Open Access
Add to BibSonomyAbstract
We demonstrate the fabrication and characterization of photonic-crystal distributed-feedback quantum cascade laser emitting at 4.7 μm. The tilted rectangular-lattice PCDFB structure was defined using a multi-exposure of two-beam holographic lithography. The devices exhibit the near-diffraction-limited beam emission with the full width at half maximum of the far-field divergence angles about 4.5° and 2.5° for stripe widths of 55 μm and 95 μm, respectively. Single-mode emission with a side mode suppression ratio of ≈20 dB is achieved in the temperature range (80-210 K). The single-facet output power is above 1 W for a 95 μm × 2.5 mm laser bar at 85 K in pulsed operation.
©2009 Optical Society of America
1. Introduction
Edge-emitting photonic-crystal distributed-feedback (PCDFB) lasers [1–4] capable of single-mode operation with high beam brightness in near- and mid-infrared semiconductor lasers have gained much attention in recent years for a number of applications in medicine, chemical sensing or free-space optical communications. A two dimensional weak-index perturbation is used to sustain a large area of spatial coherence, allowing for single-mode and high-power output with small beam divergence. The key element in the device fabrication lies in the PC lattice production which is usually achieved with electron-beam lithography (EBL). This would limit the wide application of PCDFB structure for its rather expensive and time-consuming preparation procedure. However, the holographic lithography (HL) technique [5–7] capable of producing of large-area periodic lattice with high efficiency and low cost is desirable to overcome this issue.
Recently, we demonstrated the feasibility of the HL technique in the fabrication of first-order 2D PCDFB quantum cascade lasers (QCLs), single mode emission at 7.8 μm with a side-mode suppression ratio (SMSR) about 20 dB was obtained [8]. In this work, we applied the HL technique to the fabrication of 2D PCDFB QCLs at 4.7 μm. High-power and single-mode operation with near-diffraction-limited far field profiles are observed, indicating that the PC lattice structure fabricated by the HL technique offers the designed optical feedback mechanism.
2. Design, growth and fabrication of PCDFB QCLs
The QCL structure was grown by solid-source molecular beam epitaxy on an n-InP (Si, 3 × 1017 cm−3) substrate. The structure consists of a 300 nm thick n-doped (6 × 1016 cm−3) InGaAs layer, a 1.57-µm-thick active region (average doping of 3.2 × 1016 cm−3), and a 300 nm thick n-doped (6 × 1016 cm−3) InGaAs layer. After defining the tilted PCDFB lattice structure on upper InGaAs layer using HL technique, a top waveguide cladding consisting of a 2.5 µm thick n-doped (1 × 1017 cm−3) InP layer and a 0.5 µm thick n-doped (6 × 1018 cm−3) InP layer was grown by metal-organic chemical-vapor deposition (MOCVD). The active region consists of 30 periods of a strain-compensated In0.67Ga0.33As/In0.37Al0.63As quantum wells and barriers with lasing wavelength at 4.7 µm [9].
Device processing started with the holographic fabrication of tilted rectangular lattices in the QCL material. Figure 1(a) shows a schematic of the lattice fabrication, the detailed experimental setups is reported elsewhere [8,10]. The t1 and t2 represent the two exposure time factors, corresponding to the long and short lattice constants, Λ1 and Λ2 respectively. We choose a tilted angle of 15° to simplify the alignment of cleaved wafer edge with fringes formed by two beam interference from the holographic system, and a first- and second-order Bragg coupling mechanism (denoted with (1,2)) for the transverse and longitudinal directions respectively. The effective index neff and the index contrast for 250-nm deep PC lattice fabricated in the upper InGaAs layer are estimated to be 3.249 and 0.0172. Lattice constants of Λ1 = 2.85 µm and Λ2 = 1.52 µm for the two directions is recorded accordingly by controlling the angles between the two holographic laser beams in the holographic system. Figure 1(b) and (c) show the calculated iso-intensity distributions of two-beam interference with double exposures, and the corresponding atomic force microscopy (AFM) images of PC lattices recorded on the photoresist. An elliptical-unit-cell lattice structure is realized when keeping t1/t2 = 1. The ellipticity E of the unit cell, defined by the ratio between the semi-major axes of the ellipse along the x and y direction (denoted as ax/by), is about 1.87, close to the ratio of Λ1/Λ2. Further increase the time ratio to t1/t2 = 3, E≈1.1 is obtained, as shown in Fig. 1(d) and (e). The two unit-cell structures possess a dramatic difference in the coupling coefficients defined in Ref. 11, as shown in Fig. 1(f). The elongation of elliptical unit cell in y direction for E≈1.1 enhances the ratios |κ2|/|κ1| and |κ2|/|κ3| compared with the case of E≈1.87. Furthermore, the range of by/Λ2 for E≈1.1 extends to 0.85 where the unit cell cross linked to each other in x direction, while the range of by/Λ2 for E≈1.87 just extends to 0.5. In practical device fabrication, lattice structure with E≈1.1 and 0.6≤by/Λ2≤0.85 is chosen to achieve the desired coupling coefficients.
