## Purdue Quantum Center Kickoff Workshop

Poster Session

Oct. 14 (Wed), 12:50pm – 2pm in MRGN

### 1. Towards photoassociation in ^{87}Rb BEC with raman light-induced synthetic gauge fields

David Blasing (dblasing@purdue.edu)

We present our experimental studies of photoassociation in ^{87}Rb Bose-Einstein condensate (BEC) both without and with the presence of Ramanlight-induced gauge fields. These gauge fields couple the three bare m_{f} spins in the F = 1 manifold of ^{87}Rb, with the new eigenstates being superpositions of the bare m_{f} states. Some photoassociation channels are allowed or forbidden depending the mf spin of the colliding atoms. We will report the progress in our measurements, with the goal of investigating the role of synthetic gauge fields on the photoassociation process.

### 2. Nanoscale sensing of photonic density of states with spins in diamond

Simeon Bogdanov (sbogdan@purdue.edu)

Diamond nitrogen-vacancy (NV) centers are promising solid state systems for applications in nanoscale magnetometry and quantum information processing by virtue of possessing optically readable spins which retain coherence at room temperature. When coupled to nanophotonic structures such as optical metamaterials, metasurfaces or plasmonic waveguides the radiative transition rates of NV centers experience a Purcell enhancement which affects the spin state readout. We have experimentally studied the correlation between the optical transition lifetime and spin-induced fluorescence contrast in nanodiamond NV ensembles and found good agreement with rate equations analysis. This correlation can serve as a tool for measuring the local photonic density of states. Such a procedure relies simply on measuring the intensity of the fluorescence rather than on time-correlated single-photon counting used in conventional setups.

### 3. Efficient optimization of perturbative reductions for adiabatic simulation of quantum many-body systems

Yudong Cao (cao23@purdue.edu)

Algorithms for Adiabatic Quantum Computation (AQC) are most conveniently formulated in terms of many-body systems. In particular, many applications on quantum chemistry, such as electronic structure calculation, rely on finding the ground states of many-body Hamiltonians. However, experimentally only pairwise interaction is implementable using the current technologies. Therefore it behooves to find a systematic way of reducing many-body Hamiltonian to 2-body. One of such methods referred to as perturbative gadgets has been proposed recently in the quantum information community. For a given many-body Hamiltonian on N qubits one could construct a 2-body ‘gadget’ Hamiltonian on poly(N) qubits such that the low energy effective theory of the ‘gadget’ Hamiltonian gives rise to the target Hamiltonian up to a controllable error. A major drawback of the gadget idea is that it requires the interaction strength of the Hamiltonian to grow as the system size. Here we present an algorithm that efficiently finds the smallest interaction strength required for reducing a given target Hamiltonian to 2-body. Our formalism may find applications in perturbation theory beyond the context of gadgets.

### 4. Exploring many-body physics with strong atom-light interactions

Cheng-An Chen (chengan@purdue.edu)

The possibility to achieve strong atom-light interactions with cold atoms trapped along nano-photonic structures has opened up new opportunities in quantum physics and quantum information science. Recent experimental realizations include nano-fibers [1], photonic crystal cavities [2] and waveguides [3]. These nano-photonic structures offer excellent optical confinement, and are capable of mediating strong interactions between distant atom pairs coupled to photons propagating in their guided modes. With proper photonic state engineering and quantum control on cold atoms, it is possible to synthesize a novel quantum matter with arbitrary, pairwise-tunable interactions [4, 5, 6], enabling unprecedented opportunities to study long-range quantum dynamics in a strongly interacting many-body system. In this poster, we discuss our experimental effort in constructing a new apparatus for the integration of ultracold atoms and nano-photonic optical chips. We will also discuss a candidate resonator structure that can be readily fabricated at the Birck nanocenter for our future experiment.

[1] E. Vetsch et al, Phys. Rev. Lett. 104, 203603 (2010); A. Goban et al, Phys. Rev. Lett. 109, 033603 (2012).

[2] J. D. Thompson et al, Science 340, 6137 (2013); T. G. Tiecke et al, Nature 508 (7495), 241-244 (2014).

[3] A. Goban et al, Nature Communications 5, 3808 (2014); A. Goban et al, Phys. Rev. Lett. 115, 063601 (2015).

