Pawel Mackowiak

Some aspects of designing an efficient nitride VCSEL resonator

The optical model of vertical-cavity surface-emitting lasers (VCSELs) based on the effective frequency method is applied to formulate some guidelines useful for designers of nitride VCSEL resonators. Various materials for resonator distributed Bragg reflectors mirrors are investigated, namely SiO2, TiO2, HfO2, MgO, Y2O3, ZrO2, GaN, AlN, and AlGaN. For pulse-operating nitride VCSELs, both dielectric resonator mirrors (preferably SiO2/TiO2 stacks) are recommended, whereas in the case of continuous-wave operation, the bottom dielectric mirror should be replaced by a semiconducting one (e.g. GaN/Al0.15Ga0.85N stack) to enhance heat extraction. It is shown that the active region radius in the analysed lasers should not be less than 5 µm. The simulation also revealed that the barrier width has a strong influence on the threshold gain. Some thermoelectrical aspects of design have also been addressed.

Temperature and thickness dependence of steam oxidation of AlAs in cylindrical mesa structures

The Deal-Grove model of thermal oxidation kinetics is adapted to cylindrically symmetric mesa structures and applied to study steam oxidation of AlAs. Oxidation process parameters are extracted from available experimental data as functions of temperature and the AlAs layer thickness. The oxidation rate is found to be very sensitive not only to temperature, but also to the oxidation front position inside the mesa. The oxidation rate slows down as the oxidation front moves into the mesa, reaches a minimum, and then accelerates at the final stages of the oxidation process. Complex nonmonotonic dependence of the oxidation process on layer thickness is also revealed.

Detailed threshold analysis of UV-emitting nitride vertical-cavity surface-emitting lasers

In the present paper a detailed (but simple from a mathematical point of view) threshold analysis of room-temperature pulse operation of ultraviolet light (UV) emitting GaN/AlGaN/AlN vertical-cavity electrically pumped surface-emitting lasers (VCSELs) is carried out to examine the possible use of this semiconductor injection laser configuration in future mass application. Several VCSEL structures are considered. An index-guided nitride single quantum well (SQW) VCSEL structure ensures the best laser performance, as expected, particularly in the case of small-size devices. But surprisingly, gain-guided SQW VCSELs are found to exhibit comparable thresholds with much simpler double-heterostructure (DH) VCSELs of bulk active regions. Moreover, thresholds of DH nitride VCSELs are proved to be much less sensitive to all optical losses (for example material absorption as well as diffraction, scattering, and end losses) than their SQW counterparts. Therefore the SQW VCSELs need more advanced technology (making possible achievement of lower dislocation densities and/or higher facet reflectivities) to be superior to DH ones.

Designing guidelines for possible continuous-wave-operating nitride vertical-cavity surface-emitting lasers

A detailed self-consistent threshold simulation of the continuous wave (CW) operations at room temperature (RT) of possible GaN/AlGaN/AlN vertical-cavity surface-emitting lasers (VCSELs) is developed in a simple mathematical form in order to give an opportunity for it to be carried out using only PC-level computing power. In the analysis, the mismatch-related phenomena and temperature dependences of many model parameters are included with the aid of a self-consistent approach. Multiple-quantum-well (MQW) structures are proved to be the best suited for RT CW nitride VCSEL devices. In contrast, currently available nitride technology practically excludes the possibility of an efficient RT CW operation of single-quantum-well (SQW) nitride VCSELs. Double-heterostructure (DH) nitride VCSELs are found to be less sensitive to increases in optical losses than other nitride VCSELs, therefore their RT CW operation, if possible, may occur for a wider current range than that in QW VCSELs. It is also revealed that substrate material has a critical influence on the possibility of reaching RT CW thresholds, which strongly favours the SiC substrate of very high thermal conductivity.

Design guidelines for fundamental-mode-operated cascade nitride VCSELs

Design guidelines for manufacturing fundamental-mode-operated nitride vertical-cavity surface-emitting lasers (VCSELs) are presented. Results of comparative analysis of various possible VCSEL configurations carried out with the aid of an advanced self-consistent optical-electrical-thermal device modeling are given. Our computer simulations suggest that the properly arranged design with a semitransparent contact, a double (cascade) active region, and two tunnel junctions ensures the best laser performance characteristics, in particular, single-fundamental-mode room-temperature continuous-wave operation.

