1. Introduction

Surface-emitting lasers (SELs) offer several advantages over edge-emitting lasers, such as: circular beam for simplified fiber coupling and packaging, longitudinal single-mode emission for narrow linewidth application, on-wafer testing without cleaving substrates, and IC-like array manufacturability and integration [1,2]. To compensate for very short gain length in surface emission geometry; however, one have to incorporate particular optical feedback mechanism where photonic-crystal (PC) structure is most frequently used. For example, vertical-cavity surface-emitting lasers (VCSELs) use quarter-wavelength distributed Bragg reflector (DBR) or one-dimensional (1D) PC as high-reflection mirrors. 1D cavity mode is constructed by introducing π-phase shift or PC defect between bottom and top DBR stacks. Another example is photonic-crystal surface-emitting lasers (PCSELs) in which two-dimensional (2D) cavity mode is constructed over a large area of 2D-PC without microdefect [2,3]. Coherent light emissions from optical gain media, designed around the edge of photonic band structure, propagate with zero group velocity and strongly couple with 2D-PC structure where in-plane multidirectional distributed feedback (DFB) diffract vertically [3].

GaAs-based In(Ga)As quantum dot (QD) gain media are novel and low-cost solution for near-infrared lasing emissions between 1.1 and 1.3 μm. In particular, QD VCSELs of 1.3 μm range are expected to be ultimate devices in optical fiber communication system because of minimum dispersion in silica fiber. It was not until 2006 that 1.3-μm QD VCSELs with fully and compositionally graded DBRs were demonstrated using standard VCSEL process [4]. However, critical growth conditions as well as limited modal gain of self-assembled QDs somehow restrict the commercialization of QD VCSELs. Recently, we demonstrated the first QD PCSELs based on GaAs substrate [5]. They exhibited electrical injection lasing as well as 1.3-μm wavelength emissions. The growth structure was much simpler and total thickness was greatly reduced. Moreover, our fabrication of PCSEL devices was simplified by depositing indium-tin-oxide (ITO) over “PC slab-on-substrate” structure. Previous technology of epitaxial regrowth or wafer fusion bonding, which was complicated and involved high temperature process over 600 °C, could be detrimental to QD gain characteristics.

In this paper, threshold current temperature dependence of PCSELs is investigated with respect to gain-cavity detuning. The lasing wavelengths cover the range of 1283~1318 nm. We propose that GaAs-based 1.3-μm QD PCSELs can be manufactured inexpensively in wavelength division multiplexing (WDM) arrays. This paper is organized as follows. Section 2 describes device structure of PCSELs and optical property of QDs. Section 3 presents measurement results and discussions. The concluding remarks are summarized in section 4.

2. PCSEL structure and QD property

The PCSELs were designed for the transverse electric (TE) mode in 2D square lattice operated at Γ band-edge and patterned with circular shaped air holes. Figure 1(a) shows a schematic of electrically injected InAs QD PCSEL. The device fabrication was greatly simplified by direct deposition of ITO over “PC slab-on-substrate” structure. The growth structure and device process were the same as that previously described in [5]. The PC holes was patterned by e-bean lithography and dry etching within area of 300 × 300 μm², while the current aperture atop was defined by Si₃N₄ with circular diameter of 150 μm. Figure 1(b) shows the cross-sectional image from scanning electron microscope (SEM). The etched PC depth was about 545 nm and ITO completely covered the PC structure.

 

Fig. 1 QD PCSEL (a) schematic structure and (b) cross-sectional SEM image.

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The grown sample was optically characterized by 1064-nm pulsed fiber laser (50-ns pulse width and 200-μs repetition period) at room temperature (RT) of 25 °C. As shown in Fig. 2, the ground-state (GS) and first excited-state (ES) wavelengths of InAs QDs are around 1297 nm and 1201 nm, respectively. The spectral full-width at half-maximum (FWHM) of GS-QD is as large as 44 nm. Five lattice periods of 385, 388, 390, 393, and 395 nm were patterned so that resonant cavity wavelengths were within gain spectral width. Device characterizations were performed under pulsed current with repetition period of 1 ms. The pulse durations were 1 μs, 5 μs, and 50 μs for different measurements of light-current (L-I) characteristics, lasing spectra, and photonic band structures, respectively. All measurements were temperature controlled by thermoelectric cooler.