[4] J. S. Douglas, H. Habibian, C.-L. Hung, A. V. Gorshkov, and H. J. Kimble, D. E. Chang, Nature Photonics 9 (5), 326-331 (2015).

[5] A Gonzalez-Tudela, C.-L. Hung, D. E. Chang, J. I. Cirac, and H. J. Kimble, Nature Photonics 9 (5), 320-325 (2015). [6] C.-L. Hung, A. Gonzalez-Tudela, J. I. Cirac and H. J. Kimble, In preparation.

### 5. Proposed methods to measure the anapole moment of 133 cesium in hyperfine ground states.

Joseph Choi (choi88@purdue.edu)

Parity-non-conserving (PNC) interaction provides weak interaction in forbidden transitions in atomic and molecular structures. Precise measurement of this weak interaction would lead to highly accurate derivation of anapole moment in neutral atoms and molecules. This weak transition has been most accurately measured in atomic cesium beam by the Boulder group [1] to date with much higher uncertainty (~5%) than the standard model prediction (~1%). Where their derivation of the anapole moment was based on the comparison of two atomic transitions 6S1/2F = 3 ® 7S1/2F’ = 4 and 6S1/2F = 4 ® 7S1/2F = 3, we propose to measure the weak transition amplitude in the hyperfine ground states 6S1/2F = 3®6S1/2F’ = 4 with interference between microwave fields in a high Q cavity and Raman lasers. Many aspects of the experimental setup are based on the previous framework by Gomez et al. [2] in a MOT geometry. In our atomic beam geometry setup, we propose a novel geometry with a transmission line cavity formed with parallel plates and curved reflectors to significantly reduce stray fields, maintain good polarization, and highly suppress the magnetic dipole moment transition.

[1] C. S. Wood, S. C. Bennett, D. Cho, B. P. Masterson, J. L. Roberts, C. E. Tanner, and C. E. Wieman, “Measurement of parity nonconservation and an anapole moment in cesium,” Science 275, 1759-1763 (1997).

[2] E. Gomez, S. Aubin, G. D. Sprouse, L. A. Orozco, and D. P. DeMille, “Measurement method for the nuclear anapole moment of laser-trapped alkali-metal atoms,” Phys. Rev. A 75, 033418 (2007).

### 6. Unprecedentedly confined light in biological condensed matters

Seung Ho Choi (choi184@purdue.edu)

As “queen of gems”, pearls have mysterious allure from prehistory, while the origin of optical appearance has not yet been described explicitly. Here, essence of mesoscopic physics elucidates light interactions with numerous nanoslabs of nacre that possess irregularity. Computation of resonant tunneling (i.e. transmission eigenchannels) shows that structural variations in nacre thickness reside near a critical value, maximizing multiple resonance formation. This finding allows transforming from these native accessories in amplifying media into lasing systems with strong light confinement and its special type (i.e. hybridized localized states), enabling efficient and steady emission. We postulate that the biogenic design will inspire biomimetic tolerance strategies for designing and fabricating photonic systems, which is imperfect yet efficient and occasionally comparable with periodic lattices.

### 7. Atomistic modeling of spin qubits hosted in realistic silicon quantum dots

Rifat Ferdous (rferdous@purdue.edu)

Silicon(Si) quantum dots(QD) are promising host for spin based qubits. Essential components in spin qubits: ESR control – we studied the microscopic physics of electron g-factor, in a Silicon quantum dot Qubit lifetime – we studied spin relaxation times with spin-valley coupling introduced by interface roughness in a Si QD.

### 8. Electron spin control of levitated nitrogen-vacancy centers in low vacuum

Thai Hoang (hoang0@purdue.edu)

Nitrogen-vacancy (NV) centers in nanodiamond is optically trapped inside a vacuum chamber using optical tweezers. Thermal effect on electronic spin is investigated in a low vacuum condition. With temperature directly measured from electron spin resonance (ESR) measurement, a levitated nanodiamond is a promising candidate to study non-equilibrium and thermodynamics.

### 9. Entangled photons nonlocal detection

Poolad Imany (pimany@purdue.edu)

We have demonstrated dispersion cancellation, orthogonal optical coding and spectral shaping on entangled photons. Using a pulse shaper and sum frequency generation that employs ultrafast coincidence detection, we were able to put different dispersions and optical codes on the time-frequency entangled photons; in a way they cancel each other in measuring the correlation function between them. Our last project focuses on nonlocal detection and manipulation of the entangled photons using cross-correlation with ultrashort synchronized reference pulses.

