Long-wavelength InP-based quantum-dash lasers

Self-assembled InAs quantum-dash (QD) lasers with emission wavelengths between 1.54 and 1.78 μm based on the AlGaInAs-AlInAs-InP material system were grown by gas source molecular beam epitaxy. Threshold current densities below 1 kA/cm² were achieved for 1-mm-long mirror coated broad area lasers with a stack of four QD layers. The devices can be operated up to 80°C in pulsed mode and show a high T₀ value of 84 K up to 35°C. In comparison to quantum-well lasers a much lower temperature sensitivity of the emission wavelength was achieved. The temperature shift of Δλ/ΔT = 0.12 nm/K is as low as that caused by the refractive index change     

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.     

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.     

Coherent InGaAs/GaAs laser arrays with laterally coupled distributed feedback gratings

A new approach for coherent positive index guided distributed feedback lasers array using laterally coupled gratings is demonstrated. The device fabrication requires no regrowth which makes this concept suitable for a variety of material systems. The lasers are based on an InGaAs/GaAs single quantum well structure emitting around 980 nm with high sidemode suppression ratio of more than 50 dB.     

DFB Lasers With Deeply Etched Vertical Grating Based on InAs–InP Quantum-Dash Structures

Distributed feedback lasers with first-order vertical grating based on AlInGaAs-InAs-InP quantum-dash lasers were fabricated by electron beam lithography and Cl₂-Ar reactive ion etching with an electron cyclotron resonance source. Low threshold currents and single-mode operation with sidemode suppression ratios of 48 dB and a direct modulation bandwidth of 5.5 GHz were demonstrated     

High-temperature operating 1.3-μm quantum-dot lasers fortelecommunication applications

High-performance 1.3-μm-emitting quantum-dot lasers were fabricated by self-organized growth of InAs dots embedded in GaInAs quantum wells. The influence of the number of quantum-dot layers on the device performance was investigated. Best device results were achieved with six-dot layers. From the length dependence; a maximum ground state gain of 17 cm^-1 for six dot layers could be determined. Ridge waveguide lasers with a cavity length of 400 μm and high-reflection coatings show threshold currents of 6 mA and output powers of more than 5 mV. Unmounted devices can be operated in continuous wave mode up to 85°C. A maximum operating temperature of 160°C was achieved in pulsed operation for an uncoated 2.5-mm-long ridge waveguide laser     

22-GHz modulation bandwidth of long cavity DBR laser by using a weakly laterally coupled grating fabricated by focused ion beam lithography

A 22-GHz directly modulated 3-dB bandwidth could be obtained by 1.3-mm-long weakly laterally coupled distributed Bragg reflector lasers fabricated by focused ion beam lithography. In addition to a high bandwidth, the lasers show a stable emission spectrum with side-mode suppression ratios of more than 40 dB and output powers exceeding 20 mW.     

Wide range tunable laterally coupled distributed-feedback lasers based on InGaAs-GaAs quantum dots

The authors have investigated tunable distributed feedback (DFB) lasers based on InGaAs quantum dots grown by molecular beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side-mode suppression ratios of up to 40 dB have been achieved. The tuning range covers 30 nm.     

Room temperature operation of ultra-short quantum cascade lasers with deeply etched Bragg mirrors

Mid-infrared GaAs based bound-to-continuum quantum cascade microlasers with ridge waveguide geometry are fabricated by the monolithic integration of deeply etched semiconductor-air Bragg mirrors. Devices with ultra-short cavities of 50 and 150 /spl mu/m can be operated near room temperature (260 K) or at room temperature (300 K), respectively. 50 /spl mu/m-long devices show singlemode emission up to relatively high drive currents due to the large mode spacing of about 30 cm/sup -1/ (340 nm).     

Electrical properties of ultrananocrystalline diamond/amorphous carbon nanocomposite films

The electrical surface properties of ultrananocrystalline diamond/amorphous carbon composite films have been investigated by four-point probe I/V and Hall measurements, whereas impedance spectroscopy has been used to establish the electrical bulk properties of the films. It turned out that the surface is p-type conductive with a resistivity of 0.14 Ω cm and a sheet carrier concentration of 7.6 × 10¹³ cm^?2. The bulk resistivity is higher by almost seven orders of magnitude (1.3 × 10⁶ Ω cm). The bulk conduction is thermally activated with an apparent activation energy of 0.17 eV. From Cole–Cole plots of the impedance spectra it can be concluded that there are three different contributions to the bulk conductivity. In order to try to identify these three components contributing to the electrical bulk conduction, Raman spectra have been recorded at five different wavelengths from the IR to UV region. These measurements showed that the UNCD/a-C films consist of at least three components: diamond nanocrystallites, an amorphous carbon matrix, and trans-polyacetylene-like structures probably at the interface between these two.