Grating-based far field modifications of ring quantum cascade lasers Rolf Szedlak,* Clemens Schwarzer, Tobias Zederbauer, Hermann Detz, Aaron Maxwell Andrews, Werner Schrenk, and Gottfried Strasser Vienna University of Technology, Institute for Solid State Electronics & Center for Micro- and Nanostructures, Floragasse 7, A-1040 Vienna, Austria *[email protected] http://www.qcllab.at/
Abstract: We present methods for beam modifications of ring quantum cascade lasers emitting around λ = 9 μ m, which are based on novel distributed feedback grating designs. This includes the creation of a rotationally symmetric far field with a central intensity maximum using an off-center grating as well as the generation of partial radially polarized emission beams induced by a rotation of the grating slits. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3295) Laser beam characterization; (140.3300) Laser beam shaping; (140.3490) Lasers, distributed-feedback; (140.3560) Lasers, ring; (140.5965) Semiconductor lasers, quantum cascade.
References and links 1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264, 553 (1994). 2. O. Cathabard, R. Teissier, J. Devenson, J. C. Moreno, and A. Baranov, “Quantum cascade lasers emitting near 2.6 μ m,” Appl. Phys. Lett. 96, 141110 (2010). 3. A. Wade, G. Fedorov, D. Smirnov, S. Kumar, B. S. Williams, Q. Hu, and L. Reno, “Magnetic-field-assisted terahertz quantum cascade laser operating up to 225 K,” Nat. Photonics 3, 141110 (2009). 4. Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room temperature continuous wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett. 93, 021103 (2008). 5. E. Mujagi´c, S. Schartner, L. Hoffmann, W. Schrenk, M. Semtsiv, M. Wienold, W. Masselink, and G. Strasser, “Grating-coupled surface emitting quantum cascade ring lasers,” Appl. Phys. Lett. 93, 011108 (2008). 6. E. Mujagi´c, L. Hoffmann, S. Schartner, M. Nobile, W. Schrenk, M. Semtsiv, M. Wienold, W. Masselink, and G. Strasser, “Low divergence single-mode surface emitting quantum cascade ring lasers,” Appl. Phys. Lett. 93, 161101 (2008). 7. E. Mujagi´c, M. Nobile, H. Detz, W. Schrenk, J. Chen, C. Gmachl, and G. Strasser, “Ring cavity induced threshold reduction in single-mode surface emitting quantum cascade lasers,” Appl. Phys. Lett. 96, 031111 (2010). 8. R. Szedlak, C. Schwarzer, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, and G. Strasser, “On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers,” Appl. Phys. Lett. 104, 151105 (2014). 9. Z. Liu, D. Wasserman, S. Howard, A. Hoffman, C. Gmachl, X. Wang, T. Tanbun-Ek, L. Cheng, and F.-S. Choa, “Room-temperature continuous-wave quantum cascade lasers grown by mocvd without lateral regrowth,” IEEE Photon. Technol. Lett. 18, 1347 (2006). 10. C. Schwarzer, E. Mujagi´c, S. I. Ahn, A. M. Andrews, W. Schrenk, W. Charles, C. Gmachl, and G. Strasser, “Grating duty-cycle induced enhancement of substrate emission from ring cavity quantum cascade lasers,” Appl. Phys. Lett. 100, 191103 (2012). 11. C. Schwarzer, R. Szedlak, S. I. Ahn, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, and G. Strasser, “Linearly polarized light from substrate emitting ring cavity quantum cascade lasers,” Appl. Phys. Lett. 103, 081101 (2013). 12. S. Li, G. Witjaksono, S. Macomber, and D. Botez, “Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift,” IEEE J. Sel. Top. Quant. Electron. 9, 1153 (2003).
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15829
13. L. Mahler, M. I. Amanti, C. Walther, A. Tredicucci, F. Beltram, J. Faist, H. E. Beere, and D. A. Ritchie, “Distributed feedback ring resonators for vertically emitting terahertz quantum cascade lasers,” Opt. Express 17, 13039 (2009). 14. S. H. Macomber, “Nonlinear Analysis of Surface-Emitting Distributed Feedback Lasers,” IEEE J. Quantum Electron. 26, 2065 (1990). 15. S. H. Macomber, J. S. Mott, B. D. Schwartz, R. S. Setzko, J. J. Powers, P. A. Lee, D. P. Kwo, R. M. Dixon, and J. E. Logue, “Curved-Grating, Surface-Emitting DFB Lasers and Arrays,” Proc. SPIE 3001, 42 (1997). 16. G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures (Les Editions de Physique, France, 1988). 17. Y. Bai, S. Tsao, N. Bandyopadhyay, S. Slivken, Q. Y. Lu, D. Caffey, M. Pushkarsky, T. Day, and M. Razeghi, “High power, continuous wave, quantum cascade ring laser,” Appl. Phys. Lett. 99, 261104 (2011).
1.
