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Abstract
We demonstrate high-power continuous-wave (CW) lasing oscillation of 1.3-µm wavelength InP-based photonic-crystal surface-emitting lasers (PCSELs). Single-mode operation with an output power of over 100 mW, a side-mode suppression ratio (SMSR) of over 50 dB, and a narrow single-lobe beam with a divergence angle of below 1.2° are successfully achieved by using a double-lattice photonic crystal structure consisting of high-aspect-ratio deep air holes. The double lattice is designed to enhance both the in-plane optical feedback and the surface radiation effects in the photonic crystal. The coupling coefficients for 180$^\circ $, +90$^\circ $, and -90$^\circ $ diffractions are estimated from the measurements of the photonic band structure as κ1D = 417 cm−1, κ2D+ = 135 cm−1, and κ2D− = 65 cm−1, respectively. The stable single-mode, high-beam-quality operation is attributed to these large coupling coefficients introduced by the asymmetric double-lattice structure.
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1. Introduction
Photonic-crystal surface-emitting lasers (PCSELs) operate at a singularity point (typically the Γ point) of a two-dimensional (2D) photonic crystal (PC), where fundamental and higher-order Bloch waves are coupled with each other to form stable 2D standing waves (or cavity modes) [1–3]. Accordingly, PCSELs have the potential to sustain oscillation in a single longitudinal and lateral mode even over broad cavity areas, which leads to high-power and high-beam-quality operation. Novel features of PCSELs, including not only high-beam-quality operation [3–7], but also control of polarization and beam patterns [3,8–11] and beam-scanning functionality [12,13], have already been demonstrated, mainly in the wavelength range around 940 nm aiming at applications to light detection and ranging (LiDAR) and laser processing.
On the other hand, PCSELs operating in the wavelength range of 1.3 − 1.6 µm (telecommunication wavelengths) are important for applications to telecommunications and eye-safe sensing. PCSELs at these wavelengths have been investigated using several different methods in order to form an air/semiconductor PC structure with the large refractive index contrast necessary for coupling among not only fundamental Broch waves but also higher-order Bloch waves [1,8,14–17]. For example, Imada et al. used a material system of InGaAsP/InP and formed the PC structure by employing a wafer-bonding technique [1,8]. Hsu et al. utilized a material system of InAs/InGaAs/GaAs and formed the PC structures by deep dry etching into the upper cladding layer [14]. Recently, other important approaches were reported, where a PC structure was formed by an epitaxial regrowth technique for InP-based materials [15–17]. In Ref. [17] in particular, deep air holes were formed underneath the active layer for the purpose of achieving strong optical confinement within the PC layer and avoiding dry-etching damage to the active layer. As a result, CW oscillation was achieved at room temperature. However, there were two remaining issues: low output power (below a few milliwatts) and a doughnut-like far-field pattern (FFP). These were caused by the rotationally symmetric nature of the electromagnetic field in the square-lattice PC with symmetric circular air holes. Towards optical communication and eye-safe sensing applications, not only high output power but also a narrow-divergence, symmetric, single-lobe beam is required to achieve high optical coupling to a single-mode fiber and long-distance sensing. In addition, single-mode operation with a high side-mode suppression ratio (SMSR) is required for telecommunications.
In the present work, we introduce a double-lattice PC structure [7] to InP-based PCSELs to attain high output power and single-lobe emission. Here, the two lattices of the double-lattice structure are separated to strengthen the 180$^\circ $ diffraction of light within the PC by constructive interference. In addition, the asymmetry of the double-lattice structure enhances the emission of light from the surface of the PC and leads to a single-lobe beam. We also establish dry-etching conditions to form much deeper air holes with a high aspect ratio to obtain strong optical confinement in the PC layer. As a result, a light output power of over 100 mW with the high SMSR of 56 dB is successfully achieved under CW conditions at 25°C, and a single-lobe beam with a narrow divergence angle of below 1.2° is obtained.
2. Device structure and fabrication
Figure 1(a) shows a perspective-view schematic of the PCSEL structure in this work. First, an n-InGaAsP PC layer is grown on an n-type InP substrate by metal-organic vapor phase epitaxy (MOVPE). Then, a 2D PC structure with the double-lattice air holes is formed in the n-InGaAsP PC layer by using electron-beam lithography and dry etching processes in a circular area with a diameter of 300 µm. The lattice constant (a) is set to 412 nm to match the emission wavelength (1.3 µm) inside the device material and approximately the gain peak of the active layer. After the PC formation, an InP overgrown layer (spacer layer), an InGaAsP multiple-quantum-well (MQW) active layer, a p-InP over-cladding layer, and a p+-GaInAs contact layer are grown atop the 2D PC by MOVPE regrowth.