Single-photon devices in quantum cryptography

Modern communication in absolute secrecy requires creation of new intrinsically secure quantum communication channels. It is particularly necessary during the first connection between two parties establishing then in assumed unconditional security the secret cryptographic key which is supposed to be used afterwards during normal information exchanging. This new emerging field of quantum information technology is based on a new type of light sources, in which numbers of emitted photons can be carefully controlled. Especially advantageous are sources of single photons emitted at strictly predetermined moments, so called single-photon devices. Then any possible eavesdropper activity will be followed by some unavoidable disturbance which alerts both communication parties to an event. In the present paper, the Purcell effect associated with enhancement of spontaneous emission coupled to a resonator is explained, methods used to produce streams of antibunched photons are given, mechanisms applied to control carrier injection into quantum dots are shown and some possible designs of single-photon devices are presented and described. These devices are based on taking advantage of both the Purcell effect and the atom-like energy spectrum of quantum dots.

How to reach a single-fundamental-mode operation in nitride VCSELs

An advanced 3D model of a room-temperature (RT) continuous-wave (CW) VCSEL operation is used to investigate a possibility of reaching a RT CW single-fundamental-mode operation of nitride VCSELs. A nitride VCSEL of a traditional, double-ring-contacted structure has been found to generate at RT simultaneously many higher-order transverse modes which is a direct consequence of an unavoidable strong current-crowding effect at edges of its active region. Various possible structure modifications of nitride VCSELs have been examined to improve uniformity of the current injection into active regions which seems to be crucial for achieving their RT lasing operation and improving their mode selectivity. Both semi-transparent contacts and tunnel junctions have been suggested as useful tools to enhance considerably radial current flows in the nitride VCSEL structure. They should be, however, properly arranged within the laser cavity not only to improve the radial current sperding between the top ring contact and the centrally lcoated active region but also they should not introduce additional optical losses within a semi-transparent contact and highly doped tunnel junctions.

Simulation of threshold operation of GaInNAs diode lasers

The advanced three-dimensional fully self-consistent optical-electrical-thermal-gain model of the 1.3-μm (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser (VCSEL) has been developed to simulate its room-temperature (RT) continuous-wave (CW) performance characteristics and to enable its structure optimisation. The standard GaInNAs VCSEL structure with an intracavity-contacted configuration exhibits very nonuniform current injection into its active region, whereas a uniform current injection is important in long-wavelength VCSELs for low threshold, high-efficiency and stable-mode operation. Therefore we decided to insert an additional tunnel junction within the active-region neighbourhood. The tunnel junction is shown to enhance effectively hole injection via a lateral electron current, with only a modest increase (a small penalty) in voltage drop and series resistance compared to standard devices.

Designing of possible structures of nitride vertical-cavity surface-emitting lasers

Performance of various possible designs of 400-nm nitride vertical-cavity surface-emitting lasers (VCSELs) has been analysed with the aid of the advanced three-dimensional (3D) thermal-electrical-optical-gain self-consistent threshold simulation. It has been demonstrated that it is practically impossible to reach the fundamental-mode operation in nitride VCSELs of the traditional design with two ring contacts. To enhance this desired operation, uniformity of current injection into VCSEL active regions should be dramatically improved. Therefore, we focused our research on designs with tunnel junctions and/or a semitransparent contact. In particular, it has been proved that the design with two cascading active regions, two tunnel junctions and a semitransparent contact may offer the most promising room-temperature performance characteristics for both pulse and continuous-wave operation. In particular, this design offers high mode selectivity with distinct fundamental transverse mode domination. Our simulations reveal, that the thickness and localization of a semitransparent contact as well as localization of active regions and tunnel junctions are crucial for a successful construction designing.

Three-dimensional comprehensive self-consistent simulation of a room-temperature continuous-wave operation of GaAs-based 1.3-/spl mu/m quantum-dot (InGa)As/GaAs vertical-cavity surface-emitting lasers

The comprehensive self-consistent three-dimensional optical-electrical-gain-thermal self-consistent model of the oxide-confined long-wavelength 1.3-/spl mu/m (InGa)As/GaAs quantum-dot vertical-cavity surface-emitting laser is presented. The model has been used to modify and optimise VCSEL structure to reduce its anticipated room-temperature continuous-wave lasing threshold. Some essential guidelines for designers of laser sources for the second-generation 1.3-/spl mu/m communication systems are given.