Namkyoo Park

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• Ph.D., Applied Physics, Caltech, USA
• Professor (Full) at Seoul National University

Elastic bound states in the continuum (BICs) have recently attracted significant interests due to their exceptionally high-Q-factor, which enables the confined mode to be completely decoupled from spectrally coexisting radiative channels. We report on the emergence of a state that induces a slow vibration phenomenon, which exhibits a multiphysics a...

Unitarity serves as a fundamental concept for characterizing linear and conservative wave phenomena in both classical and quantum systems. Developing platforms that perform unitary operations on light waves in a universal and programmable manner enables the emulation of complex light–matter interactions and the execution of general-purpose function...

Light behaviours in disordered materials have been of research interest primarily at length scales beyond or comparable to the wavelength of light, because order and disorder are often believed to be almost indistinguishable in the subwavelength regime according to effective medium theory (EMT). However, it was demonstrated that the breakdown of EM...

Finding hidden order within disorder is a common interest in material science, wave physics, and mathematics. The Riemann hypothesis, stating the locations of nontrivial zeros of the Riemann zeta function, tentatively characterizes statistical order in the seemingly random distribution of prime numbers. This famous conjecture has inspired various c...

Photonics has unlocked the potential for energy-efficient acceleration of deep learning. Most approaches toward photonic deep learning have diligently reproduced traditional deep learning architectures using photonic platforms, separately implementing linear-optical matrix calculations and nonlinear activations via electro-optical conversion, optic...

Unitarity serves as a fundamental concept for characterizing linear and conservative wave phenomena in both classical and quantum systems. Developing platforms that perform unitary operations on light waves in a uni-versal and programmable manner enables the emulation of complex light-matter interactions and the execution of general-purpose functio...

Light behaviours in disordered materials have been of research interest primarily at length scales beyond or comparable to the wavelength of light, because order and disorder are often believed to be almost indistinguishable in the subwavelength regime according to effective medium theory (EMT). However, it was recently demonstrated that the breakd...

Non-Abelian physics, originating from noncommutative sequences of operations, unveils novel topological degrees of freedom for advancing band theory and quantum computation. In photonics, significant efforts have been devoted to developing reconfigurable non-Abelian platforms, serving both as classical testbeds for non-Abelian quantum phenomena and...

Ultrahigh field magnetic resonance imaging (UHF MRI) has become an indispensable tool for human brain imaging, offering excellent diagnostic accuracy while avoiding the risks associated with invasive modalities. When the radiofrequency magnetic field of the UHF MRI encounters the multifaceted complexity of the brain, characterized by wavelength-sca...

The presence of long-range interactions is crucial in distinguishing between abstract complex networks and wave systems. In photonics, because electromagnetic interactions between optical elements generally decay rapidly with spatial distance, most wave phenomena are modeled with neighboring interactions, which account for only a small part of conc...

Finding hidden order within disorder is a common interest in material science, wave physics, and mathematics. The Riemann hypothesis, stating the locations of nontrivial zeros of the Riemann zeta function, tentatively characterizes statistical order in the seemingly random distribution of prime numbers. This famous conjecture has inspired various c...

Field transformation, as an extension of the transformation optics, provides a unique means for nonreciprocal wave manipulation, while the experimental realization remains a substantial challenge as it requires stringent material parameters of the metamaterials, e.g., purely nonreciprocal bianisotropic parameters. Here, we develop and demonstrate a...

Programmable photonic circuits (PPCs) have garnered substantial interest for their potential in facilitating deep learning accelerations and universal quantum computations. Although photonic computation using PPCs offers ultrafast operation, energy-efficient matrix calculations, and room-temperature quantum states, its poor scalability hinders inte...

Reducing geometrical complexity while preserving desired wave properties is critical for proof-of-concept studies in wave physics, as evidenced by recent efforts to realize photonic synthetic dimensions, isospectrality, and hyperbolic lattices. Laughlin’s topological pump, which elucidates quantum Hall states in cylindrical geometry with a radial m...

Ultrasound technology is widely utilized in applications, including tumor treatments, drug delivery, and skin care. However, improvements are required to prevent unwanted damage to non‐targeted tissues. The ultrasound focusing technology, represented by the highly intensive focused ultrasound (HIFU) technology, is actively researched to handle this...

