Choloong Hahn

Observation of exceptional points in reconfigurable non-Hermitian vector-field holographic lattices

Recently, synthetic optical materials represented via non-Hermitian Hamiltonians have attracted significant attention because of their nonorthogonal eigensystems, enabling unidirectionality, nonreciprocity and unconventional beam dynamics. Such systems demand carefully configured complex optical potentials to create skewed vector spaces with a desired metric distortion. In this paper, we report optically generated non-Hermitian photonic lattices with versatile control of real and imaginary sub-lattices. In the proposed method, such lattices are generated by vector-field holographic interference of two elliptically polarized pump beams on azobenzene-doped polymer thin films. We experimentally observe violation of Friedels law of diffraction, indicating the onset of complex lattice formation. We further create an exact parity-time symmetric lattice to demonstrate totally asymmetric diffraction at the spontaneous symmetry-breaking threshold, referred to as an exceptional point. On this basis, we provide the experimental demonstration of reconfigurable non-Hermitian photonic lattices in the optical domain and observe the purest exceptional point ever reported to date.

Extremely broadband, on-chip optical nonreciprocity enabled by mimicking nonlinear anti-adiabatic quantum jumps near exceptional points

Time-asymmetric state-evolution properties while encircling an exceptional point are presently of great interest in search of new principles for controlling atomic and optical systems. Here, we show that encircling-an-exceptional-point interactions that are essentially reciprocal in the linear interaction regime make a plausible nonlinear integrated optical device architecture highly nonreciprocal over an extremely broad spectrum. In the proposed strategy, we describe an experimentally realizable coupled-waveguide structure that supports an encircling-an-exceptional-point parametric evolution under the influence of a gain saturation nonlinearity. Using an intuitive time-dependent Hamiltonian and rigorous numerical computations, we demonstrate strictly nonreciprocal optical transmission with a forward-to-backward transmission ratio exceeding 10 dB and high forward transmission efficiency (∼100%) persisting over an extremely broad bandwidth approaching 100 THz. This predicted performance strongly encourages experimental realization of the proposed concept to establish a practical on-chip optical nonreciprocal element for ultra-short laser pulses and broadband high-density optical signal processing.

Plasmonic gain in long-range surface plasmon polariton waveguides bounded symmetrically by dye-doped polymer

The plasmonic gain of a top-pumped active symmetric metal slab waveguide is investigated theoretically and experimentally. The structure consists of a thin Ag film cladded above and below by gain media (IR140-doped poly (methyl methacrylate)), and operating with long-range surface plasmon polaritons (LRSPPs) at near-infrared wavelengths. We consider the spatial distribution of the pump intensity and the position dependence of the dipole lifetime within the claddings when computing the LRSPP gain. We find that the bottom cladding provides significant gain to the LRSPP, despite the low pump transmittance through the Ag film, as long as the pump intensity is strong enough to saturate the gain material (∼4 MW/cm2). In this situation, the LRSPP gain is doubled compared to the case where the top cladding only is active. The LRSPP gain was measured in a fabricated structure using the variable stripe length method, yielding gmod = 16.7 cm−1 at a pump intensity of ∼4 MW/cm2. The measured LRSPP gain agrees very wel...

Unidirectional Bragg Gratings Using Parity-Time Symmetry Breaking in Plasmonic Systems