Introduction
During the last two decades quantum cascade lasers [1] (QCLs) have become sophisticated, reliable and attractive light sources in the infrared and terahertz spectral region. To this day, QCLs emitting between 2.6 μ m and 440 μ m have been demonstrated [2, 3]. This large wavelength range covers the molecular fingerprint region of the electromagnetic spectrum, where many materials show characteristic absorption due to vibrational and rotational resonances. The possibility to excite such resonances with QCLs qualifies these unipolar semiconductor lasers for several applications in chemical and gas sensing, spectroscopy, industry and medicine. Due to the presence of two atmospheric windows at around 8 − 9 μ m and 10 − 12 μ m, free-space telecommunication and other long distance utilizations within the atmosphere are possible. For such applications a light source with a high performance in terms of power, continuous wave and single-mode operation at room temperature is desirable. All these requirements can be met by QCLs [4]. Beyond that, QCLs convince with their compactness and can replace bulky spectrometers and other large measurement setups. Typically, QCLs are fabricated as Fabry-P´erot (FP) ridge lasers with two cleaved facets for the light outcoupling. However, special applications can require a more advanced laser design. Vertically emitting ring QCLs [5, 6] show enhanced performance compared to FP devices, due to their circular resonator and therefore facetless nature. The absence of facet losses reduces the threshold and increases the maximum operation temperature [7]. A second order distributed feedback (DFB) grating on top of the ring waveguide allows vertical light outcoupling. Since the emitting area of a ring QCL is larger than the facet of a FP device, the ring laser exhibits a much more collimated far field. It consists of concentric interference rings with an intensity minimum in the center and azimuthal polarization. Modifications of the properties of this beam facilitate an improved implementation in measurement setups and could even clear the way for completely new applications [8]. In this paper we present strategies and methods for such kind of beam modifications of ring QCLs emitting around λ = 9 μ m. These modification strategies are based on novel DFB grating designs, which are a grating with rotated slits and a slightly off-centered grating. 2. 2.1.
Fabrication and experiment Growth and top-down processing
The heterostructure of the device is based on an In0.52 Al0.48 As/In0.53 Ga0.47 As two phonon resonance active region [9] grown on top of an InP substrate by molecular beam epitaxy. The emission wavelength is located around 9 μ m. Top-down processing starts with the definition of the distributed feedback (DFB) grating by e-beam lithography followed by evaporation of a Ti/Au layer, which serves as a hard mask for the etching of the 1.2 μ m deep grating into the upper cladding layer. This step is performed by using reactive ion etching (RIE). The 10 μ m wide and 6 μ m deep waveguide is formed by optical lithography and subsequent RIE. For the final steps a SiN isolation layer and a Ti/Au top contact are deposited. This contact is opened #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15830
above the grating by a lift-off process to ensure surface light emission. The outer circumference of the ring is 400 μ m. A sketch of the heterostructure and the waveguide is given in Fig. 1. The 2nd order DFB grating Gold contact InAlAs
InGaAs
Isolation Active region InP substrate
Fig. 1. Schematic illustration of the heterostructure and the waveguide of a ring QCL. The active region is sandwiched between two thin InGaAs layers, the upper InAlAs cladding layer and the InP substrate. Electric contact is provided by a Ti/Au layer on top of a SiN isolation layer. The second order DFB grating on top of the waveguide enables vertical light outcoupling. The inset shows a sketch of the complete ring laser.
inset shows the complete ring QCL. The device modifications we discuss in this paper are all related to the DFB grating. This means that only the e-beam lithography is varied but all other processes do not change. The e-beam lithography of the off-center grating is moved linearly in one direction in order to introduce an offset between the center points of the grating and the waveguide. For the rotated grating slits a simple rotation would change the grating duty cycle and therefore the emitted output power [10]. Hence, the width of the slits was adapted in order to ensure a constant duty cycle and at the same time a uniform grating constant for all tilting angles. 2.2.
Measurement setup
The lasers were characterized in pulsed mode with a pulse duration of 100 ns and a repetition rate of 5 kHz. They are mounted on a Peltier cooler and operate at room temperature. For the far field measurements a liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detector is used in combination with an x-y translational stage positioned at a distance of 15 cm in front of the laser. The recorded section of the far field is a square of 24 × 24 mm. 3. 3.1.
Results and discussion Off-center grating
Recently, it has been shown that an abrupt π -phase shift in the grating period of a ring QCL introduces a far field with a central intensity maximum due to constructive interference from op-
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15831
1
π 2
1 2
0
0
− π2
1
(b) ϕ
0.1
Intensity (norm.)
(a)
Amplitude (a.u.)
