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Development of novel quantum dot based materials for compact laser devices for potential telecommunication and biophotonic applications

Final Report Summary - QDLASER (Development of novel quantum dot based materials for compact laser devices for potential telecommunication and biophotonic applications)

InAs/InGaAsP quantum dot growth and laser devices based on QD active material at 1.5 μm

The main part of the project was dedicated to the development and optimization of quantum dot (QD) active material epitaxial growth for the InAs/InP material system for a wavelength range around 1.5 μm. The main approach for the synthesis of QDs was chosen to be the self-assembling Stranski-Krastanow growth method. The grown QD material was implemented as an active medium in laser devices and lasing was demonstrated in the continues wave regime for both narrow ridge single mode lasers and photonic crystal cavity lasers. As an alternative method to Stranski-Krastanow growth, the selective area growth approach assisted by diblock copolymer lithography was implemented.

The main results achieved on QD growth in the frame of this project are:

• Self-assembled (SA) quantum dots (QDs) grown by metalorganic vapour phase epitaxy (MOVPE) was demonstrated for the material system InAs/InGaAsP/InP [1],[2]. The morphology of resulting QDs was studied by aberration-corrected scanning transmission electron microscopy [1]. The strain field maps in the InAs QDs in InP was investigated using the dark field electron holography method. A strain of 5.4% was determined [3]. The 3D shape of the QD was reconstructed from 61 HR STEM images acquired at 2° intervals over a ±60° tilt range using the SIRT algorithm [4]. However, due to their large lateral size the emission wavelength of this kind of QDs exceeds 1.65 µm and hence a new design of QDs was suggested and implemented in order to shift the emission wavelength of QDs toward the 1.55 µm wavelength range, which is of high interest for of telecommunication applications. A layer consisting of 1.5-1.7 mono-layer of GaAs was included in the growth to cover the InAs QD array before InP or InGaAsP overgrowth. Due to strain issues of the GaAs material together with InAs, the QD tops are redistributed around the QD base and form a ring of InGaAs. These morphological changes lead to the reduction of the emission wavelength of the QDs to the desired value of 1.5 µm [1],[2].
• In order to evaluate the properties of the assembled QDs as a gain medium, narrow ridge lasers with an active region based on five layers of InAs/GaAs/InGaAsP/InP QDs were fabricated. Continuous wave lasing of a 2 µm wide ridge waveguide QD laser structures at a wavelength of 1.5 µm at room temperature was demonstrated. The threshold current for the 5 arrays of QDs is around 400 mA for 4 mm long devices and around 275 mA for 2 mm long devices. The maximum output power for the device with a cavity length of 4 mm about 7.4 mW [2],[5]. The wide spectral bandwidth is attributed with the significant broad QD size distribution. Since all QDs are pumped but only some of them, the ones with a certain size, contribute to stimulated emission, the resulting threshold current is rather high. The net modal gain spectra of the SOA structure was investigated as a function of current density. The spectra were obtained by a segmented contact method at room temperature for transverse electric (TE) polarization under continuous wave bias conditions [6], [7]. A significant blue-shift of the gain peak wavelength with increasing carrier density is observed. This, together with a weakly pronounced ground and exited states separation typically reflects the shallow shape of these QDs. An inherently dense energy structure and affinity to a wetting layer accompanied by strong inhomogeneous broadening leads to smearing of the spectrum over energy [7]. To overcome this problem the volume and the aspect ratio of QDs should be changed.
• Photonic crystal defect waveguides with embedded active layers containing single or multiple quantum wells or quantum dots have been fabricated. Spontaneous emission spectra are enhanced close to the bandedge, consistently with the enhancement of gain by slow light effects. These are promising results for future compact devices for terabit/s communication, such as miniaturised semiconductor optical amplifiers and mode-locked lasers [8], [9].
• Investigation of epitaxial growth dynamics of self-assembled InAs/InP QDs allow us to find a tailored growth regime with suppressed surface migration of adatoms. The QDs formed under this regime, which includes low deposition temperature and low V/III ratio, exhibit higher QD density with almost twice reduced size and low aspect ratio, which results in emission at the desired 1.55 µm wavelength and significant energy level separation (~70 meV), necessary for a temperature stability and low noise performance in device applications [7].
• The 8 band kp theory calculations of the QD electronic properties, shown in the figure 2, were performed for the QDs formed under different growth conditions and with different design. The QD shape and size were extracted from the STEM images. The calculation results are in good agreement with optical measurements performed for these types of dots [7].
• A membrane photonic nanolaser with three layers of “small” InAs/InP QDs was fabricated with different cavity length. Lasing at a wavelength of 1.55 µm with optical pumping was demonstrated [in preparation].
• A first experiment on a new approach for the synthesis of QDs was carried out, with the aim to narrow the size distribution of the resulting QDs. This synthesis is referred to as selective area growth (SAG) of QDs based on diblock copolymer lithography. The optimization of glass mask fabrication, ICP dry etching and epitaxial growth allows us to achieve the formation of SAG QDs with an emission wavelength around 1.55 µm with high optical quality, which are comparable with the best SA QDs on InP [7]. More investigations of this method are required.

