Gain and noise saturation of wide-band InAs-InP quantum dash optical amplifiers: model and experiments

We present a theoretical model for gain and noise saturation in quantum dash (QDash) semiconductor optical amplifiers. The model is based on the density matrix formalism and addresses static saturation spectra. The calculations are confirmed by a series of experiments which highlight the unique properties of these amplifiers. We demonstrate a high gain, a wide bandwidth, and high saturation power. The saturation spectrum is shown to be asymmetric, emphasizing saturation at short wavelength. The asymmetry stems from the high energy tail of the density of state function in those quantum wire (QWire) like gain media as well as from the interactions with the wetting layer.     

GaAs-based four-channel photonic crystal quantum dot laser module operating at 1.3 /spl mu/m

A GaAs-based laser module, which combines the output of four singlemode lasers operating at 1.3 /spl mu/m by using photonic crystal mirrors and combiners, has been developed. The complete device size is around 0.2 mm/sup 2/. The lasers can be operated individually or in parallel with sidemode suppression ratios better than 20 dB.     

Wavelength stabilized single-mode lasers by coupled micro-square resonators

Wavelength stabilized single-mode lasers were realized by on-chip coupling of a ridge waveguide laser with two micro-ring resonators. The micro-ring resonators were fabricated by deeply etching to achieve high lateral optical confinement and to allow small ring diameters in the range between 20-60 /spl mu/m. A square-like ring geometry with 45/spl deg/ facets was used to improve design control and side-wall quality. By using micro rings with two different diameters of 50 and 60 /spl mu/m, a free spectral range of about 18 nm could be obtained which allow a stable single-mode operation with a side-mode suppression ratio of more than 40 dB and a continuous-wave output power of 30 mW per facet at room temperature.     

Semiconductor quantum dots

Since the invention of semiconductor lasers, huge improvements in device performance have been achieved, and a large variety of specialized designs for different applications were conceived. Two major steps have played a key role in the improvement of device properties. The first step was the application of semiconductor heterostructures that allowed the separate optimization of optical and carrier confinement. The second step was the introduction of quantum films, also called quantum wells, in the carrier recombination zone (started in the 1980s). This permitted a strong reduction of threshold current density due to an increased density of states at the laser energy. This effect of increased density of states is related to the partial discretization of the allowed energy states of carriers, i.e., electrons and holes, and is based on quantum mechanical principles. One major advantage of quantum-dot structures results from the full three-dimensional carrier confinement on a nanometer scale. Therefore, a semiconductor quantum dots, InAs dots embedded in GaAs, behave like non- or weakly interacting single atoms. In addition, the realization of device-quality quantum dot structures became possible by the introduction of self-organized growth. Both, molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used.     

50 mW CW-operated single-mode surface-emitting AlGaAs lasers with45° total reflection mirrors

The authors report on the fabrication and testing of surface-emitting AlGaAs 3.5 μm ridge lasers with etched mirrors and 45° internal deflectors. The 45° mirror coupling coefficient and the resulting threshold current penalty have been analyzed theoretically and experimentally. A surface-emitted optical power of 50 mW CW at 26 mA threshold current and external differential efficiency of 57% has been achieved in lateral fundamental-mode operation. The optical power density of 14 mW CW per micrometer ridge width is the highest reported to date and produces two-dimensional surface-emitting laser arrays of diffraction-limited beam quality suitable for optical storage applications     

Focused ion-beam implantation induced thermal quantum-wellintermixing for monolithic optoelectronic device integration

By focused ion beam implantation induced thermal intermixing the bandgap of quantum-well layer structures can be selectively changed. This allows lateral bandgap engineering and gives a new degree of freedom for lateral structuring. The principle technological aspects like the dependence of the bandgap shift on implantation parameters and the spatial resolution are investigated and applied to the fabrication of photonic and optoelectronic devices. Lateral waveguiding in InP-based materials, the possibility of monolithic integration of bandgap shifted waveguide areas into active devices and the improvement of the lateral carrier confinement in ridge waveguide lasers are demonstrated. Due to the high spatial resolution, modulated bandgap gratings could be realized with periods down to 90 mn. These bandgap gratings were used to create gain-coupled distributed-feedback lasers in different material systems with well controlled single-mode emission     

Structural and optical properties of ultrananocrystalline diamond / InGaAs/GaAs quantum dot structures