### 10. Super resolution measurements of the near-field coupling of the polarized modes of gold nanorods to single quantum emitters

Benjamin Isaacoff (isaacoff@umich.edu)

The localized surface plasmon resonances of metal nanoparticles result in complex light-matter interactions that depend strongly on the nanoparticle geometry. In this work, we use single-molecule super-resolution imaging and single-particle spectroscopy to study the polarization-dependent response of gold nanorods (GNRs), which support two orthogonal plasmon modes. Furthermore, we measure the emission intensity and polarization of single fluorescent molecules coupled to the GNR as a function of excitation polarization and spectral overlap with the GNR modes. Based on such differential excitation, we demonstrate polarization control of plasmon-enhanced fluorescence from single molecules coupled to single nanoparticles. These experiments are compared with broadband finite difference time domain (FDTD) simulations studying the role of fluorophore position and orientation, revealing the underlying mechanisms of this coupling. These super-resolution measurements and the associated simulations demonstrate how polarization can be used to actively control nanoparticle plasmonics and opens the door to a new framework for controlling single quantum emitters.

### 11. Atoms to devices lab

Zubin Jacab (zjacob@ualberta.ca)

I will overview the recent developments in my group on three intertwined themes of I) super-coulombic virtual photon interaction between quantum emitters II) quantum photonics with extreme skin-depth waveguides and III) negative frequency resonance in moving media.

### 12. Density-to-potential inversions in TDDFT

Daniel Jensen (jensend@purdue.edu)

Time-dependent density functional theory (TDDFT) is a formally exact theory for solving the quantum many-body problem. Most implementations of TDDFT define a fictitious non-interacting system governed by the Kohn-Sham potential that exactly reproduces the time-dependent density of the interacting system. The Kohn-Sham potential is usually approximated but sometimes an exact solution is needed to help develop new approximations. We present a multilevel optimization method to accurately and efficiently perform density-to-potential inversions in TDDFT. This method uses the time-dependent Kohn-Sham equations of TDDFT as a constraint to recover the Kohn-Sham potential from a given target density. We present numerically exact results of our density-to-potential inversions for one‑dimensional interacting and non-interacting model systems.

### 13. Metasurface: Novel architecture for quantum nanophotonics

Pankaj Jha (pkjha@berkeley.edu)

We introduce and theoretical demonstrate a two-dimensional judiciously designed array of subwavelength-scale nanoantennas -a metasurface- as a novel architecture for quantum nanophotonics. We tailor the quantum vacuum over macroscopic distances and harness this engineered vacuum to induce quantum interference among spontaneous emission channels within an atom. Metasurface will provide opportunities, beyond conventional settings, in the field of quantum metaphotonics.

### 14. Nanophotonics with alternative plasmonic materials: impacting quantum photonics

Nathaniel Kinsey (nkinsey@purdue.edu)

As the field of plasmonics has progressed into many new areas, it has become clear that gold and silver are not the ideal materials for every application. In particular, they are limited due to their inability to grow continuous thin films (< 10nm), lack of tunable optical properties, and lack of CMOS-compatibility. Many materials have been proposed as alternatives to the noble metals, but two classes of plasmonic ceramics have stood out above the rest: transition metal nitrides (TMNs), and transparent conductive oxides (TCOs). We wish to show the great flexibility of this technology and stimulate a constructive dialogue about possible future collaborations. In addition, we hope to illustrate several potential areas in which these new materials may have an impact on Quantum Optics in the future.

### 14. Experimental studies of collective excitations of a BEC in light-induced gauge fields

Chuan-Hsun Li (li841@purdue.edu)

We present our experimental studies of collective modes including spin dipole mode and scissors mode of a ^{87}Rb Bose-Einstein condensate (BEC) in the presence of Raman light-induced gauge fields and synthetic spin-orbit coupling (SOC). By Raman dressing the mf spin states within the F=1 manifold, we engineer atoms’ energy-momentum dispersion to create synthetic SOC, and spin dependent synthetic electric and magnetic fields. We have used spin dependent synthetic electric fields to make two BECs with different spins oscillate and collide in the optical trap. We have studied the effects of SOC on both the momentum damping and thermalization behaviors of the BECs when undergoing such spin dipole oscillations. We have also used spatially dependent synthetic electric fields to excite the scissors mode, which has been used as a probe for superfluidity. We have investigated the effects of the synthetic gauge fields and SOC on the measured scissors mode.