Programmable photonic circuits (PPCs) have garnered substantial interest in achieving deep learning accelerations and universal quantum computations. Although photonic computation using PPCs offers critical advantages, including ultrafast operation, energy-efficient matrix calculation and room-temperature quantum states, its poor scalability impede...

We propose a concept called acoustic amplifying diode combining signal isolation and amplification in a single device. The signal is exponentially amplified in one incident direction with no reflection and is perfectly absorbed in another. The reflection is eliminated from the device in both directions with impedance matching, preventing backscatte...

Developing hardware for high-dimensional unitary operators plays a vital role in implementing quantum computations and deep learning accelerations. Programmable photonic circuits are singularly promising candidates for universal unitaries owing to intrinsic unitarity, ultrafast tunability and energy efficiency of photonic platforms. Nonetheless, wh...

Metalenses are attractive alternatives to conventional bulky refractive lenses owing to their superior light-modulating performance and sub-micrometre-scale thicknesses; however, limitations in existing fabrication techniques, including high cost, low throughput and small patterning area, have hindered their mass production. Here we demonstrate low...

Recently, there has been increasing interest in the temporal degree of freedom in photonics due to its analogy with spatial axes, causality and open-system characteristics. In particular, the temporal analogues of photonic crystals have allowed the design of momentum gaps and their extension to topological and non-Hermitian photonics. Although rece...

We experimentally confirm the non-Hermitian band structures of a photonic temporal crystal in the microwave. These non-Hermitian band structures are shown to be two measurable distinct subsets of complex eigenfrequency surfaces defined in complex momentum space.

We propose a theoretical approach for the realization of unidirectional light scattering without spatial patterning, enabled by correlated photonic disorder in time domain. Our study enables novel photonic devices such as spatially pattern-free color filters.

We propose a perturbative design method for engineering quasi-isospectrality in multidimensional photonic systems. Our study provides platform-transparency alleviating mathematical strictness of supersymmetric transformation.

We investigate modal localization of light in disordered hyperbolic lattices. We examine modal area at the bulk of a disordered hyperbolic lattice, which demonstrates that high degree in the lattice leads to the delocalization.

Lyubarov et al. (Research Articles, 22 July 2022, p. 425) claim that the spontaneous emission rate of an atom vanishes at the momentum gap edges of photonic Floquet media. We show that their theoretical prediction is based on assumptions that result in misleading interpretations on the spontaneous emission rate in photonic Floquet media.

We propose a concept called acoustic amplifying diode in combining both signal isolation and amplification in a single device. The signal is exponentially amplified in one direction with no reflection and is completely absorbed in another. In this case, the reflection is eliminated from the device in both directions due to impedance matching, preve...

All‐Optical Memories Realizing photonic in‐memory processors is critical for light‐based deep learning accelerators. In article number 2200579 Namkyoo Park, Sunkyu Yu, and co‐workers demonstrate an all‐optical building block for in‐memory processors by exploiting topological states of dynamical parity‐time‐symmetric systems. The proposed memory ach...

Periodically driven systems are ubiquitously found in both classical and quantum regimes. In the field of photonics, these Floquet systems have begun to provide insight into how time periodicity can extend the concept of spatially periodic photonic crystals and metamaterials to the time domain. However, despite the necessity arising from the presen...

The temporal degree of freedom in photonics has been a recent research hotspot due to its analogy with spatial axes, causality, and open-system characteristics. In particular, the temporal analogues of photonic crystals have stimulated the design of momentum gaps and their extension to topological and non-Hermitian photonics. Although recent studie...

Developing hardware for high-dimensional unitary operators plays a vital role in implementing quantum computations and deep learning accelerations. Programmable photonic circuits are singularly promising candidates for universal unitaries owing to intrinsic unitarity, ultrafast tunability, and energy efficiency of photonic platforms. Nonetheless, w...

The effect of deep subwavelength disorder in one-dimensional dichromic multilayer films on the optical transmission, localization length, and Goos–Hänchen shift around the critical angle is analyzed using sets of disordered multilayer films with different degrees of order metric τ. For each Gaussian-perturbed multilayer film designed by a Metropoli...

