C. Y. Ngo

Broadband terahertz plasmonic response of touching InSb disks.

The spectral characteristics of localized surface plasmons (LSPs) depend strongly on the geometry of the metal nanostructure sustaining them. [ 1 , 2 ] This fact makes the tuning of their lightharvesting and focusing abilities across the whole visible and infra-red regimes possible. Lately, the tailoring of the spectral width of LSP resonances through design has attracted much attention due to its many potential applications in technological areas as diverse as sensing, imaging and photovoltaics. [ 3 , 4 ] The excitation of Fano-like resonances in composite nanoparticles has been shown to be useful for controlling their optical properties within an extremely narrow frequency window. [ 5 ] In contrast, transformation optics (TO) has recently been proposed as a route to transfer the broad bandwidth behavior of plasmonic waveguides to nanoantennas using geometric singularities. [ 6 , 7 ]

Tuning InAs quantum dots for high areal density and wideband emission

The authors report the effect of growth temperature and monolayer coverage on areal density and photoluminescence spectral width of InAs quantum dot (QD). Areal density and spectral width were found to be strongly dependent on growth temperature and monolayer coverage, respectively. Upon proper tuning, both high areal density and large photoluminescence spectral width were obtained. Areal density of 1.5×1011cm−2 is four times higher than those previously reported, while spectral width of 136nm is the broadest spectral width obtained without any forms of band gap engineering. These results will contribute to an improvement in the performance of QD superluminescent diode.

Narrow ridge waveguide high power single mode 1.3-μm InAs/InGaAs ten-layer quantum dot lasers

Ten-layer InAs/In0.15Ga0.85As quantum dot (QD) laser structures have been grown using molecular beam epitaxy (MBE) on GaAs (001) substrate. Using the pulsed anodic oxidation technique, narrow (2 μm) ridge waveguide (RWG) InAs QD lasers have been fabricated. Under continuous wave operation, the InAs QD laser (2 × 2,000 μm2) delivered total output power of up to 272.6 mW at 10 °C at 1.3 μm. Under pulsed operation, where the device heating is greatly minimized, the InAs QD laser (2 × 2,000 μm2) delivered extremely high output power (both facets) of up to 1.22 W at 20 °C, at high external differential quantum efficiency of 96%. Far field pattern measurement of the 2-μm RWG InAs QD lasers showed single lateral mode operation.

An investigation of growth temperature on the surface morphology and optical properties of 1.3 µm InAs/InGaAs/GaAs quantum dot structures

We have investigated the surface morphology and optical properties of InAs/InGaAs/GaAs quantum dots grown at 450, 480 and 510 °C. While the performances of QD devices utilizing InAs/InGaAs/GaAs structures have been well-documented, there have not been many research efforts on the growth optimization of InAs/InGaAs/GaAs QDs. We found that, unlike InAs/GaAs QD structures, InAs/InGaAs/GaAs QD structures benefit from a lower QD growth temperature. Evidence from the photoluminescence (PL) spectra suggests a decreasing presence of nonradiative islands and a multi-modal size distribution following the decrease in the growth temperature. Strong room temperature PL emission at 1.32 µm with a narrow full-width at half-maximum (FWHM) of 27.8 meV was obtained from the InAs/InGaAs/GaAs QD structure grown at 450 °C, as verified by the large areal density and narrow dot size distribution. This work indicates the feasibility of obtaining good surface morphology and optical properties at a low growth temperature for InAs/InGaAs/GaAs QD structures.

Two-state competition in 1.3 μm multilayer InAs/InGaAs quantum dot lasers

The competition of ground state (GS) and excited state (ES) is investigated from the as-grown and thermally annealed 1.3 μm ten-layer p-doped InAs/GaAs quantum dot (QD) lasers. The modal gain competition between GS and ES are measured and analyzed around the ES threshold characteristics. Our results show that two-state competition is more significant in devices with short cavity length operating at high temperature. By comparing the as-grown and annealed devices, we demonstrate enhanced GS and suppressed ES lasing from the QD laser annealed at 600 °C for 15 s.

