Metallic periodic structure in subwavelength scale offers an exciting way to couple light into surface plasmons (SPs), thus manipulating the properties of near-field optics. We show that subwavelength metallic grating (SMG) defined on the substrate side of substrate emitting quantum cascade lasers enables far-field improvement in mid-infrared spectrum. The SMG is designed to tailor the interaction of SPs with single mode transverse magnetic light. The experiment results are in good agreement with the simulated model. A far-field full width at half maximum (FWHM) divergence angle of 3.9 ° in the direction perpendicular to the laser waveguide layers is obtained, improved by a factor of 8.5 compared with traditional surface emitting device.
© 2015 Optical Society of America
The far-field of a semiconductor laser is determined by spatial Fourier transform of near-field intensity distribution on the emitting aperture . There is an inverse correlation between emitting aperture and beam divergence angle. As with other semiconductor lasers being limited by small edge emitting size, quantum cascade lasers (QCLs) have to suffer a poor beam quality. Indirect method for reducing divergence angle is applying an optical component (lens or curved mirror) to focus or collimate the output light, which is not conducive to system miniaturization and needs delicate optical alignment. By comparison, direct improvements usually centered on broadening the emitting aperture such as tapered waveguide  and surface emitting configuration [3–6]. However, those inherent alterations may introduce extra optical losses or heat dissipation problems.
Yu et al. reported a series of results [7–9] that low divergence far-field QCLs were achieved by plasmonic collimator defined on the edge facet. The periodic metallic structure was designed in subwavelength scale to be momentum matched with input light and thus couple it into surface plasmons (SPs). Actually the subwavelength metallic structure plays a role as antenna that increases the radiating area of light, which corresponds to an expanded near-field, then transfers into a low divergence far-field. However, due to the particularity of focused ion beam (FIB) technology, it is unpractical and time-consuming to realize mass fabrication of subwavelength pattern on the edge facet. In contrast, substrate emitting configuration provides a natural broad area for traditional mask planar lithography process, representing an easier implementable way to fabricate subwavelength metallic pattern.
Here we design substrate emitting quantum cascade lasers (SEQCLs) with subwavelength metallic grating (SMG) defined on the substrate facet to improve the beam quality, as shown in Fig. 1(a). First of all, we define that x axis direction is perpendicular to the laser waveguide layers and y axis direction is along the laser waveguide layers. The substrate pattern consists of two parts: central periodic metallic grating with period being subwavelength scale and contact layer on both sides for current injection. On account of inherent low divergence in y axis direction , we only need to design and fabricate SMG along x axis direction. Furthermore, in order to obtain fundamental transverse mode (TM00) far-field in x axis direction and efficient heat dissipation, the devices should be processed into narrow ridge (the order of wavelength). Figure 1(b) shows the scanning electron microscope (SEM) image of substrate pattern. The width of SMG along x axis direction is 300 μm. SEQCLs incorporated with SMG represent a potential approach to realize flawless mid-infrared laser source with small divergence, single mode spectrum and continues wave (CW) operation.
2. Structure and simulation
SPs can be resonantly exited at the interface between a dielectric (εd) and a metal (εd) such as air and Au by the interaction between free metal surface charges and the incident electromagnetic waves. With the development of subwavelength optics [10–13], it is widely believed that periodic structure on the metal surface can lead to an enhancement of SPs excitation and light transmission. Actually, the metallic pattern offers the missing momentum km to satisfy the equation for conservation of momentum :
Where ki and θ is the wavevector and angle of incident light respectively, ksp is the momentum of SPs, n is the order of Bragg diffraction. Theoretically, for vertical incident light (θ = 0) with only considering the main first Bragg diffraction (n = 1), the strong coupling of vertical light into SPs will occur when km = ksp, which means that a fluctuant metal surface with period p would compensate the missing momentum ksp for SPs waves propagation along the metal surface. Accordingly, the metallic grating period p satisfied the Eq. (1) can be derived as p = λsp = λ[(εd + εm)(εdεm)−1]1/2, where λsp is the corresponding wavelength of SPs mode, λ is the wavelength of light in vacuum. Since the coefficient [(εd + εm)(εdεm)−1]1/2, for air and Au material, is constantly greater than 1, therefore the p should be in subwavelength scale.