Grating phase shift (rad.)
posing sides of the ring waveguide [11]. A similar breaking of the grating symmetry is used for surface emitting DFB ridge lasers in order to create single-lobe emission beams [12]. Another method to fabricate ring lasers which emit a central intensity peak is to intentionally relocate the DFB grating [13]. The grating offset results in an effective chirp of the grating. Such chirped gratings were used for single-lobe emission from straight near-infrared ridge lasers [14, 15]. In our approach the offset between grating and ring waveguide is chosen to be a quarter of the grating constant in the horizontal direction, which is around 0.7 μ m. The generated grating phase shift due to the chirp is given in Fig. 2(a). Its maximum value is +π /2 at 90◦ (top of
0.01 0
90 180 270 360 ◦ Azimuthal coordinate ϕ ( )
1140 1150 1160 1170 Wavenumbers (1/cm)
Fig. 2. (a) Grating phase shift (solid black) produced by the effective chirp due to the offset of the grating. The dashed red curve shows the amplitude of the antisymmetric mode with a continuous π -shift between the top and the bottom of the ring. This marks the symmetry breaking due to the off-center grating. (b) Spectra of an off-center ring QCL, which proves that the near and far field characteristics are produced by solely one mode. The inset shows the homogenous intensity distribution of the near field.
the ring) and its minimum value is −π /2 at 270◦ (bottom of the ring). This causes a continuous π -shift between the top and bottom of the ring which results in phase-matched wavefronts, constructive interference and a central single-lobe farfield pattern along the vertical direction. Figure 2(b) shows the single-mode spectrum of the off-center ring laser. This proves that the near and far field characteristics are not produced by an overlap of several different modes but by one mode around 1148 cm−1 only. The near field in the inset of Fig. 2(b) shows a constant intensity distribution along the ring waveguide in contrast to the abrupt π -phase shift with dark spots at the grating phase shifts [11]. In Fig. 3 the far field of an off-center ring laser is given. A scanning electron microscopy (SEM) image of the grating and the waveguide is given in the inset. In contrast to a standard ring QCL far field with an inherent intensity minimum in the center, it displays a circular symmetric far field with a central-lobe intensity pattern. Both, the π -phase shift and the off-center grating ring QCL have a central maximum in the emission beam but only the latter exhibits a circular symmetric far field with a uniform intensity distribution. Furthermore, the polarization characteristics of the central lobe are different for the two approaches. Due to the abrupt phase shifts in the grating period, the π -phase shift ring QCL shows destructive interference directly at the phase shifts. This destroys the circular symmetry. Beyond that, the far field of the laser exhibits a linearly polarized central lobe [11]. In contrast to that, the central lobe of the off-center grating device contains all polarizations. Figure 4 and Media 1 show differently polarized far fields of such a ring QCL. The polarized far fields were recorded with an external wire grid polarizer. For all orientations of the polarizer the central maximum is visible, which proves that it is formed by all polarization components. This means that for all angles ϕ it holds that the area at ϕ interferes constructively with the area at ϕ + 180◦ . #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15832
4
Experiment
Calculation
1.0
Polar angle (◦ )
0.6 0 0.4 −2
Intensity (norm.)
0.8 2
0.2 6 m
−4
0.0 −4
−2
0
2
4
Azimuthal angle (◦ ) Fig. 3. Far field of a ring QCL with an off-center grating, captured with an MCT detector. The far field shows circular symmetric interference rings with an intensity maximum in the center. This is in contrast to standard ring QCLs with an intensity minimum in the far field center. In previous methods [11] an abrupt π -phase shift was used to create a central lobed emission beam. The present method utilizes a shifted grating. The dashed square depicts the section of the polarized far fields in Fig. 4. The inset shows a SEM image of the off-center grating. The offset is approximately 0.7 μ m in the horizontal direction. The long green and the short red lines on top mark the position of the waveguide and the grating, respectively.
This is only possible with a phase shift in between, which would be visible in the near field as a darker region. Due to the fact that we measure a constant near field intensity for all ϕ , the mode has to move in azimuthal direction. In our measurements we integrate over time, which gives us circular symmetric near and far fields. We calculated the far field of such a rotating mode by shifting the near field amplitude (Fig. 2(a), dashed red) by multiples of Δϕ . For every shift a separated far field is calculated by a two dimensional Fourier transformation. In order to simulate the rotation of the mode, the separate far fields for all the multiples of Δϕ are summed up. The resulting far field is given on the left side of Fig. 3. It shows a good agreement with our experimental data. 3.2.
Tilted grating slits
Due to quantum mechanical selection rules, the electric field of the light generated in a QCL is always polarized in growth direction [16]. A DFB grating on top of the waveguide refracts the light in vertical direction, which means that the electric field vector faces along the waveguide. For a circular waveguide this results in azimuthally polarized near and far fields [11, 17]. The creation of a radially polarized emission beam could give a more detailed understanding of how the mode can be altered in the cavity and be favorable for some applications. The approach we present in this paper is based on a rotation of the DFB grating slits. In this approach standard ring QCLs without an off-center grating were utilized. This means that the far field contains two central lobes with an intensity minimum in the center. An important issue is to obtain a constant grating duty cycle for different tilting angles. This is necessary to preserve similar
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15833
0.8
Polar angle (◦ )
0.6 0.4
1 0
Intensity (norm.)
