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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 13 — Jul. 1, 2013
  • pp: 15869–15877
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Enhanced light output power of quantum cascade lasers from a tilted front facet

Sangil Ahn, Clemens Schwarzer, Tobias Zederbauer, Hermann Detz, Aaron M. Andrews, Werner Schrenk, and Gottfried Strasser  »View Author Affiliations


Optics Express, Vol. 21, Issue 13, pp. 15869-15877 (2013)
http://dx.doi.org/10.1364/OE.21.015869


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Abstract

We present a technique for enhancing the light output power of quantum cascade lasers (QCLs) by tilting of the front facet, which leads to a change of the modal reflectivity, resulting in an asymmetric light intensity distribution along the laser cavity. This asymmetry provides most of the light being emitted through one facet of the laser. An experimental study of threshold current, slope efficiency and light output power as a function of the front facet angles were performed and compared to conventional QCLs. The lasers with a front facet angle of 8° shows a 20% improved power output from the front facet.

© 2013 OSA

1. Introduction

Quantum cascade lasers (QCLs) are semiconductor heterostructure lasers built from multiple quantum wells and are reliable optical light sources in the mid-infrared (MIR) and terahertz (THz) regions of the electromagnetic spectrum [1

1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum Cascade Laser,” Science 264(5158), 553–556 (1994). [CrossRef] [PubMed]

, 2

2. S. Kumar, “Recent progress in Terahertz Quantum Cascade Lasers,” IEEE J. Quantum Electron. 17(1), 38–47 (2011). [CrossRef]

]. They have many attractive features, such as freely designable emission wavelengths, and the emission of multiple photons per electron [3

3. Y. Yao, A. J. Hoffman, and C. F. Gmachl, “Mid-infrared quantum cascade lasers,” Nat. Photonics 6(7), 432–439 (2012). [CrossRef]

]. A wide range of applications for medical, optical communication, and environment monitoring have been realized with QCLs [4

4. M. Hannemann, A. Antufjew, K. Borgmann, F. Hempel, T. Ittermann, S. Welzel, K. D. Weltmann, H. Völzke, and J. Röpcke, “Influence of age and sex in exhaled breath samples investigated by means of infrared laser absorption spectroscopy,” J. Breath. Res. 5(2), 027101 (2011). [CrossRef] [PubMed]

7

7. B. Schwarz, P. Reininger, H. Detz, T. Zederbauer, A. M. Andrews, S. Kalchmair, W. Schrenk, O. Baumgartner, H. Kosina, and G. Strasser, “A bi-functional quantum cascade device for same frequency lasing and detection,” Appl. Phys. Lett. 101(19), 191109 (2012). [CrossRef]

].

Most applications of semiconductor lasers require the emission from only one facet of the laser cavity. If identical facets are used, one facet emits only half of the total light power. Usually, the other facet is covered by high reflectivity materials to improve the feedback of the optical modes into the cavity and to insure most of the light escapes from only one facet, resulting in an improved optical output power. Under these circumstances, the enhancement of the optical power performance can be achieved by optimizing the reflectivity of the laser facets. An anti-reflection (AR) coating at the front facet and a high-reflection (HR) coating at the back facet are the most established techniques to manipulate the reflectivity [8

8. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, C. Kumar, and N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. 95(15), 151112 (2009). [CrossRef]

, 9

9. Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys. 109(5), 053103 (2011). [CrossRef]

].

Tilting the laser facet is also a useful approach to tune the reflectivity [10

10. C. E. Zah, J. S. Osinski, C. Caneau, S. G. Menocal, L. A. Reith, J. Salzman, F. K. Shokoohi, and T. P. Lee, “Fabrication and performance of 1.5µm GaInAsP travelling-wave laser amplifiers with angled facets,” Electron. Lett. 23(19), 990 (1987). [CrossRef]

]. However, the tilted facet produces light losses, since a portion of reflected light at the tilted facet is not coupled back into the laser mode [11

11. M. Troccoli, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Mid-infrared (λ≈7.4 µm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality,” Appl. Phys. Lett. 80(22), 4103 (2002). [CrossRef]

]. For this reason, the tilted facet techniques have been less desirable for optical power improvement of light sources and, instead, used limitedly for laser applications such as traveling wave amplifiers (TWA) and superluminescent light emitting diodes (SLED) [11

