arXiv subjects

Mehmet Emre Tasgin

Publications and source records attributed to Mehmet Emre Tasgin.

At least 37 records · Page 2Linked to original sources

arXiv2022

On-demand continuous-variable quantum entanglement source for integrated circuits

Integration of devices generating nonclassical states~(such as entanglement) into photonic circuits is one of the major goals in achieving integrated quantum circuits~(IQCs). This is demonstrated successfully in recent decades. Controlling the nonclassicality generation in these micron-scale devices is also crucial for the robust operation of the IQCs. Here, we propose a micron-scale quantum entanglement device whose nonlinearity (so the generated nonclassicality) can be tuned by several orders of magnitude via an \textit{applied voltage} without altering the linear response. Quantum emitters~(QEs), whose level-spacing can be tuned by voltage, are embedded into the hotspot of a metal nanostructure~(MNS). QE-MNS coupling introduces a Fano resonance in the ``nonlinear response''. Nonlinearity, already enhanced extremely due to localization, can be controlled by the QEs' level-spacing. Nonlinearity can either be suppressed (also when the probe is on the device) or be further enhanced by several orders. Fano resonance takes place in a relatively narrow frequency window so that $\sim$meV voltage-tunability for QEs becomes sufficient for a \textit{continuous} turning on/off of the nonclassicality. This provides as much as 5 orders of magnitude modulation depths.

physics.optics↗

arXiv2022

Silent-enhancement of multiple Raman modes via tuning optical properties of graphene nanostructures

Raman scattering signal can be enhanced through localization of incident field into sub-wavelength hot-spots through plasmonic nano-structures (Surface-enhanced Raman scattering-SERS). Recently, further enhancement of SERS signal via quantum objects are proposed by [1] without increasing the hot-spot intensity (\textit{silent-enhancement}) where this suggestion prevents the modification of vibrational modes or the breakdown of molecules. The method utilizes path interference in the non-linear response of Stokes-shifted Raman modes. In this work, we extend this phenomenon to tune the spectral position of \textit{silent-enhancement} factor where the multiple vibrational modes can be detected with a better signal-to-noise ratio, simultaneously. This can be achieved in two different schemes by employing either (i) graphene structures with quantum emitters or (ii) replacing quantum emitters with graphene spherical nano-shell in \cite{Postaci2018}. In addition, the latter system is exactly solvable in the steady-state. These suggestions not only preserve conventional non-linear Raman processes but also provide flexibility to enhance (silently) multiple vibrational Raman modes due to the tunable optical properties of graphene.

physics.optics↗

arXiv2021

Quantization of magnetic flux and electron-positron pair creation

An electron-positron pair ($e^- e^+$) is created in vacuum above a critical electric field strength $E_{crt}$ which is quite large in the laboratory scale. The photon, thus the field, annihilates in the pair creation process. Here, we question if the pair creation (at $E=E_{crt}$) introduces a boundary condition in the electromagnetic state, e.g., similar to the one in angular or linear momentum. We show that introduction of such a reasonable condition yields approximately the well-known magnetic flux quanta which normally one obtains using different arguments.

hep-ph↗

arXiv2020

Ultra-large actively tunable photonic band gaps via plasmon-analog of index enhancement

We present a novel method for active continuous-tuning of a band gap which has a great potential to revolutionize current photonic technologies. We study a periodic structure of x and y-aligned nanorod dimers. Refractive index of a y-polarized probe pulse can be continuously-tuned by the intensity of an x-polarized auxiliary (pump) pulse. Order of magnitude index-tuning can be achieved with a vanishing loss using the plasmon-analog of refractive index enhancement [Phys. Rev. B 100, 075427 (2019)]. Thus, a large band gap can be created from a non-existing gap via the auxiliary pulse. We also present a "proof of principle" demonstration of the phenomenon using numerical solutions of Maxwell equations. The new method, working for any crystal dimensions, can also be utilized as a linear photonic switch operating at tens of femtoseconds.

physics.optics↗

arXiv2020

Negative superluminal velocity and violation of Kramers-Kronig relations in "causal" optical setups

