Jiho Sohn

Invisible plasmonic meta-materials through impedance matching to vacuum

We report on perfect transmission in two-dimensional plasmonic matamaterials in the terahertz frequency range, in which zeroth order transmittance becomes essentially unity near specific resonance frequencies. Perfect transmission may occur when the plasmonic metamaterials are perfectly impedance matched to vacuum, which is equivalent to designing an effective dielectric constant around epsilonr = -2. When the effective dielectric constant of the metamaterial is tuned towards epsilonr and the hole coverage is larger than 0.2, strong evanescent field builds up in the near field, making perfect transmission possible.

Femtosecond and picosecond nondegenerate four-wave mixing in GaAs/AlGaAs multiple quantum wells by two independently tunable lasers

We have performed femtosecond and picosecond nondegenerate two-beam and three-beam (ω1, ω1;ω2) four-wave mixing experiments by using two partially synchronized, independently tunable lasers with a simple sample-and-hold switch. When the jitter is compensated for externally, heavy- and light-hole beating is observed with these two mutually incoherent lasers, and the synchronous phases of oscillations with the detuning show that these beats are true quantum beats. We also see the strong completely off-resonant signals, and the below-bandgap signals are stronger than the signals at the continuum for equivalent detunings. This off-resonance signal has a Lorentzian dependence on detuning in both the femtosecond and the picosecond experiments. When ω2 is completely above or below the excitonic resonance, the spectrum of the diffracted beam is determined mostly by the laser spectrum, which can be explained by semiconductor Bloch equations.

Light out-coupling efficiency enhancement in organic light-emitting diodes using a multilayer stacked electrode with sol-gel processed Ta 2 O 5

We demonstrate enhanced light out-coupling efficiency of organic light-emitting diodes by applying a multilayer stacked electrode structure consisting of fast and cost-effective sol-gel processed tantalum pentoxide (Ta2O5), thin layer of Au and molybdenum trioxide (MoO3). The application of the Ta2O5/Au/MoO3 electrode can modulate the optical characteristics of the device due to the optical microcavity effect. The refractive index of the sol-gel processed Ta2O5 thin film varied depending on the annealing temperature and reached a maximum at 400 °C (n = 2.2 at 512 nm). The influence of the refractive index of the Ta2O5 layer and the thickness of the multilayer electrode stack on the optical microcavity effect was systematically investigated. The device with the Ta2O5/Au/MoO3 electrode, fabricated at an optimum condition based on the simulation result by calculating the photon flux, exhibited 52% enhancement in light out-coupling efficiency at 1000 cd/m2 and improved color stability with the viewing angle, having near-Lambertian emission.

Crystallinity dependent thermal degradation in organic solar cell

An operating solar cell undergoes solar heating; thus, the degradation study of organic photo-voltaic (OPV) with a thermal stress is required for their practical applications. We present a thermal degradation study on OPVs fabricated with photo-active polymers having different crystalline phase. Light intensity dependent analysis for different thermal stress duration is performed. In crystalline, the degradation majorly occurs due to drop in open-circuit voltage while in amorphous one it is due to drop in short-circuit current. Physical mechanism in both systems is explained and supported by the X-ray diffraction, morphological and optical characterization.

From exciton resonance to frequency mixing in the femtosecond nondegenerate four-wave mixing

We demonstrate a transition from exciton resonance to frequency mixing for the first time, in the nondegenerate femtosecond four wave mixing (FWM) of a GaAs-AlGaAs multiple quantum well. The frequency mixing which is dominant at high intensity and high detuning survives only within the pulse-width.

How to perform two color femtosecond experiments with two independently tunable, partially synchronized lasers with 5-10 ps timing jitters

Even in this day of proliferation of widely tunable femtosecond laser sources, severe limitations exist when one tries to perform two color femtosecond experiments with two independently tunable femtosecond lasers, One can generate, by clever cavity manipulation, two perfectly synchronized femtosecond pulses, but with very limited tunability. On the other hand, commercially available systems deliver completely independent tunability over the entire tuning range, but with timing jitters in the range of 5-10 ps. Here, we have developed a novel scheme which allows true two color femtosecond experiments with independent tunability of the two lasers.