11. M. Troccoli, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Mid-infrared (λ≈7.4 µm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality,” Appl. Phys. Lett. 80(22), 4103 (2002). [CrossRef]

, 12

12. E. A. Zibik, W. H. Ng, D. G. Revin, L. R. Wilson, J. W. Cockburn, K. M. Groom, and M. Hopkinson, “Broadband 6 µm < λ < 8 µm superluminescent quantum cascade light-emitting diodes,” Appl. Phys. Lett. 88(12), 121109 (2006). [CrossRef]

]. Also, the tilted facet was introduced to achieve spatial beam quality control, without the power improvement, for broad area QCLs [13

13. Y. Bai, S. Slivken, Q. Y. Lu, N. Bandyopadhyay, and M. Razeghi, “Angled cavity broad area quantum cascade lasers,” Appl. Phys. Lett. 101(8), 081106 (2012). [CrossRef]

]. However, tilting one facet of two laser facets was successfully demonstrated as a competent approach to achieve light escaping predominantly from one facet [14

14. C. F. Lin, “Superluminescent diodes with angled facet etched by chemically assisted ion beam etching,” Electron. Lett. 27(11), 968 (1991). [CrossRef]

]. The difference in the reflectivities between tilted and normal facets induces an asymmetric light intensity distribution along the laser cavity, resulting in asymmetric light power emission from the facets. When the ratio of the reflectivities is high enough, most of the light will emit through the facet which has the lower reflectivity. This effect is now applied to QCLs and will be discussed in further detail later in this paper.

In this work, we investigate the effect of the tilted front facet on the power performance of QCLs. The tilted facets were fabricated by focused ion beam (FIB) milling. The FIB technique has been successfully used to demonstrate advanced optical semiconductor devices [15

15. A. O. Dirisu, G. Silva, Z. Liu, C. F. Gmachl, F. J. Towner, J. Bruno, and D. L. Sivco, “Reduction of facet reflectivity of quantum-cascade lasers with subwavelength grating,” IEEE Photon. Technol. Lett. 19(4), 221–223 (2007). [CrossRef]

, 16

16. N. Yu, R. Blanchard, J. Fan, F. Capasso, T. Edamura, M. Yamanishi, and H. Kan, “Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators,” Appl. Phys. Lett. 93(18), 181101 (2008). [CrossRef]

]. This technique provides an important advantage for experimental studies that it is possible to record the characteristics of the very same device before and after FIB milling. If a more scalable approach is demand, the same structure could be also achieved, without a usage of FIB process, by using the combination of a cleaved front facet and an etched back facet at a tilted waveguide.

2. Light intensity distribution along a laser cavity

A semiconductor laser consists of a gain medium in a cavity, including front and back facets with reflectivity Rfront and Rback. The optical modes, which are confined to the cavity, bounce back and forth between the facets, thereby enhancing the amplification process through the feedback mechanism with a constructive interference of the modes. The amplification process will finally reach a state where the optical gain equals the optical losses in the system. This is a threshold condition and the corresponding threshold current density Jth can be written as
Jth×GΓ=αwln(Rfront×Rback)/2L
(1)
where G, Γ, αw and L are indicating the gain coefficient, optical confinement factor, waveguide loss and cavity length, respectively.

2.1 As-cleaved facets and a coated front facet

2.2 A tilted front facet

3. Fabrication, measurement set up and focused ion beam (FIB)

The QCL heterostructure in this work was grown by molecular beam epitaxy (MBE). It consists of thirty-five periods of an In0.53Ga0.47As/In0.52Al0.48As two-phonon resonance active core [19

19. Z. Liu, D. Wasserman, S. S. Howard, A. J. Hoffman, C. F. 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(12), 1347–1349 (2006). [CrossRef]

]. The low-doped n-type (n = 2 × 1017 cm−3) InP substrate is acting as the lower waveguide cladding layer. The active region is sandwiched between two 500 nm InGaAs (n = 5 × 1016 cm−3) core layers. The upper waveguide cladding consists of a 1500 nm InAlAs layer (n = 1 × 1017 cm−3) followed by a 800 nm InAlAs-layer (n = 2 × 1017 cm−3) grown on top. The structure is covered by a 350 nm highly doped InGaAs layer (n = 4 × 1018 cm−3). A 100 nm InGaAs (n = 2 × 1019 cm−3) contact layer completed the QCL growth [20