Structural and optical properties of stacked self-assembled InAs∕InGaAs quantum dots on graded Si1−xGex∕Si substrate

We have investigated the effects of InAs monolayer coverage on the structural and optical characteristics of stacked InAs quantum dot (QD) layers on graded Si1−xGex∕Si substrate. No preferential InAs QDs nucleation was observed along the cross-hatched lines on the graded Si1−xGex∕Si substrate. Employing alternate-beam molecular beam epitaxy, InAs QDs with areal density as high as 7×1010cm−2 was achieved. Temperature dependence of the InAs QDs optical properties is discussed. The InAs QDs show room-temperature photoluminescence at 1.3μm with full width at half-maximum of 65nm. The results are significant for potential realization of III-V QD devices on silicon-based platforms.

Temperature-dependent photoluminescence study of 1.3μm undoped InAs∕InGaAs∕GaAs quantum dots

InAs∕InGaAs∕GaAs quantum dot (QD) structures are commonly employed for 1.3μm emission. However, reduction in the thermal stability of the undoped InAs∕InGaAs∕GaAs QD lasers has been observed upon inclusion of the InxGa1−xAs strain-reducing layer. In this work, the effect of QD growth temperature on the temperature-dependent photoluminescence of the 1.3μm undoped InAs∕InGaAs∕GaAs QD samples was investigated. Due to higher confining potential of QD samples grown at lower growth temperature, enhancement in the thermal stability was observed. We believe that our findings will be beneficial to those working on improving the uncooled performance of 1.3μm undoped InAs∕InGaAs∕GaAs QD photonic devices.

Effect of thermal annealing on properties of InSbN grown by molecular beam epitaxy

We study the annealing effects on the properties of as-grown InSbN films. The annihilation of donor defects in the form of N-N interstitials is suggested by the shifting of N induced tensile strain and a decrease in free electron concentration from ∼1×1018 to device level of ∼2×1016 cm−3. These findings support the argument that N interstitials thermally dissociate into single N. Corresponding signatures for the reduced Sb antisites and N-N interstitial defects are apparent in Raman spectra. This work will benefit those working on long wavelength infrared photodetectors.

Investigation of Semiconductor Quantum Dots for Waveguide Electroabsorption Modulator

In this work, we investigated the use of 10-layer InAs quantum dot (QD) as active region of an electroabsorption modulator (EAM). The QD-EAM is a p-i-n ridge waveguide structure with intrinsic layer thickness of 0.4 μm, width of 10 μm, and length of 1.0 mm. Photocurrent measurement reveals a Stark shift of ~5 meV (~7 nm) at reverse bias of 3 V (75 kV/cm) and broadening of the resonance peak due to field ionization of electrons and holes was observed for E-field larger than 25 kV/cm. Investigation at wavelength range of 1,300–1320 nm reveals that the largest absorption change occurs at 1317 nm. Optical transmission measurement at this wavelength shows insertion loss of ~8 dB, and extinction ratio of ~5 dB at reverse bias of 5 V. Consequently, methods to improve the performance of the QD-EAM are proposed. We believe that QDs are promising for EAM and the performance of QD-EAM will improve with increasing research efforts.

InSb1−xNx/InSb/GaAs alloys by thermal annealing for midinfrared photodetection

InSb1−xNx alloys on GaAs substrates are prepared by molecular beam epitaxy and in situ thermal annealed at different temperatures in Sb ambience. X-ray diffraction indicates that the amount of N incorporation in Sb lattice sites is dependent on the annealing temperature. Low annealing temperature increases the N incorporation and extends the absorption to long wavelength infrared range. InSb1−xNx photoconductors operating near 10 μm at 77 K are realized. The measured wavelengths are in good agreement with band gaps of the alloys calculated using a two-level band anticrossing model with Varshni relation. This work will benefit those working on midinfrared photodetectors.