Fig. 1 (a) The schematic of the lattice fabrication. (b,c) and (d,e) are calculated iso-intensity distributions of two-beam interference with double exposures, and the corresponding AFM images of PC lattices recorded on photoresist with t1/t2 = 1 and 3, respectively. (f) The coupling coefficients as a function of by/Λ2 with E≈1.1 and 1.8.
In order to define PCDFB structure on the upper InGaAs layer, a 120-nm-thick layer of Si3N4 was deposited on top of the InGaAs layer by plasma-enhanced chemical vapor deposition (PECVD). The rectangular lattice was recorded on the photoresist in the holographic system with t1/t2 = 3. This lattice pattern was transferred to Si3N4 using a CF4-based dry etch. Then the 250-nm deep PC lattice was etched in upper InGaAs layer by Ar/Cl2/BCl3 plasma in an inductively coupled plasma etching system. Figure 2(a) shows the top view of the InGaAs PC lattice. Following the removal of the residual Si3N4, the InP top waveguide cladding was grown over the lattice pattern by MOCVD. The growth cross section is shown in Fig. 2(b), where the buried lattice can be clearly observed. After the InP regrowth, W = 55 μm and 95 μm wide laser ridges were defined on the wafer with buried PC lattice using conventional photolithography and nonselective wet chemical etching. A 350-nm thick SiO2 layer was subsequently deposited around the ridges and 20-µm and 30-µm windows were opened on the two ridges for current injection. Then the Ti/Au (30 nm/120 nm) was deposited for the top contact, and a thick Au layer of 5~6 μm was electroplated on the top contact to efficiently spread the current and heat on the laser surface. The wafer was then lapped down to about 150 µm, and polished for back Au-Ge-Ni alloy (200 nm total) contact deposition. Finally, L = 2.5 mm long laser bars were cleaved and indium soldered to cooper heat sinks with the epilayer side up. The scanning electron microscopy (SEM) of the PCDFB device near the front facet is shown in Fig. 2(c). The inset shows the stripe edge near the cleaved facet. The round edge of the channel can effectively reflect the light out of the active-region plane, leaving the effective ridge reflectivity to a few percents. This would substantially reduce the FP-like filamentation in the lasing spectra.
Fig. 2 The scanning electron microscopy (SEM) pictures for (a) the PC lattice transferred to the InGaAs layer, (b) cross section of the lattice after MOCVD re-growth, (c) top view of PCDFB device near the front facet. Inset: the stripe edge after wet-etching process.
3. Experimental results and discussions
For far-field characterization, lasers were mounted on a cold head of a liquid-nitrogen cryostat which was fixed to a rotation stage with a step resolution of 0.05°. A nitrogen-cooled HgCdTe detector was located 15-cm away to collect the lasing light. A repetition rate of 5 kHz at a pulse width of 2 µs was used for the driving current. Figure 3 shows the far field distributions of the PCDFB QCLs with W = 55 μm and 95 μm at 4.5 A and 5.0 A, respectively. The full widths at half maximum (FWHM) were 4.5° and 2.5° for the two ridge widths. The inset shows that the far-field’ FWHM and position for W = 55 μm PCDFB QCLs remain nearly unchanged at different currents. The slight expansion of the far-field angles from 4.5° at 3.5 A to 4.7° at 6.5 A demonstrates that near-diffraction limited lasing beam was obtained at all working-current conditions.