The in‐memory processor has played an essential role in overcoming the von Neumann bottleneck, which arises from the partition of memory and a processing unit. Although photonic technologies have recently attracted attention for ultrafast and power‐efficient in‐memory computing, the realization of an all‐optical in‐memory processor remains a challe...

A novel data‐driven approach to examine “active disorder” for light is introduced by Jungmin Kim, Seungkyun Park, Sunkyu Yu, and Namkyoo Park (see article number 2102642). This work demonstrates a wave‐oriented evaluation and design of active disorder by deep neural networks mapping the disorder state of a multilayer structure and the corresponding...

Resolving spatial and temporal complexities in wave–matter interactions is essential for controlling the light behavior inside disordered and nonstationary systems and therefore achieving high capacity devices. Although these complexities have usually been studied separately, a few examples exploiting both degrees of freedom have derived intriguing...

The intrinsic geometry of wavevector diagrams describes electronic or photonic transport at a given energy level. Lifshitz transition is an intriguing example of the topological transition in wavevector diagrams, which plays a critical role in abnormal transport with enhanced magnetoresistance or superconductivity. Here, we develop the spatial anal...

We propose data-driven engineering of active light-disorder interactions. Neural networks generate the family of disorders for active multilayer structures having similar modulation sensitivity, enabling the independent controls of multiple wave-material characteristics.

We investigate supersymmetric transformations for engineering the short-range order of material. In crystals and quasicrystals, the weak value momentum of the ground state determines the control of short-range order while preserving long-range fluctuations.

We experimentally reveal the unique non-Hermitian band structures of a photonic Floquet medium emulated in the microwave regime. We observe the presence of the momentum gaps and the subharmonic parametric oscillations therefrom.

The independent tailoring of wave quantities lays the foundation for controlling wave phenomena and designing wave devices. The concept of isospectrality, which suggests the existence of systems that provide identical spectra, has inspired a novel route to the spectrum-preserved engineering of wave–matter interactions in photonics, acoustics, and q...

Periodically driven systems, characterised by their inherent non-equilibrium dynamics, are ubiquitously found in both classical and quantum regimes. In the field of photonics, these Floquet systems have begun to provide insight into how time periodicity can extend the concept of spatially periodic photonic crystals and metamaterials to the time dom...

Abstract We describe the quantum mechanical rotation of a photon state, the Wigner rotation—a quantum effect that couples a transformation of a reference frame to a particle’s spin, to investigate geometric phases induced by Earth’s gravitational field for observers in various orbits. We find a potentially measurable quantum phase of the Wigner rot...

We propose a polarization selective transparent electrode (PSTE) utilizing a rectangular-shaped metal nano-wire structure. The performance of the PSTE was evaluated with simulation based on finite element method (COMSOL Multiphysics). Parametric studies have been carried out to give a guide for geometric parameter selection to meet the requirement...

The concept of topology is universally observed in various physical objects when the objects can be described by geometric structures. Although a representative example is the knotted geometry of wavefunctions in reciprocal space for quantum Hall family and topological insulators, topological states have also been defined for other physical quantit...

Willis metamaterial enables exotic manipulations of acoustic waves with a precise combination of bulk modulus, mass density, and Willis parameters. While the realization of unrestricted and completely decoupled constitutive parameters would extend the horizon of future applications, the restriction of passivity and reciprocity dictate a hard bound...

In article number 2007831, Sungjun In, Namkyoo Park, and Jusung Park report a dispersion‐controlled gold–aluminum–silicon dioxide–aluminum nanopawn structure, utilizing a novel Au nanocolloidal lithography process. With the complementary spectral responses between reflected and scattered light of the nanopawn structure, information encryption based...

The ideals of reconfigurable metasurfaces would be operation in a broad frequency range with a high extinction ratio and fatigue resistivity. In this paper, all the above is achieved in the microwave regime by transforming a bare metallic film into well‐controlled nanometer sized gaps in a fully reversible manner. It is shown that adjacent metallic...

One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible...

