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Vertical Cavity Surface Emitting Quantum-Dot Laser (QD-VCSEL)

VCSEL stands for Vertical Cavity Surface Emitting Laser (spoken: “vixel”). Unlike à edge emitting lasers, the laser light is emitted perpendicular to the wafer surface (see Fig. 1).

The two mirrors that define the laser cavity have to be grown from semiconductor material. As the distance between both mirrors is only about a micron, light inside the cavity is amplified over a very short distance until it hits one of the mirrors and is reflected. In order to gain enough amplification for lasing, the mirrors must have a very high reflectivity, typically in the range of 99.99 % or better! Facet mirrors of edge emitters have a reflectivity of 32 %, a bathroom mirror typically 97 %. The high reflectivity is achieved by Bragg reflectors, that means pairs of layers of translucent semiconductors (or oxides) with two different indices of refraction and suitable (~100 nm) thickness. Depending on the difference in refractive index, up to dozens of layers have to grown epitaxially to from the bottom or top mirror. For GaAs VCSELs, a possible pair of materials is Aluminium oxide / GaAs.

Fig. 1: Schematic cross-section of a VCSEL

Electric current flows through the device via two ring-shaped metal contacts (so-called intracavity contact design, Fig. 1). Alternatively, the current can also be send through the mirrors if they are conductive. Inside the mirror cavity lies a p-i-n-junction, were the carriers (electrons and holes) recombine.

The gain medium between the p- and n-doped regions is of particular interest since we use quantum dots instead of conventional quantum wells for the conversion of current (electron-hole-pairs) into light (photons). The main advantage of quantum dots is their discrete, atom-like emission spectrum. Since quantum dots differ in their size due to growth statistics, their ground-state emission wavelength varies by 2-5 %. Therefore, an ensemble of several million quantum dots shows a broadened spectrum. In order to achieve more gain inside a laser cavity, layer of quantum dots can be stacked (see inset in Fig. 2 showing three layers of QDs).

Quantum dots offer a low threshold current density, large temperature stability and minimum chirp. These advantages combine with the design benefits of VCSELs.

Fig. 2: Schematic cross section of a GaAs-QD-VCSEL with oxide mirrors

VCSELs are ideal light sources for parallel optical data transmission: They can be easily fabricated in arrays right on the wafer and coupled into optical fibers (see Fig. 3). Due to their low beam divergence they can also be used for short-range free space optical transmission - e.g. optical interconnects between chips.

Fig. 3: Array of QD-VCSELs

Surface emitting Quantum-Dot Laser

MPEG-Movie (6.5 MB)

This movie illustrates the processes inside a quantum-dot surface emitting laser. The laser in this animation contains a single sheet of pyramidal shaped quantum dots. The movie starts with a camera flight over the quantum dot layer. The red spheres symbolize photons that are moving up and down inside the laser.

Later we see the VCSEL from the outside, where another advantage becomes obvious: the emitted light beam is circular and not diverging (staying parallel).

This movie shows rather complex physical processes in a very simple form. The physic entities displayed here are actually invisible: single photons, electrons, holes etc. The movie shall give a simple impression of the processes taking place in a quantum dot VCSEL.