### 16. Hybrid electrothermoplasmonic nanotweezer

Justus Ndukaife (jndukaif@purdue.edu)

Plasmon-enhanced optical trapping is being actively researched by several groups around the world as a means for stable trapping of nanoscale objects, which cannot be addressed by conventional diffraction-limited laser tweezers. In this approach, plasmonic nanoantennas made of noble metals such as gold and silver are illuminated to generate highly localized and enhanced electromagnetic field in the vicinity of the nanoantenna. This highly enhanced and localized field also known as a plasmonic hotspot generates short range (over tens of nanometers) gradient force and tight potential well for confining nanoscale objects. However a long standing problem in this field is how to rapidly load the potential well without relying on Brownian diffusion. Conventional design rely on Brownian diffusion to load the trap, which is very slow and could take several minutes to hours depending on the concentration of the nanoscale objects. Recently we have addressed this challenge by introducing a novel design paradigm known as the Hybrid Electroplasmonic Nanotweezer (HENT), where a large scale microfluidic vortex is induced to enable rapid delivery and trapping of nanoscale objects within a few seconds. In this poster I will discuss the working principle of HENT, as well as the advancements over the state-of-the-art nanotweezers. I will also discuss how to ‘print’ trapped particles on plasmonic hotspots thus providing additive nanomanufactuing functionality on-chip. The HENT device holds promise for numerous applications including selection, and analysis of nanoparticles, label free biosensing, single nanoparticle spectroscopy such as Surface Enhanced Raman Scattering (SERS), and deterministic coupling of quantum emitters to plasmonic cavities for efficient generation of non-classical light.

### 17. Charge transfer at a metal-atom interface via partition theory

Kelsie Niffenegger (kniffene@purdue.edu)

Standard approximations to the exchange-correlation (XC) functional of Kohn-Sham Density Functional Theory are insufficiently accurate to describe charge transfer at metal-atom interfaces. The root of the problem is connected to the lack of derivative discontinuities in the approximate XC functionals at integer numbers of electrons. Partition Theory (PT) is a promising method to correct this issue. We study the simplest model for an atom adsorbed at a metal surface: A one-dimensional step potential separated a distance R from an attractive well that admits only one bound state of energy E0 when isolated. The semi‑infinite metal is populated with non-interacting electrons up to the Fermi energy, E_{f}. By calculating the total ground-state density as an energy integral, we illustrate how the interplay of the three parameters in the model (R, E_{0}, E_{f}) determines the physics of charge transfer. In particular, we find the parameter regimes in which the “atom” gains a fraction of an electron from the “surface”, and vice versa. We derive the PT-equations for this problem and indicate how the associated partition potential can be calculated.

### 18. Temporal position modulation of entangled photons through pump frequency tuning

Ogaga Odele (oodele@purdue.edu)

Recent years have witnessed a substantial interest in manipulating time-frequency entangled photons (biphotons) using classical optical signal-processing techniques. In some of our earlier contributions to this area, we applied Fourier-transform pulse shaping to demonstrate dispersion cancellation of entangled photons for arbitrary spectral orders and encoding and decoding of biphoton wave packets. Currently, we are exploring manipulation of biphotons through pump frequency tuning; our latest contribution is a novel scheme for controlling the relative arrival time of an entangled photon pair. We believe our framework serves as a model for rapid delay control of entangled photons and hope that it would be useful in quantum key distribution for mitigating delay issues.

### 19. Single-particle spectra and two-particle interactions in two-dimensional topological insulators

Marius Radu (radu@purdue.edu)

HgTe forms a two-dimensional topological insulator when sandwiched between CdTe barriers for a HgTe layer wider than the critical thickness. We derive single-particle and two-particle interaction Hamiltonians describing physics of these compounds by using k · p theory and extended Kane model. We include contributions from upper conduction bands with orbital states of p-symmetry that bring about the terms describing lack of inversion symmetry in host semiconductors. A crucial ingredient is hetero-interface contribution to intrinsic spin-orbit interactions that drives significant anticrossing gaps in spectra. Single-particle Hamiltonian and two-particle Hamiltonian contain important spin-dependent terms. The spin-dependent interaction couples orbital motion of one particle with evolution of spin of the other particle. Such particle-particle interactions do not conserve spin and lower the symmetry of exchange interactions, leading, e.g., to Dzyaloshinskii-Moriya exchange term. We study the effects of new interactions on Berry curvature, spin-Hall conductance, and magnetotransport phenomena in CdTe/HgTe structures.

