studying photo\-electron angular distributions (PADs) in the case where the ionization pathway of a LIS is difficult to resolve energetically allows for an unambiguous determination of the dominant LIS. The IR intensity and relative polarization between the lasers are varied to control the ionization pathway. Numerical solutions of the time-dependent Schr\"odinger equation within a single-active electron model with a local potential completely support the experimental findings in this project.
[1] S. Meister $\textit{et al}$., Phys. Rev. A $\bf{102}$ (2020) 062809; Phys. Rev. A $\bf{103}$ (2021) in press.
[2] S. Chen $\textit{et al}$., Phys. Rev. A $\bf{86}$ (2012) 063408.
[3] S. Meister $\textit{et al}$., Applied Sciences $\bf{10}$ (2020) 2953. 
*work supported by NSERC, NSF, and XSEDE

Emergence of light-induced states in the few-photon ionization of atomic helium*

Lattice gauge theory is an indispensible tool for non-Abelian fields, such as those in quantum chromodynamics where lattice results have been of central importance for several decades. Recent studies suggest that quantum computers could extend the reach of lattice gauge theory in dramatic
ways, but the usefulness of quantum annealing hardware for lattice gauge theory has not yet been explored. In this work, we implement SU(2) pure gauge theory on a quantum annealer for lattices comprising a few plaquettes in a row with a periodic boundary condition. Numerical results are obtained from calculations on D-Wave Advantage hardware for eigenvalues, eigenvectors, vacuum expectation values, and time evolution. The success of this initial exploration indicates that the quantum annealer might become a useful hardware platform for some aspects of lattice gauge
theories.

SU(2) lattice gauge theory on a quantum annealer

We combined information from Fluorescence Correlation Spectroscopy (FCS), Nuclear Magnetic Resonance (NMR), Small-Angle X-ray Scattering (SAXS) and Single-Molecule Förster Energy Transfer (smFRET) to calculate conformational ensembles for the neuronal transcription factor initiation protein 4E-BP2. This is a 120-residue intrinsically disordered protein which toggles between an active, non-phosphorylated (NP) state and an inactive multi-phosphorylated (5P) state. An initial pool of 4E-BP2 structures was generated using Trajectory Directed Ensemble Sampling (TraDES) (Feldman & Hogue, Proteins: Structure, Function, and Bioinformatics, 46, 8-23, 2002). The ENSEMBLE method (Krzeminski et al, Bioinformatics, 29, 398-399, 2013) was used to search and select conformations within the initial pool that are consistent with the experimental data. NMR and SAXS data were used as restraints for the conformational search and FCS and smFRET data were used as validation. 

The NP 4E-BP2 ensemble calculated with SAXS-only restraints showed a bimodal distribution of protein global dimensions such as the radius of gyration and the end-to-end distance. The back-calculated FRET efficiency and the hydrodynamic radius for this ensemble were lower than the respective experimental values. These inconsistencies are likely due to lack of short-range (5-20 Å) distance restraints, which are currently addressed by acquiring Paramagnetic Resonance Enhancement (PRE) data. Such data is available for the phosphorylated state (5P 4E-BP2), however this is partly folded, with a 4-stranded beta domain spanning residues 18-62. To address this challenge, we used the FastFloppyTail (FFT) program (Ferrie & Petersson, The Journal of Physical Chemistry B, 124, 5538-5548, 2020) to generate structures in the initial pool. As such, the folded region was modelled using the 5P 4E-BP2 folds from the Protein Databank, while the disordered tails were generated through the FFT algorithm. By incorporating the more realistic FFT-built prior in ENSEMBLE calculations, more accurate conformational ensembles of both phospho- and non-phospho- variants of 4E-BP2 were obtained. Analysis of these ensembles in terms of polymer physics and contact maps reveal new molecular level details lead to new insights into the biological function of 4E-BP2.

Protein Conformational Ensembles for a Disordered Protein Restricted by Separate Biophysical Experiments

Chemical warfare agents (CWAs) are potential threats to civil society and defence personnel. In recent years, many efforts has been deployed to develop a scalable, rapid and accurate detection system to identify trace amount of CWAs. Here we report a graphene-based field-effect transistor (GFET) sensor able to detect 800 ppb of dimethyl methyl phosphonate (DMMP), a simulant of the nerve agent sarin. We observe enhanced sensitivity when the GFET sensor is exposed to few mWs of UV light. Back gate measurements performed before and during exposures to the analyte allow us to investigate the sensing mechanism while monitoring the induced changes in carrier concentration and mobility in graphene.

Development of graphene-based field-effect transistor (GFET) sensors for a simulant of chemical warfare agents

Cosmology presupposes that on scales of $10^{8}$ light years the universe is the same at every point and in every direction. This is observationally supported by the cosmic microwave background (CMB) which has a temperature of 2.7 Kelvin in all directions. However, there exist small perturbations on this symmetric background - for example the CMB has perturbations of 0.001 Kelvin. A study of these fluctuations is cosmological perturbation theory. In this talk, I will review the standard theory of cosmological perturbations, explain our framework which is different from the standard method and then generalize our framework to include a matter clock.

Cosmological perturbation theory with matter time.

