Yong-Shik Park

Photoinduced handedness switching in terahertz chiral metamolecules

Switching the handedness, or the chirality, of a molecule is of great importance in chemistry and biology, as molecules of different handedness exhibit dramatically different physiological properties and pharmacological effects. Here we experimentally demonstrate handedness switching in metamaterials, a new class of custom-designed composites with deep subwavelength building blocks, in response to external optical stimuli. The metamolecule monolayer flips the ellipticity and rotates the polarization angle of light in excess of 10° under optical excitation, a much stronger electromagnetic effect than that of naturally available molecules. Furthermore, the experimentally demonstrated optical switching effect does not require a structural reconfiguration, which is typically involved in molecular chirality switching and is inherently slow. The handedness switching in chiral metamolecules allows electromagnetic control of the polarization of light and will find important applications in manipulation of terahertz waves, such as dynamically tunable terahertz circular polarizers and polarization modulators for terahertz radiations.

Split ring resonator sensors for infrared detection of single molecular monolayers

We report a surface enhanced molecular detection technique with zeptomole sensitivity that relies on resonant coupling of plasmonic modes of split ring resonators and infrared vibrational modes of a self-assembled monolayer of octadecanthiol molecules. Large near-field enhancements at the gap of split ring resonators allow for this resonant coupling when the molecular absorption peaks overlap spectrally with the plasmonic resonance. Electromagnetic simulations support experimental findings.

Compact Magnetic Antennas for Directional Excitation of Surface Plasmons

Plasmonics is considered as one of the most promising candidates for implementing the next generation of ultrafast and ultracompact photonic circuits. Considerable effort has been made to scale down individual plasmonic components into the nanometer regime. However, a compact plasmonic source that can efficiently generate surface plasmon polaritons (SPPs) and deliver SPPs to the region of interest is yet to be realized. Here, bridging the optical antenna theory and the recently developed concept of metamaterials, we demonstrate a subwavelength, highly efficient plasmonic source for directional generation of SPPs. The designed device consists of two nanomagnetic resonators with detuned resonant frequencies. At the operating wavelength, incident photons can be efficiently channeled into SPP waves modulated by the electric field polarization. By tailoring the relative phase at resonance and the separation between the two nanoresonators, SPPs can be steered to predominantly propagate along one specific direction. This novel magnetic nanoantenna paves a new way to manipulate photons in the near-field, and also could be useful for SPP-based nonlinear applications, active modulations, and wireless optical communications.

Maskless Plasmonic Lithography at 22 nm Resolution

Optical imaging and photolithography promise broad applications in nano-electronics, metrologies, and single-molecule biology. Light diffraction however sets a fundamental limit on optical resolution, and it poses a critical challenge to the down-scaling of nano-scale manufacturing. Surface plasmons have been used to circumvent the diffraction limit as they have shorter wavelengths. However, this approach has a trade-off between resolution and energy efficiency that arises from the substantial momentum mismatch. Here we report a novel multi-stage scheme that is capable of efficiently compressing the optical energy at deep sub-wavelength scales through the progressive coupling of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Combining this with airbearing surface technology, we demonstrate a plasmonic lithography with 22 nm half-pitch resolution at scanning speeds up to 10 m/s. This low-cost scheme has the potential of higher throughput than current photolithography, and it opens a new approach towards the next generation semiconductor manufacturing.

Optical negative refraction by four-wave mixing in thin metallic nanostructures

The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.

Reflective interferometry for optical metamaterial phase measurements

The unambiguous determination of optical refractive indices of metamaterials is a challenging task for device applications and the study of new optical phenomena. We demonstrate here simple broadband phase measurements of metamaterials using spectrally and spatially resolved interferometry. We study the phase response of a π-shaped metamaterial known to be an analog to electromagnetically induced transparency. The measured broadband interferograms give the phase delay or advance produced by the metamaterial in a single measurement. The presented technique offers an effective way of characterizing optical metamaterials including nonlinear and gain-metamaterial systems.

Far-field measurement of ultra-small plasmonic mode volume

Light-matter interaction can be greatly enhanced in nano-scale plasmonic cavities with tightly confined optical mode, where the mode volume determines the interaction strength. The experimental determination of the mode volume of plasmonic elements is therefore of fundamental importance. Mapping the electric field distribution using near-field scanning optical microscopy (NSOM) may disturb the field distribution hence prevent a reliable measurement of the mode volume. Here, we develop a non-pertubative technique to experimentally determine the mode volume of plasmonic resonators in the far field through a unique optical force method.

Fluorescence enhancement by a two-dimensional dielectric annular Bragg resonant cavity

We show that photons can be efficiently extracted from fluorescent molecules, utilizing the strongly enhanced local field of a two-dimensional dielectric annular Bragg resonant cavity. Due to the diffraction and constructive interference together with the annular focusing, the periodic ring structure converts the normal incident light into planar guided modes and forms a hot spot at the center of the structure. Theoretically, the field can be enhanced more than 40 times, which leads to the averaged 20-fold enhancement of the fluorescence signal observed in experiments. Compared with fluorescence enhancement by plasmonic structures, this dielectric approach does not suffer from pronounced quenching that often occurs near metallic structures. These results not only can be applied as ultrasensitive sensors for various biological systems, but also have broad potential applications, such as optical trapping and fluorescent microscopy.