Johannes Michael Ostermann

Reliable polarization control of VCSELs through monolithically integrated surface gratings: a comparative theoretical and experimental study

Vertical cavity surface-emitting lasers (VCSELs) with a well-defined and predictable polarization of the emitted light are sought for a number of applications. In this paper, we show that one can define and stabilize the polarization of single- and multimode oxide-confined VCSELs with a monolithically integrated dielectric surface grating. In recent years, we have developed a three-dimensional, fully vectorial model for VCSELs, which proved to nicely reproduce the experimental results of quite complex structures, such as noncircular devices and phase-coupled VCSEL arrays. This software allows for the first time to analyze the effects of a dielectric grating in the output facet cap layer and its capability to fix the polarization of the emitted light. It is here employed as a design tool, yielding excellent agreement with the experimental data. Since the simulations predict the polarization behavior to be sensitively dependent on the grating parameters, hundreds of VCSELs with 99 different parameter sets, two grating orientations and active diameters of 4 and 7 /spl mu/m have been analyzed. Even VCSELs with eight or more coexisting modes turned out to be linearly polarized with an orthogonal polarization suppression ratio in excess of 15 dB. Theoretical and experimental emission far-fields are compared, and it is shown that diffraction side lobes can be prevented with properly chosen grating parameters which simultaneously ensure full polarization stability.

Investigation of polarization properties of VCSELs subject to optical feedback from an extremely short external cavity-part I: theoretical analysis

A two modes rate equation model for vertical-cavity surface-emitting lasers (VCSELs) subject to optical feedback from an extremely short external cavity (ESEC) is presented. By making use of it we develop a map of bistability to investigate the parametric dependence of polarization properties of VCSELs in such configuration, finding out a periodic dependence of the polarization switching (PS) currents on the ESEC length. By increasing the external mirror reflectivity we can make this periodic dependence stronger and strongly asymmetric providing the possibility to prevent PS for any injection current achieving polarization stabilization in VCSELs. Further numerical simulations with isotropic and non isotropic feedback show how parameters as the compression coefficients, the frequency splitting between the linearly polarized (LP) modes, the differential gain and the top mirror reflection coefficient, affect the map of bistability.

Optimized integrated surface grating design for polarization-stable VCSELs

Monolithically integrated surface gratings have proven to control the polarization of single-mode and even multimode vertical-cavity surface-emitting lasers (VCSELs) very effectively. Unfortunately, up until now, the grating parameters have had to be known and realized very accurately for proper performance. The simulations and experimental results presented in this paper show in very good agreement that by changing the thickness of the cap layer of the VCSEL structure, the dependence of the polarization control on the grating parameters can be strongly reduced. With this modification, for multimode devices, we have achieved a stable polarization of all modes orthogonal to the grating grooves independent of the investigated grating period, the grating depth, and the orientation of the grating itself. The orthogonal polarization suppression ratio is, on average, 17.1 dB and exceeds 12 dB for 117 out of 120 highly multimode VCSELs. At the same time, the optimized layer design significantly reduces the diffraction in the far field, which occurs for grating periods larger than the emission wavelength of the laser

Polarization switching induced by phase change in extremely short external cavity vertical-cavity surface-emitting lasers

We experimentally investigate the spectral and polarization properties of vertical-cavity surface-emitting lasers (VCSELs) subject to polarization insensitive optical feedback from an extremely short external cavity (few microns) and find that the wavelength and the current at which the light polarization switches between the two linearly polarized fundamental modes of the VCSEL are periodically modulated with the length of the external cavity. High contrast polarization switching is demonstrated for a quarter-wavelength change of the external cavity length. To explain our experimental findings we suggest a two-mode rate equation VCSEL model that considers the feedback optical field as instantaneous and accounts for multiple roundtrips in the external cavity with different coupling efficiencies back into the laser. Our numerical results are in very good agreement with the experiments.

Polymer-assisted self-assembly of superparamagnetic iron oxide nanoparticles into well-defined clusters: controlling the collective magnetic properties.

The combination of superstructure-forming amphiphilic block copolymers and superparamagnetic iron oxide nanoparticles produces new nano/microcomposites with unique size-dependent properties. Herein, we demonstrate the controlled clustering of superparamagnetic iron oxide nanoparticles (SPIOs) ranging from discretely encapsulated SPIOs to giant clusters, containing hundreds or even more particles, using an amphiphilic polyisoprene-block-poly(ethylene glycol) diblock copolymer. Within these clusters, the SPIOs interact with each other and show new collective properties, neither obtainable with singly encapsulated nor with the bulk material. We observed cluster-size-dependent magnetic properties, influencing the blocking temperature, the magnetoviscosity of the liquid suspension, and the r2 relaxivity for magnetic iron oxide nanoparticles. The clustering methodology can be expanded also to other nanoparticle materials [CdSe/CdS/ZnS core/shell/shell quantum dots (QDs), CdSe/CdS quantum dots/quantum rods (QDQRs), gold nanoparticles, and mixtures thereof].

