arXiv subjects

Alpan Bek

Publications and source records attributed to Alpan Bek.

At least 19 recordsLinked to original sources

arXiv2025

Disorder-Engineered Hybrid Plasmonic Cavities for Emission Control of Defects in hBN

Defect-based quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for scalable quantum photonics due to their stable single-photon emission at room temperature. However, enhancing their emission intensity and controlling the decay dynamics remain significant challenges. This study demonstrates a low-cost, scalable fabrication approach to integrate plasmonic nanocavities with defect-based quantum emitters in hBN nanoflakes. Using the thermal dewetting process, we realize two distinct configurations: stochastic Ag nanoparticles (AgNPs) on hBN flakes and hybrid plasmonic nanocavities formed by AgNPs on top of hBN flakes supported on gold/silicon dioxide (Au/SiO2) substrates. While AgNPs on bare hBN yield up to a two-fold photoluminescence (PL) enhancement with reduced emitter lifetimes, the hybrid nanocavity architecture provides a dramatic, up to 100-fold PL enhancement and improved uniformity across multiple. emitters, all without requiring deterministic positioning. Finite-difference time-domain (FDTD) simulations and time-resolved PL measurements confirm size-dependent control over decay dynamics and cavity-emitter interactions. Our versatile solution overcomes key quantum photonic device development challenges, including material integration, emission intensity optimization, and spectral multiplexity. Future work will explore potential applications in integrated photonic circuits hosting on-chip quantum systems and hBN-based label-free single-molecule detection through such quantum nanoantennas.

physics.optics↗

arXiv2025

QUPERS: Quantum Enhanced Raman Spectroscopy

We present experimental demonstration of a recently predicted path interference phenomenon [Nanophotonics 7, 1687 (2018)]. A SERS process becomes resonant when both incident and the converted frequencies match with two plasmon resonances -- a condition which is hard to satisfy experimentally. Yet, presence of a quantum dot~(QD) coupled to the lower-energy plasmon mode has been predicted to introduce interference effect in the conversion paths and to bring the SERS process into resonance. Here, we experimentally demonstrate a 7-fold additional enhancement factor~(EF) {\it multiplying} the conventional SERS enhancement owing to the presence of QDs in addition to metal nanoparticles~(MNPs). We eliminate influence of alternative enhancement mechanisms and show that observed additional EF of 7 is solely because of the path interference~(Fano) effect taking place in the presence of the QDs. As the most significant evidence: we observe that when QD/MNP ratio exceeds $1$, EF starts to decrease. Two or more QDs sitting at different~(random) positions on a nanostar MNP surface starts to degrade the path interference effect. Even though the theoretically predicted maximum EFs of $\sim$100--1000 when QD is in the MNP hotspot are not always experimentally feasible in stochastic nature of large area SERS substrate, still, observation of a 7-fold overall increase in such a setting is significant for fabricating Raman-based sensor applications. More importantly, our scheme allows for designing selective SERS substrates. As the QD resonances can actively be shifted by external factors, such as by applying an electric field (Stark effect), it provides a previously-not-existing tool: active selective enhancement of Raman bands via an applied voltage.

physics.optics↗

arXiv2024

Continuous and Reversible Electrical Tuning of Fluorescent Decay Rate via Fano Resonance

We demonstrate that the decay rates of a fluorescent molecule can be controlled by electrically shifting a transparency introduced by a Fano resonance. An auxiliary quantum object (QO), located at the hotspot of a plasmonic nanoparticle, suppresses plasmonic excitation at its level spacing {\omega}_QO. As a result, the local density of states (LDOS) associated with the plasmonic spectrum is also suppressed at {\omega}={\omega}_QO. By shifting {\omega}_QO via an applied voltage, we continuously tune the radiative and nonradiative decay rates of the fluorescent molecule by up to two orders of magnitude. This mechanism offers a valuable tool for integrated quantum technologies, enabling on-demand entanglement and single-photon sources, voltage-controlled quantum gate operations, and electrical control of superradiant-like phase transitions. The approach also holds promise for applications in super-resolution microscopy and surface-enhanced Raman spectroscopy (SERS).

physics.optics↗

arXiv2023

Voltage-controlled extraordinary optical transmission in the visible regime

Control of components in integrated photonic circuits is crucial in achieving programmable devices. Operation bandwidth of a plasmonic device cannot be generally tuned once it is manufactured, especially in the visible regime. Here, we demonstrate the electrical control of such a device for extraordinary optical transmission~(EOT) in the visible regime. (i) Operation frequency of the EOT device can be tuned via a bias voltage applied through nanowires. (ii) Or, at a given frequency, the EOT signal (normalized to the incident field) can be tuned continuously, e.g., between $10^{-4}$ and $0.4$. This corresponds to a 3-orders of magnitude modulation depth. We utilize Fano resonances induced by a quantum emitter~(QE) that is embedded into the nanoholes. The external bias-voltage tunes QE's resonance. We also discuss the lifetime extensions of surface plasmon polaritons as a response to an ultra-short optical pulse. Our proposed method provides the active electronic control of EOT signal which makes it a feasible and compact element in integrated photonic circuits, for bio-sensing, high resolution imaging, and molecular spectroscopy applications.

physics.optics↗

arXiv2023

Development of a Selective Wet-Chemical Etchant for 3D Structuring of Silicon via Nonlinear Laser Lithography