1.0
0.2
−1 0.0 −1
0
1
Azimuthal angle (◦ ) Fig. 4. Polarized far fields of the area given by the dashed square in Fig. 3 (see Media 1). The white arrows denote the transmitted polarization component. The intersection of the black lines shows the far field center, which is defined by the intensity maximum of the unpolarized far field. All polarization components show a clear intensity maximum in the center. This proves that, in contrast to a π -phase shift ring, the central lobe of an offcenter grating ring QCL contains all polarization components. For different polarizations this central lobe is rotating around the center point. The asymmetry in the beam pattern may be due to the influence of the extended contact, which was not shifted and is therefore misaligned to the grating.
DFB properties. Figure 5(a) shows a SEM image of a tilted grating with a rotation angle of αt = 30◦ . In Fig. 5(b) the principle of the varying duty cycle compensation is explained with (a)
(b) seff
αt
b
Λ Bar Slit
10 m Tilted
Standard
Fig. 5. (a) SEM image of a grating with tilted slits. The rotation angle is αt = 30◦ . (b) Comparison between standard and tilted grating. The bar width is given by b and the grating period by Λ. In order to ensure an equal grating duty cycle b/Λ for tilted and non-tilted structures, the slit width is reduced according to the rotation angle.
the grating bar width b and the grating period Λ. The slit width is reduced according to the rotation angle in order to ensure a constant effective slit width seff , resulting in a constant duty cycle b/Λ along the azimuthal direction for all rotation angles. Far fields were measured with an MCT detector and a wire grid polarizer with vertically oriented wires placed between the device and the detector. The results are shown in Fig. 6. The vertical green line shows the symmetry #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15834
0◦
1.0
10◦
0.6
Polar angle (◦ )
4
20◦
30◦
αt
0.4
2 0
Intensity (norm.)
0.8
0.2
−2 −4
0.0 −4 −2
0
2
4
Azimuthal angle (◦ ) Fig. 6. Horizontally polarized far fields of ring QCLs with a tilted grating. The rotation angle is given in the upper left corner of each MCT scan and varies from 0◦ to 30◦ . The horizontal and oblique lines show the symmetry axis of the standard and the tilted grating far fields, respectively. Presumably due to a reduced coupling strength of the grating, the 30◦ -device exhibits a noisy far field compared to devices with a lower rotation angle.
axis for a standard device and the oblique blue line shows the symmetry axis according to the rotation angle αt . This suggests that the electric field vector of the emitted light is always perpendicular to the grating slits, which act as a grid polarizer. This means that the degree of radial polarization of the emission beam is given by sin(αt ). The laser with αt = 30◦ exhibits a comparatively noisy far field due to a reduced output power, shown in the light-current (LI) plot in Fig. 7. The standard (0◦ ) and the 10◦ -device show almost the same output power. The other two devices exhibit decreasing power with increasing rotation angles. This is attributed to a distortion of the mode, a suboptimal feedback and therefore a reduction of the coupling strength of the DFB grating. This assumption is supported by the fact that for rotation angles above 30◦ all fabricated devices showed multi-mode emission. Since the duty cycle was kept constant, the power decrease cannot be attributed to power scaling due to duty cycle variations [10]. The inset in Fig. 7 displays the spectra of the four lasers. From 0◦ to 20◦ all the devices show single-mode emission at 1148.5 cm−1 . In the 30◦ -device a neighboring mode is excited.
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15835
Current (A) 1
1.5 αt αt αt αt
1130
1140
1150
2 = 0◦ = 10◦ = 20◦ = 30◦
50
40
1160
30
Wavenumbers (1/cm)
20
Optical output power (mW)
0.5
Intensity (norm.)
0
10
0
2
4
6
8
10
12
14
16
0
Current density (kA/cm2 ) Fig. 7. LI characteristics of devices with rotation angles from 0◦ to 30◦ . A reduction in output power and a slight increase of the threshold is observed for increasing angles, except for the 0◦ - and 10◦ -devices which have a similar maximum optical power. The inset shows the spectra of the devices. Up to 20◦ all lasers show single-mode emission around 1148.5 cm−1 which corresponds to a wavelength of 8.7 μ m. Solely for the 30◦ -device a neighboring mode is excited. These measurements were performed without an external polarizer. The dots in the LI mark the current densities at which the spectra in the inset and the MCT scans in Fig. 6 were recorded.
4.
Conclusion
In this paper we introduce two approaches to modify the far field pattern of a ring QCL. It has been shown that beside previous methods like the π -phase shift there is another possibility to create a central-lobe emission beam. To do so, the grating was intentionally shifted by 0.7 μ m. This acts as an effective grating chirp and introduces a continuous phase shift between the mode and the grating. The resulting far field shows an intensity maximum in the center, which contains all polarization components, suggesting a time-dependent behavior of the emission beam. In the second approach a ring QCL with a centered grating but rotated grating slits was utilized to introduce a certain degree of radial polarization. This degree depends on the tilting angle of the slits. The LI curves show that the output power is decreasing with an increasing rotation of the slits. This is attributed to a reduced coupling strength of the grating. Acknowledgments The authors acknowledge the support by the Austrian Science Fund projects Next-Lite (FWF: F49-P09) and Solids4Fun (FWF: W1243).