We investigate nonanalyticities (e.g., zeros and poles) of refractive index $n(ω)$ and group index $n_g(ω)$ in different optical setups. We first demonstrate that: while a Lorentzian dielectric has no nonanalyticity in the upper half of the complex frequency plane (CFP), its group index -- which governs the pulse-center propagation -- violates the Kramers-Kronig relations (KKRs). Thus, we classify the nonanalyticities as in the (a) first-order (refractive index or reflection) and (b) second-order (group index or group delay). The latter contains the derivative of the former. Then, we study a possible connection between the negative superluminal velocities and the presence of nonanalyticities in the upper half of the CFP. We show that presence of nonanalyticities in the upper half of the CFP for (a) the first-order response and (b) the second-order response are accompanied by the appearance of negative (a) phase velocity and (b) group velocity, respectively. We also distinguish between two kinds of superluminosity, $v>c$ and $v<0$, where we show that the second one ($v<0$) appears with the violation of KKRs.

physics.optics↗

arXiv2020

Single-molecule-resolution ultrafast near-field optical microscopy via plasmon lifetime extension

A recent study shows that: when a long lifetime particle is positioned near a plasmonic metal nanoparticle, lifetime of plasmon oscillations extends, but, "only" near that long-life particle [PRB 101, 035416 (2020)]. Here, we show that this phenomenon can be utilized for ultrahigh (single-molecule) resolution ultrafast apertureless (scattering) SNOM applications. We use the exact solutions of 3D Maxwell equations. We illuminate a metal-coated silicon tip, a quantum emitter (QE) placed on the tip apex, with a femtosecond laser. The induced near-field in the apex decays rapidly except in the vicinity of the sub-nm-sized QE. Thus, the resolution becomes solely limited by the size of the QE. As positioning of a QE on the tip apex is challenging, we propose the use of a newly-discovered phenomenon; stress-induced defect formation in 2D materials. When a monolayer, e.g., transition metal dichalcogenide (TMD) is transferred to the AFM tip, the tip indentation of 2D TMD originates a defect-center located right at the sharpest point of the tip; that is exactly at its apex. Moreover, the resonance of the defect is tunable via a voltage applied to the tip. Our method can equally be used for background-noise-free nonlinear imaging and for facilitating single-molecule-size chemical manipulation.

physics.optics↗

arXiv2020

Continuously-tunable Cherenkov-radiation-based detectors via plasmon index control

A recent study [PRB 100, 075427 (2019)], finally, demonstrated plasmon-analog of refractive index enhancement in metal nanostructures, which has already been studied in atomic clouds for several decades. Here, we simply utilize this phenomenon for achieving continuously-tunable enhanced Cherenkov radiation in metal nanostructures. Beyond enabling Cherenkov radiation from slow-moving particles, or increasing its intensity, the phenomenon can be used in continuous-tuning the velocity cutoff of particles contributing to the Cherenkov radiation. More influentially, this allows a continuously-tunable analysis of the contributing particles as if the data is collected from many different detectors, which enables data correction. The phenomenon can also be integrated into lattice metal nanostructures, for continuous medium tuning, where a high density of photonic states is present and the threshold for the Cherenkov radiation can even be lifted. Additionally, vanishing absorption can heal radiation angle distortion effects caused by the metallic absorption.

physics.optics↗

arXiv2020

Enhanced spontaneous down-conversion in a nonlinear crystal embedded with plasmonic-quantum emitter hybrid structures

Control of nonlinear response of nanostructures via path interference effects, i.e. Fano resonances, has been studied extensively. In such materials, a frequency conversion process which takes place near a hot spot has been considered. Here, we study a different case. The frequency conversion process takes place along the body of a nonlinear crystal. Metal nanoparticle-quantum emitter dimers control the down-conversion process, taking place throughout the crystal body, via introducing interfering conversion paths. Dimers behave as interaction centers. We show that a 2 order of magnitude enhancement is possible, beyond an enhancement due to localization effects. This factor multiplies the enhancement taking place due to the field localization.

physics.optics↗

arXiv2020

Plasmon lifetime enhancement in a bright-dark mode coupled system

Metallic nanoparticles can localize the incident light to hotspots as plasmon oscillations, where the intensity can be increased up to four orders of magnitude. Even though the lifetime of plasmons are typically short, it can be increased via interactions with quantum emitters, e.g. spaser nano-lasers. However, molecules can bleach in days. Here, we study the lifetime enhancement of plasmon excitations due to the coupling with longer lifetime dark plasmon modes. Exact solutions of the 3D Maxwell equations, i.e. FDTD, demonstrates that the lifetime of the coupled system increases, as also predicted by a basic oscillator model. We report an optimum bright-dark plasmon mode coupling, where lifetime enhancement becomes maximum, and show that no precise positioning of the nanostructures is required to obtain enhanced lifetime.