Recently-demonstrated high-quality three-dimensional (3D) subsurface laser processing inside crystalline silicon (c-Si) wafers opens a door to a wide range of novel applications in multidisciplinary research areas. Using this technique, a novel maskless micro-pillars with precise control on the surface reflection and coverage are successfully fabricated by etching the laser processed region of c-Si wafer. To achieve this, a particular selective wet chemical etching is developed to follow subsurface laser processing of c-Si to reveal the desired 3D structures with smooth surfaces. Here, we report the development of a novel chromium-free chemical etching recipe based on copper nitrate, which yields substantially smooth surfaces at high etch rate and selectivity on the both laser-processed Si surface and subsurface, i.e., without significant etching of the unmodified Si. Our results show that the etch rate and surface morphology are interrelated and strongly influenced by the composition of the adopted etching solution. After an extensive compositional study performed at room temperature, we identify an etchant with a selectivity of over 1600 times for laser-modified Si with respect to unmodified Si. We also support our findings using density functional theory calculations of HF and Cu adsorption energies, indicating significant diversity on the c-Si and laser-modified surfaces.

physics.app-ph↗

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↗

arXiv2020

Fano-control of 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 studies, a frequency conversion process takes place near a hot spot. Here, we study the case where the frequency conversion process takes place \textit{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 orders of magnitude enhancement is possible, on top of the enhancement due to localization effects. That is, this factor multiplies the enhancement taking place due to the field localization.

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

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

Single-Photon Nanoantenna with in Situ Fabrication of Plasmonic Ag Nanoparticle at an hBN Defect Center

We present a practical new method for fabricating a coupled single quantum emitter-plasmonic nanoantenna system. Emission characteristics of a single defect center embedded in hexagonal Boron Nitride (hBN) multilayers is modified using plasmonic nanoantennas. By dewetting thin silver films on hBN multilayers, plasmonic nanoantennas are obtained in a size controlled way with no adverse effects on the defects. A very same single defect center is investigated with and without nanoantenna in order to demonstrate the modification of its emission characteristics. Based on the initial silver film thickness in dewetting process, on-demand enhancement and quenching effects are observed. For attaining deterministic coupling strengths, an electromagnetic simulation model is employed in the light of experiments. Fluorescence lifetime, radiative and nonradiative emission rate calculations are used for estimating the spatial configuration of the defect-nanoantenna system as well as for confirming the experimental findings. Our approach provides a low-cost and uncomplicated coupling scheme as an alternative to the scanning probe tip antenna technique.

physics.optics↗

arXiv2019

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↗

arXiv2019

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↗

arXiv2019

Fano Enhancement of Unlocalized Nonlinear Optical Processes

Field localization boosts nonlinear optical processes at the hot spots of metal nanostructures. Fano resonances can further enhance these "local" processes taking place at the hot spots. However, in conventional nonlinear materials, the frequency conversion takes place along the entire crystal body. That is, the conversion process is "unlocalized". The path interference (Fano resonance) schemes developed for localized processes become useless in such materials. Here, we develop Fano enhancement schemes for unlocalized nonlinear optical processes. We show that 3 orders of magnitude Fano enhancement multiply the enhancements achieved via field trapping techniques, e.g., in epsilon-near-zero~(ENZ) materials. We demonstrate the phenomenon both analytically and by numerical solutions of Maxwell's equations. The match between the two solutions is impressive. We observe that the interference scheme for unlocalized processes is richer than the one for the local processes. The method can be employed for any kind of nonlinear optical conversion. Moreover, the Fano enhancement can be continuously controlled by an applied voltage.

physics.optics↗

arXiv2018

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↗

arXiv2018

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 $\lambda_{\rm p}=581$ nm can be effectively operated off-resonantly at $\lambda_{\rm exc}=532$ 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↗

arXiv2018

Controlling steady-state second harmonic signal via linear and nonlinear Fano resonances

Nonlinear signal even from a single molecule becomes visible at hot spots of plasmonic nanoparticles. In these structures, Fano resonances can control the nonlinear response in two ways. \textit{(i)} A linear Fano resonance can enhance the hot spot field, resulting enhanced nonlinear signal. \textit{(ii)} A nonlinear Fano resonance can enhance the nonlinear signal without enhancing the hot spot. In this study, we compare the enhancement of second harmonic signal at the steady-state obtained via these two methods. Since we are interested in the steady-state signal, we adapt a linear enhancement which works at the steady-state. This is different than the dark-hot resonances that appears in the transparency window due to enhanced plasmon lifetime.

physics.optics↗

arXiv2017

Fano enhancement of SERS signal without increasing the hot spot intensity

Plasmonic nanostructures enhance nonlinear response, such as surface enhanced Raman scattering (SERS), by localizing the incident field into hot spots. The localized hot spot field can be enhanced even further when linear Fano resonances (FR) take place in a double resonance scheme. However, hot spot enhancement is limited with the modification of the vibrational modes, the break-down of the molecule and the tunnelling regime. Here, we present a method which can circumvent these limitations. Our analytical model and solutions of 3D Maxwell equations show that: enhancement due to the localized field can be multiplied by a factor of $10^2$ to $10^3$. Moreover, this can be performed without increasing the hot spot intensity which also avoids the modification of the Raman modes. Unlike linear Fano resonances, we create a path interference in the nonlinear response. We demonstrate on a single equation that enhancement takes place due to cancellation of the contributing terms in the denominator of the SERS response.

physics.optics↗

arXiv2016

Fluorescence Excitation by Enhanced Plasmon Upconversion under Continuous Wave Illumination

We demonstrate effective background-free continuous wave nonlinear optical excitation of molecules that are sandwiched between asymmetrically constructed plasmonic gold nanoparticle clusters. We observe that near infrared photons are converted to visible photons through efficient plasmonic second harmonic generation. Our theoretical model and simulations demonstrate that Fano resonances may be responsible for being able to observe nonlinear conversion using a continuous wave light source. We show that nonlinearity enhancement of plasmonic nanostructures via coupled quantum mechanical oscillators such as molecules can be several orders larger as compared to their classical counterparts.

physics.optics↗