### 20. Nonlinear optics at low powers: new mechanism of on-chip optical frequency comb generation

Andrei Rogov (arogov@purdue.edu)

Nonlinear optical effects provide a natural way of light manipulation and interaction, and form the foundation of applied photonics -- from high-speed signal processing and telecommunication, to ultra-high bandwidth interconnects and information processing. However, relatively weak nonlinear response at optical frequencies calls for operation at high optical powers, or boosting efficiency of nonlinear parametric processes by enhancing local field intensity with high quality factor resonators near cavity resonance, resulting in reduced operational bandwidth and increased loss due to multi-photon absorption. Here, we present an alternative to this conventional approach, with strong nonlinear optical effects at substantially lower local intensities, based on period-doubling bifurcations near nonlinear cavity anti-resonance, and apply it to low-power optical comb generation in a silicon chip.

### 21. Electrostatic simulations of quantum dots for quantum computing

Harshad Sahasrabudhe (harshad.sahasrabudhe@gmail.com)

NEMO5[1] and NEMO-3D[2] are quantum transport simulation tools for nanoelectronic devices. NEMO-3D also provides tight binding and effective mass Hamiltonian solvers coupled with Poisson equation solver on a finite element grid. These solvers can be used to simulate electron wavefunctions, charge density and potential inside quantum dots in metal-oxide-semiconductor (MOS) interfaces, GaAs/AlGaAs heterostructures, Si/SiGe herostructures etc with realistic gates and geometry. This information can be used to predict the behavior of these devices and extract observable quantities such as gate capacitance. Similar simulations using the QCAD[3] software have helped design devices. In this poster, we present the methodology and some previous results obtained from electrostatic quantum dot simulations using NEMO5 and NEMO-3D. One of the previous results shown is the single atom transistor simulation.

[1] S. Steiger, M. Povolotskyi, H.-H. Park, T. Kubis, and G. Klimeck, IEEE Trans. Nano., vol. 10, no. 6, pp. 1464–1474, Nov. 2011.

[2] Klimeck, G., Oyafuso, F., Boykin, T. B., Bowen, R. C. & von Allmen, P. Comput. Model. Eng. Sci. 3, 601–642 (2002).

[3] X. Gao, E. Nielsen, R. P. Muller, R. W. Young, A. G. Salinger, N. C. Bishop, M. P. Lilly, and M. S. Carroll, J. Appl. Phys. 114, 164302 (2013).

### 22. Enhancement of single-photon emission from nitrogen-vacancy centers with hyperbolic metamaterials

Mikhail Shalaginov (shalaginov@purdue.edu)

The broadband enhancement of single–photon emission from nitrogen-vacancy centers in nanodiamonds coupled to a planar multilayer metamaterial with hyperbolic dispersion is studied experimentally. The metamaterial is fabricated as an epitaxial metal/dielectric superlattice consisting of CMOS-compatible ceramics: titanium nitride (TiN) and aluminum scandium nitride (Al_{x}Sc_{1-x}N). It is demonstrated that employing the metamaterial results in significant enhancement of collected single–photon emission and reduction of the excited-state lifetime. Our results could have an impact on future CMOS-compatible integrated quantum sources.

### 23. Non-abelian states

George Simion (simion@purdue.edu)

The key element of topologically protected quantum computing is the existence of non-Abelian quasiparticle excitations. Such states are expected to appear in some fractional quantum Hall states. Valence band holes have properties that makes them fundamentally different from electrons. In the presence of magnetic field, their spectrum is not a fan-like diagram and they exhibit level crossings. These crossings provide a knob that can tune LL mixing, enhancing the 3-body interaction. A states with 1/2 filling factor has been observed in hole liquid. The properties of two dimensional charged quantum hole liquid in the presence of magnetic field are studied using the spherical geometry. The properties of novel 1/2 state are discussed.