physics.optics↗

arXiv2020

A quantum emitter coated with graphene interacting in the strong coupling regime

We demonstrate the strong coupling of a quantum dot and a graphene spherical shell coating it. Our simulations are the exact solutions of 3D Maxwell equations. Interaction produces sharp hybrid modes, even when the two are off-resonant, which are voltage-tunable (continuously) in an 80 meV interval. Despite a voltage-tunable quantum dot, the coupling of the light to these "very sharp" plexcitonic resonances is an order of magnitude larger than its coupling to a quantum dot. Hence, our results are very attractive for sensing applications and graphene display technologies with sharper colors. Moreover, on a simple theoretical model, we explain why such sharp, highly tunable, resonances emerge.

physics.optics↗

arXiv2020

Measuring nonclasicality of single-mode systems

A beam splitter can generate entanglement if the input field is nonclassical, e.g. squeezed. Thus, beam splitter transformation can be utilized for obtaining single-mode nonclassicality conditions from two-mode entanglement criteria. Here, we demonstrate that (i) the generalized quadrature-squeezing condition $\langle\hat{a}^\dagger\hat{a}\rangle < |\langle \hat{a}^2\rangle|$ can be obtained from the two-mode entanglement criterion of Duan-Giedke-Cirac-Zoller~[PRL 84, 2722]. (ii) Single-mode nonclassicality condition, obtained from the two-mode entanglement criterion of Simon~[PRL, 84 2726], can detect (also accompanies) the nonclassicality of a superradiant ground state where no quadrature-squeezing takes place. (iii) We present an explicit form for a single-mode nonclassicality measure, for Gaussian states, via maximization of the beam splitter output with respect to the beam splitter parameters. We measure the nonclassicality of a squeezed state and the one generated by a damped second harmonic crystal cavity. We determine pump strengths where single-mode nonclassicality onsets, i.e. resists against the vacuum noise.

quant-ph↗

arXiv2019

Ultra-high resolution aSNOM imaging at off-resonant wavelengths

An atomic force microscope~(AFM) tip, with a few nm-thick noble metal coating, gives rise to strong electric-field at the near-field of tip apex, i.e. hot spot, when illuminated with a beam of light linearly polarized in the axial direction. This strong near-field enables resolving molecular landscape or nano-scale defects on crystal surfaces in apertureless scanning near field optical microscopy or tip enhanced Raman spectroscopy applications. However, strong near fields appear only at certain illumination wavelengths at which material and geometry dependent plasmon resonances take place. Once the metal coated tip is manufactured, optimal operation wavelength remains fixed since the material and geometry of the tip apex remains fixed. Here, we show for the first time a method which renders an AFM tip useful at wavelengths off-resonant to its plasmon resonances. The technique relies on decoration of the tip with appropriate auxiliary molecules. For instance, a tip originally bearing a plasmon resonance at $λ_{\rm p}=581$ nm can be effectively operated off-resonantly at $λ_{\rm exc}=532$ nm, when it is decorated by an appropriate auxiliary molecule. Furthermore, the near-field is found to be strongest just below the auxiliary molecule which enables a single-molecule-size ultra-high spatial resolution imaging. We demonstrate the phenomenon with exact solutions of 3D Maxwell equations. We also show why such an enhancement takes place.

cond-mat.mes-hall↗

arXiv2019

Entanglement criteria for two strongly interacting ensembles

Two interacting atomic ensembles display a Dicke-like quantum phase transition above a critical coupling strength. We show that an ensemble-ensemble entanglement accompanies the quantum phase transition. We derive entanglement criteria, which can witness the entanglement of the two interacting ensembles. We observe that all criteria are successful in the thermodynamic limit, while only the newly introduced ones~(number squeezing-like criteria) can witness the ensemble-ensemble entanglement for a finite number of particles. We also mention about implementations of these criteria to two-component condensate systems and nanoscale quantum plasmonics.

