Photonic Crystal Surface-emitting Lasers
While many semiconductor lasers are edge-emitting lasers, some of them are surface-emitting, i.e., the output beam is perpendicular to the wafer surface. Originally, such lasers have always been realized as vertical cavity surface-emitting lasers (VCSELs), and partially as vertical cavity surface-emitting lasers (VECSELs) where the laser resonator contains at least one external mirror. However, it is also possible to obtain vertical emission in combination with a horizontal (lateral) cavity, i.e., a device where the intracavity laser radiation propagates essentially in directions along the wafer surface. One of the ways to realize that is to utilize a two-dimensional photonic crystal structure . Such devices are called photonic crystal surface-emitting lasers (PCSELs). Although that technology has not yet become mature, it exhibits promising features.
Basic Architecture and Operation Principles
The basic architecture of a photonic crystal surface-emitting laser is explained in the following:
- The central part is a two-dimensional photonic crystal structure, functioning as the lateral cavity. It essentially consists of a thin layer of semiconductor material (e.g gallium arsenide = GaAs) containing some pattern (e.g. square or triangular pattern) of air holes spanning a certain area. The semiconductor material must be transparent (non-absorbing) for the generated laser radiation.
- Laser gain by stimulated emission is provided by coupling the photonic crystal structure to a thin active layer (amplifying layer) beneath the photonic crystal layer within the evanescent waves of the modes. The active region is separated from the photonic crystal structure only by a thin electron blocking layer for keeping the electrical carriers confined in the active region.
- Above and below that structure, there is an optically transparent and electrically conducting cladding layer made of doped semiconductor.
- An electric current for pumping the active region is applied through metallic electrodes on the top and the bottom. On the laser emission side (top side), that electrode covers only a small part of the area, e.g. a rectangular region with dimensions of the order of 10 μm to 100 μm. It is also possible to use a top electrode where one rectangular area in the middle has been removed. This leads to pumping of the photonic crystal mode in its outer region, while output coupling is possible in the central region.
Such devices can be fabricated with various methods for epitaxial semiconductor growth, usually involving metal–organic chemical vapor deposition (MOCVD). Partly, wafer bonding methods are also employed.
The photonic crystal has a photonic bandgap structure which generally provides several different modes, essentially by coupling multiple plane waves together by the effect of the distributed reflection. (For example, for a square lattice one usually exploits a Γ (Gamma) singularity point of the bandgap structure to get such modes.) The hole pattern should be designed such that one of those modes has clearly the lowest threshold pump power, and consequently only that mode will exhibit lasing over the full range of operation currents. One then obtains a radiation pattern with very high spatial coherence and also relatively high temporal coherence.
Important design aspects of the photonic crystal structure include not only the positions of the air holes but also their detailed shape (e.g. triangular in the plane of the mode), because that has a profound effect on the photonic bandgap structure and on the strength of coupling to the output beam.
Another important aspect is that the photonic crystal structure also reflects some of the light such as to form the output beam, which leaves the device in a direction which is perpendicular to the wafer surface.
The principle of mode formation has some similarity to that of a distributed feedback laser (DFB laser), because there is a distributed reflection within the active region. In contrast to an ordinary DFB laser, however, one utilizes a distributed reflection in two dimensions and couples out the light in a perpendicular direction.
Comparison with Other Surface-emitting Semiconductor Lasers
A PCSEL is in many aspects similar to a VCSEL. The most essential difference, however, is related to the size of the active region:
- For a VCSEL, one needs to strongly restrict the diameter of the active region when single-mode operation is required. That usually limits the possible output power to a couple of milliwatts. Much higher output powers are possible with larger active areas, but then with spatially multimode operation and accordingly poor beam quality.
- A PCSEL, however, can have a much larger active area while maintaining single-mode operation, if it is based on a suitably designed photonic crystal structure. Therefore, it allows for much higher single-mode output powers of the order of 1 W . The resulting radiance (brightness) is then correspondingly much higher.
In terms of single-mode output power, PCSELs are more similar to VECSELs than to VCSELs. In contrast to those, they mainly have the advantage that it is not necessary to resort to optical pumping. However, fabrication of a PCSEL is substantially more difficult than that of an optically pumped VECSEL gain chip.
In principle, one may realize power scaling by further increasing the size of the active area . However, this strategy soon hits limitations e.g. concerning the stability of single-mode operation. Therefore, PCSELs should not really be considered as power-scalable lasers.
For applications like optical data transmission, very high modulation speeds for the output power are required (when not using an external optical modulator). VCSELs are fairly strong in this respect, offering modulation frequencies of tens of gigahertz. However, PCSELs can also be optimized for very high modulation frequencies by realizing them with a relatively small diameter of the active area. As that reduces the available laser gain, one then needs to optimize the photonic crystal structure basically for stronger reflections. That way, tens of gigahertz are also possible .
It is possible to modify the photonic crystal structure with a special modulation pattern such that one obtains two output beams in different directions instead of a single one. It is even possible to tune the emission direction(s) through an electrical voltage applied to additional electrodes . Although further optimizations will be required, e.g. for obtaining a single beam with variable direction and high power conversion efficiency, this approach appears promising for important applications like 3D sensing (e.g. automobile LIDAR) or laser printing, where it is highly desirable to have a rapid beam steering capability, ideally in two dimensions, with high speed and without using additional optics involving movable parts.
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