Y. D. Jang

Gain recovery in a quantum dot semiconductor optical amplifier and corresponding pattern effects in amplified optical signals at 1.5 μm

Fast gain recovery observed in quantum-dot semiconductor-optical-amplifiers (QDSOAs) is useful for amplifying high-speed optical signals. The small but finite slow recovery component can deteriorate the signal amplification due to the accumulation of gain saturation during 10 Gb/s operation. A study of the gain recoveries and pattern effects in signals amplified using a 1.5 μm InAs/InGaAsP QDSOA reveals that the gain recovery is always fast, and pattern-effect-free amplification is observed at the ground state. However, at the excited state, the slow component increases with the current, and significant pattern effects are observed. Simulations of the pattern effects agreed with the observed experimental trends.

Design for an efficient single photon source based on a single quantum dot embedded in a parabolic solid immersion lens

We have designed a single photon emitter based on a single quantum dot embedded within a single mode parabolic solid immersion lens (pSIL) and a capping low-index pSIL. Numerical simulations predicted that the emitter performance should exhibit a high photon collection efficiency with excellent far-field emission properties, broadband operation, and good tolerance in its geometric (spatial configuration) parameters. Good geometric tolerance in a single-mode pSIL without yielding significant losses in the photon collection efficiency is advantageous for device fabrication. The low-index top pSIL layer provided this structure with a high photon collection efficiency, even in the case of a small numerical aperture (NA). Photon collection efficiencies of 64% and 78% were expected for NA values of 0.41 and 0.5, respectively. In addition to the benefits listed above, our combined pSIL design provided excellent broadband performance in a 100 nm range.

Estimation of relative defect densities in InGaN laser diodes by induced absorption of photoexcited carriers.

Defects are one of the most important factors influencing the optical properties of groups III-V nitride semiconductor materials and thereby their applicability to light-emitting diodes. In this paper, we demonstrate that it is possible to estimate the presence of defects in InGaN laser diodes by performing pump-probe measurements and observing the induced absorptions. We have confirmed that the induced absorption originates from defects by performing experiments in which the pump intensity is varied. We believe that our method provides a powerful tool for evaluating the optical quality of InGaN materials before processing them into device fabrications.

Deterministic coupling of epitaxial semiconductor quantum dots to hyperbolic metamaterial

Hyperbolic metamaterial (HMM) can enhance the radiative recombination rate of a near-field coupled emitter due to its large photonic density of states (PDOS). Thus far, this enhancement has only been demonstrated qualitatively on account of the difficulties to achieve an equal emitter–HMM distance as well as the same polarization for all emitters. Here, we report on the deterministic coupling of epitaxially grown quantum dots (QDs) with HMM, for which a quantitative investigation of the rate enhancement is possible. Advantages of epitaxial QDs over other emitters to investigate HMM coupling include a precise QD–HMM distance, the same polarization direction, and single-exponential decay of photoluminescence. In order to isolate metal-related effects, we have fixed the thickness of the silver (Ag) layer and have varied only that of the germanium (Ge) layer in the HMM to investigate the effect of the PDOS, which depends on the dispersion relation, which in turn depends on the thickness ratio. The recombination rate enhancement, as measured by lifetime reduction, varied from 2.2 to 4.2, depending on the thickness ratio of Ag and Ge in the HMM. These findings match well with simulation results, clearly supporting the role of HMM in the rate enhancement. The coupling of high-quality epitaxial QDs with HMM, demonstrated here for the first time, to the best of our knowledge, may bring about diverse future applications.

Analysis of In/Ga Inter-Diffusion Effect on the Thermodynamical Properties of InAs Quantum Dot

Debye temperature is an important thermodynamical factor in quantum dots (QDs); it can be used to determine the degree of homogeneity of a QD structure as well as to study the interdiffusion mechanism during growth. Direct estimation of the Debye temperature can be obtained using the Varshni relation. The Varshni relation is an empirical formula that can interpret the change of emission energy with temperature as a result of phonon interaction. On the other hand, phonons energy can be calculated using the Fan Expression. The Fan expression and Varshni relation are considered equivalent at a temperature higher than Debye temperature for InAs quantum dot. We investigated InAs quantum dot optically, the photoluminescence spectra and peak position dependency on temperature has been discussed. We applied a mathematical treatment using Fan expression, and the Varshni relation to obtain the Debye temperature and the phonon energy for InAs quantum dots sample. Debye temperature increase about double compared to bulk crystal. We concluded that the In/Ga interdiffusion during growth played a major role in altering the quantum dot thermodynamical parameters.

Comparison of Carrier Lifetime for InAs Quantum Dots in the Quaternary Barriers on InP Substrate

We have found that the carrier lifetime of an InAs/InGaAsP quantum dot (QD) on an InP substrate is twice that of an InAs/InAlGaAs QD on the same substrate, although the ground‐state energy levels and barrier heights of these QDs are comparable. The carrier lifetime of the InAs/InAlGaAs QD within the ground state PL band is shorter as the detection wavelength is longer. On the contrary, it shows the same carrier lifetime for the InAs/InGaAsP QD. The difference is interpreted in terms of the smaller conduction band‐offset in InAs/InGaAsP QDs compared to InAs/InAlGaAs QDs.

Direct Observation of Electronic Couplings between 1.5 μm Emitting InGaAs/InGaAsP Quantum Dots on InP

We have estimated the vertical and lateral electronic couplings between 1.5 μm emitting InGaAs/InGaAsP quantum dots (QDs) by the carrier lifetime dependence on energy position over the ground state photoluminescence (PL) band. The areal dot density is over 1011/cm2. For a QD sample with 40 nm barrier spacing, the measured carrier lifetimes are almost the same across the entire PL band, indicating the negligible lateral and vertical electronic couplings between QDs at this high dot density. However, for a QD sample with the 15 nm barrier spacing between QD layers the lifetime increases with increasing wavelength, clearly indicating the significant vertical electronic coupling between QDs.