 

Fig. 2 Emission spectrum of QD sample under pulsed optical pumping at RT of 25 °C.

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3. Results and discussions

Figure 3 shows the L-I characteristics and lasing spectra of QD PCSELs with five different lattice periods at RT of 25 °C. The lowest threshold current is achieved for 390-nm PCSEL as shown in Fig. 3(a). The lasing wavelengths around threshold shown in Fig. 3(b) are about 1283, 1292.2, 1298, 1307.2 and 1313.3 nm for PCSELs with lattice periods of 385, 388, 390, 393 and 395 nm, respectively. The lasing linewidths are less than 0.1 nm for all PCSEL devices, and quality factors (Q) are over 1.3 × 10⁴. The radiation constants are estimated to be less than 15 cm⁻¹, and the calculated cavity lifetime is not larger than 10 ps. By defining the gain-cavity detuning (Δλ) between GS gain peak of QDs (λ_QD = 1297 nm) and resonant cavity wavelength of PC (λ_PC), i.e. Δλ ≡ λ_QD - λ_PC, the lowest threshold current PCSEL is associated with near-zero detuning, which follows same design rule for GaAs-based QW VCSELs [6].

 

Fig. 3 (a) L-I characteristics and (b) lasing spectra of QD PCSELs @ RT of 25 °C.

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The temperature-dependent lasing characteristics of QD PCSELs are measured and analyzed in terms of lattice periods. All electrically injected devices exhibit consistent lasing peak shift with temperature at a rate of ~0.1 nm/K, which is the same as our previously published results [7,8]. However, anomalous negative threshold temperature behavior is observed for 395-nm device. Figure 4(a) shows the L-I curves of 395-nm PCSEL with temperature varied from 20 °C to 70 °C. They are measured in steps of 5 °C but shown in steps of 10 °C for clarity. Threshold currents are determined and plotted versus temperature in Fig. 4(b). The threshold current is defined by drawing a tangent line to L-I curve at relative power level of 0.05 and by taking intersection of the straight line with current axis. Almost infinite characteristic temperature (T₀ > 1000 K) is extracted in temperature range of 20~50 °C. However, T₀ decreases to 40 K at higher temperature of 50~70 °C. Moreover, the lasing wavelength at 70 °C is as long as 1318 nm.

 

Fig. 4 (a) Temperature dependent L-I cures and (b) threshold current versus temperature for QD PCSEL with 395-nm period.

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The temperature dependence of threshold current for the other PCSELs is plotted in Fig. 5. At temperature range below 50 °C, the extracted T_o is about 28 K, 35 K, 45 K, 80 K and >1000 K for PCSELs with lattice periods of 385, 388, 390, 393 and 395 nm, respectively. As reported in [8], the rate of wavelength shift for QD gain (~0.59 nm/K) was about six times higher than that for PC cavity (~0.1 nm/K). Therefore, the positively detuned 385-nm PCSEL renders an increasing deviation away from gain peak with increasing temperature and exhibits lowest T₀ among compared devices. In the same scenario, the negatively detuned 395-nm PCSEL renders an decreasing deviation to gain peak with increasing temperature and estimates to be zero detuned at temperature around 60 °C. However, the threshold current of 395-nm PCSEL is insensitive to temperature variation only at temperature range of 20~50 °C, which is determined by interplay between reduced detuning and decreased material gain. Nonetheless, 395-nm device has larger T₀ than 393-nm device above 50 °C. The performance characteristics of investigated QD PCSELs are summarized in Table 1. The wide lasing wavelength range from 1283 nm to 1318 nm can potentially be applied in laser arrays for WDM system.

 

Fig. 5 Threshold current temperature dependence of QD PCSELs with lattice periods of (a) 385 nm, 388 nm, 390 nm, and (b) 393 nm.