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15836
Abstract: We present methods for beam modifications of ring quantum cascade lasers emitting around λ = 9 μ m, which are based on novel distributed feedback grating designs. This includes the creation of a rotationally symmetric far field with a central intensity maximum using an off-center grating as well as the generation of partial radially polarized emission beams induced by a rotation of the grating slits. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3295) Laser beam characterization; (140.3300) Laser beam shaping; (140.3490) Lasers, distributed-feedback; (140.3560) Lasers, ring; (140.5965) Semiconductor lasers, quantum cascade.
References and links 1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264, 553 (1994). 2. O. Cathabard, R. Teissier, J. Devenson, J. C. Moreno, and A. Baranov, “Quantum cascade lasers emitting near 2.6 μ m,” Appl. Phys. Lett. 96, 141110 (2010). 3. A. Wade, G. Fedorov, D. Smirnov, S. Kumar, B. S. Williams, Q. Hu, and L. Reno, “Magnetic-field-assisted terahertz quantum cascade laser operating up to 225 K,” Nat. Photonics 3, 141110 (2009). 4. Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, “Room temperature continuous wave operation of quantum cascade lasers with 12.5% wall plug efficiency,” Appl. Phys. Lett. 93, 021103 (2008). 5. E. Mujagi´c, S. Schartner, L. Hoffmann, W. Schrenk, M. Semtsiv, M. Wienold, W. Masselink, and G. Strasser, “Grating-coupled surface emitting quantum cascade ring lasers,” Appl. Phys. Lett. 93, 011108 (2008). 6. E. Mujagi´c, L. Hoffmann, S. Schartner, M. Nobile, W. Schrenk, M. Semtsiv, M. Wienold, W. Masselink, and G. Strasser, “Low divergence single-mode surface emitting quantum cascade ring lasers,” Appl. Phys. Lett. 93, 161101 (2008). 7. E. Mujagi´c, M. Nobile, H. Detz, W. Schrenk, J. Chen, C. Gmachl, and G. Strasser, “Ring cavity induced threshold reduction in single-mode surface emitting quantum cascade lasers,” Appl. Phys. Lett. 96, 031111 (2010). 8. R. Szedlak, C. Schwarzer, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, and G. Strasser, “On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers,” Appl. Phys. Lett. 104, 151105 (2014). 9. Z. Liu, D. Wasserman, S. Howard, A. Hoffman, C. Gmachl, X. Wang, T. Tanbun-Ek, L. Cheng, and F.-S. Choa, “Room-temperature continuous-wave quantum cascade lasers grown by mocvd without lateral regrowth,” IEEE Photon. Technol. Lett. 18, 1347 (2006). 10. C. Schwarzer, E. Mujagi´c, S. I. Ahn, A. M. Andrews, W. Schrenk, W. Charles, C. Gmachl, and G. Strasser, “Grating duty-cycle induced enhancement of substrate emission from ring cavity quantum cascade lasers,” Appl. Phys. Lett. 100, 191103 (2012). 11. C. Schwarzer, R. Szedlak, S. I. Ahn, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, and G. Strasser, “Linearly polarized light from substrate emitting ring cavity quantum cascade lasers,” Appl. Phys. Lett. 103, 081101 (2013). 12. S. Li, G. Witjaksono, S. Macomber, and D. Botez, “Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift,” IEEE J. Sel. Top. Quant. Electron. 9, 1153 (2003).
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15829
13. L. Mahler, M. I. Amanti, C. Walther, A. Tredicucci, F. Beltram, J. Faist, H. E. Beere, and D. A. Ritchie, “Distributed feedback ring resonators for vertically emitting terahertz quantum cascade lasers,” Opt. Express 17, 13039 (2009). 14. S. H. Macomber, “Nonlinear Analysis of Surface-Emitting Distributed Feedback Lasers,” IEEE J. Quantum Electron. 26, 2065 (1990). 15. S. H. Macomber, J. S. Mott, B. D. Schwartz, R. S. Setzko, J. J. Powers, P. A. Lee, D. P. Kwo, R. M. Dixon, and J. E. Logue, “Curved-Grating, Surface-Emitting DFB Lasers and Arrays,” Proc. SPIE 3001, 42 (1997). 16. G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures (Les Editions de Physique, France, 1988). 17. Y. Bai, S. Tsao, N. Bandyopadhyay, S. Slivken, Q. Y. Lu, D. Caffey, M. Pushkarsky, T. Day, and M. Razeghi, “High power, continuous wave, quantum cascade ring laser,” Appl. Phys. Lett. 99, 261104 (2011).
1.