Fig. 1. (a) Perspective-view schematic of the PCSEL structure in this work and (b) 3D image constructed from slice-and-view SEM observations of the air holes after regrowth.
As a feature of our InP-based PCSEL, the air holes of the PC are formed under the active layer with the thin InP spacer layer (thickness < 100 nm) to avoid dry-etching damage to the active layer. In addition, deep air holes with a high aspect ratio are formed to enhance optical confinement within the PC layer. Toward the formation of deep air holes with a high aspect ratio, we have optimized the dry-etching conditions, such as plasma density, chamber pressure, and bias power considering the extension of the mean free path for plasma ions, to reduce the influence of microloading effects. A 3D image constructed from slice-and-view scanning-electron microscope (SEM) observations of the air holes after regrowth is shown in Fig. 1(b). The formation of double-lattice air holes was observed, and the depths of the large and small air holes after regrowth are 1060 nm and 670 nm, respectively. The aspect ratio of both air holes is over 6, which is around twice as large as that in previous work [17]. Hence, this approach is very suitable for forming the PC structure with deep air holes in the vicinity of the active layer.
After the regrowth, a circular isolation mesa with a diameter of 200 µm is formed by photolithography and dry etching processes to remove the highly doped contact layer, and this is followed by chemical vapor deposition of an insulating film. The current injection area is restricted by this isolation mesa. After the formation of the mesa, a square p-electrode is deposited on the top of the device, and a n-electrode with a circular window is deposited on the back side of the device using photolithography and evaporation processes. The emitted light is measured through the circular window of the n-electrode on the back side of PCSEL as shown in Fig. 1(a).
3. Device characteristics
First, we measured lasing characteristics under pulsed conditions to avoid thermal effects. Figure 2(a) shows the current (I) - light output (L) characteristic of the PCSEL with the double-lattice PC under pulsed condition with a pulse width of 1 µs and a duty cycle of 0.1% at 25°C; the I-L characteristic of a single-lattice PCSEL with a circular air hole and the same device configuration is also shown for reference. The optical power is detected using an InGaAs photodiode. The light output power of the double-lattice PCSEL is much higher than that of the single-lattice PCSEL, and the slope efficiency of the double-lattice PCSEL is 25 times higher than that of the single-lattice PCSEL. This result indicates that the introduction of the double-lattice PC is effective to enhance the slope efficiency of surface emission owing to the asymmetric nature of the electromagnetic field in the double lattice structure. Figure 2(b) shows the FFPs of the double- and single-lattice PCSELs under pulsed conditions at an injection current of 400 mA. In the case of the single-lattice PC, a doughnut-like beam shape was observed due to the rotationally symmetric nature of the electromagnetic field [3]. On the other hand, the double-lattice PCSEL showed a single-lobe beam owing to the asymmetric nature of the electromagnetic field. The beam divergence angle was as narrow as 1.0° for the double-lattice PCSEL.
Fig. 2. (a) I-L characteristics of PCSELs with double- and single-lattice PCs consisting of the same device structure under pulsed conditions with a pulse width of 1 µs and a duty cycle of 0.1% at 25°C. (b) FFPs of PCSELs with double- and single-lattice PCs under pulsed conditions at an injection current of 400 mA.
The lasing characteristics of the double-lattice PCSEL were then evaluated under CW conditions at a temperature of 25°C. Figures 3(a) shows current - optical output - voltage (I-L-V) characteristics, and Fig. 3(b) shows lasing spectra at injection currents of 400 mA and 800 mA, respectively. The optical power of the I-L characteristic is detected using the InGaAs photodiode, and the lasing spectra are measured with a spectrum analyzer using a single-mode fiber. Lasing oscillation occurs at an injection current (threshold current: Ith) of 320 mA. A light output power of over 100 mW is successfully achieved under CW conditions. The slope efficiency (ηSE) and differential resistance at injection currents of 400–600 mA are 0.20 W/A and 0.69 Ω, respectively. From the lasing spectra, single-mode lasing with the high SMSR of over 50 dB is obtained, and the lasing occurs in the mode of band-edge B, which is described later, without mode hopping even at a high injection current of over 800 mA. We consider that the formation of deep air holes and the introduction of the double-lattice PC structure led to lasing operation with a high SMSR. The lasing wavelength shifts to longer wavelengths with increasing injection current due to the effect of heating under CW conditions. The thermal resistance, which was estimated from the shift of the lasing spectra, is ∼19 K/W.
