J. W. Jang

Continuous-wave operation of 1.5μm InGaAs∕InGaAsP∕InP quantum dot lasers at room temperature

Continuous-wave operation at room temperature from InGaAs∕InGaAsP∕InP quantum dot (QD) laser diodes (LD) has been achieved. A ridge waveguide QD LD with 7 QD-stacks in the active region lases at 1.503μm at 20°C and that with 5 QD-stacks lases at 1.445μm at room temperature. The shift in lasing wavelength is believed to be due to the difference in the quantized energy states involved in producing gain for lasing. With smaller number of QD stacks and shorter cavity length, the lasing wavelength shifts to shorter wavelength indicating that more of higher excited states are involved in producing gain. By increasing the number of QD stacks to 15, lasing at 1.56μm has been achieved under pulsed mode.

Room temperature operation of InGaAs∕InGaAsP∕InP quantum dot lasers

The growth conditions for InGaAs∕InGaAsP∕InP quantum dots (QDs) have been optimized and QDs of high luminescence efficiency and the room temperature operation of QD lasers emitting at ∼1.5μm have been demonstrated. Lattice-matched InGaAsP (λg=1.0–1.1μm) was used as a barrier layer for the InGaAs QDs and the emission wavelength was controlled by the QD composition. High-density InGaAs QDs with an areal density as high as 1.13×1011cm−2 have been grown. The integrated and peak intensity of the photoluminescence (PL) spectra at room temperature are as high as 25% and 10% of those at 10K, respectively. The room temperature PL peak intensity is about 50% that of a high-quality InGaAs∕InP quantum well. Room temperature, pulsed operation at ∼1.5μm has been achieved from broad area lasers with a 1mm cavity length. Threshold current density per QD stack of ∼430A∕cm2 is measured for the five-, seven-, and ten-stack QD lasers.

Gain characteristics of InAs∕InGaAsP quantum dot semiconductor optical amplifiers at 1.5μm

The authors have fabricated ridge waveguide quantum dot (QD) semiconductor optical amplifiers (SOAs) on InP substrates that operate in the 1.5μm region. The active layer consists of InAs∕InGaAsP QD layers with a high dot density, but which still have good isolation between dots in the lateral and vertical directions, as confirmed by time-resolved photoluminescence measurements. One of these QD SOAs exhibited a fiber-to-fiber gain of 22.5dB and a chip gain of 37dB at 1.51μm. The spectral gain shape was found to be maintained for variations of the peak gain from 12to22dB, reflecting the zero-dimensional density of states at room temperature.

Photoluminescence and lasing characteristics of InGaAs∕InGaAsP∕InP quantum dots

The InGaAs quantum dots (QDs) were grown with InGaAsP(λg=1.0–1.1μm) barrier, and the emission wavelength was controlled by the composition of InGaAs QD material in the range between 1.35 and 1.65μm. It is observed that the lateral size increases and the height of the QDs decreases with the increase in relative concentration of trimethylgallium to trimethylindium supplied during InGaAs QD growth. It is seen that the higher concentration of group III alkyl supply per unit time leads to higher QD areal density, indicating that the higher concentration causes more QDs to nucleate. By optimizing the growth conditions, the QDs emitting at around 1.55μm were grown with an areal density as high as 8×1010cm−2. The lasing action between the first excited subband states at the wavelength of 1.488μm has been observed from the ridge waveguide lasers with five QD stacks up to 260K. The threshold current density of 3.3kA∕cm2 at 200K and a characteristic temperature of 118K were measured.

Unambiguous observation of electronic couplings between InGaAs∕InGaAsP quantum dots emitting at 1.5μm

We have unambiguously estimated the vertical and lateral electronic couplings between quantum dots (QDs) by comparing the carrier lifetimes at different energy positions inside the ground state band. InGaAs∕InGaAsP QDs on InP(100) substrate give photoluminescence around 1.55μm and have the dot density over 1011∕cm2. The measured carrier lifetimes are almost the same across the entire photoluminescence band, indicating negligible lateral electronic coupling between QDs at this high dot density. However, for a QD sample with the 15nm barrier spacing between QD layers the lifetime increases with increasing wavelength, clearly indicating the significant vertical electronic coupling between QDs.

Reliable strain determination method for InGaAsN/GaAs quantum wells using a simple photoluminescence measurement

We present a reliable method for determining the strain in InGaAsN quantum wells. The method uses the fact that the splitting between heavy hole and light hole energy levels depends mostly on the strain. We also found that the strain was largely relaxed in an In0.34Ga0.66As/GaAs quantum well, but recovered when a small amount of nitrogen was added to the In0.34Ga0.66As layer.

An efficient in-plane energy level shift in InAs/InGaAsP/InP quantum dots by selective area growth

Selective area growth was adopted to grow high-quality quantum dots (QDs) of different energy levels on the same plane at 1.5 μm. At room temperature, the photoluminescence (PL) peak of InAs/InGaAsP QDs on InP substrate was shifted from 1445 to 1570 nm for sample 1 (from 1385 to 1485 nm for sample 2) in a plane, with a PL intensity comparable to those of regular samples grown without dielectric patterns. The dot shape was a round dome, with the density reduced by 28% and the height increased by 17%. Time-resolved PL indicated that the selectively grown QDs behaved similarly to regular QDs. These results open up a practical method for in-plane integration of QD devices.

Strong photoluminescence at 1.3μm with a narrow linewidth from nitridized InAs∕GaAs quantum dots

Nitridized quantum dots (QDs) were prepared by metal-organic chemical vapor deposition. These QDs all showed strong photoluminescence (PL) emission at 1.3μm at room temperature, narrow spectral widths of 30meV, and large separations of 98meV between the ground and first excited states. Interestingly, the PL peak positions of the nitridized QDs were all around 1.3μm, despite the QDs having been prepared using significantly different amounts of nitrogen. Time-resolved PL revealed no electronic coupling between the QDs. These properties could potentially make these nitridized QDs very useful candidates for the fabrication of devices emitting at 1.3μm.

Gain and high speed transmission characteristics of InAs/InP quantum dot Semiconductor Optical Amplifiers

High performance quantum dot semiconductor optical amplifiers with penalty free transmission of 10 Gb/s signal with gain of -8.2 dB and 40 Gb/s transmission with pattern-effect-free, wide open eye have been demonstrated.

Characteristics of InAs/InGaAsP quantum dot laser diodes lasing at 1.55um

We have measured I-V, L-I curves and electroluminescence spectra from InAs/InGaAsP quantum dot (QD) laser diodes (LDs) to investigate how to optimize QD LDs for high output power. The slope of an L-I curve, which is proportional to the differential quantum efficiency, decreased rapidly after lasing due to heat in cw mode. Since the heat problem is not significant in pulse mode, the efficiency is constant up to a rather high current level. In spite of the heat problem, the maximum output power is over 79 mW from a single facet in cw mode at 20 °C. At the same temperature, the lowest threshold current is 132 mA with cavity length, width and QD layers of 500 um, 5 um and 7 stacks, respectively. The characteristic temperatures of QD LD are 188 K and 111 K under pulse and cw mode, respectively. Typical lasing wavelength is around 1.55 um. The slope efficiency, internal loss and gain are 0.368 W/A, 5.2 cm-1 and 15 cm-1, respectively.