Introduction
During the last two decades quantum cascade lasers [1] (QCLs) have become sophisticated, reliable and attractive light sources in the infrared and terahertz spectral region. To this day, QCLs emitting between 2.6 μ m and 440 μ m have been demonstrated [2, 3]. This large wavelength range covers the molecular fingerprint region of the electromagnetic spectrum, where many materials show characteristic absorption due to vibrational and rotational resonances. The possibility to excite such resonances with QCLs qualifies these unipolar semiconductor lasers for several applications in chemical and gas sensing, spectroscopy, industry and medicine. Due to the presence of two atmospheric windows at around 8 − 9 μ m and 10 − 12 μ m, free-space telecommunication and other long distance utilizations within the atmosphere are possible. For such applications a light source with a high performance in terms of power, continuous wave and single-mode operation at room temperature is desirable. All these requirements can be met by QCLs [4]. Beyond that, QCLs convince with their compactness and can replace bulky spectrometers and other large measurement setups. Typically, QCLs are fabricated as Fabry-P´erot (FP) ridge lasers with two cleaved facets for the light outcoupling. However, special applications can require a more advanced laser design. Vertically emitting ring QCLs [5, 6] show enhanced performance compared to FP devices, due to their circular resonator and therefore facetless nature. The absence of facet losses reduces the threshold and increases the maximum operation temperature [7]. A second order distributed feedback (DFB) grating on top of the ring waveguide allows vertical light outcoupling. Since the emitting area of a ring QCL is larger than the facet of a FP device, the ring laser exhibits a much more collimated far field. It consists of concentric interference rings with an intensity minimum in the center and azimuthal polarization. Modifications of the properties of this beam facilitate an improved implementation in measurement setups and could even clear the way for completely new applications [8]. In this paper we present strategies and methods for such kind of beam modifications of ring QCLs emitting around λ = 9 μ m. These modification strategies are based on novel DFB grating designs, which are a grating with rotated slits and a slightly off-centered grating. 2. 2.1.
Fabrication and experiment Growth and top-down processing
The heterostructure of the device is based on an In0.52 Al0.48 As/In0.53 Ga0.47 As two phonon resonance active region [9] grown on top of an InP substrate by molecular beam epitaxy. The emission wavelength is located around 9 μ m. Top-down processing starts with the definition of the distributed feedback (DFB) grating by e-beam lithography followed by evaporation of a Ti/Au layer, which serves as a hard mask for the etching of the 1.2 μ m deep grating into the upper cladding layer. This step is performed by using reactive ion etching (RIE). The 10 μ m wide and 6 μ m deep waveguide is formed by optical lithography and subsequent RIE. For the final steps a SiN isolation layer and a Ti/Au top contact are deposited. This contact is opened #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15830
above the grating by a lift-off process to ensure surface light emission. The outer circumference of the ring is 400 μ m. A sketch of the heterostructure and the waveguide is given in Fig. 1. The 2nd order DFB grating Gold contact InAlAs
InGaAs
Isolation Active region InP substrate
Fig. 1. Schematic illustration of the heterostructure and the waveguide of a ring QCL. The active region is sandwiched between two thin InGaAs layers, the upper InAlAs cladding layer and the InP substrate. Electric contact is provided by a Ti/Au layer on top of a SiN isolation layer. The second order DFB grating on top of the waveguide enables vertical light outcoupling. The inset shows a sketch of the complete ring laser.
inset shows the complete ring QCL. The device modifications we discuss in this paper are all related to the DFB grating. This means that only the e-beam lithography is varied but all other processes do not change. The e-beam lithography of the off-center grating is moved linearly in one direction in order to introduce an offset between the center points of the grating and the waveguide. For the rotated grating slits a simple rotation would change the grating duty cycle and therefore the emitted output power [10]. Hence, the width of the slits was adapted in order to ensure a constant duty cycle and at the same time a uniform grating constant for all tilting angles. 2.2.
Measurement setup
The lasers were characterized in pulsed mode with a pulse duration of 100 ns and a repetition rate of 5 kHz. They are mounted on a Peltier cooler and operate at room temperature. For the far field measurements a liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detector is used in combination with an x-y translational stage positioned at a distance of 15 cm in front of the laser. The recorded section of the far field is a square of 24 × 24 mm. 3. 3.1.
Results and discussion Off-center grating
Recently, it has been shown that an abrupt π -phase shift in the grating period of a ring QCL introduces a far field with a central intensity maximum due to constructive interference from op-
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15831
1
π 2
1 2
0
0
− π2
1
(b) ϕ
0.1
Intensity (norm.)
(a)
Amplitude (a.u.)