20. E. Mujagić, 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(3), 031111 (2010). [CrossRef]

]. The QCL ridges were defined from a single die using contact lithography and then SiCl4/Ar reactive ion etching. Thereafter, a 300 nm thick silicon nitride (SiN) insulating layer was deposited by plasma-enhanced chemical vapor deposition. The SiN was then removed from the top of the ridges. A 500 nm Ti/Au contact finished the topside processing and a backside ohmic contact Ge/Au/Ni/Au (15/30/14/150 nm) completed the standard QCL fabrication. Ridges with cleaved facets were soldered with indium to copper heat sinks, wire bonded, and installed on a Peltier cooler to stabilize the heat sink at 293 K. The length for all the fabricated QCLs is 2.4 mm. To suppress higher order lateral modes, the width of the fabricated QCLs was chosen to be 10 µm since narrower ridges exhibit a larger discrimination against higher order lateral modes.

The lasers were operated in pulsed mode with a pulse length of 100 ns at a repetition rate of 5 kHz (0.05% duty-cycle). Optical output for the power and spectrum were measured with a calibrated deuterated triglycine sulfate detector in a Fourier transform infrared spectrometer. The emission wavelength (λ) of the QCLs is ~8 µm at 293 K. The far field measurements were carried out using a liquid-nitrogen cooled mercury cadmium telluride (MCT) detector mounted on a computer-controlled rotational stage. The active size of the detector is 1 mm. The distance between the detector and laser facet is 50 mm. All the measurements were performed at room temperature.

After the fundamental characterization steps are completed, tilted facets with angles of 4, 8, 12, 17 and 22° where fabricated (Fig. 3
Fig. 3 (a) Sketch of a QCL with a tilted front facet. Only the front facet of the QCLs was milled, while the back facet was left as cleaved. (b) A scanning electron microscope picture shows the fabricated tilted front facet (θF = 12°). ( + ) and (–) signs are defined for the far field measurement in the following section.
). Only the front facet of the QCLs was milled, while the back facet was left as cleaved. The FIB milling was performed using a ZEISS NEON 40ESB with a Ga + liquid ion source. The ion beam was accelerated at the energy of 30 keV with a beam current of 300 pA, focused onto the surface of the ridge end and raster scanned in the areas to be milled. The angle of the milled facet is defined by a computerized rotational milling pattern control system. After the FIB process, The angle of the milled facets was measured by a scanning electron microscope (SEM) imaging system. We consider that an error of ± 0.5° is possible. The FIB process is similar to the one presented in [21

21. Z. Y. Zhang, I. J. Luxmoore, C. Y. Jin, H. Y. Liu, Q. Jiang, K. M. Groom, D. T. Childs, M. Hopkinson, A. G. Cullis, and R. A. Hogg, “Effect of facet angle on effective facet reflectivity and operating characteristics of quantum dot edge emitting lasers and superluminescent light-emitting diodes,” Appl. Phys. Lett. 91(8), 081112 (2007). [CrossRef]

], Z. Y. Zhang et al. An example of a milled QCL facet is shown in Fig. 3(b), where the facet has been tilted by 12°.

Typical drawbacks of the FIB milling process are an implantation of the Ga+ ions and the damage of the surface, attributed to the exposure of the surface to high dosage Ga+ ions during the milling process. However, in our work, the direction of focused ion beam is parallel to the laser facet where the laser beam emits. Hence, the laser facet is less affected from the Ga+ ions implantation and the damage. In other words, the most damaged part is the substrate of the QCL which is not influencing to the laser performance. In order to study the effect of the tilted facets, all measurements are done on the emission from the tilted facets at room temperature before and after milling. To check for damage at the surface by the FIB milling, we compared the output power performance between cleaved and milled 0° facet angle (θF). They show nearly the same output power performance. Therefore, the FIB induced surface damage is negligible in this experiment.