The solid polymer light-emitting electrochemical cell (PLEC) possesses a polymer homojunction that is reminiscent of a conventional p-n junction but also exhibits distinct features that are profoundly intriguing. The PLEC junction is formed under bias when the propagating p- and n-doping fronts in the semiconducting polymer make contact. The PLEC junction can be immobilized by cooling after the initial junction formation. Once the junction is fixed by cooling, the “frozen-junction” PLEC exhibits a unipolar electroluminescence (EL) and photovoltaic response. Further, the resulting frozen junction can be relaxed, or partially de-doped into a p-i-n junction by controlled manipulation of ion motion. The as-frozen p-n junction can be relaxed with repeated thermal cycles. More precisely, the de-doping was carried out in a single heating/cooling cycle during which a constant reverse bias (RB) current was applied to monitor the extent of de-doping and the magnitude of RB EL. It is on a frozen polymer p-i-n junction that the most puzzling electroluminescent phenomenon was observed. A model is developed that explains the reverse bias EL as caused by the tunnel injection of electrons and holes from bandgap states into a de-doped “intrinsic” region between the p- and n-doped regions. Moreover, the RB EL exhibits hypersensitivity to temperature when the de-doped cell was cooled. There is currently no explanation to this drastic EL enhancement by cooling. Several possible causes have been ruled out through a series of control experiments.

Temperature Sensitive Electroluminescence Observed in a Reverse Biased, Frozen Polymer P-I-N Junction

Recently, there has been considerable interest to study the nonlinear properties of ensembles of
metallic nanoparticles and quantum dots [1,2]. Nonlinear optical properties can be used for
processing the information content of data images, on which the research can potentially induce a
revolution in electronic as well as photonic nanotechnology and nanomedicine. We have studied
the energy transport due to the Kerr nonlinearity in plasmonic nanohybrids. The Kerr coefficient
has been calculated by using the quantum density matrix method in the dipole-dipole coupling
between quantum dots. Induced dipoles are created when the probe photon falls on the metallic
nanoparticles. These metallic nanoparticles interact with each other via dipole-dipole interaction.
We showed that the power is transferred from the metallic nanoparticles to the quantum dots
through surface plasmon polaritons. During this process, enhancement in the energy transfer in the
quantum dots is found. We have also predicted that the power spectrum peak would split into two
peaks due to the strong coupling between excitons and the dipole-dipole interaction. Considering
one peak as ON position and two peaks as OFF position, the present findings can be applied to
fabricate nanoswitches. The Kerr nonlinear plasmonics in metallic nanohybrids can also be used
for medical applications since there will be no damaging effect on the body.
[1] J Guo, K Black, J Hu, M Singh, Journal of Physics: Condensed Matter 30 (18), 185301 (2018).
[2] M.R. Singh; Phys. Rev. A102, 013708 (2020).

Kerr Nonlinearity and Energy Transport in Quantum Dots and Metallic Nanoparticles

Entropy stabilized oxides, containing a large number of different elements, exhibiting simple crystal structure have shown interesting properties such as colossal dielectric permittivity, superionic conductivity and enhanced exchange coupling.
To study how the ionic character can be tuned by using different mixtures of oxides, the effective charge for the rocksalt structure high entropy oxide (CuCoMgZnNi)O and medium entropy oxides including (CoMgZnNi)O, (CuMgZnNi)O, (CuCoZnNi)O, (CuCoMgNi)O were calculated by using Scott formula for effective charge and the data from infrared reflectance measurements. The data suggest that (CoMgZn Ni)O has the highest ionic character and the lowest ionic character belongs to (CuCoZnNi)O which has the the lowest effective charge.

Effective Charges in Entropy Stabilized Oxides

During the Covid-19 pandemic, face masks have become the new norm with their widespread use in public as part of a multi-barrier approach for infection control, including physical distancing, hand hygiene, and altered social behaviour. Masks provide benefits to both the mask wearer and to those in their proximity when they are worn by all individuals in a common area. The gold standard in personal protective equipment (PPE) remains the N95 respirator, made of synthetic materials with electrostatic properties that filter and retain more than 95% of aerosols <1 µm and larger in size. N95 respirators degrade during washing and disinfection, and as such are single use disposable PPE. Similarly, surgical-style masks made of polypropylene and non-woven materials are unsuited to frequent washing/decontamination with heat or detergents. Owing to their disposable nature, most commercially available PPE such as the above are unsustainable for supplying the public due to supply interruptions, high cost over time, and a lack of aesthetic attributes (colour, pattern) to encourage use.

As a result, textile manufacturers and a new cottage industry of homemade mask makers, including volunteers making donated masks for vulnerable populations, can provide fabric face masks to the public. Recently, the U.S. CDC indicated that commercial manufacturers of face masks will require testing although the conditions of such standards have yet to be outlined. Recently, Dr. Tam has made recommendations for mask materials to elevate the quality of masks being worn by the public, however these recommendations are not easily translatable to actual mask construction and such recommendations are based on very limited testing of fabric masks.

This presentation will discuss the development of a test apparatus for assessing mask efficacy by measuring the aerosols transmitted through the masks. We use a laser-based system, using relatively inexpensive diode lasers to illuminate the exhaled particles, a webcam for data acquisition, and Python-based particle tracking software. We make the approximation that the intensity of the scattered light from the droplets is proportional to the size of the droplet, but we will be able to quantify the droplet size by analyzing the data with Mie scattering theory.

Reference:
https://advances.sciencemag.org/content/6/36/eabd3083?te=1&nl=running&emc=edit_ru_20200822

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