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 (see Figure 1):
- 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, gallium nitride = GaN or indium phosphide = InP) 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.
- One may also use a distributed Bragg reflector (Bragg mirror) on one side of the active region for more efficient power extraction.
Such devices can be fabricated with various methods for epitaxial semiconductor growth, usually involving metal–organic chemical vapor deposition (MOCVD). Originally, wafer bonding methods were also employed.
The photonic crystal has a photonic band 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 (see Figure 2). The lattice points 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 lattice points but also their detailed shape (e.g. triangular in the plane of the mode), because that has a profound effect on the photonic band structure and on the strength of coupling to the output beam.
Another important aspect is that the photonic crystal structure also diffracts 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. Particularly for devices with a large active area, the beam divergence becomes rather small. Effectively, the laser emits a collimated beam, not requiring any collimation lens.
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 two-dimensional coupling of light and couples out the light in a perpendicular direction.
An important technical detail is that one needs to realize a photonic crystal structure with large refractive index contrast, using air holes (voids) in the semiconductor material, in order to achieve high-power single mode operation with well-defined polarization and beam shape.
Further design improvements are achieved by realizing two-dimensionally arranged gain and loss sections, which help to achieve even higher peak powers and shorter pulse durations (tens of picosecond or less) while maintaining spatially single-mode operation .
Comparison with Other Surface-emitting Semiconductor Lasers
A PCSEL is in some aspects similar to a VCSEL: both are surface-emitting semiconductor lasers which can be electrically pumped, and which generally emit a circular beam with high beam quality. However, there are important differences. The most essential difference 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; 1 W has been demonstrated in continuous operation , and about 10 W in pulsed operation [13, 17]. Far higher continuous-wave output powers appear to be possible with single-mode or at least few-mode characteristics, provided that sufficiently effective cooling is applied, or alternatively pulsed operation with small duty cycle. The resulting radiance (brightness) is correspondingly much higher than for VCSELs, and the beam divergence is very small.
In terms of single-mode output power, PCSELs are more similar to VECSELs than to VCSELs. In contrast to VECSELs, PCSELs mainly have the advantage that it is not necessary to resort to optical pumping. However, fabrication of a PCSEL is more difficult than that of an optically pumped VECSEL gain chip.
One may try to realize power scaling by further increasing the size of the active area [13, 16]. However, for this to work, one will have to implement a cooling strategy which essentially establishes a 1D heat flow, so that the operation temperature does not substantially increase for increased active areas. Only if this is successfully done, PCSELs can be considered as power-scalable continuous-wave lasers, possibly generating continuous-wave outputs of 100 W or even 1 kW. However, one may at least operate such devices with pulsed emission and high peak power at a low duty cycle, much reducing the heat generation in that way. Note that pulsed operation is quite suitable for some applications such as LIDAR .
Concerning the length of the laser resonator, a PCSEL is similar to a VCSEL (much in contrast to a VECSEL). That implies that single-frequency operation (i.e., lasing on a single longitudinal mode) is relatively easily achieved. That leads to a rather small emission linewidth. The temperature coefficient of the emission wavelength is also similarly small as for VCSELs.
Possible Applications of PCSELs
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, as long as small devices with stable single-mode output are used. PCSELs can also be optimized for very high modulation frequencies by realizing them with a relatively small diameter of the active area, which requires an optimization of the mode confinement for achieving sufficiently high gain. That way, PCSELs with modulation bandwidths of tens of gigahertz are possible .
Even in the area of laser material processing, PCSELs may become useful, particularly if their output powers can be further increased. The same applies to the pumping of solid-state lasers including fiber lasers.
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 [12, 19]. Although further optimizations will be required, 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.
Control of Polarization and Beam Shape
The output beam shape and polarization of PCSELs can be tailored in various ways. Essentially, this is done by configuring and/or modulating the features of the lattice, such as the shape and arrangement of the lattice points [20, 21]; external optical elements are not required. For example, it is possible to generate cylindrical vector vortex beams (also called topological beams), which exhibit azimuthal polarization.
Acknowledgement: The author thanks Prof. Susumu Noda for various useful comments and for contributing various illustrative figures to this article.
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