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Table 1. Performance Characteristics of QD PCSELs

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Figure 6 shows below-threshold spectra of two PCSELs along Γ-X and Γ-M directions with angular step of 2° at heatsink temperature of 20 °C. They are normalized and shifted in vertical axis. Four photonic band-edge modes in the vicinity of Γ point were labeled A, B, C and D in order of decreasing wavelength. Note that modes C and D were degenerate. Noda’s group reported in experiment and simulation [9–13] that square-lattice PC with circular air holes exhibited mode-A lasing based on their multilayer structure where PC layer was buried between symmetric cladding layers. However, our “PC slab-on-substrate” structure was asymmetric with reduced top cladding thickness and etched PC layers were neither regrown nor wafer-bonded with high index semiconductor materials. We have previously revealed the photonic band structure by angle-dependent spectra measurement and identified the lasing mode of near-zero detuned 390-nm PCSEL as mode B in [5]. Here we present same measurements for positively detuned 385-nm PCSEL [see Fig. 6(a)] and negatively detuned 395-nm PCSEL [see Fig. 6(b)]. By examining the emission spectra below and above threshold, the lasing mode can be identified. All PCSELs exhibit consistent mode-B lasing whether detuning is positive or negative, large or small. New model should be constructed for “PC slab-on-substrate” structure to simulate our experimental finding.

 

Fig. 6 Angle-dependent spectra along Γ-X and Γ-M with angular step of 2° for (a) 385-nm and (b) 395-nm QD PCSELs.

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From Fig. 6(a), the respective wavelengths of band-edge modes A, B, and C/D were 1286.3, 1284.1 and 1278.5 nm, while the normalized frequencies, in unit of lattice period over wavelength, were 0.29931, 0.29982, and 0.30113, respectively. From Fig. 6(b), the respective wavelengths of band-edge modes A, B, and C/D were 1317, 1314.5, and 1308.5 nm, while the normalized frequencies were 0.29992, 0.30049, and 0.30187, respectively. The band-edge modes are analyzed, in first approximation, by the analytical formula revealed in [11]. The coupling constant κ₁, which describes 2D coupling via intermediate coupling of basic and high order waves propagating at 45° to each other, is calculated to be 2350~2450 cm⁻¹. Another coupling constant κ₃, which describes coupling of counter-propagating waves, is about 420~440 cm⁻¹. The coupling constant of our asymmetric “PC slab-on-substrate” structure is smaller than that of regrown or bonded PCSELs [2,11].

The measured and calculated far-field patterns (FFPs) exhibited azimuthal polarization and no noticeable intensity variation could be observed by merely rotating the linear polarizer; however, the imaged FFPs superimposing different linear polarization were evidently different [12,13]. Figure 7 shows the FFP of 390-nm QD PCSEL with polarizing filter inserted to Hamamatsu A3267-12 FFP system. The arrows indicate direction of linear polarizer used to filter the beam. The images without and with pseudocolor are shown in Figs. 7(a) and 7(b), respectively. The rightmost images, without polarizing filter, is analyzed to have narrow divergence angle < 1.3° at 150 mA. The doughnut-shaped beam with azimuthal polarization is therefore confirmed for our band-edge mode B lasing. Moreover, PCSELs with large area coherence were reported to exhibit single transverse mode with almost ideal Gaussian beam even under high current injection [2]. The good beam quality facilitates itself to very high coupling efficiency into single-mode fiber in a WDM system.

 

Fig. 7 Polarized FFP (a) without and (b) with pseudocolor. The rightmost images are FFP without polarizing filter.

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4. Summary

In this paper, temperature-dependent lasing characteristics of 1.3-μm-range QD PCSELs are investigated with respect to gain-cavity detuning. The cavity resonance wavelength is varied by changing the lattice period of PC. Temperature-insensitive threshold current is obtained by detuning PC cavity resonance to long-wavelength side of QD gain peak at room temperature. Almost infinite characteristic temperature is realized at certain temperature range for PCSEL with large negative gain-cavity detuning. Moreover, the lasing band-edge mode is identified as mode B instead of mode A and is irrelevant to gain-cavity detuning. The far-field distribution of lasing mode B is further characterized as doughnut-shaped beam with azimuthal polarization.

The spectral width of GS emissions from QD sample is about 44 nm; however, QD PCSEL devices exhibit lasing spectral range over 35 nm. The broad lasing range is ascribed to broad gain spectra associated with self-assembled QDs as well as high quality factor associated with photonic band-edge effect. The 1.3-μm-range QD PCSELs have advantages over QD VCSELs as the associated sample growth is less critical and device fabrication is simplified with our “PC slab-on-substrate” structure. Moreover, PCSELs with narrow and single lobed beam is superior to conventional VCSELs in fiber coupling. With continuous technological advances and improvements, GaAs-based QD PCSELs of 1.3-μm range will emerge as promising light sources in WDM arrays of optical fiber communication network.