Se-Heon Kim

Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Bottom-up Photonic Crystal Lasers

The directed growth of III-V nanopillars is used to demonstrate bottom-up photonic crystal lasers. Simultaneous formation of both the photonic band gap and active gain region is achieved via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. The nanopillars implement a GaAs/InGaAs/GaAs axial double heterostructure for accurate, arbitrary placement of gain within the cavity and lateral InGaP shells to reduce surface recombination. The lasers operate single-mode at room temperature with low threshold peak power density of ∼625 W/cm2. Cavity resonance and lasing wavelength is lithographically defined by controlling pillar pitch and diameter to vary from 960 to 989 nm. We envision this bottom-up approach to pillar-based devices as a new platform for photonic systems integration.

Modelling and fabrication of GaAs photonic-crystal cavities for cavity quantum electrodynamics

In this paper, we present recent progress in the growth, modelling, fabrication and characterization of gallium arsenide (GaAs) two-dimensional (2D) photonic-crystal slab cavities with embedded indium arsenide (InAs) quantum dots (QDs) that are designed for cavity quantum electrodynamics (cQED) experiments. Photonic-crystal modelling and device fabrication are discussed, followed by a detailed discussion of different failure modes that lead to photon loss. It is found that, along with errors introduced during fabrication, other significant factors such as the presence of a bottom substrate and cavity axis orientation with respect to the crystal axis, can influence the cavity quality factor (Q). A useful diagnostic tool in the form of contour finite-difference time domain (FDTD) is employed to analyse device performance.

Photoinduced luminescence enhancement from anodically oxidized porous Si

We have investigated the phenomenon of photoluminescence (PL) increase in anodically oxidized porous Si with increasing laser illumination time by transmission Fourier transform infrared spectroscopy (FTIR), PL spectroscopy, and electron paramagnetic resonance. The adsorption of oxygen without hydrogen loss was observed during laser illumination by FTIR. The PL intensity increased linearly, while the dangling bond (DB) density decreased with increasing illumination time. By assuming that the decrease of DB density has a linear response to the illumination time, we identify that the change in DB density is mainly responsible for the observed PL increase after laser illumination.

Room Temperature, Continuous-Wave Coupled-Cavity InAsP/InP Photonic Crystal Laser with Enhanced Far-field Emission Directionality

We demonstrate room temperature, continuous-wave lasing with enhanced far field emission directionality in coupled-cavity photonic crystal lasers, made with InAsP/InP quantum well material. These surface-emitting lasers can have a very low effective threshold power of 14.6 μW, with a linewidth of 60 pm, and 40% of the surface emitted power concentrated within a small divergence angle of ±30°.

Hybrid Single Quantum Well InP/Si Nanobeam Lasers for Silicon Photonics

We report on a hybrid InP/Si photonic crystal nanobeam laser emitting at 1578 nm with a low threshold power of ~14.7 μW. Laser gain is provided from a single InAsP quantum well embedded in a 155 nm InP layer bonded on a standard silicon-on-insulator wafer. This miniaturized nanolaser, with an extremely small modal volume of 0.375(λ/n)(3), is a promising and efficient light source for silicon photonics.

Photonic Crystal Nanocavity Laser in an Optically Very Thick Slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that when T≥800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route toward an efficient in-plane laser for photonic integrated circuits.

From Vertical-Cavities to Hybrid Metal/Photonic-Crystal Nanocavities: Towards High-Efficiency Nanolasers

We provide a numerical study showing that a bottom reflector is indispensable to achieve unidirectional emission from a photonic-crystal (PhC) nanolaser. First, we study a PhC slab nanocavity suspended over a flat mirror formed by a dielectric or metal substrate. We find that the laser’s vertical emission can be enhanced by more than a factor of 6 compared with the device in the absence of the mirror. Then, we study the situation where the PhC nanocavity is in contact with a flat metal surface. The underlying metal substrate may serve as both an electrical current pathway and a heat sink, which would help achieve continuous-wave lasing operation at room temperature. The design of the laser emitting at 1.3 μm reveals that a relatively high cavity Q of over 1000 is achievable assuming room-temperature gold as a substrate. Furthermore, linearly polarized unidirectional vertical emission with the radiation efficiency over 50% can be achieved. Finally, we discuss how this hybrid design relates to various plasmonic cavities and propose a useful quantitative measure of the degree of the “plasmonic” character in a general metallic nanocavity.

A Photonic-Crystal Optical Antenna for Extremely Large Local-Field Enhancement

We propose a novel design of an all-dielectric optical antenna based on photonic-band-gap confinement. Specifically, we have engineered the photonic-crystal dipole mode to have broad spectral response (Q ~70) and well-directed vertical-radiation by introducing a plane mirror below the cavity. Considerably large local electric-field intensity enhancement ~4,500 is expected from the proposed design for a normally incident planewave. Furthermore, an analytic model developed based on coupled-mode theory predicts that the electric-field intensity enhancement can easily be over 100,000 by employing reasonably high-Q (~10,000) resonators.

High-Q Impurity Photon States Bounded by a Photonic Band Pseudogap in an Optically Thick Photonic Crystal Slab

We show that, taking a two-dimensional photonic crystal slab system as an example, surprisingly high quality factors (Q) over 10^5 are achievable, even in the absence of a rigorous photonic band gap. We find that the density of in-plane Bloch modes can be controlled by creating additional photon feedback from a finite-size photonic-crystal boundary that serves as a low-Q resonator. This mechanism enables significant reduction in the coupling strength between the bound state and the extended Bloch modes by more than a factor of 40.