表面发射二维光子晶体二极管激光器的室温连续波操作

We achieved room temperature continuous wave operation of a  surface emitting two-dimensional photonic crystal diode laser by current  injection. This is the first time ever that room temperature continuous wave  operation of a photonic crystal diode laser has been realized. This laser  features single mode oscillation over a large area, which is impossible for  conventional lasers. In this work, we optimized the epitaxial layer  composition for better carrier confinement and clarified the relationship  between the diameter of the air holes in the photonic crystal and the  threshold current of the laser in order to estimate the optimized threshold  current.

Photonic crystals , which have a periodic refractive index change, possess great potential  for realizing new optical devices. The photonic band-gap is a well-known property of  photonic crystals that allows them to block light waves selectively. Many types of two dimensional (2D) photonic crystal lasers, such as defect-mode lasers using the photonic bandgap and artificially-introduced defects or multi-directional distributed feedback (DFB)  lasers  have been demonstrated. In particular, surface-emitting 2D photonic crystal lasers  (Fig. 1) are operated by current injection and have the capability for single mode  oscillation over a broad area due to the 2D DFB effect. Broad-area lasers have several  advantages, not only in terms of high output power and heat sinking, but they also exhibit a  narrow divergence angle.

The basic characteristics of the device as a semiconductor diode laser have also been  improved. A remarkable reduction in the laser threshold current was successfully realized by  reducing the distance between the active layer and the photonic crystal layer to make the 2D  photonic crystal effect stronger. The threshold current in this case is 1/25 of the value that  we measured in our previous work . Consequently, continuous wave (CW) operation by  current injection was achieved. However, the operating temperature was below -20ºC, due  to carrier overflow from the active layer. In this work, to produce an increase in the operating  temperature of the laser, we confined carriers into the active layer by using a higher band gap  AlGaAs layer (aluminum composition: 0.30), which was introduced between the  InGaAs/GaAs active layer and the photonic crystal layer. Consequently, the temperature  characteristics of the laser are improved and the realization of CW operation at temperatures  up to room temperature (RT) can be expected. In addition, it is known that the size of the air  holes in the photonic crystal affects the characteristics of photonic crystal devices, though  their effect on a laser of this type has not clarified so far. Therefore, in this work, the  relationship between the relative volume occupied by the photonic crystal air holes, (known as  the air-filling factor) and the threshold current of the laser is revealed, along with  improvements in the processing technique.

Wafers of type-B consisted of a GaAs SCH layer, a p-type Al0.4Ga0.6As cladding layer, a  GaAs contact layer and an AlGaAs etch stop layer on a p-type GaAs substrate. Air rods (an  essential element of the 2D photonic crystal) were formed on wafer A by electron beam  lithography and plasma etching. To obtain the as-designed diameter of the photonic crystal,  we changed our previous method of drawing hole patterns on resist by electron beam  lithography from painting out circular patterns one by one to plotting one dot per each circle.  Furthermore, we used inductively-coupled plasma etching for the plasma etching process  because we could more easily obtain vertical and uniform photonic crystal air holes and lower  sidewall roughness  with this technique compared with the reactive ion etching process.  The arrangement of the air rods was a square lattice with a depth of about 100 nm. The depth  and the distribution of the photonic crystal were determined in order to obtain a coupling  coefficient κ  of over 1000 cm^-1 with sufficient optical confinement in the active layer.  The lattice constant was 286.25 nm, which is equal to the lasing wavelength in the material.  After the formation of the photonic crystal, one each of wafers A and B were stacked and  fused at high temperature, as shown in Fig. 1. The p-type GaAs substrate and the AlGaAs etch  stop layer were removed by a mechanical lapping process and chemical etching. After that, an  insulating layer of silicon nitride and a square-shaped Ti/Au electrode whose side length was  50 µm, were formed on the surface of the exposed p-type GaAs contact layer.

Fig1

In summary, we have succeeded for the first time in operating a surface-emitting 2D photonic  crystal diode laser in CW mode at RT by current injection. We used an Al_0.3Ga_0.7As subcladding layer between the active layer and the photonic crystal layer to block carriers into the  active layer. By using the sub-cladding layer, carriers are confined in the active layer and RTCW operation is successfully realized in this work. We also investigated the relationship  between the airfilling factor and the threshold current of the device experimentally, and  estimated the threshold current when the air-filling factor is optimized.

The authors would like to thank Dr. Masayuki Fujita, Mr. Shinpei Ogawa, Dr. Eiji Miyai, Mr.  Kyosuke Sakai, Mr. Takui Sakaguchi and Mr. Mitsuru Yokoyama for helpful discussions and  assistance. This work was partly supported by Core Research for Evolutional Science and  Technology - Japan Science and Technology Agency (CREST-JST), 21st Century COE  Program - Kyoto University, Kyoto Nanotechnology Cluster (Kyo-nano).