### 24. Time-resolved circular dichroic spectroscopy for photosynthesis research

Valentyn Stadnytskyi (valentyn@purdue.edu)

There are multiple examples of light harvesting complexes in photosynthetic organisms that have light absorbing pigments closely spaced. Such structural organization leads to strong interaction between pigments and causes excitonic delocalization of electronic excited states. Excitonic nature of the excited states gives a rise to coherences and quantum beats between different states and it has been suggested that such quantum interference can enhance the efficiency of energy transfer (Fleming, Shulten). This phenomenon relies on a particular realization of the excitonic excited state structure within light harvesting proteins, defined by the energies of individual noninteracting pigments and interactions between the pigments. Since the individual pigment energies are not directly observable, conclusions about their values are made using exciton modeling of available experimental data. Currently, we are developing and applying timeresolved circular dichroism (CD) nanosecond spectroscopy to study exciton structure of Fenna-Matthews-Olson(FMO) light harvesting complex (via triplet-state dynamics) and refine the exciton model parameters. Unlike unpolarized optical spectra, excitonic CD is extremely sensitive to mutual orientation of interacting pigments, which will allow to greatly enhance the sensitivity of the model to model parameters. With modifications, this new technique will allow us to measure changes in CD spectrum of ΔAR-L~10^{-6}, which should be sufficient to measure a change in CD absorption of the FMO protein.

### 25. Line shifts due to coherent interactions for different sizes and shapes of an atomic cloud at constant density

Tyler Sutherland (rsutherl@purdue.edu)

The line-broadening of a weakly-driven atomic sample due to the coherent build up of long-range dipole-dipole interactions is simulated numerically for a medium to low-density cloud of atoms with essentially no approximation. This is done using the classical equations of motion for N electromagnetically driven dipoles. We show that given an axially symmetric Gaussian cloud, the emitted photon spectra can be highly dependent on the shape of the cloud along the line of laser propagation. We also show that ‘contorting’ the cloud along the line of laser propagation can lead to a deviation from a Lorentzian line-profile and an overall red-shift of the spectral peak. Lastly, we show that a cloud will emit a spectra with a line-width proportional to N^{1/3} * density^{2/3} that is seen for all densities for up to 20,000 stationary atoms.

### 26. Quantum computing with spin torque

Brian Sutton (bmsutton@purdue.edu)

The objective of this poster is to describe a quantum computing architecture that uses non-equilibrium spin potentials enabled by modern spintronics to perform all basic qubit operations including initialization, arbitrary single qubit rotation, single-qubit ensemble readout, and a square root of SWAP on selected pairs of qubits. The use of spin torque as a substitute for magnetic fields is now well established for classical operations like the switching of a nanomagnet. Here we present what could be viewed as an application of spin torque like effects to quantum processes involving single qubit rotations as well as two qubit entanglement. A key ingredient of this scheme is the use of a large number of itinerant electrons whose cumulative effect is to produce the desired qubit operations on static spins. Each interaction involves entanglement and collapse of wavefunctions so that the operation is only approximately unitary. We will first show that standard single qubit operations can all be effected through interactions with an itinerant non-equilibrium spin population. The spin induced rotation is only approximately unitary, but we will show that the non-unitary component can be made arbitrarily small. We then show that two qubit gates can also be implemented through separate interactions with the itinerant spin population, which can be conveniently controlled even when the qubits are spatially separated. The two qubit rotation too is only approximately unitary, but the non-unitary component can be kept below tolerable limits with proper design. As a capstone example, we present the implementation of a complete CNOT gate using the proposed spin potential based architecture, and show that the fidelity under ideal conditions can be made acceptably close to one.

### 27. Precision measurements in cesium

Yao De George Toh (ytoh@purdue.edu)

Atomic cesium has played a central role in parity non conservation (PNC) measurements over the past 40 years. Currently the most precise determination of the weak charge Qw of any atomic system is derived from an experiment in cesium from 1997, together with theoretical models. The aim of our program is to apply the two-color, two-pathway coherent control techniques that we have pioneered and demonstrated to conduct new, higher precision studies on the PNC amplitude. In addition, we carried out related precision measurements of relevant electric dipole transition amplitudes for comparison with theory for the atom. We have plans to carry out more of these measurements.

