“The beam quality of our laser is so good that it can burn a small hole in a piece of paper – even though the laser does not contain any lens,” says team leader Susumu Noda of Kyoto University. “You can see this phenomenon in a recent Japanese TV show broadcast.”

Producing high power – that is, greater than 1 W output – from conventional semiconductor lasers generally means increasing their size. However, doing this significantly degrades beam quality.

So-called surface-emitting lasers containing photonic crystals (PCSELs) could help overcome this problem because they coherently lase over a large area, something that results in a narrow single-lobed beam. PCSELs could be used in single-chip light sources thanks to the fact that their polarization, beam pattern and beam direction can be controlled. Until now, however, no-one had ever made a high-power PCSEL laser.

Vertical and in-plane asymmetry

Noda and colleagues’ device produces an almost ideal Gaussian laser beam with a power of up to 0.5 W at room temperature – which is a record for this type of laser. The improvement comes thanks to the vertical and in-plane asymmetry of the air-hole structure of the 2D photonic crystal employed, which acts as a broad area (0.2 × 0.2 mm) laser cavity. Thanks to the 2D diffraction effect from the photonic crystal, the laser beam is emitted along the plane of the crystal. “The result is 1000 times greater light emission compared to that possible from conventional vertical-cavity surface-emitting lasers,” Noda told nanotechweb.org.

A photonic crystal is a nanostructured material in which periodic variation of the refractive index on the length scale of visible light produces a photonic "band gap". This gap affects how photons propagate through the material and is similar to the way in which a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In Noda’s team’s work, the edge of the band gap is exploited to form a 2D large area optical cavity mode, which is a gain medium for amplifying light.

Isosceles triangle makes good PCSEL

The researchers made their PCSEL in the following way. First, they grew an n-AlGaAs cladding layer, an active layer containing InGaAs/AlGaAs multiquantum wells (MQWs), an AlGaAs carrier blocking layer and a GaAs layer on an n-GaAs substrate using a technique called MOCVD. They then fabricated a photonic crystal structure using electron beam lithography and dry etching on the top GaAs layer.

The 2D crystal consists of a square lattice in which each lattice point is associated with an isosceles triangle-shaped hole (see figure). The 2D crystal measures 400 µm on both sides and the lattice constant (the period of the photonic crystal in the x- and y-directions) is 287 nm. The air-hole filling factor (the area of the unit cell occupied by air-holes) is 48%. In the third fabrication step, the team grew a p-AlGaAs cladding layer and a p-GaAs contact layer on the 2D PC by MOCVD regrowth, so confining the air-holes inside the 2D crystal layer.

Avoiding defects, and increasing power output

So why is the laser so high-power? One reason is that most earlier studies employed a wafer-bonding technique to incorporate a photonic crystal structure inside the device, explained Noda. This bonding process induces many defects at the bonded interface that absorb laser light. “In our work, we avoid introducing such defects by using MOCVD,” he said. “The second important difference between our experiments and previous ones is the in-plane shape of the air holes, which are asymmetric right-isosceles-triangle-shaped. These triangles play an important role.”

With a symmetrically shaped air hole, such as a circle, for example, the in-plane resonant electromagnetic field becomes symmetric. As a result of this symmetry, the field cancels out when it is emitted in the vertical direction by the so-called first-order Bragg diffraction effect of the 2D photonic crystal, which reduces the laser output power. “The right-isosceles-triangle shape we used ‘breaks’ this cancellation and allows us to obtain higher power outputs.”

Our studies will advance the field of lasers because they could help overcome limitations in applications that suffer from low laser beam quality, he added. A wide range of applications, such as materials processing, laser medicine, nonlinear optics and sensing, to name but a few, could benefit.

The work is detailed in Nature Photonics doi:10.1038/nphoton.2014.75.