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Room temperature passive mode-locked laser based on InAs/GaAs quantum-dot superlattice
© Sobolev et al.; licensee Springer. 2012
Received: 10 July 2012
Accepted: 22 September 2012
Published: 2 October 2012
Passive mode-locking is achieved in two sectional lasers with an active layer based on superlattice formed by ten layers of quantum dots. Tunnel coupling of ten layers changes the structural polarization properties: the ratio between the transverse electric and transverse magnetic polarization absorption coefficients is less by a factor of 1.8 in the entire electroluminescence spectrum range for the superlattice.
In recent years, intense efforts have been devoted to the studies of effects of tunneling coupling between electron states in semiconductor heterostructures with quantum dots (QDs), which offer much promise in the development of high-speed lasers , optical modulators , and amplifiers . For optical amplifiers and modulators, it is desirable to have polarization-independent characteristics. Thus, dependencies of gain and absorption have been studied in quantum well structures  and QDs . However, in standard uncoupled QD structures, the absorption coefficient at the lasing wavelength for transverse electric (TE)-polarized light differs by an order . It is known that in structures with coupled QDs, the intensity of transverse magnetic (TM) polarization increases with the number of QD layers [2, 5, 6].
Direct current modulation of semiconductor lasers does not meet the needs of modern high-speed communication lines, so systems consisting of a laser and modulator are used. As more broadband alternative to the direct current modulation can be laser with integrated electro-optical modulator based on the Stark effect, high-speed performance of the Stark modulator is fundamentally limited by physical processes, namely, carrier escape from QDs and carrier removal from the p‐n junction area. Because the same processes are crucial for the passive mode-locking (PML) regime, the modulation frequency ceiling can be determined by the largest feasible PML frequency in a laser fabricated from the same structure. It should be noted that the implementation of two sectional PML lasers is technically easier than creating a high-speed modulator, because there is no need to eliminate parasitic capacitance and inductance. The modulation frequency ceiling can be determined by the largest feasible frequency of the of the PML regime in a laser fabricated from the same structure.
In this communication, we report on a room-temperature study a ten-layer system of tunnel-coupled In(Ga)As/GaAs QD. As shown in [7, 8], the structure with ten tunnel-coupled layers of In(Ga)As/GaAs QDs exhibits the Wannier-Stark effect and is a quantum dot superlattice (QDSL). We have observed the EL and absorption spectra for light polarized in the plane perpendicular to the growth axis (x and y) in the same spectral range as that for light polarized along the growth direction (z) of the structure. No transitions involving light holes were observed in the electroluminescence and absorption spectra. The observed behavior of the measured signals allows one to conclude that the optical transitions for light polarized in the plane perpendicular to the growth axis and in the plane along the structure growth direction involve ground states of heavy holes, whose wave functions have, in addition to the x and y components, a z component. In this system, the ratio between the light absorption coefficients for TE and TM polarizations is close to 1 in contrast to structures with unbounded QDs, where the ratio is about 10. This makes it a promising structure for optical polarization-independent modulators used in fiber-optic communication lines (FOLs). Two sectional PML laser diodes with an absorbing section acting as modulator were made from the SLQD structure. It shows the fundamental possibility of implementing a laser and modulator in a monolithically integrated design.
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, 8–10] 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.
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.
Results and discussion
At the lasing wavelength in the PML regime, the absorption coefficient for TM-polarized light is only 1.6 times smaller than that for TE polarization (Figure 4). The maximum ratio of absorption coefficients reaches 1.8 and at energies less than 1.012 eV and more than 1.156 eV; absorption for TM polarization is more (Figure 4, lines 1 and 2). The electroluminescence spectrum width in the laser structure at a current density of J = 0.3J th is about 130 nm, where J th is the threshold current density. It is due to two factors: the QD size dispersion and energy level splitting with QD coupling. The absorption coefficient front is about 90 nm, which is comparable to the electroluminescence spectrum width.
In conclusion, based on a structure containing ten layers of coupled QDs, two sectional lasers were created in which PML realization needs a rather small reverse bias on the absorber section. The absorption value both for TM and TE polarizations exceeds 50 cm–1, which is sufficient for modulators used in FOLs. In contrast to the structure with uncoupled QDs, where TM polarization can be neglected, the luminescence intensity and absorption coefficients for TE and TM polarizations in SLQD are comparable.
This study was supported by the Russian Foundation for Basic Research (12-02-00388-а), grant of Russian Academy of Sciences and grant for young researchers of SPb government.
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