A schematic including periodic metallic grating is shown in Fig. 2(a). Beam collimation occurs as follows: the vertical radiation propagates from laser core, through the substrate, to SMG. Then, one part of light directly transmits through the slits and scatters into free space due to metal diffraction. The other part couples into SPs waves and propagates along the interface between metal and air. Finally the scattered radiations from discrete silts are coherently converted into far-field. In order to investigate the effect of SPs at interface between air and SMG in detail, we performed simulation using finite-element methods (wave optics module of COMSOL Multiphysics). Two-dimension (2D) models for SMG-SEQCLs and traditional SEQCLs were created as shown in Fig. 2(b) and Fig. 2(c). The geometric size of interface between metal and air was set to be close to real devices. Moreover, to save computational memory, the light source was set as a port and the width of InP substrate was set to the same as ridge width with scattering boundary conditions. Figure 2(b) shows 2D simulation of the electric field intensity distribution of a SMG-SEQCL with 4.6 μm vertical incident laser beam. The related parameters metallic grating period p = 4.0 μm, duty cycle d/p = 70% and the refractive index of Au was set to be 3.08 + 29i . The SPs waves are strongly localized to the slits of SMG facet as shown in the left inset of Fig. 2(b), indicating strong coupling between incident light and SMG. The scattered radiations from the slits, associating with the central transmission light, form a set of coherent light array which corresponds to an expanded near-field distribution. For comparison, a similar simulation for tradition SEQCL is shown in the Fig. 2(c). The light directly transmits from the substrate facet into the air.
Parameter SPs conversion efficiency I was defined by Eq. (2) to quantitatively characterize the proportion of the incident light coupling into SPs, which equals to the ratio between WL1+L2 and the input power Winput. Another parameter total optical transmittance T was also defined by Eq. (3), which is related to the substrate emitting power.
Figure 3(a) shows the SPs conversion efficiency I changes with the metallic grating period p and Fig. 3(b) shows the corresponding optical transmittance T. The input optical power Winput is set to 1 W. Two types of coupling, unmatched coupling in blue label and matched coupling in red label, are marked in Fig. 3(a). In the unmatched region from 3.5 μm to 3.8 μm and 4.1 μm to 4.6 μm, the value of I is as small as 3-5%, indicating that the momentum provided by periodic metallic grating is insufficient or weighted to compensate the missing momentum for SPs waves propagation. Accordingly the value of T for SMG-SEQCL is about 50%, smaller than 62% for traditional SEQCL. When the grating period p locates at the matched region from 3.8 μm to 4.1 μm, I experiences a fast increase and can reach up to 41%. At the maximum value point of I when p = 4.02 μm, the SMG plays an anti-reflection role with optical transmittance T of 82% compared to the traditional SEQCL of 62%.
Figure 4 shows the electric field intensity distribution of 2 μm above the metal surface at different metallic grating period p. We choose p = 3.7 μm and p = 4.1 μm belongs to the unmatched region as example. The electric field intensity distributions for those two periods are located in the central of substrate facet as shown in Fig. 4(a) and Fig. 4(d), indicating a little conversion from light waves into SPs waves. The slight perturbation of the electric field intensity is due to the metal scattering of vertical incident light. The electric field intensity distribution are dramatically expanded when the p = 4.0 μm and p = 4.02 μm belong to the matched region, which are beneficial to far-field improvement. Considering the process accuracy, finally we choose p = 4.0 μm as the optimized metallic grating period.
3. Device fabrication
The fabrication of our devices presented in this paper started from a 4.6 μm QCL wafer. The core detail structure description can be found in . A second order distributed feedback (DFB) grating with a period of 1.45 μm was defined on the highly doped InP cladding layer using holographic lithography technique and transferred to depth of 350 nm by wet chemical etching. Then the wafer was etched into double-channel waveguide laser with an average core width of 10 μm. The following processing procedure was similar to the description in . An extra processing that SMG with period of 4.0 μm and duty cycle d/p = 70%, along x axis direction, was defined on substrate contact layer (Ge/Au/Ni/Au) with 300 μm width by conventional optical photolithography and metal lift-off process. In addition, traditional SEQCLs without SMG on the substrate were also fabricated for comparison. After the above processing, the wafer was cleaved to 2.5-mm-long bars with a high-reflectivity (HR) coating consisting of Al2O3/Ti/Au/Ti/Al2O3 (200/10/100/10/120 nm) deposited on both edge facets. Finally, the laser bars were epilayer-down bonded to copper heat sink for testing.