physics.optics↗

arXiv2019

Single-mode nonclassicality criteria via Holstein-Primakoff transformation

We study different representations of a many-particle system. We reveal the stunning beauty of the connections among the nonclassicalities of a many-particle, a single-mode and a two-mode systems. First, we demonstrate that (separable) atomic coherent states~(ACSs), of a many-particle system, mimic the coherent states of single-mode light. This enables the utilization of a many-particle entanglement~(MPE) criterion as a single-mode nonclassicality~(SMNc) condition via Holstein-Primakoff transformation. As an example, we show that spin-squeezing criterion, for MPE, can be transformed into the quadrature-squeezing, a nonclassicality condition for a single-mode system. Second, we demonstrate a similar connection between the two-mode separable states and separable ACSs of a many-particle system. The second connection makes us be able to obtain a single-mode nonclassicality condition from a two-mode entanglement criterion, using the many-particle system as a bridge. As an example, we obtain Mandel's Q-parameter from the two-mode entanglement criterion of Hillery{\&}Zubairy.

quant-ph↗

arXiv2019

Entanglement, violation of Kramers-Kronig relation and curvature in spacetime

Recent studies show that a maximally entangled Schwinger pair creates a nontraversable Einstein-Rosen~(ER) bridge (wormhole) in the gravitational theory, which bridges causally not connected parts of the AdS spacetime~[PRL 111, 211603 and 211602]. Same authors raise the possibility of a general correspondence between a weaker entanglement and curvature. Here, we provide some clues for such a generalized relation. First, we show that (i) entanglement and (ii) violation of Kramers-Kronig~(KK) relations appear at the same critical parameters for a standard two-mode (squeezing) entanglement interaction. We also include the dampings. Second, we bring a study into attention: Presence of a spacetime-curvature in vacuum polarization, electron-positron Schwinger pairs, makes QED also violate the KK relations without violating the causality. Then, we discuss that these findings are possible to provide new clues for a generalized relation between entanglement and spacetime-curvature. Such an interpretation may also save violation of KK relations from implying the violation of the causality.

physics.gen-ph↗

arXiv2019

Quantifications for multi-mode entanglement

We introduce two independent quantifications for 3-mode and 4-mode entanglement. We investigate the conversion of one type of nonclassicality, i.e. single-mode nonclassicality, into another type of nonclassicality, i.e. multi-mode entanglement, in beam-splitters. We observe parallel behavior of the two quantifications. The methods can be generalized to the quantification of any multi-mode entanglement.

quant-ph↗

arXiv2019

Nonclassicality and entanglement for wavepackets

Mode-entanglement based criteria and measures become insufficient for broadband emission, e.g. from spasers (plasmonic nano-lasers). We introduce criteria and measures for the (i) total entanglement of two wavepackets, (ii) entanglement of a wavepacket with an ensemble and (iii) total nonclassicality of a wavepacket~(WP). We discuss these criteria in the context of (i) entanglement of two WPs emitted from two initially entangled cavities (or two initially entangled atoms) and (ii) entanglement of an emitted WP with the ensemble/atom for the spontaneous emission and the single-photon superradiance. We also show that, (iii) when the two constituent modes of a WP are entangled, this creates nonclassicality in the WP as a noise reduction below the standard quantum limit. The criteria we introduce are, all, compatible with near-field detectors.

quant-ph↗

arXiv2018

Coherent control of optical bistability in Rydberg electromagnetically-induced transparency atomic system

We study optical bistable behavior of a Rydberg electromagnetically-induced transparency (EIT) atomic medium in a unidirectional optical ring-cavity. Due to strong van del Waal (vdW) interactions between the atoms, both optical nonlinear dispersion and nonlinear absorption coefficients are enhanced substantially. Under the condition of two-photon on resonance, we show that probe one-photon detuning can change the phase of the third order nonlinearity coefficient, which tunes the character of the optical bistability within different ratios of dispersive and absorptive types. This enables the single-photon control over photonic devices for further manipulation of light other than switches and transistors. More interestingly, we predict appearance of a scaling phenomena for optical bistabilities with the factors of coupling Rabi frequency and atomic density. Additionally, we also discuss the influence of the cavity detuning and the mirror transmission coefficient on the optical bistable behavior. The strong bistable feature provides a good ingredient for realizing all-optical logic gate devices in optical computing.

quant-ph↗