1. Introduction

Photonic crystals [1–3], 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 [4] or multi-directional distributed feedback (DFB) lasers [5,6] have been demonstrated. In particular, surface-emitting 2D photonic crystal lasers [5] (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.

 

Fig.1.Schematic . iagram of the device structure.

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Fig. 2. (a) Schematic of a square lattice photonic crystal. The two narrow arrows indicate two particular directions G-X and G-M, and the broad arrows indicate propagating light waves. (b) Schematic showing the propagating directions of the coupled waves.

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Figure 2 shows a schematic diagram of a laser cavity composed of a 2D square lattice photonic crystal structure whose pitch in the G-X direction corresponds to one lasing wavelength, and in Fig. 2(a), one arrow corresponds to one wavelength. When the light wave propagating in one specific G-X direction (0°) is considered, the light wave is diffracted to the reverse direction (180°) by Bragg diffraction. The light wave is also diffracted in two other Γ-X directions (+90° and -90°) as shown in Fig. 2(a) because they also satisfy the Bragg diffraction condition. Consequently, light waves propagating in four equivalent G-X directions are coupled with each other, and a 2D large area cavity is formed. In addition, the light wave is also diffracted toward the vertical direction by first-order Bragg diffraction, as shown in Fig. 2(b). Therefore, this device works as a surface-emitting laser. As mentioned above, the electromagnetic field distribution of the laser is determined by the photonic crystal structure. Coherent single-mode lasing oscillation over a large area can therefore be achieved by this laser, a phenomenon that has not been realized by conventional lasers. In fact, we successfully demonstrated large-area single mode oscillation (over 500 µm) with narrow beam divergence [5] in our previous work. Furthermore, we can control the oscillation mode of the laser by careful design of the photonic crystal structure. For example, unification of the lasing polarization could be realized by a photonic crystal structure with an elliptic lattice point located on a square lattice position [7].

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 [8]. The threshold current in this case is 1/25 of the value that we measured in our previous work [5,7]. Consequently, continuous wave (CW) operation by current injection was achieved [8]. However, the operating temperature was below -20°C, 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 [9], 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.

2. Design and fabrication

 

Fig. 3. Schematic conduction band diagram of the layer structure near the active layer.

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Figure 1 shows a schematic diagram of the lasers fabricated in this work. Two types of wafers (labeled A and B) were prepared. Wafers of type-A consisted of an n-type Al_0.4Ga_0.6As cladding layer, three InGaAs/GaAs quantum wells (TQWs), GaAs separate confinement heterostructure (SCH) layers with an additional AlGaAs layer on an n-type GaAs substrate. The schematic conduction band diagram of the epitaxial layer structure near the active layers is shown in Fig. 3. The additional AlGaAs layer above the TQWs layer is tentatively named the ‘sub-cladding layer’ in this paper. We prepared and fabricated two variants of wafer A, where the only difference between the wafers was in the aluminum composition of the sub-cladding layer. One was Al_0.25Ga_0.75As, while the other was Al_0.30Ga_0.70As, and we expected to observe a difference in the carrier confinement effect between these two wafers. In our previous work [8], only device characteristics for a sub-cladding layer consisting of Al_0.25Ga_0.75As were reported.

Wafers of type-B consisted of a GaAs SCH layer, a p-type Al_0.4Ga_0.6As 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 [10] 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 [11] of over 1000 cm^-1 with sufficient optical confinement in the active layer [8]. 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.

After all of these processes, we made an analysis of the devices by secondary ion mass spectrometry (SIMS). The thickness (20 nm) and the aluminum composition of the Al_0.25Ga_0.75As and Al_0.30Ga_0.70As sub-cladding layers were confirmed as meeting the design criteria. Therefore, the sub-cladding layer was not thin enough for carriers to overflow by the tunneling effect and the band gap was as expected, which would not have been the case if aluminum diffused during the wafer fusion process at high temperature.

3. Results and discussion

 

Fig. 4. Temperature characteristic of the devices under pulsed conditions (1kHz-500ns).