Optical systems following concepts of parity-time (PT) symmetry have attracted significant attention because of their extraordinary behavior such as unidirectional reflectance or power oscillation. PT symmetric optical systems are realized by judiciously manipulating the complex refractive index to produce even- and odd-symmetric distributions for the real and imaginary indices, respectively. We propose two PT symmetric Bragg gratings based on step-in-width metal stripes and dielectric-loaded metal stripes operating with long-range surface plasmon polaritons. The gratings are designed to operate near 880 nm because optical gain can be conveniently provided by IR140-doped PMMA. Asymmetric reflectance is predicted in the proposed gratings based on modal and transfer matrix method computations. Moreover, we analyzed pulse reshaping and energy transport in generic gratings, and in the proposed plasmonic gratings, in terms of group and energy velocities. It is found that the group and energy velocities are dispersionless at the PT symmetry breaking point. Also, the group velocity dispersion can be inverted by changing the PT symmetric state from broken to unbroken or vice versa. Our designs are practical because a large left-right asymmetric reflectance contrast is produced for a wide range of physical dimensions. The proposed gratings are suitable as on-chip devices for optical processing providing new functionality such as switching and controlling the time delay of a data pulse without distortion.

Surface Plasmon Modes Confined in the Gap Between Metal Nanowire and Dielectric Slab

We propose a metal-dielectric hybrid waveguide structure consisting of a single metal nanowire placed on a flat dielectric slab. Mode size and propagation loss of the surface-plasmons confined in the metal-dielectric gap are compared with those of the complementary structure with a dielectric nanowire on a metal surface. In the case of the nanowire`s diameter much smaller than the wavelength the two structures reveal quite different characteristics; the dielectric nanowire-on-metal has longer propagation distance, but only the metal nanowire-on-dielectric exhibits a mode size two fold smaller than the diffraction limit. The proposed hybrid structure may therefore be more suitable for realization of nanocavity lasers.

Observation of an anti-PT-symmetric exceptional point and energy-difference conserving dynamics in electrical circuit resonators

Parity-time (PT) symmetry and associated non-Hermitian properties in open physical systems have been intensively studied in search of new interaction schemes and their applications. Here, we experimentally demonstrate an electrical circuit producing key non-Hermitian properties and unusual wave dynamics grounded on anti-PT (APT) symmetry. Using a resistively coupled amplifying-LRC-resonator circuit, we realize a generic APT-symmetric system that enables comprehensive spectral and time-domain analyses on essential consequences of the APT symmetry. We observe an APT-symmetric exceptional point (EP), inverse PT-symmetry breaking transition, and counterintuitive energy-difference conserving dynamics in stark contrast to the standard Hermitian dynamics keeping the system’s total energy constant. Therefore, we experimentally confirm unique properties of APT-symmetric systems, and further development in other areas of physics may provide new wave-manipulation techniques and innovative device-operation principles.The study of parity-time (PT) symmetric optical systems has recently attracted much attention. Here, the authors experimentally study an anti-PT symmetric circuit system and observe an exceptional point with an inverse PT symmetry breaking transition and energy-difference conserving dynamics.

Time-asymmetric loop around an exceptional point over the full optical communications band

Topological operations around exceptional points1–8—time-varying system configurations associated with non-Hermitian singularities—have been proposed as a robust approach to achieving far-reaching open-system dynamics, as demonstrated in highly dissipative microwave transmission3 and cryogenic optomechanical oscillator4 experiments. In stark contrast to conventional systems based on closed-system Hermitian dynamics, environmental interferences at exceptional points are dynamically engaged with their internal coupling properties to create rotational stimuli in fictitious-parameter domains, resulting in chiral systems that exhibit various anomalous physical phenomena9–16. To achieve new wave properties and concomitant device architectures to control them, realizations of such systems in application-abundant technological areas, including communications and signal processing systems, are the next step. However, it is currently unclear whether non-Hermitian interaction schemes can be configured in robust technological platforms for further device engineering. Here we experimentally demonstrate a robust silicon photonic structure with photonic modes that transmit through time-asymmetric loops around an exceptional point in the optical domain. The proposed structure consists of two coupled silicon-channel waveguides and a slab-waveguide leakage-radiation sink that precisely control the required non-Hermitian Hamiltonian experienced by the photonic modes. The fabricated devices generate time-asymmetric light transmission over an extremely broad spectral band covering the entire optical telecommunications window (wavelengths between 1.26 and 1.675 micrometres). Thus, we take a step towards broadband on-chip optical devices based on non-Hermitian topological dynamics by using a semiconductor platform with controllable optoelectronic properties, and towards several potential practical applications, such as on-chip optical isolators and non-reciprocal mode converters. Our results further suggest the technological relevance of non-Hermitian wave dynamics in various other branches of physics, such as acoustics, condensed-matter physics and quantum mechanics.Time-asymmetric light transmission over the entire optical communications band is achieved using a silicon photonic structure with photonic modes that dynamically encircle an exceptional point in the optical domain.