As an efficient patterning method for nanostructures, nanocolloidal lithography (NCL) presents a controllable and scalable means for achieving a uniform and good sidewall profile, and a high aspect ratio. While high selectivity between the etching mask and targeted materials is also essential for NCL-based precision nanophotonic structures, its rea...

We introduce the data-driven design of multilayers with enhanced tunability. By applying the machine-learning-designed claddings to a phase-changeable core, we obtain the deterministic realization of on-off states in the angular transmittance, achieving tunable engineered disorder.

We introduce coexisting oscillation quenching states in parity-time-symmetric systems. The degrees of freedom in the triatomic system including nonlinear resonators allow multiple dynamical stabilities with different optical energy distributions at a given system.

Disorder, which qualitatively describes some measure of irregularities in spatial patterns, is ubiquitous in many-body systems, equilibrium and non-equilibrium states of matter, network structures, biological systems and wave–matter interactions. In photonics, the introduction of order and disorder for device applications has traditionally been tre...

Dynamic Light Modulation In article number 2000050, Namkyoo Park and co‐workers report on electrically controlled photonic logic gates covering all six fundamental operations: NOR/OR, NAND/AND, and x = y/XOR. As a building block for photonic‐thermodynamic or optoelectronic devices in nano‐photonics, metamaterials, and materials science, a new litho...

Composites of phase change material (PCM) and structured metal (SM) are promising templates for photonic applications combining advantages of both insulator‐metal transition (IMT) hysteresis and strong light‐matter interactions. While significant progress has been made for applications including active modulators, photonic memories, and thermal emi...

The vast amount of design freedom in disordered systems expands the parameter space for signal processing. However, this large degree of freedom has hindered the deterministic design of disordered systems for target functionalities. Here, we employ a machine learning approach for predicting and designing wave-matter interactions in disordered struc...

Supplementary Note 1. Training datasets from collective and individual deformations / Supplementary Note 2. Training process and cost functions / Supplementary Note 3. Dependence of power law distributions on data size / Supplementary Note 4. Scale invariance for different degrees of localization / Supplementary Note 5. Fitting with other heavy-ta...

Dynamic control of light based on gate‐tunable metasurfaces has revolutionized traditional optoelectronic devices due to its unprecedented compactness and versatile functionalities. However, these devices are typically based on metal‐insulator‐metal geometries that enable field‐effect modulation of only reflected light. Transmittance modulation tec...

Non-Euclidean geometry, discovered by negating Euclid’s parallel postulate, has been of considerable interest in mathematics and related fields for the description of geographical coordinates, Internet infrastructures, and the general theory of relativity. Notably, an infinite number of regular tessellations in hyperbolic geometry—hyperbolic lattic...

Acoustic bianisotropy, also known as the Willis parameter, expands the field of acoustics by providing nonconventional couplings between momentum and strain in constitutive relations. Sharing the common ground with electromagnetics, the realization of acoustic bianisotropy enables the exotic manipulation of acoustic waves in cooperation with a prop...

The notion of mode shaping based on evanescent coupling has been successfully applied in various fields of optics, such as in the dispersion engineering of optical waveguides. Here, we show that the same concept provides an opportunity for the seemingly different field of ultra-high-field MRI, addressing transmit RF magnetic field (B1+) inhomogenei...

Elastic waves are complex mixtures of transverse and longitudinal oscillations even in isotropic and homogeneous media, in contrast to the quantum, electromagnetic, or acoustic waves which could share the same formalism of Hamiltonian and application techniques. Here, we reformulate the elastic wave equation into a set of polarization-dependent dec...

Band engineering near Dirac points is an emerging topic in condensed matter physics and photonics, enabling multifaceted devices of record-high conductivity and zero refractive index in both electronic and photonic structures. Recently, an extended class of Dirac cone, type I, II, or III, has attracted much attention with its controlled directional...

The vast amount of design freedom in disordered systems expands the parameter space for signal processing, allowing for unique signal flows that are distinguished from those in regular systems. However, this large degree of freedom has hindered the deterministic design of disordered systems for target functionalities. Here, we employ a machine lear...

Non-Euclidean geometry, discovered by negating Euclid's parallel postulate, has been of considerable interest in mathematics and related fields for the description of geographical coordinates, Internet infrastructures, and the general theory of relativity. Notably, an infinite number of regular tessellations in hyperbolic geometry-hyperbolic lattic...