VCSELs with enhanced single-mode power and stabilized polarization for oxygen sensing

Vertical-cavity surface-emitting lasers (VCSELs) with a single-mode single-polarization emission at a wavelength of approximately 763 nm have become attractive for oxygen sensing. Up to now, VCSELs used for this application are single-mode because of a small active diameter which correspondingly leads to small optical output power. Employing the surface relief technique and in particular the surface grating relief technique, we have increased the single-mode output to more than 2.5 mW averaged over a large device quantity. To the best of our knowledge, this is the highest single-mode power ever reported for VCSELs in this wavelength range. Through the grating relief simultaneously, we were able to stabilize the light polarization.

Amphiphilic, cross-linkable diblock copolymers for multifunctionalized nanoparticles as biological probes

Nanoparticles (NPs) play an increasingly important role in biological labeling and imaging applications. However, preserving their useful properties in an aqueous biological environment remains challenging, even more as NPs therein have to be long-time stable, biocompatible and nontoxic. For in vivo applications, size control is crucial in order to route excretion pathways, e.g. renal clearance vs. hepato-biliary accumulation. Equally necessary, cellular and tissue specific targeting demands suitable linker chemistry for surface functionalization with affinity molecules, like peptides, proteins, carbohydrates and nucleotides. Herein, we report a three stage encapsulation process for NPs comprised of (1) a partial ligand exchange by a multidentate polyolefinic amine ligand, PI-N3, (2) micellar encapsulation with a precisely tuned amphiphilic diblock PI-b-PEG copolymer, in which the PI chains intercalate to the PI-N3 prepolymer and (3) radical cross-linking of the adjacent alkenyl bonds. As a result, water-soluble NPs were obtained, which virtually maintained their primal physical properties and were exceptionally stable in biological media. PEG-terminal functionalization of the diblock PI-b-PEG copolymer with numerous functional groups was mostly straightforward by chain termination of the living anionic polymerization (LAP) with the respective reagents. More complex affinity ligands, e.g. carbohydrates or biotin, were introduced in a two-step process, prior to micellar encapsulation. Advantageously, this pre-assembly approach opens up rapid access to precisely tuned multifunctional NPs, just by using mixtures of diverse functional PI-b-PEG polymers in a combinatorial manner. All constructs showed no toxicity from 0.001 to 1 μM (particle concentration) in standard WST and LDH assays on A549 cells, as well as only marginal unspecific cellular uptake, even in serum-free medium.

Polarization-stable oxide-confined VCSELs with enhanced single-mode output power via monolithically integrated inverted grating reliefs

It has been proven experimentally that the single-mode output power of vertical-cavity surface-emitting lasers (VCSELs) can be increased significantly with an inverted surface relief and that a reliable polarization control can be achieved by a monolithically integrated surface grating for even highly multimode VCSELs. In this paper, for the first time we report on a combination of these two techniques, namely, an inverted grating relief to realize VCSELs with a stable polarization and an increased single-mode output power. A statistically relevant number of monolithic, oxide-confined 850-nm VCSELs is investigated. All devices with a grating relief have a stable polarization with its orientation defined by the grating grooves. On the other hand, 64% of the reference devices exhibit polarization switches. Concurrently, the maximum single-mode output power is enhanced by more than a factor of three. Within the entire range of implemented grating parameters, the worst grating relief device still delivers higher single-mode output power than the best reference device.

Shallow surface gratings for high-power VCSELs with one preferred polarization for all modes

Monolithically integrated full-aperture surface gratings are shown to control the polarization of all modes of even highly multimode 850-nm oxide-confined standard industrial vertical-cavity surface-emitting lasers (VCSELs). An orthogonal polarization suppression ratio (OPSR) of more than 11 dB up to thermal rollover is achieved for an output power of 23 mW. For devices with 8-mW output power, an OPSR of more than 20 dB at thermal rollover is observed, without major drawbacks to the overall laser performance.

A universal approach to ultrasmall magneto-fluorescent nanohybrids.

Seeded emulsion polymerization is a powerful universal method to produce ultrasmall multifunctional magnetic nanohybrids. In a two-step procedure, iron oxide nanocrystals were initially encapsulated in a polystyrene (PS) shell and subsequently used as beads for a controlled assembly of elongated quantum dots/quantum rods (QDQRs). The synthesis of a continuous PS shell allows the whole construct to be fixed and the composition of the nanohybrid to be tuned. The fluorescence of the QDQRs and magnetism of iron oxide were perfectly preserved, as confirmed by single-particle investigation, fluorescence decay measurements, and relaxometry. Bio-functionalization of the hybrids was straightforward, involving copolymerization of appropriate affinity ligands as shown by immunoblot analysis. Additionally, the universality of this method was shown by the embedment of a broad scale of NPs.