Jong-Moo Lee

Temperature Dependence of Silicon Nanophotonic Ring Resonator With a Polymeric Overlayer

In this paper, we investigate the temperature dependence of a silicon-on-insulator-based silicon nanophotonic ring resonator covered with a polymeric overlayer. Temperature-dependent wavelength shift is measured to be as low as 5 pm/degC for the TM mode in a silicon ring resonator composed of a 500 times 220 nm2 channel waveguide. We also show through simulations and experiments that the temperature dependence can be reduced for the TE mode or for both the TE and TM modes by adjusting the mode volume of a silicon nanophotonic waveguide.

Controlling temperature dependence of silicon waveguide using slot structure.

We show that the temperature dependence of a silicon waveguide can be controlled well by using a slot waveguide structure filled with a polymer material. Without a slot, the amount of temperature-dependent wavelength shift for TE mode of a silicon waveguide ring resonator is very slightly reduced from 77 pm/ degrees C to 66 pm/ degrees C by using a polymer (WIR30-490) upper cladding instead of air upper cladding. With a slot filled with the same polymer, however, the reduction of the temperature dependence is improved by a pronounced amount and can be controlled down to -2 pm/ degrees C by adjusting several variables of the slot structure, such as the width of the slot between the pair of silicon wires, the width of the silicon wire pair, and the height of the silicon slab in our experiment. This measurement proves that a reduction in temperature dependence can be improved about 8 times more by using the slot structure.

Crosstalk Reduction in a Shallow-Etched Silicon Nanowire AWG

We introduced shallow-etched silicon nanowire into the arrayed waveguides to decrease the random phase error due to the core width fluctuation. A fairly improved crosstalk value of 18 dB was achieved in the fabricated arrayed-waveguide grating with the on-chip loss of 3 dB.

Sub-dB/cm propagation loss in silver stripe waveguides

We demonstrate sub-dB/cm propagation losses in polymer-based silver stripe waveguides at the wavelength of 1.31 microm. The silver stripe waveguides were fabricated in a low-loss fluorinated polymer clad. To form uniform metal stripe patterns, which are essential for reducing propagation loss, we developed a lift-off process using double layers of photoresist and SiNx. A propagation loss of less than 1.0 dB/cm was obtained with the 11- nm-thick silver stripes in the width range of 1.5 - 4.5 microm. A coupling loss of approximately 1.0 dB with a polarization maintaining single mode fiber was achieved for a width of 4.5 microm. For a width of 2.0 microm, we recorded a minimum propagation loss of 0.4 dB/cm, which is comparable with that of dielectric multi-mode waveguides.

Effects of two-step Mg doping in p-GaN on efficiency characteristics of InGaN blue light-emitting diodes without AlGaN electron-blocking layers

A light-emitting diode (LED) structure containing p-type GaN layers with two-step Mg doping profiles is proposed to achieve high-efficiency performance in InGaN-based blue LEDs without any AlGaN electron-blocking-layer structures. Photoluminescence and electroluminescence (EL) measurement results show that, as the hole concentration in the p-GaN interlayer between active region and the p-GaN layer increases, defect-related nonradiative recombination increases, while the electron current leakage decreases. Under a certain hole-concentration condition in the p-GaN interlayer, the electron leakage and active region degradation are optimized so that high EL efficiency can be achieved. The measured efficiency characteristics are analyzed and interpreted using numerical simulations.

Efficiency and Electron Leakage Characteristics in GaN-Based Light-Emitting Diodes Without AlGaN Electron-Blocking-Layer Structures

The authors investigate efficiency and electron leakage characteristics in GaN-based light-emitting diodes (LEDs) without AlGaN electron-blocking-layer (EBL) structures. Both simulation and electroluminescence (EL) measurement results show that the internal quantum efficiency decreases rapidly as the thickness of an undoped GaN interlayer between active layers and a p-GaN layer increases, which is caused by electron leakage from active layers to the p-GaN due to inefficient hole injection. However, photoluminescence (PL) measurement results show that the quality of active layers deteriorates as the interlayer thickness decreases. The EL and PL results imply that the optimization of the undoped GaN interlayer thickness is important for achieving high internal quantum efficiency in AlGaN-EBL-free LEDs.

Polymer wavelength channel selector composed of electrooptic polymer switch array and two polymer arrayed waveguide gratings

An all-polymer wavelength channel selector is proposed and fabricated using chip-to-chip bonding of 16-channel electrooptic polymer switch array between two polymer arrayed waveguide gratings for the first time to our knowledge. A low power penalty of 0.1 dB at 10 Gb/s shows its potential as a wavelength channel selector for a packet switching system. Further efforts, however, should be paid to improve upon the large insertion loss of about 28 dB and bias phase drift problem for the proposed polymer channel selector to be a practical device.

Low bending loss metal waveguide embedded in a free-standing multilayered polymer film

Very low vertical bending loss is demonstrated in a flexible metal waveguide. The waveguide consists of an 8 nm-thick and 68 mm-long Ag strip embedded in a free-standing multilayered low-loss polymer film. The polymer film is composed of a 10 microm-thick inner cladding with a refractive index of 1.524, and a pair of 20 microm-thick outer claddings which both have a refractive index of 1.514, resulting in a total thickness of 50 microm. The measured vertical bending loss is lower than 0.3 dB/180 masculine at a wavelength of 1310 nm for the bending radii down to 2 mm.

Silver Stripe Optical Waveguide for Chip-to-Chip Optical Interconnections

Polymer-based silver stripe optical waveguides were developed for chip-to-chip optical interconnections, and their optical characteristics were investigated. The lowest propagation loss of 0.8 dB/cm was achieved with a 14-nm-thick 1.5-mum-wide stripe, at a wavelength of 1.3 mum . The 2.5-Gb/s optical signals were successfully transmitted via 10-cm-long silver stripe optical waveguides.

Highly fluorinated and photocrosslinkable liquid prepolymers for flexible optical waveguides

We evaluate highly fluorinated liquid prepolymers and their UV-cured films for flexible optical waveguide applications. Two liquid prepolymers, 2,3,5,6,2′,3′,5′,6′-octafluoro-4,4′-bis-[2-{2-[1,1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-2,2-difluoro-ethoxy]-biphenyl (7) and 2,3,5,6,2′,3′,5′,6′-octafluoro-4,4′-bis-[2-(2-{2-[1,1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-difluoro-ethoxy]-biphenyl (8), are synthesized with decafluorobiphenyl, 2,3,4,5,6-pentafluoro styrene, and fluorinated triethylene glycol or fluorinated tetraethylene glycol. The prepolymers are photocurable to give flexible and transparent films. The refractive indices of the films made from the prepolymers are controlled with their blending composition, 1.417–1.437 at a wavelength of 1.31 μm. The optical losses of the films prepared from prepolymers 7 and 8 are 0.25 and 0.23 dB cm−1 at a wavelength of 1.31 μm, respectively. The films are thermostable at temperatures over 400 °C. The ultimate stress, initial modulus, and elongation of the films prepared from prepolymers 7 and 8 are 17.1 MPa, 1.25 GPa, and 1.5% and 12.3 MPa, 0.44 GPa, and 6.9%, respectively. Optical waveguides fabricated from the prepolymers are flexible without any substrate film and the optical losses at a bending radius of 1.5 mm are under 0.2 dB. We identify the liquid prepolymers as possible good candidate materials for flexible optical waveguide applications by studying the optical transmission in flexible optical waveguides fabricated from them.