4. Result and discussion

4.1 Modal reflectivity of a tilted front facet

4.2 A comparison of slope efficiencies between the front and back facets

Figure 5(a)
Fig. 5 (a) A comparison of LIV characteristics (the linear part of the LI curve just above threshold) for the emission from front and back facets of the QCL with a facet angle of 17°. (b) Ratios of the slope efficiencies between front and back facet (triangles), as a function of facet angles compared to the ratios (Eq. (2) of the power arriving at the laser facets (stars).
shows an example of light-current-voltage (LIV) characteristics for the emission from the front facet (tilted) and the back facet (as-cleaved) of the QCL with a front facet angle of 17°. In this case, 87% of the total output power is out-coupled through the tilted front facet, showing successfully the asymmetry of the light intensity distribution. In Fig. 5(b), the slope efficiency (ηs = dP / dI) ratios of front (dPfront / dI) to back (dPback / dI) facets as a function of facet angles are shown (triangles). The Pfront and Pback are light output power from the front and back facets, respectively. And they are compared to the ratios (stars) of the power arriving at the laser facets (Eq. (2)). Equation (2) can be valid for the tilted facet since (Tfront / Tback) is negligible as mentioned in a section 2.2. In order to avoid additional phenomenon in the comparison of the slope efficiencies, such as gain saturation induced by higher current injection, only the linear parts (just above threshold) of the LI curves are taken for the comparison. Up to 17°, the measured ratio of the slope efficiencies follows the ratio of power arriving at the facets. However, for the facet angle of 22°, the ratio of the slope efficiency is decreased. The emission from the tilted front facet is decreased strongly by the reduced transmission of the front facet. In the case of a plane wave, the transmission would already be zero.

4.3 Enhancement of light output power by a tilted front facet

4.4 Lateral far field profiles

In Fig. 7(a)
Fig. 7 (a) Far field profiles measured along the lateral (Y) direction [see Fig. 3(a)] of various facet angles of QCLs, driven nearly at the peak optical power. The dash line at 0° represents the ridge normal direction. (b) The beam emission angle from the ridge normal (circles) and FWHM (squares) as a function of the facet angles are shown.
, the lateral far field mode profiles of the QCLs are shown. The profiles were measured nearly at the peak optical power along the lateral (Y) direction [see Fig. 3(a)]. In order to compare precisely the emitted beam angles from the tilted facets, all measured lasers are located side by side on the same chip. The data have been normalized to allow for a comparison. The dash line at 0° represents the ridge normal direction. Positive and negative angles are defined in Fig. 3(b). The far field profiles reveal two remarkable behaviors. First, the profiles from all given facet angles are single-lobed with decreasing a full width at half maximum (FWHM) for larger facet angles, showing improved beam quality up to 17°. Second, the beam emission angles from the ridge direction are small for the facet angles up to 17°.

In Fig. 7(b), the beam emission angles (θE) from the ridge direction for θF of 4°, 8°, 12° and 17° are −4°, −3°, −6°, and −4°, respectively (−8°, −11°, −18°, and −21° from the facet normal). Based on Snell’s law, using the emission angles in the air and neff of 3.2 for the device, the incident angles into the tilted facets inside the cavity are calculated to be around 2.5°, 3.4°, 5.5° and 6.4° for θF of 4°, 8°, 12° and 17°, respectively. The far field for the θF of 22° shows an emission angle of −40° from the ridge direction (−62° from the facet normal), which would results in an incident angle of 16° into the tilted facets. These results indicate that the mode inside the cavity no longer propagates linearly along the waveguide due to the nonparallel facets. For a better understanding of the beam emission angle from the tilted front facet, better knowledge of the optical mode profile inside the laser cavity would be required. However, this is beyond the scope of this paper.

5. Conclusion

Acknowledgments

The authors acknowledge the support by the Austrian projects IR-ON (Austrian Science Fund (FWF): F2503-N17), the Austrian Nanoinitiative project PLATON, the “Gesellschaft für Mikro- und Nanoelektronik” GMe.

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(5158), 553–556 (1994). [CrossRef] [PubMed]

2.

S. Kumar, “Recent progress in Terahertz Quantum Cascade Lasers,” IEEE J. Quantum Electron. 17(1), 38–47 (2011). [CrossRef]

3.