### 28. Few body treatment of the quantum Hall effect

Bin Yan (yanbin@purdue.edu) and Rachel Wooten (wootenr@purdue.edu)

The fractional quantum Hall effect is considered the ultimate strongly-interacting quantum system. We treat this many-body two dimensional electron system using few-body theoretical method of hyperspherical coordinates, which maps the relative particle coordinates to a (2N-2) dimensional hypersphere. This treatment introduces a new quantity, the grand angular momentum, that is approximately conserved even in the presence of interactions. The technique yields calculations that are more precise than standard perturbation theory calculations, but also reveals evidence that the low energy quantum states corresponding to experimentally observable filling fractions can often be identified through exceptionally high degeneracy in the non-interacting limit. Our findings suggest the possibility of finding other, previously unobserved, fractional quantum Hall states and also of finding novel excitation frequencies that may be experimentally observable.

### 29. The radical pair mechanism and the avian chemical compass: Quantum coherence and entanglement

Yiteng Zhang (zhyt@purdue.edu)

The radical pair mechanism can help to explain avian orientation and navigation. Some evidence indicates that the intensity of external magnetic fields plays an important role in avian navigation. In this paper, using a two-stage model, we demonstrate that birds could reasonably detect the directions of geomagnetic fields and gradients of these fields using a yield-based chemical compass that is sensitive enough for navigation. Also, we find that the lifetime of entanglement in this proposed compass is angle dependent and long enough to allow adequate electron transfer between molecules.

### 30. Ultrafast sub-diffraction thermoreflectance thermal imaging microscopy

Amir Ziabari (aziabari@purdue.edu)

Thermoreflectance (TR) thermal imaging microscopy is a non-invasive full field optical method for temperature mapping of active electronic and photonic devices with submicron spatial, 10 mK temperature and 800 picosecond time resolution. The measurements can been done in 10K-800K temperature range. Temperature non-uniformity has been characterized in high speed transistors, nanowire devices, semiconductor lasers and light emitting diodes. Defects in solar cells and interconnect vias have also been identified. The current work focus on detecting temperature rise in devices with sub-diffraction limit feature sizes.

### 31. Time-varying metasurfaces and lorentz non-reciprocity

Amr Shaltout (aemadeld@purdue.edu)

A cornerstone equation of optics – Snell's law – relates the angles of incidence and refraction for light passing through an interface between two media. It is built on two fundamental constrains: the conservation of tangential momentum and the conservation of energy. By relaxing the classical Snell’s law photon momentum conservation constrain when using space-gradient phase discontinuity, optical metasurfaces enabled an entirely new class of ultrathin optical devices. Here, we show that by eradicating the photon energy conservation constrain when introducing time-gradient phase discontinuity, we can further empower the area of flat photonics and obtain a new genus of optical devices. With this approach, classical Snell’s relations are developed into a more universal form not limited by Lorentz reciprocity, hence, meeting all the requirements for building magnetic-free optical isolators. Furthermore, photons experience inelastic interaction with time-gradient metasurfaces, which modifies photonic energy eigenstates and results in a Doppler-like wavelength shift. Consequently, metasurfaces with both space- and time-gradients can have a strong impact on a plethora of photonic applications and provide versatile control over the physical properties of light.

### 32. Optical constants of gold thin films at elevated temperatures

Harsha Reddy (heragamr@purdue.edu)

Understanding the influence of temperature on the optical properties of thin metal films is particularly important in modelling high temperature applications in a variety of research fields, such as plasmonics and near field radiative heat transfer. The optical properties of bulk metals at elevated temperatures have been studied in the past. But the same studies on thin metal films of thicknesses ranging from few tens to few hundreds of nanometers, which have different optical properties compared to their bulk counterparts, are relatively scarce. In this report we present our results on the optical constants of 200 nm, 50 nm and 30 nm poly crystalline and 200 nm single crystalline gold films at elevated temperatures in the wavelength range 370 - 2000 nm. The optical constants at elevated temperatures were measured by integrating a heating stage into the Variable Angle Spectroscopic Ellipsometer set up. The 200 nm and the 50 nm films were heated from room temperature to 500 ^{0}C and the 30 nm films were heated from room temperature to 450 ^{0}C. Furthermore, the 200 nm films were subjected to multiple heating cycles. Our results show that the real part of the dielectric function changes only marginally with temperature but the imaginary part, which contributes to losses, increase monotonically. At 500 ^{0}C, the imaginary part becomes nearly twice as large as it is at room temperature for 200 nm thick SC and PC films. For the case of 50 nm and 30 nm films the losses display a fundamentally different trend compared to the 200 nm samples. Specifically the imaginary part of the dielectric function, depending on the temperature range, shows increasing and decreasing behavior with increasing temperature. The huge variation in the imaginary part at elevated temperatures significantly reduces the surface plasmon polariton propagation length and the quality factor of localized surface plasmon resonance of a spherical particle by about 45%, 50% and 70% in 200 nm, 50 nm and 30 nm films respectively. Further the optical properties of 50 nm and 30 nm films degrade significantly after heat treatment. The Ellipsometer data were fitted using a Drude + 2 Critical Point model and the various terms in the model are compared with the theoretical predictions.