Fig. 3 The far field distributions of the PCDFB QCLs with W = 55 μm and 95 μm respectively. Inset: the far-field distributions for W = 55 μm device at different driving currents.
Note that the off-facet-normal angles ∆θ of the far field distribution for both devices are about −1.5°, which are largely reduced compared with the strong off normal emission with ∆θ = −18.4°reported by Y. Bai, et al [3]. We believe the small ∆θ in this work is caused by the slight misalignment of the PC lattice with the designed tilted angle with respect to the cleaved facet, which is estimated to be −0.46° according to the Snell’s law. Further aligning the lattice with the designed tilted angle would eliminate this off normal emission.
The spectral measurements were carried out using a Fourier transform infrared (FTIR) spectrometer (Bruker Equinox 55) with a resolution of 0.5 cm−1. Single mode operation was obtained for both ridge widths. Figure 4 (a) shows the emission spectra of the PCDFB QCLs with W = 55 μm. Two-mode oscillation was observed near threshold with wavelength difference estimated as ∆λ = 7.3 nm. The value of ∆λ is very close to the stopband width of the (1, 2) order coupling at the Brillouin zone edge of point X1, as shown in the inset of Fig. 4 (a). The mode at the longer wavelength became dominate with increasing pump current. The corresponding current-wavelength-tuning coefficients for lasers with W = 55 μm and 95 μm were found to be about 2.2 nm/A and 3.4 nm/A. The about 55% higher current tuning for 95-µm ridge width is caused by the more prominent heating effect of the 2 µs long electric pulse. This wavelength-shift phenomenon related to thermal accumulation has also been observed by Faist et al [12]. Besides, the line width is almost unchanged, ranging from 1.9 nm to 2.1 nm for the two ridge widths. Figure 4(b) depicts the emission spectra at different working temperatures. Single mode operations with SMSR≈20 dB were obtained over the range of 85-210 K. The temperature-wavelength-tuning coefficients were about 0.62 nm/K and 0.79 nm/K for the two ridge widths.
Fig. 4 (a) and (b) The emission spectra of the PCDFB QCLs with W = 55 μm at different driving currents and working temperatures. Inset: TM-polarized photonic band structure for the lattice structure calculated with an effective-index based plane wave expansion method.
Figure 5(a) and (b) show the light-current-voltage (L-I-V) characteristics of PCDFB QCLs measured at different temperatures. The threshold current density (Jth) is 2.1 kA/cm2 (1.8 kA/cm2) at 85 K and increases to 3.7 kA/cm2 (3.3 kA/cm2) at 210 K for W = 55 μm (95 μm). The slope efficiencies for the two ridge widths are about 220 mW/A and 270 mW/A at 85 K. The larger Jth and the lower slope efficiency for W = 55 μm device is caused by the relatively smaller value of |κ2|W which induces significant diffraction losses at the stripe edges. Compared with the total loss of 7.6 cm−1 for a 22 µm × 2 mm Fabry-Pérot laser with the same waveguide structure [9], the loss of a 95 µm × 2.5 mm PCDFB QCL is estimated to be 24 cm−1. The most of losses originate from the diffraction loss induced by the ridge edges and the experimental error in the PC-lattice fabrication. Further improve the lattice fabrication and heating-dissipation management would improve the device performance. Despite the low slope efficiency and high loss, the PCDFB QCLs with W = 55 μm and 95 μm emitted up to 520 mW and 1030 mW at 85 K with single spectral mode operation and diffraction-limited beam qualities.