Grating phase shift (rad.)
posing sides of the ring waveguide [11]. A similar breaking of the grating symmetry is used for surface emitting DFB ridge lasers in order to create single-lobe emission beams [12]. Another method to fabricate ring lasers which emit a central intensity peak is to intentionally relocate the DFB grating [13]. The grating offset results in an effective chirp of the grating. Such chirped gratings were used for single-lobe emission from straight near-infrared ridge lasers [14, 15]. In our approach the offset between grating and ring waveguide is chosen to be a quarter of the grating constant in the horizontal direction, which is around 0.7 μ m. The generated grating phase shift due to the chirp is given in Fig. 2(a). Its maximum value is +π /2 at 90◦ (top of
0.01 0
90 180 270 360 ◦ Azimuthal coordinate ϕ ( )
1140 1150 1160 1170 Wavenumbers (1/cm)
Fig. 2. (a) Grating phase shift (solid black) produced by the effective chirp due to the offset of the grating. The dashed red curve shows the amplitude of the antisymmetric mode with a continuous π -shift between the top and the bottom of the ring. This marks the symmetry breaking due to the off-center grating. (b) Spectra of an off-center ring QCL, which proves that the near and far field characteristics are produced by solely one mode. The inset shows the homogenous intensity distribution of the near field.
the ring) and its minimum value is −π /2 at 270◦ (bottom of the ring). This causes a continuous π -shift between the top and bottom of the ring which results in phase-matched wavefronts, constructive interference and a central single-lobe farfield pattern along the vertical direction. Figure 2(b) shows the single-mode spectrum of the off-center ring laser. This proves that the near and far field characteristics are not produced by an overlap of several different modes but by one mode around 1148 cm−1 only. The near field in the inset of Fig. 2(b) shows a constant intensity distribution along the ring waveguide in contrast to the abrupt π -phase shift with dark spots at the grating phase shifts [11]. In Fig. 3 the far field of an off-center ring laser is given. A scanning electron microscopy (SEM) image of the grating and the waveguide is given in the inset. In contrast to a standard ring QCL far field with an inherent intensity minimum in the center, it displays a circular symmetric far field with a central-lobe intensity pattern. Both, the π -phase shift and the off-center grating ring QCL have a central maximum in the emission beam but only the latter exhibits a circular symmetric far field with a uniform intensity distribution. Furthermore, the polarization characteristics of the central lobe are different for the two approaches. Due to the abrupt phase shifts in the grating period, the π -phase shift ring QCL shows destructive interference directly at the phase shifts. This destroys the circular symmetry. Beyond that, the far field of the laser exhibits a linearly polarized central lobe [11]. In contrast to that, the central lobe of the off-center grating device contains all polarizations. Figure 4 and Media 1 show differently polarized far fields of such a ring QCL. The polarized far fields were recorded with an external wire grid polarizer. For all orientations of the polarizer the central maximum is visible, which proves that it is formed by all polarization components. This means that for all angles ϕ it holds that the area at ϕ interferes constructively with the area at ϕ + 180◦ . #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15832
4
Experiment
Calculation
1.0
Polar angle (◦ )
0.6 0 0.4 −2
Intensity (norm.)
0.8 2
0.2 6 m
−4
0.0 −4
−2
0
2
4
Azimuthal angle (◦ ) Fig. 3. Far field of a ring QCL with an off-center grating, captured with an MCT detector. The far field shows circular symmetric interference rings with an intensity maximum in the center. This is in contrast to standard ring QCLs with an intensity minimum in the far field center. In previous methods [11] an abrupt π -phase shift was used to create a central lobed emission beam. The present method utilizes a shifted grating. The dashed square depicts the section of the polarized far fields in Fig. 4. The inset shows a SEM image of the off-center grating. The offset is approximately 0.7 μ m in the horizontal direction. The long green and the short red lines on top mark the position of the waveguide and the grating, respectively.
This is only possible with a phase shift in between, which would be visible in the near field as a darker region. Due to the fact that we measure a constant near field intensity for all ϕ , the mode has to move in azimuthal direction. In our measurements we integrate over time, which gives us circular symmetric near and far fields. We calculated the far field of such a rotating mode by shifting the near field amplitude (Fig. 2(a), dashed red) by multiples of Δϕ . For every shift a separated far field is calculated by a two dimensional Fourier transformation. In order to simulate the rotation of the mode, the separate far fields for all the multiples of Δϕ are summed up. The resulting far field is given on the left side of Fig. 3. It shows a good agreement with our experimental data. 3.2.
Tilted grating slits
Due to quantum mechanical selection rules, the electric field of the light generated in a QCL is always polarized in growth direction [16]. A DFB grating on top of the waveguide refracts the light in vertical direction, which means that the electric field vector faces along the waveguide. For a circular waveguide this results in azimuthally polarized near and far fields [11, 17]. The creation of a radially polarized emission beam could give a more detailed understanding of how the mode can be altered in the cavity and be favorable for some applications. The approach we present in this paper is based on a rotation of the DFB grating slits. In this approach standard ring QCLs without an off-center grating were utilized. This means that the far field contains two central lobes with an intensity minimum in the center. An important issue is to obtain a constant grating duty cycle for different tilting angles. This is necessary to preserve similar
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15833
0.8
Polar angle (◦ )
0.6 0.4
1 0
Intensity (norm.)