Fig. 3. (a) I-L-V characteristics of the double-lattice PCSEL under CW conditions at a temperature of 25°C and (b) lasing spectra of the double-lattice PCSEL under CW conditions at a temperature of 25°C (injection current: 400 mA and 800 mA).
Figure 4(a) shows the measured photonic band structure of the double-lattice PCSEL near the Γ point at an injection current below threshold at room temperature (RT) under pulsed conditions with a pulse width of 1 µs and a duty cycle of 0.1%. The method for measuring the photonic bands is described in Ref. [18]. There exist four band-edge modes at the Γ point for the square-lattice PC, labelled A, B, C, and D in the order of increasing frequency [19]. We have observed the formation of these four band-edge modes in the photonic band measurement. To determine the lasing band edge, the spectra at the Γ point below and above threshold have been measured as shown in Fig. 4(b). From the comparison of these spectra, it has been verified that the lasing oscillation occurs in the mode of band-edge B. Additionally, to evaluate the strength of optical coupling within the PC structure, we have calculated the in-plane optical coupling coefficients for 180° (κ1D) and 90° (κ2D+ and κ2D-) diffractions as shown in Fig. 4(c) [7,20,21]. The calculated κ1D, κ2D+ and κ2D- are 417 cm-1, 135 cm−1, and 65 cm−1, respectively. Strong optical coupling, which results in robust single-mode stability, is obtained not only for 180° but also for 90° diffraction, owing to the formation of a PC with high-aspect ratio air holes as well as to the enhancement of 180$^\circ $ diffraction by the suitably separated double-lattice [7]. Accordingly, single-mode lasing with a high SMSR has been achieved. In addition, it is indicated that the asymmetric nature of the double-lattice PC brings about the high slope efficiency even under CW conditions.
Fig. 4. (a) Photonic band structure of the double-lattice PCSEL near the Γ point at an injection current below threshold at room temperature under pulsed conditions with a pulse width of 1 µs and a duty cycle of 0.1%, (b) spectra at the Γ point below (red line) and above (black line) threshold, and (c) directions of in-plane diffraction for the κ1D, κ2D+, and κ2D- coupling coefficients and their values for the measured double-lattice PCSEL.
Figure 5 shows the FFPs and polarization states of the double-lattice PCSEL at injection currents of 300 mA and 800 mA. A single-lobe beam with a narrow divergence angle of below 1.2° is observed even under CW conditions. Although a slight side lobe appears at an injection current of 800 mA, we consider that this is due to the window effect of the n-electrode on the back side of the device and can be removed by adjusting the size of this window relative to that of the effective active area of the PCSEL (see Fig. 1). On the polarization state, a linear polarization in the Γ-M direction is observed at an injection current of 800 mA under CW conditions at 25°C. We have verified from three-dimensional coupled-wave theory [20] that this polarization state reflects the feature of the asymmetric electromagnetic field of the double-lattice PC.
Fig. 5. FFPs of the double-lattice PCSEL at injection currents of 300 mA and 800 mA, including polarization states in the +45° and −45° directions (Γ-M directions) at an injection current of 800 mA under CW conditions (measurement temperature: 25°C).
Figure 6 shows the dependence of the beam divergence angle in the FFPs for the double-lattice PCSEL as a function of injection current under CW conditions. A beam divergence angle of below 1.2° (evaluated using the beam’s 1/e2 width) is obtained, and this narrow beam divergence is maintained even at an injection current of 900 mA. This result indicates that a single-lobe beam with a narrow divergence angle is maintained even at a high output power of 100 mW owing to uniform 2D oscillation over a broad device area, which is enabled by the high optical coupling coefficients κ1D and κ2D.
Fig. 6. Dependence of the beam divergence angle on injection current for the double-lattice PCSEL under CW conditions at 25°C.
4. Conclusion
We have demonstrated high-power, single-mode lasing oscillation of InP-based PCSELs at 1.3-µm wavelength. A light output power of over 100 mW and a high SMSR of over 50 dB have been achieved through the combination of the introduction of the double-lattice PC structure and the formation of deep air holes with a high aspect ratio. A single-lobe beam with a narrow divergence angle of below 1.2° is also achieved as a feature of the double-lattice PCSEL. These results pave the way toward 1.3-µm-wavelength InP-based PCSELs for various applications including optical telecommunication and sensing.
Funding
Council for Science, Technology and Innovation; Core Research for Evolutional Science and Technology (JP MJCR17N3).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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