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
We report on the theoretical analysis and fabrication of a novel type of vertical-cavity surface-emitting laser
(VCSEL) that provides selection of a certain higher-order transverse mode. This selection is based on a spatial
variation of the threshold gain by adding an antiphase layer with an etched relief structure. The field intensity
profile emitted from this laser is calculated numerically as well as with an analytical approach. The main factors
that control the selected mode such as the threshold gain, the confinement factor, and the phase parameter are
calculated as a function of the active aperture, aiming to achieve single higher-order transverse mode emission.
For a given aspect ratio of a rectangular oxide aperture, the threshold gain difference between the selected
and neighboring modes is maximized via the relief diameter and the size of the aperture. The fabrication
process involves selective etching of the antiphase layer, passivation of the relief, oxidation of an AlAs layer to
the desired aperture after reaching this layer using wet-chemical etching. N- and p-metalization processes are
applied, followed by polyimide passivation. Finally, bondpad metalization is carried out for electrical contacting.
Mode selection is successfully achieved. Attractive applications for such devices are found in optical manipulation
of micro-particles such as sorting and separation.
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.
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.
High-performance vertical-cavity surface-emitting lasers (VCSELs) with an emission wavelength of approximately 764 nm are demonstrated. This wavelength is very attractive for oxygen sensing. Low threshold currents, high optical output power, single-mode operation, and stable polarization are obtained. Using the surface relief technique and in particular the grating relief technique, we have increased the single-mode output power to more than 2.5mW averaged over a large device quantity. The laser structure was grown by molecular beam epitaxy (MBE) on GaAs (100)-oriented substrates. The devices are entirely based on the AlGaAs mixed compound semiconductor material system. The growth process, the investigations of the epitaxial material together with the device fabrication and characterization are discussed in detail.
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.
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.
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.