1.0
0.2
−1 0.0 −1
0
1
Azimuthal angle (◦ ) Fig. 4. Polarized far fields of the area given by the dashed square in Fig. 3 (see Media 1). The white arrows denote the transmitted polarization component. The intersection of the black lines shows the far field center, which is defined by the intensity maximum of the unpolarized far field. All polarization components show a clear intensity maximum in the center. This proves that, in contrast to a π -phase shift ring, the central lobe of an offcenter grating ring QCL contains all polarization components. For different polarizations this central lobe is rotating around the center point. The asymmetry in the beam pattern may be due to the influence of the extended contact, which was not shifted and is therefore misaligned to the grating.
DFB properties. Figure 5(a) shows a SEM image of a tilted grating with a rotation angle of αt = 30◦ . In Fig. 5(b) the principle of the varying duty cycle compensation is explained with (a)
(b) seff
αt
b
Λ Bar Slit
10 m Tilted
Standard
Fig. 5. (a) SEM image of a grating with tilted slits. The rotation angle is αt = 30◦ . (b) Comparison between standard and tilted grating. The bar width is given by b and the grating period by Λ. In order to ensure an equal grating duty cycle b/Λ for tilted and non-tilted structures, the slit width is reduced according to the rotation angle.
the grating bar width b and the grating period Λ. The slit width is reduced according to the rotation angle in order to ensure a constant effective slit width seff , resulting in a constant duty cycle b/Λ along the azimuthal direction for all rotation angles. Far fields were measured with an MCT detector and a wire grid polarizer with vertically oriented wires placed between the device and the detector. The results are shown in Fig. 6. The vertical green line shows the symmetry #209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15834
0◦
1.0
10◦
0.6
Polar angle (◦ )
4
20◦
30◦
αt
0.4
2 0
Intensity (norm.)
0.8
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−2 −4
0.0 −4 −2
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Azimuthal angle (◦ ) Fig. 6. Horizontally polarized far fields of ring QCLs with a tilted grating. The rotation angle is given in the upper left corner of each MCT scan and varies from 0◦ to 30◦ . The horizontal and oblique lines show the symmetry axis of the standard and the tilted grating far fields, respectively. Presumably due to a reduced coupling strength of the grating, the 30◦ -device exhibits a noisy far field compared to devices with a lower rotation angle.
axis for a standard device and the oblique blue line shows the symmetry axis according to the rotation angle αt . This suggests that the electric field vector of the emitted light is always perpendicular to the grating slits, which act as a grid polarizer. This means that the degree of radial polarization of the emission beam is given by sin(αt ). The laser with αt = 30◦ exhibits a comparatively noisy far field due to a reduced output power, shown in the light-current (LI) plot in Fig. 7. The standard (0◦ ) and the 10◦ -device show almost the same output power. The other two devices exhibit decreasing power with increasing rotation angles. This is attributed to a distortion of the mode, a suboptimal feedback and therefore a reduction of the coupling strength of the DFB grating. This assumption is supported by the fact that for rotation angles above 30◦ all fabricated devices showed multi-mode emission. Since the duty cycle was kept constant, the power decrease cannot be attributed to power scaling due to duty cycle variations [10]. The inset in Fig. 7 displays the spectra of the four lasers. From 0◦ to 20◦ all the devices show single-mode emission at 1148.5 cm−1 . In the 30◦ -device a neighboring mode is excited.
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15835
Current (A) 1
1.5 αt αt αt αt
1130
1140
1150
2 = 0◦ = 10◦ = 20◦ = 30◦
50
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Current density (kA/cm2 ) Fig. 7. LI characteristics of devices with rotation angles from 0◦ to 30◦ . A reduction in output power and a slight increase of the threshold is observed for increasing angles, except for the 0◦ - and 10◦ -devices which have a similar maximum optical power. The inset shows the spectra of the devices. Up to 20◦ all lasers show single-mode emission around 1148.5 cm−1 which corresponds to a wavelength of 8.7 μ m. Solely for the 30◦ -device a neighboring mode is excited. These measurements were performed without an external polarizer. The dots in the LI mark the current densities at which the spectra in the inset and the MCT scans in Fig. 6 were recorded.
4.
Conclusion
In this paper we introduce two approaches to modify the far field pattern of a ring QCL. It has been shown that beside previous methods like the π -phase shift there is another possibility to create a central-lobe emission beam. To do so, the grating was intentionally shifted by 0.7 μ m. This acts as an effective grating chirp and introduces a continuous phase shift between the mode and the grating. The resulting far field shows an intensity maximum in the center, which contains all polarization components, suggesting a time-dependent behavior of the emission beam. In the second approach a ring QCL with a centered grating but rotated grating slits was utilized to introduce a certain degree of radial polarization. This degree depends on the tilting angle of the slits. The LI curves show that the output power is decreasing with an increasing rotation of the slits. This is attributed to a reduced coupling strength of the grating. Acknowledgments The authors acknowledge the support by the Austrian Science Fund projects Next-Lite (FWF: F49-P09) and Solids4Fun (FWF: W1243).
#209299 - $15.00 USD (C) 2014 OSA
Received 31 Mar 2014; revised 9 May 2014; accepted 19 May 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015829 | OPTICS EXPRESS 15836