Experimental demonstration of a broadband time-asymmetric waveguide architecture enabled by dynamically encircling an exceptional point in the optical domain (Conference Presentation)

We experimentally demonstrate a robust Si-photonic waveguide architecture that realizes dynamically encircling an exceptional point (EP) in the optical domain and broadband asymmetric modal transmission as an essential consequence. The structure consists of a pair of coupled channel waveguides and an adjacent slab-waveguide patch that enable precise lithographic controls on the phase velocities and radiation rates of the guided photonic modes. Complex modal index and inter-mode coupling constant profiles required for the encircling-an-EP parametric control are precisely coded in the geometry of those elements. The device created on this basis induces the symmetry-exchanging adiabatic state flip for one transmission direction and symmetry-preserving anti-adiabatic state-jump for the transmission in the opposite direction. In fabrication, we use a state-of-the-art electron-beam lithography for creating mm-long devices with nm-scale transversal precision. A comprehensive spectral measurement for the intensity and phase distributions of the transmitted optical states is obtained with a specially designed phase-sensitive infrared microscopy integrated with a tunable diode-laser system and spectrum analyzer. On this basis, we confirm in the experiment the highly asymmetric modal transmission persisting over a broad spectral band exceeding 100 nm in the telecommunications window around 1,550 nm. Hence, we establish a substantive experimental step toward broadband non-reciprocal photonic devices based on the unique non-Hermitian dynamics.

Surface plasmon distributed feedback lasers and parity-time symmetric gratings

This paper discusses the amplification of surface plasmons (SPs) via propagation through one or two optically-pumped dipolar gain materials incorporated into the claddings adjacent to a thin metal plane or stripe. Then, cavity designs to ensure single-mode SP lasing are discussed, emphasizing distributed feedback (DFB) concepts. Single-mode DFB lasers are then implemented as a step-in-width metal stripe waveguide Bragg grating to produce distributed SP modal feedback, which in conjunction with SP modal gain, enables lasing in one SP mode. By also structuring the gain medium it is possible to interleave a gain grating with a loss grating such that a parity-time (PT) symmetric grating is enabled. Parity-time (PT) symmetric optical materials or devices are synthetic structures where the refractive index distribution is judiciously synthesized such that n(r) = n * (-r) about an axis or plane of symmetry. By altering the refractive index of the medium at a certain critical threshold, PT symmetry breaks down sharply. This breaking threshold is referred to as an exceptional point or a spontaneous phase transition, where many fascinating optical phenomena can be observed. One phenomenon of particular interest is asymmetric reflectance produced in waveguide Bragg gratings where the period of the grating satisfies this relationship. In a carefully-designed structure, the asymmetry can be ideal, i.e., the reflectance from one end is non-zero but the reflectance from the other is zero.

Active asymmetric plasmonic Bragg gratings

We discuss asymmetric reflectance in surface plasmon Bragg gratings incorporating optical gain, referred to as active asymmetric surface plasmon Bragg gratings. It is shown that balanced modulation of index and gain/loss with quarter pitch spatial shift causes unidirectional coupling between contra-propagating modes in long-range surface plasmon polariton Bragg gratings. Such gratings operate at the breaking threshold of parity-time symmetry (exceptional point). Two active asymmetric surface plasmon Bragg gratings designs are proposed and their performance is examined through modal and transfer matrix method computations.