### 33. Plasmon resonance in multilayer graphene nanoribbons

Di Wang (wang1292@purdue.edu)

Plasmon resonances in nanopatterned single-layer graphene nanoribbons (SL-GNR), double-layer graphene nanoribbons (DL-GNR) and triple-layer graphene nanoribbons (TL-GNR) are studied experimentally using ’realistic’ graphene samples. The existence of electrical tunable plasmons in stacked multilayer graphene nanoribbons was first experimentally verified by infrared microscopy. We find that the strength of the plasmonic resonance increases in DL-GNR when compared to SL-GNR. However, further increase was not observed in TL-GNR when compared to DL-GNR. We carried out systematic full-wave simulations using a finite-element technique to validate and fit experimental results, and extract the carrier-scattering rate as a fitting parameter. The numerical simulations show remarkable agreement with experiments for an unpatterned SLG sheet, and a qualitative agreement for a patterned graphene sheet. We conclude with our perspective of the key bottlenecks in both experiments and theoretical models.

### 34. Second harmonic generation with plasmonic metasurfaces: Direct comparison of electric and magnetic resonances

Rohith Chandrasekar (chandrar@purdue.edu)

Plasmonic resonances in metallic nanostructures have been shown to drastically enhance local electromagnetic fields, and thereby increase the efficiency of nonlinear optical phenomena, such as second harmonic generation (SHG). While it has been experimentally observed that enhanced fields can significantly boost SHG, to date it proved difficult to probe electrical and magnetic resonances in one and the same nanostructure. This however is necessary to directly compare relative contributions of electrical and magnetic components of SHG enhancement. In this paper we report an experimental study of a metasurface capable of providing electrical and magnetic resonant SHG enhancement for TM polarization. Our metasurface could be engineered such that the peak frequencies of electrical and magnetic resonances could be adjusted independently. We used this feature to distinguish their relative contributions. Experimentally it was observed that the magnetic resonance provides only 50% as much enhancement to SHG as compared to the electric resonance. In addition aligning both resonances in frequency results in conversion efficiency of 1.32 x 10^{-10}.

### 35. The role of epsilon-near-zero substrates in the optical response of plasmonic antennas

Jongbum Kim (kim668@purdue.edu)

Radiation patterns and resonance wavelengths of a plasmonic antenna are significantly influenced by its local environment, particularly its substrate. Here, we experimentally explore the role of dispersive substrates, such as aluminum- or gallium-doped zinc oxide in the near infrared and 4H-silicon carbide in the mid-infrared wavelengths, upon Au plasmonic antennas, extending from the dielectric to metal-like regimes, crossing through epsilon-near-zero (ENZ) conditions.

### 36. Alternative plasmonic materials (TCOs) for the optical application in the near infrared

Jongbum Kim (kim668@purdue.edu)

Research on alternative plasmonic materials is becoming an important step to improve the performance of plasmonic and nanophotonic devices for many applications. Our study focuses on transparent conducting oxides which have great potential for enabling high performance, tunable, semiconductor compatible plasmonic devices.

### 37. Nanolasers: spaser and random laser

Xiangeng Meng (meng14@purdue.edu)

The nanolaser project aims at developing compact light sources by using plasmonic nanostructures as resonant cavities. Here, we are combining top-down and bottom-up approaches to fabricate various artificial plasmonic nanostructures which serve as optical cavities for lasing. We have developed several nanolaser sources with controllable figures of merit such as emission efficiency, lasing threshold, emission directionality, and lasing mode confinement.