Andrea Kroner

Optical spin manipulation of electrically pumped vertical-cavity surface-emitting lasers

We analyze the potential for the spin manipulation of vertical-cavity surface-emitting lasers (VCSELs) by operating them electrically and injecting additional spin-polarized carriers by polarized optical excitation. The output polarization of the VCSELs can be easily controlled by the spin orientation of the optically injected carriers when the injection current does not exceed the threshold current.

Surface Relief Versus Standard VCSELs: A Comparison Between Experimental and Hot-Cavity Model Results

We present a detailed experimental as well as theoretical study of vertical-cavity surface-emitting lasers (VCSELs) with and without etched surface modifications. The so-called inverted surface relief leads to a suppression of higher-order transverse modes, where measurements of output power and optical spectra show a maximum single-mode output power of 6.1 mW. For simulations, a hot-cavity model is applied, which can handle the complex electrical, thermal, and electromagnetic problems in a VCSEL structure in a fully 3-D manner. The optical characteristics of both structures, including current-dependent output power and spectral properties up to thermal rollover, are very well reproduced by the simulations. Furthermore, the evolution of the beam profile is investigated by simulations as well as spectrally resolved near-field measurements at various distances to the laser surface. Here, the simulations confirm the significantly stronger thermal guiding in the relief device indicated in the measurements.

Monolithic polarization control of multimode VCSELs by a dielectric surface grating

Based on design guidelines from a three-dimensional, fully vectorial model, we have fabricated vertical-cavity surface-emitting lasers (VCSELs) with a monolithically integrated dielectric surface grating for polarization control. For VCSELs with emission wavelengths of 850 and 980 nm we have achieved orthogonal polarization suppression ratios (OPSRs) above 15 dB for all modes up to thermal rollover, which very well agrees with theory. It is shown both theoretically and experimentally that the grating has no influence on the emission far-field. The surface grating has also been combined with a surface relief to stabilize the polarization and to increase the fundamental mode output power at the same time.

VCSEL-based optical trapping for microparticle manipulation

In recent years, research into microfluidic devices has attracted much interest in the fields of biology and medicine, since they promise cheap and fast sample analysis with drastically reduced volume requirements. The combination of various analysis steps on one chip forms a small-sized biomedical system, where handling, fixing, and sorting of particles are major components. Here, it was demonstrated that optical manipulation is an efficient tool; in particular it is accurate, contactless, and biocompatible. However, the commonly required extensive optical setup contradicts the concept of a miniaturized system. We present a novel particle manipulation concept based on vertical-cavity surface-emitting lasers (VCSELs) as light sources. The small dimensions and the low power consumption of these devices enable a direct integration with microfluidic systems. The symmetric geometry of VCSELs leads to a high-quality, circular output beam, which we additionally shape by an etched surface relief in the laser output facet and an integrated photoresist microlens. Thus, a weakly focused output beam with a beam waist of some micrometers is generated in the microfluidic channel. With this configuration we were able to demonstrate particle deflection, trapping, and sorting with a solitary VCSEL with output powers of only 5mW. Furthermore, the surface emission of VCSELs allows a comparatively easy fabrication of two-dimensional laser arrays with arbitrary arrangement of pixels. Smart particle sorting and switching schemes can thus be realized. We have fabricated densely packed VCSEL arrays with center-to-center spacings of only 24 μm. Equipped with integrated microlenses, these arrays are integrated with microfluidic chips based on polydimethylsiloxane (PDMS), enabling ultra-compact particle sorting and fractionation.

Novel concepts of vertical-cavity laser-based optical traps for biomedical applications

Using vertical-cavity surface-emitting lasers (VCSELs) as light sources in optical traps offers various advantages compared to the common approaches. In particular, these are small dimensions, a circularly symmetric output beam, and the simple fabrication of two-dimensional laser arrays. We investigate the application of VCSELs in a standard tweezers setup, where trapping forces of up to 4.4 pN are achieved with 15 μm polystyrene particles and a transverse multi-mode VCSEL. The latter has improved trapping characteristics compared to a single-mode device. By introducing a small-spaced array of three VCSELs in the setup, non-mechanical movement with average velocities of up to 3 μm/s is demonstrated with 10 μm particles. Furthermore, the novel concept of the integrated optical trap is presented. By integrating a microlens directly on the VCSEL output facet, two-dimensional optical trapping is achieved in a small-sized system without any external optics. Elevation and trapping of 10 μm polystyrene particles is demonstrated at optical output powers of about 5 mW. In order to improve the beam quality of the lasers, the inverted surface relief technique is applied, which eliminates a previously observed offset between laser center and trapped particle.