The combination of the unique properties of ultrananocrystalline diamond (UNCD) films and of semiconductor quantum dot (QD) structures could significantly improve the performance of different electronic and optoelectronic devices, where e.g. good thermal management and advanced mechanical parameters are required. In the current work quantum dot InGaAs/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) with different densities between 1.6 × 10¹⁰ cm^− 2 and 1.6 × 10¹¹ cm^− 2 controlled by the substrate temperature in the range between 490 and 515 °C. These structures were overgrown with UNCD by microwave plasma chemical vapor deposition (MWCVD) using methane/nitrogen mixtures at 570 °C. Scanning electron microscopy (SEM) reveals that without ultrasonic pretreatment the diamond nucleation density on QD structures is low and only separate islands of UNCD are deposited, while after pretreatment thin closed films are formed. From the cross-section SEM images a growth rate of ca. 3 nm/min is estimated which is very close to that on silicon at the same deposition conditions. The UNCD coatings exhibit a morphology consisting of two types of structures as shown by atomic force microscopy (AFM). The first one includes nodules with diameters between 180 and 350 nm varying with the density of the underlying QDs; the second is formed by a kind of granular substructure of these nodules with diameters of about 40 nm for all QD densities. The optical properties were investigated by photoluminescence (PL) spectroscopy before and after the deposition of UNCD. The PL signals of QD structures overgrown with UNCD, although with decreased intensity, remain almost unchanged with respect to the peak positions and widths, revealing that the UNCD/QD structures retain the optical properties of uncoated InGaAs/GaAs quantum dots.     

Plasma amination of ultrananocrystalline diamond/amorphous carbon composite films for the attachment of biomolecules

Ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) have been deposited by microwave plasma chemical vapour deposition at 600 °C from 17% CH₄/N₂ mixtures. The as-grown films turned out to be hydrogen terminated and very stable. Photochemical amination of H-terminated diamond is a well-established route to attach functional groups to such surfaces for applications in biosensors. Here we report on experiments to aminate UNCD surfaces directly by exposure to ammonia plasmas. Thereafter the surfaces were reacted with the heterobifunctional crosslinker molecule SSMCC bearing a N-hydroxysuccinimide (NHS) ester group which should react with the surface NH₂ groups. By means of X-ray photoelectron spectroscopy (XPS), contact angle measurements and fluorescence microscopy it is shown that both steps, plasma amination and SSMCC attachment lead to the desired aims. On the other hand, experiments to attach a thiol-bearing fluorescein molecule directly to H-terminated UNCD films turned out to be partially successful although according to literature such a reaction should be very unlikely.     

12 μm long edge-emitting quantum-dot laser

Highly reflecting Bragg mirrors in combination with GaInAs/AlGaAs laser structures with two layers of self-organised GaInAs quantum-dots are used to realise CW-operating edge-emitting microlasers with cavity lengths down to 12 μm. Owing to the large spacing of the longitudinal modes of 8.2 nm for 12 μm long lasers, quasi-singlemode operation is obtained     

UNCD/a-C nanocomposite films for biotechnological applications

Ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) have been deposited by microwave plasma chemical vapor deposition from a 17% CH₄/N₂ mixture. The films consist of diamond nanocrystallites of 3-5 nm embedded in an amorphous carbon matrix of 1-1.5 nm width. In a first series of experiments it is shown that as-grown UNCD/a-C films are hydrogen-terminated, conductive and very stable. Furthermore, by plasma- and photochemical treatments the H-termination can either be improved or replaced by terminating OH or F functionalities, whereas chemical room temperature processes to change the termination failed. A second set of investigations concerns the functionalization of differently terminated UNCD surfaces. Processes are discussed to bind DNA on H-terminated UNCD and to deposit an anti-fouling poly(ethylene glycol) layer on OH-terminated films. A third series of experiments shows that UNCD surfaces are not prone to unspecific interactions with highly-fouling proteins such as bovine serum albumin (BSA) but nevertheless some interaction will take place. However, the amount of adsorption and also the ratio of BSA and fibrinogen adsorption, which is of importance for the hemocompatibility of a surface, can be adjusted by the surface termination. Finally, it will be shown that continuous as-grown UNCD surfaces are bioinert and not cytotoxic for a variety of different cell lines.