Fig. 5 (a) and (b) The light-current-voltage (L-I-V) characteristics of PCDFB QCLs measured at different temperatures for W = 55 μm and 95 μm with a driving duty cycle of 1%
4. Conclusion
To summarize we have fabricated PCDFB QCL using the holographic lithography. Compared with the electron-beam lithography, the holographic technique provides a rapid and large-area processing capability. The devices with 55-μm and 95-μm ridge widths both operate in single spectral modes with near-diffraction-limited beams (FWHM = 4.5° and 2.5° respectively) and an off-facet-normal angle about −1.5°. The spectral current- and temperature-tuning coefficients are 2.2 nm/A (3.4 nm/A) and 0.62 nm/K (0.79 nm/K) for lasers with W = 55 μm (95 μm). High-power performances with single-facet powers up to 520 mW and 1030 mW for the two devices are realized in pulsed operation at 85 K.
Acknowledgments
This work was supported by the National Research Projects of China (Grant numbers are 60525406, 60736031, 60806018, 60906026, 2006CB604903, 2007AA03Z446, and 2009AA03Z403, respectively).
References and links
1. H. Hofmann, H. Scherer, S. Deubert, M. Kamp, and A. Forchel, “Spectral and spatial single mode emission from a photonic crystal distributed feedback laser,” Appl. Phys. Lett. 90(12), 121135 (2007). [CrossRef]
2. L. Zhu, X. Sun, G. A. DeRose, A. Scherer, and A. Yariv, “Room temperature continuous wave operation of single-mode, edge-emitting photonic crystal Bragg lasers,” Opt. Express 16(2), 502–506 (2008). [CrossRef] [PubMed]
3. Y. Bai, S. R. Darvish, S. Slivken, P. Sung, J. Nguyen, A. Evans, W. Zhang, and M. Razeghi, “Electrically pumped photonic crystal distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 91(14), 141123 (2007). [CrossRef]
4. C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, C. L. Canedy, J. A. Nolde, D. C. Larrabee, I. Vurgaftman, and J. R. Meyer, “Broad-stripe, single-mode, mid-IR interband cascade laser with photonic crystal distributed feedback grating,” Appl. Phys. Lett. 92(7), 071110 (2008). [CrossRef]
5. V. Berger, O. Gauthier-Lafaye, and E. Costard, “Photonic band gaps and holography,” J. Appl. Phys. 82(1), 60–64 (1997). [CrossRef]
6. N. D. Lai, W. P. Liang, J. H. Lin, C. C. Hsu, and C. H. Lin, “Fabrication of two- and three-dimensional periodic structures by multi-exposure of two-beam interference technique,” Opt. Express 13(23), 9605–9611 (2005). [CrossRef] [PubMed]
7. Z. Poole, D. Xu, K. P. Chen, I. Olvera, K. Ohlinger, and Y. Lin, “Holographic fabrication of three dimensional orthorhombic and tetragonal photonic crystal templates using a diffractive optical element,” Appl. Phys. Lett. 91(25), 251101 (2007). [CrossRef]
8. Q. Lu, W. Zhang, L. Wang, F. Q. Liu, and Z. Wang, “Photonic crystal distributed feedback quantum cascade laser fabricated with holographic technique,” Electron. Lett. 45(1), 53–54 (2009). [CrossRef]
9. L. Li, Y. Shao, J. Q. Liu, F. Q. Liu, and Z. G. Wang, “High-power operation of uncoated strain-compensated quantum cascade laser at 4.8 μm,” Chin. Phys. Lett. 24(12), 3428–3430 (2007). [CrossRef]
10. Q. Lu, W. Zhang, L. Wang, F. Q. Liu, and Z. Wang, “Transition control from circular to elliptical unit cells for 2D photonic crystals by interference lithography,” The 34th international conference on infrared, millimeter, and Terahertz Waves, accepted, (Busan, Korea, 2009), T5E42.
11. I. Vurgaftman and J. R. Meyer, “Photonic-Crystal distributed-feedback quantum cascade lasers,” IEEE J. Quantum Electron. 38(6), 592–602 (2002). [CrossRef]
12. J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D. Sivco, J. Baillargeon, and A. Cho, “Distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 70(20), 2670–2672 (1997). [CrossRef]