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Figure 4 shows the differences in the temperature characteristics of the two devices that had Al_0.25Ga_0.75As and Al_0.30Ga_0.70As as their sub-cladding layers, measured under pulsed conditions. The threshold current measured at various temperatures is divided by the value of the threshold current of each device at 20°C. The only difference between two devices is the aluminum composition of the sub-cladding layer, and the devices have roughly the same peak wavelength for spontaneous emission (940 nm) and the same lasing wavelength (957 nm), which is determined by the pitch of the photonic crystal at 20°C. The lasing wavelength is determined by grating pitch, just as it is for DFB lasers or photonic crystal lasers. In general, gain peak moves from short to long wavelength about three times as fast as the lasing wavelength with increasing ambient temperature. According to this logic, the gain peak closes in on the lasing wavelength with increasing ambient temperature, so the normalized threshold current should decrease. Since the normalized threshold current does not decrease with increasing ambient temperature in the case of the Al: 0.25 device, it is clear that carriers must be overflowing from the active layer. It is also clear that carriers overflowed from the active layer in our previous work [8] according to the results as shown in Fig. 4. In addition, the most probable dissipation route for overflowed carriers is considered to be heat generation due to non-radiative recombination at in-plane crystal defects on the wafer-bonding interface. Heat generation near the active layer makes matters worse, since other carriers can then overflow more easily from the active layer to the wafer-bonding interface. On the other hand, inhibition of carrier overflow from the active layer is realized by the Al_0.30Ga_0.70As sub-cladding layer, since the normalized threshold current of this device decreases with increasing ambient temperature, as we expected. Therefore, we measured the laser characteristics of the device with the Al_0.30Ga_0.70As sub-cladding layer under CW mode at RT.

 

Fig. 5. Lasing characteristics of the device under RT-CW condition. (a) Lasing spectrum. The operation current was 70 mA. (b) Light output power-current characteristic.

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The lasing spectrum and the light output power-current characteristic of the device under CW condition at RT are shown in Figs. 5(a) and (b) respectively. According to Figs. 5(a) and (b), lasing oscillation at 959.44 nm and light output power of over 4 mW under CW operation at RT is successfully obtained with this device. Full width at half maximum of the spectrum can be estimated to be 0.35 nm, which is determined by the resolution limit of our measurement system. The threshold current was 65 mA.

The near-field pattern of the device as detected by a CCD camera is shown in Fig. 6(a). The length of one side of the square-shaped electrode shown in the center of Fig. 6(a) is 50 µm, and this picture indicates that two-dimensional lasing oscillation is obtained in four equivalent G-X directions. The doughnut-shaped far field pattern of the device is shown in Fig. 6(b). According to the far field pattern and the polarization characteristics of the device, the transverse mode of the surface-emitted beam was the lowest order transverse electric mode (TE₀₁) in terms of propagation mode in the multimode optical fiber [12]. The mode produced by this semiconductor laser is unique, and new applications for such a mode can be expected. We can also choose the lasing mode by designing the photonic crystal structure [7], which is one of the most important merits of this laser. The angle of divergence of the output light was about 1.1°. Such a narrow divergence angle is also unique, and has similar repercussions to the phenomenon described above.

 

Fig. 6. (a) Near field pattern and polarization characteristics of the device. The blue open circles indicate the measurement points with a diameter of about 10 µm, and red double-headed arrows show the direction of polarization at each point. The operating current was 76 mA. (b) Far field pattern of the device. The operating current was 66 mA.

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Fig. 7. Relationship between the air-filling factor and the normalized threshold current at 20°C.

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RT-CW operation of a photonic crystal laser was successfully demonstrated for the first time, as described above. Next, we discuss the optimized threshold current of the laser. In this work, we explicitly investigated the relationship between the air-filling factor and the threshold current of the laser, and the experimental result for the Al_0.25Ga_0.75As sub-cladding layer is shown in Fig. 7. The threshold currents are normalized along the lowest threshold current in Fig. 7 and the dashed line in Fig. 7 is a fitting curve of the experimental result. The optimal region of the air-filling factor was between about 10% and 15%, as shown in Fig.7. However, in the case of the device shown in Fig. 5 and Fig. 6, the diameter obtained from the cross-sectional scanning electron microscope image is about 133 nm, and the calculated air-filling factor is about 17%. Although the designed diameter was smaller, diameter that was outside of the optimal region was obtained due to mass transport during the wafer fusion process at high temperature. When an air-filling factor is achieved that is within the optimal region by optimizing the wafer-bonding conditions, including the fusion temperature, the threshold current of the device with the Al_0.30Ga_0.70As sub-cladding layer can be reduced to about 15 mA. This indicates the capacity for further reduction of the threshold current in this device.

4. Summary

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 sub-cladding 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 RT-CW operation is successfully realized in this work. We also investigated the relationship between the air-filling factor and the threshold current of the device experimentally, and estimated the threshold current when the air-filling factor is optimized.