Polarization-controlled monolithic oxide-confined VCSELs

We report on advances in the fabrication and performance of monolithic 850 nm, linearly polarized vertical-cavity surface-emitting lasers (VCSELs) incorporating a semiconductor surface grating at the outcoupling facet. Depending on the grating parameters, the light is polarized either parallel or perpendicular to the grating grooves. Deep-etched gratings enable complete polarization pinning even in directions that are 45 degrees off the preferred crystal axes. On the other hand, such devices can show strong side-lobes in the far-field which may limit the available output power for some applications. Shallow-etched VCSELs with almost undistorted far-fields deliver output powers as high as 29 mW with about 12 dB orthogonal polarization suppression ratio. A combination of surface relief and grating is used to increase the transverse single-mode output power while maintaining polarization stability.

Polarization-Controlled Surface Grating VCSELs Under Externally Induced Anisotropic Strain

The polarization control of surface grating vertical-cavity surface-emitting lasers (VCSELs) is tested under externally applied anisotropic stress and is compared to that of a nominally identical standard VCSEL without a surface grating. This is done by bending the VCSEL chip in a well-defined way. The anisotropic in-plane strain thus induced in the VCSELs leads to a polarization switch of the standard VCSEL for rather moderate strain. In contrast, the polarization of the surface grating VCSELs is fixed by the grating and remains unchanged despite a high strain which causes a wavelength splitting of the two polarization modes of about 130 pm. Such a result is of high practical relevance, since strain is unavoidably induced during VCSEL fabrication and mounting and counteracts any method applied for polarization control.

Towards ultra-compact optical tweezers without external optics

In this paper, demonstration shows that the optical power level necessary for particle trapping can be achieved by lensed VCSELs. Therefore, the next step will be to adopt lensed VCSELs in integrated optical tweezers. Nowadays, commercial optical tweezers are mainly based on Nd:YAG lasers or edge-emitting laser diodes which are either bulky or need careful external beam correction. Here, the use of vertical-cavity surface-emitting lasers (VCSELs) yields various benefits like micrometer size dimensions, a Gaussian output beam and the fabrication of monolithic two-dimensional laser arrays with which multiple tweezers can be easily realized

Application of vertical-cavity laser-based optical tweezers for particle manipulation in microfluidic channels

The combination of microfluidics and optical manipulation offers new possibilities for particle handling and sorting on a single-cell level in the field of biophotonics. We present particle manipulation in microfluidics based on vertical-cavity surface-emitting lasers (VCSELs) which constitute a new low-cost, high beam quality nanostructured laser source for optical trapping, additionally allowing easy formation of small-sized, two-dimensional laser arrays. Single devices as well as densely packed linear VCSEL arrays with a pitch of only 24 μm are fabricated. Microfluidic channels with widths of 50 to 150 μm forming T- and Y-junctions are made of PDMS using common soft-lithography. With a single laser, selected polystyrene particles are trapped in the inlet channel and transferred to the desired outlet branch by moving the chip relatively to the optical trap. In a second approach, a tilted, linear laser array is introduced into the setup, effectively forming an optical lattice. While passing the continuously operating tweezers array, particles are not fully trapped, but deflected by each single laser beam. Therefore, non-mechanical particle separation in microfluidics is achieved. We also show the route to ultra-miniaturization of the system avoiding any external optics. Simulations of an integrated particle deflection and sorting scheme as well as first fabrication steps for the integrated optical trap are presented.

Toward more efficient fabrication of high-density 2-D VCSEL arrays for spatial redundancy and/or multi-level signal communication

We present flip-chip attached high-speed VCSELs in 2-D arrays with record-high intra-cell packing densities. The advances of VCSEL array technology toward improved thermal performance and more efficient fabrication are reviewed, and the introduction of self-aligned features to these devices is pointed out. The structure of close-spaced wedge-shaped VCSELs is discussed and their static and dynamic characteristics are presented including an examination of the modal structure by near-field measurements. The lasers flip-chip bonded to a silicon-based test platform exhibit 3-dB and 10-dB bandwidths of 7.7 GHz and 9.8 GHz, respectively. Open 12.5 Gbit/s two-level eye patterns are demonstrated. We discuss the uses of high packing densities for the increase of the total amount of data throughput an array can deliver in the course of its life. One such approach is to provide up to two backup VCSELs per fiber channel that can extend the lifetimes of parallel transmitters through redundancy of light sources. Another is to increase the information density by using multiple VCSELs per 50 μm core diameter multimode fiber to generate more complex signals. A novel scheme using three butt-coupled VCSELs per fiber for the generation of four-level signals in the optical domain is proposed. First experiments are demonstrated using two VCSELs butt-coupled to the same standard glass fiber, each modulated with two-level signals to produce four-level signals at the photoreceiver. A four-level direct modulation of one VCSEL within a triple of devices produced first 20.6 Gbit/s (10.3 Gsymbols/s) four-level eyes, leaving two VCSELs as backup sources.