Optimization of laser waveguide design and integration of all-active InP/AlGaAs/InGaAsP optical components

• To improve the temperature stability of the threshold current density in QW lasers as well as investigate the factors affecting the characteristic temperature and its dependence on optical losses, asymmetric potential barriers were implemented to the waveguide region. These barriers prevent the formation of bipolar carrier population in the waveguide region and lead to weakening of the temperature dependences of the transparency current density, the gain saturation parameter and, consequently, to a higher characteristic temperature for both long and short cavity laser diodes [10], [11].
• To explore the possibilities for tailoring the temporal response of semiconductor materials in order to obtain femtosecond pulses from monolithic devices it is necessary to combine the following key elements: Low loss integration, tailored epitaxial structures and high-quality QDs. In collaboration with the project FLASH, supported by Danish research council for technology and production) the technology needed to integrate materials with different dynamics were developed by the PhD student, which was co-supervised by Dr. Semenova [12]. Several methods for evaluating the material properties (segmented gain and loss measurements) were implemented, which allow for a qualified design of the integrated lasers, optimization of the individual nanostructures and comparison to theory.
• Quantum dot and quantum dash mode-locked lasers exhibiting new physics were demonstrated [13], [14].

Selective area growth of quantum wells and quantum wires.

MOVPE selective area growth (SAG) and etching of InGaAs/InP quantum wells and quantum wires on non-planar (001) InP surfaces was demonstrated in collaboration with the the Villum Kann Rasmussen Center of Excellence: Nanophotonics for Terabit Communication (NATEC) by a PhD student supervised by Dr. Semenova. To form the protection mask with e-beam lithography the negative tone resist HSQ was used. The formation of self-limited structures was observed during in-situ etching of the InP surface in the mask openings, carried out in the MOVPE reactor. The groove shape depends on the mask orientation along the crystallographic directions on the (001) surface. As a result of this investigation, the profile of SAG grown material could be tailored as demanded by the etching/growth conditions [15]. The optical properties of the resulting SAG material were investigated at temperatures between 77-300K with micro-photoluminescence and cathodoluminescence measurements [15], [16], [17].
SAG QWs active material was placed into a photonic crystal (PhC) waveguide, achieved by aligning two e-beam lithography steps. Optical measurements investigated both the transverse electric (TE) and the transverse magnetic (TM) polarized modes of the emitted light. A strong photoluminescence signal ranging from below 1500 nm up to 1540 nm was observed in the slow light regime of the PhC with a group index of 18 [10].

The expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far)

Although the main aim of the project, which was femto-second laser operation at and emission wavelength of 1.5 µm, was not accomplished completely, it is expected that we will achieve this in the near future. The reason for this is the broad size distribution of the resulting QDs, which we are improving at the moment. The results obtained during the project provide the bases for future studies and prove that the main targets mentioned in the project description and objectives can be put through. This research might have noticeable impact to device applications for our everyday life in the near future, as improved device properties are clearly within reach. Diverse areas like telecommunication, optical coherence tomography including medical applications, sensing, computer and network clock-distribution, THz generation, and metrology can benefit from the materials investigated.