The conservation of quantities or dynamics under coordinate transformations, known as gauge invariance, has been the foundation of theoretical frameworks in physics. The finding of gauge-invariant quantities has provided a new way of understanding physical problems, as shown in geometric and topological interpretations of quantum phenomena with the...

Topological phases exhibit properties that are conserved for continuous deformations, as demonstrated in topological protections in condensed-matter physics and electromagnetic waves. Despite its ubiquitous nature and recent extensions to synthetic dimensions, non-Hermitian Hamiltonians, and nonlinear dynamics, topological protection has generally...

By designing tailor-made resonance modes with structured atoms, metamaterials allow us to obtain constitutive parameters outside their limited range from natural materials. Nonetheless, tuning the constitutive parameters depends on our ability to modify the physical structure or external circuits attached to the metamaterials, posing a fundamental...

Ultra high field (UHF) brain MRI has proved its value by providing enhanced SNR, contrast, and higher resolution derived from the higher magnetic field (B0). Nonetheless, with the increased B0 of UHF MRI, the transmit RF magnetic field (B1+) inhomogeneity also became one of the critical issues requiring attention. As the effective wavelength of RF...

In this chapter, we introduce the inverse design of disordered photonic structures with the target modal response. Starting from the concept of metadisorder, which is defined by order-random interactions in the multivariable parameter space, we discuss the realization of anomalous wave transport and localization in photonic structures as an example...

In Chaps. 2 and 3, we investigated the independent controls of wave quantities in disordered photonic structures, revealing the existence of anomalous types of disorder that are distinct from both order and randomness.

In this chapter, we introduce the inverse design of disordered photonic structures with target spectral information. Starting from the concept of isospectrality, which originated from the old question of “Can one hear the shape of a drum?”, we discuss the derivation of isospectral relations between different shapes of photonic structures. Two mathe...

Chirality is ubiquitous in physics and biology from microscopic to macroscopic phenomena, such as fermionic interactions and DNA duplication. In photonics, chirality has traditionally represented differentiated optical responses for right and left circular polarizations. This definition of optical chirality in the polarization domain includes hande...

To achieve ultrafast neuromorphic signal processing, the realization of photonic equivalents to neuronal functions has been challenged. In article number 1900771, Namkyoo Park and co‐workers present a neuromorphic photonic system ruled by parity‐time symmetry as a building block for light‐based neural signal processing. Inspired by neuronal ion cha...

By designing tailor-made resonance modes with structured atoms, metamaterials allow us to obtain constitutive parameters outside their limited range from natural or composite materials. Nonetheless, tuning the constitutive parameters relies much on our capability in modifying the physical structures or media in constructing the metamaterial atoms,...

We propose an approach of steering the second harmonic (SH) emission from a single plasmonic structure, through local excitations of plasmon. The proposed idea is confirmed experimentally, by adjusting the incident beam position at the fundamental frequency, on a single plasmonic antenna. A significant directivity change ( ± 52°) for the SH emissio...

As an elementary processor of neural networks, a neuron performs exotic dynamic functions, such as bifurcation, repetitive firing, and oscillation quenching. To achieve ultrafast neuromorphic signal processing, the realization of photonic equivalents to neuronal dynamic functions has attracted considerable attention. However, despite the nonconserv...

Since the first observation of second harmonic generation (SHG), there have been extensive studies on this nonlinear phenomenon not only to clarify its physical origin but also to realize unconventional functionalities. Nonetheless, a widely accepted model of SHG with rigorous experimental verification that describes the contributions of different...

Chirality is ubiquitous from microscopic to macroscopic phenomena in physics and biology, such as fermionic interactions and DNA duplication. In photonics, chirality has traditionally represented differentiated optical responses for right and left circular polarizations. This definition of optical chirality in the polarization domain includes hande...

We exploit non-resonant inclusions under the metamaterial scheme to enable full analysis of the effective properties with different shapes and lattice structures using the S-parameter retrieval method. Various constraints on the extraction of effective properties are investigated to quantify their dependence on the number of unit cells, the frequen...