Laser structures were grown by molecular beam epitaxy on n+
-GaAs (001) substrate and are similar to the structure described in [6
]. The structure consisted of an n
-doped bottom Al0.35
As layer with a thickness of 1.5 μm, a waveguide undoped GaAs layer with a thickness of 480 nm containing ten layers of In(Ga)As QD, a p
-doped upper Al0.35
As layer with a thickness of 1.5 μm, and a p+
-doped contact GaAs layer. QD ensembles were grown ten times by InAs 2.3 monolayer deposition with GaAs barrier layers with a thickness of 6 nm between QD layers. Thus, layers of self-organized In(Ga)As QDs were built into the central part of the undoped GaAs matrix. The refractive index of the upper and lower Al0.35
As layers differed from that of the central layer, which confined light within the central part of the undoped SLQD-containing region. The vertical alignment of QDs was observed by transmission electron microscopy (see Figure ) [6
Transmission electron micrographs of the cross section of the sample. The sample has ten InAs QD layers and iswith a GaAs spacer layer 6.0-nm thick between them.
Two sectional lasers were fabricated from SLQD structures. Standard lithography techniques were used to make a 5-Âµm strip forming a single-mode waveguide. The cavity length was 3.5 mm, the absorber length was 10% of the cavity length, and the sections were electrically isolated by the gap in the contact. This laser design is in fact standard and is described in various publications [2
] but differs from them in that the active layer is SLQD, formed by ten QD layers and thin barrier layers between them. The devices were mounted on a copper heat sink; all measurements were performed at room temperature.
Absorption measurements were provided as described in [8
] using this device. The experimental setup is shown on Figure . A sample with two equal sections was used. The emission in waveguide was excited by the current injection in one of the sections; the pumping current is far below the threshold current. On the first stage (Figure ), the emission spectrum (I0
) from the closest section to the monochromator section (the right setion on Figure ) is measured, nothing is applied to the other section. Thus, the spectrum of source light is obtained. Next, the closest section is reverse-biased, and the other section is pumped with the same current as the right section in the first stage (Figure ). In this waveguided setup, radiation from the left section penetrates into the right section almost without loss, then experiences partial absorption by SLQDs in the right section, and reaches measuring setup through low-reflectance facet. Hence, the emission spectra of passed light through the absorption section (I0
) is obtained. Since both sections have the same length and the optical scheme of the experimental setup was not changed, one can assume that the intensity of the emission reaching the absorber section on the second stage is approximately equal to the intensity measured on the first stage. This allows the derivation of the SLQD absorption spectra in absolute values.
Figure 2 Schematic of a double-section laser design and the experimental setup for SLQD absorption measurements. (a) Schematic of a double-section laser design with amplifying and absorbing sections, used to measure the electroluminescence, and absorption spectra (more ...)
PML investigation was under pulsed current injection (pulse duration 1 μs) and direct current (DC) reverse bias. An autocorrelation setup based on a Michelson interferometer was used for pulse duration measurements, controlled by an oscilloscope with a 50-GHz bandwidth, an electrical spectrum analyzer with a 22-GHz bandwidth, and a 20-GHz photodetector. The devices were mounted to copper heat sink; all measurements were done at room temperature.