4. Results and discussion
Measurements for 2D far-field were done by placing a mercury cadmium telluride (MCT) detector on a 2D stepped motor control translation stage with a minimum step of 5 μm, placed at 25 cm away from the laser. Subsequently the obtained data with distance coordinate were processed into data with angle coordinate. 1D far-field intensity distribution was directly acquired by placing the laser on a rotational stage with a detector placed 90 cm away. The emitted optical power was measured with a calibrated thermopile detector that collected laser emission via a metallic tube right in front of the device. The lasing spectra measurement was performed using a Fourier transform infrared spectrometer with a resolution of 0.125 cm−1 in rapid scan mode. In all the measurements, the lasers were operated in pulsed mode with pulse width of 1 μs and duty cycle of 1%.
Based on the upper simulations of SPs wave propagation model, a broadening near-field permeating into a long range of substrate metallic pattern is obtained. Actually, the SMG structure integrated on the device plays a role in beam shaping as an optical antenna that increases the near-field radiating area of light. Hence a very low divergence far-field beyond the fundamental diffraction limit is predicted to accompany. Figure 5 shows the 2D far-field intensity distributions image of a SMG-SEQCL. An improved far-field divergence angle in x axis direction has approached the natural low divergence angle in y axis direction. The pattern of the 2D far-field for this device seems more harmonious, which represents a further improvement towards practical use for mid-infrared laser source. In order to know the accurate far-field intensity distributions, we did 1D angle scans measurement with resolution of 0.05 ° for θx and 0.005 ° for θy respectively. Figure 5 shows the 1D high resolution far-field measurement for our device. The fitted curves (θx) show the far-field of the same laser used in 2D far-field measurement in x axis direction. For comparison, a traditional SEQCL is also measured marked with dotted curve. The full width at half maximum (FWHM) divergence angle is improved from 33.4 ° to 3.9 ° for the optical modulation happened on the surface of periodic metallic grating, indicating an expaned near-field induced by the designed SMG structure.
As a narrow band wavefront engineering component, the coupling of SPs from SMG structure is sensitive to the wavelength of vertical incident light. Figure 6 shows a decreased θ(1/e2) (the far-field apertures at 1/e2) for the far-field in x axis direction when the temperature arises, corresponding to a red shift of wavelength. The inset shows the wavelength versus temperature from 290 K to 315 K with a step of 5 K. At 290 K, the θ(1/e2) is 10.7 ° with a beam quality factor  M2 = 1.6. When temperature increases to 315 K, the θ(1/e2) and M2 down to 7.2 ° and 1.3 respectively, indicating that the beam pattern of SMG-SEQCL can be regulated by the light wavelength. In addition, marked with θy, a single-lobed far-field in y axis direction is observed with an expected low FWHM divergence angle of 0.13 °.
Figure 7 shows power-current-voltage (P-I-V) characteristics of SMG-SEQCL at room temperature. As an external modulation of output light, the incorporated SMG structure should has no impact on the internal laser emission. Recently, CW mode operations [17,19] for this substrate emitting configuration have been demonstrated. Therefore, it is feasible to design and fabricate CW operation SMG-SEQCLs with low divergence in x axis direction, which is very convenient for system integration and miniaturization. In pulsed mode, SMG-SEQCL with a maximum output power of 40 mW and slope efficiency of 130 mW/A was obtained. The emitting power of SMG-SEQCL presented here is about one third of traditional SEQCL due to strong absorption of substrate metallic grating in mid-infrared spectrum. The inset shows spectra of the same laser. Single-mode emission was observed at λ~4.6 μm.
In conclusion, we have presented detailed simulations on collimation of vertical incident light by subwavelength metallic grating and demonstrated an effective improvement of the far-field in x axis direction for SMG-SEQCLs. The FWHM divergence angle reduced to 3.9 ° by a factor of 8.5 compared with traditional SEQCL. An improved beam quality factor M2 = 1.3 was measured for wavelength of 4.6 μm by increasing the operating temperature. This micro-optics integration technology is compatible with traditional planar semiconductor laser process, which means a promising way to produce high beam quality SEQCLs with short period of time and low cost.
This work was supported by the National Basic Research Program of China (Grant Nos. 2013CB632801, 2013CB632803), National Natural Science Foundation of China (Grant Nos.61435014, 61306058, 61274094) and National Scientific Instrument Developing Project of China (Grant No.2011YQ13001802-04). The authors would like to acknowledge the help of Ping Liang and Ying Hu in device processing.
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