Y. Yao, A. J. Hoffman, and C. F. Gmachl, “Mid-infrared quantum cascade lasers,” Nat. Photonics 6(7), 432–439 (2012). [CrossRef]

4.

M. Hannemann, A. Antufjew, K. Borgmann, F. Hempel, T. Ittermann, S. Welzel, K. D. Weltmann, H. Völzke, and J. Röpcke, “Influence of age and sex in exhaled breath samples investigated by means of infrared laser absorption spectroscopy,” J. Breath. Res. 5(2), 027101 (2011). [CrossRef] [PubMed]

5.

F. Capasso, R. Paiella, R. Martini, R. Colombelli, C. Gmachl, T. L. Myers, M. S. Taubman, R. M. Williams, C. G. Bethea, K. Unterrainer, H. Y. Hwang, D. L. Sivco, A. Y. Cho, A. M. Sergent, H. C. Liu, and E. A. Whittaker, “Quantum Cascade Lasers: Ultrahigh-speed operation, optical wireless communication, narrow linewidth, and far-infrared emission,” IEEE J. Quantum Electron. 38(6), 511–532 (2002). [CrossRef]

6.

G. Wysocki, R. Lewicki, R. F. Curl, F. K. Tittel, L. Diehl, F. Capasso, M. Troccoli, G. Hofler, D. Bour, S. Corzine, R. Maulini, M. Giovannini, and J. Faist, “Widely tunable mode-hop free external cavity quantum cascade lasers for high resolution spectroscopy and chemical sensing,” Appl. Phys. B 92(3), 305–311 (2008). [CrossRef]

7.

B. Schwarz, P. Reininger, H. Detz, T. Zederbauer, A. M. Andrews, S. Kalchmair, W. Schrenk, O. Baumgartner, H. Kosina, and G. Strasser, “A bi-functional quantum cascade device for same frequency lasing and detection,” Appl. Phys. Lett. 101(19), 191109 (2012). [CrossRef]

8.

R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, C. Kumar, and N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett. 95(15), 151112 (2009). [CrossRef]

9.

Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys. 109(5), 053103 (2011). [CrossRef]

10.

C. E. Zah, J. S. Osinski, C. Caneau, S. G. Menocal, L. A. Reith, J. Salzman, F. K. Shokoohi, and T. P. Lee, “Fabrication and performance of 1.5µm GaInAsP travelling-wave laser amplifiers with angled facets,” Electron. Lett. 23(19), 990 (1987). [CrossRef]

11.

M. Troccoli, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Mid-infrared (λ≈7.4 µm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality,” Appl. Phys. Lett. 80(22), 4103 (2002). [CrossRef]

12.

E. A. Zibik, W. H. Ng, D. G. Revin, L. R. Wilson, J. W. Cockburn, K. M. Groom, and M. Hopkinson, “Broadband 6 µm < λ < 8 µm superluminescent quantum cascade light-emitting diodes,” Appl. Phys. Lett. 88(12), 121109 (2006). [CrossRef]

13.

Y. Bai, S. Slivken, Q. Y. Lu, N. Bandyopadhyay, and M. Razeghi, “Angled cavity broad area quantum cascade lasers,” Appl. Phys. Lett. 101(8), 081106 (2012). [CrossRef]

14.

C. F. Lin, “Superluminescent diodes with angled facet etched by chemically assisted ion beam etching,” Electron. Lett. 27(11), 968 (1991). [CrossRef]

15.

A. O. Dirisu, G. Silva, Z. Liu, C. F. Gmachl, F. J. Towner, J. Bruno, and D. L. Sivco, “Reduction of facet reflectivity of quantum-cascade lasers with subwavelength grating,” IEEE Photon. Technol. Lett. 19(4), 221–223 (2007). [CrossRef]

16.

N. Yu, R. Blanchard, J. Fan, F. Capasso, T. Edamura, M. Yamanishi, and H. Kan, “Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators,” Appl. Phys. Lett. 93(18), 181101 (2008). [CrossRef]

17.

M. Ettenberg, H. S. Sommers, H. Kressel, and H. F. Lockwood, “Control of facet damage in GaAs laser diodes,” Appl. Phys. Lett. 18(12), 571 (1971). [CrossRef]

18.

K. Petermann, Laser diode modulation and noise, (KTH Scientific Publishers, Dordrecht, 1991), Chap. 2.4. “Lasing characteristic of Fabry-Pérot Type Laser”.

19.

Z. Liu, D. Wasserman, S. S. Howard, A. J. Hoffman, C. F. 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(12), 1347–1349 (2006). [CrossRef]

20.

E. Mujagić, 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(3), 031111 (2010). [CrossRef]

21.

Z. Y. Zhang, I. J. Luxmoore, C. Y. Jin, H. Y. Liu, Q. Jiang, K. M. Groom, D. T. Childs, M. Hopkinson, A. G. Cullis, and R. A. Hogg, “Effect of facet angle on effective facet reflectivity and operating characteristics of quantum dot edge emitting lasers and superluminescent light-emitting diodes,” Appl. Phys. Lett. 91(8), 081112 (2007). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3410) Lasers and laser optics : Laser resonators
(140.3295) Lasers and laser optics : Laser beam characterization
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: April 18, 2013
Revised Manuscript: May 25, 2013
Manuscript Accepted: May 28, 2013
Published: June 25, 2013

Citation
Sangil Ahn, Clemens Schwarzer, Tobias Zederbauer, Hermann Detz, Aaron M. Andrews, Werner Schrenk, and Gottfried Strasser, "Enhanced light output power of quantum cascade lasers from a tilted front facet," Opt. Express 21, 15869-15877 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-13-15869


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References

  1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum Cascade Laser,” Science264(5158), 553–556 (1994). [CrossRef] [PubMed]
  2. S. Kumar, “Recent progress in Terahertz Quantum Cascade Lasers,” IEEE J. Quantum Electron.17(1), 38–47 (2011). [CrossRef]
  3. Y. Yao, A. J. Hoffman, and C. F. Gmachl, “Mid-infrared quantum cascade lasers,” Nat. Photonics6(7), 432–439 (2012). [CrossRef]
  4. M. Hannemann, A. Antufjew, K. Borgmann, F. Hempel, T. Ittermann, S. Welzel, K. D. Weltmann, H. Völzke, and J. Röpcke, “Influence of age and sex in exhaled breath samples investigated by means of infrared laser absorption spectroscopy,” J. Breath. Res.5(2), 027101 (2011). [CrossRef] [PubMed]
  5. F. Capasso, R. Paiella, R. Martini, R. Colombelli, C. Gmachl, T. L. Myers, M. S. Taubman, R. M. Williams, C. G. Bethea, K. Unterrainer, H. Y. Hwang, D. L. Sivco, A. Y. Cho, A. M. Sergent, H. C. Liu, and E. A. Whittaker, “Quantum Cascade Lasers: Ultrahigh-speed operation, optical wireless communication, narrow linewidth, and far-infrared emission,” IEEE J. Quantum Electron.38(6), 511–532 (2002). [CrossRef]
  6. G. Wysocki, R. Lewicki, R. F. Curl, F. K. Tittel, L. Diehl, F. Capasso, M. Troccoli, G. Hofler, D. Bour, S. Corzine, R. Maulini, M. Giovannini, and J. Faist, “Widely tunable mode-hop free external cavity quantum cascade lasers for high resolution spectroscopy and chemical sensing,” Appl. Phys. B92(3), 305–311 (2008). [CrossRef]
  7. B. Schwarz, P. Reininger, H. Detz, T. Zederbauer, A. M. Andrews, S. Kalchmair, W. Schrenk, O. Baumgartner, H. Kosina, and G. Strasser, “A bi-functional quantum cascade device for same frequency lasing and detection,” Appl. Phys. Lett.101(19), 191109 (2012). [CrossRef]
  8. R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, C. Kumar, and N. Patel, “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings,” Appl. Phys. Lett.95(15), 151112 (2009). [CrossRef]
  9. Y. Bai, S. R. Darvish, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Optimizing facet coating of quantum cascade lasers for low power consumption,” J. Appl. Phys.109(5), 053103 (2011). [CrossRef]
  10. C. E. Zah, J. S. Osinski, C. Caneau, S. G. Menocal, L. A. Reith, J. Salzman, F. K. Shokoohi, and T. P. Lee, “Fabrication and performance of 1.5µm GaInAsP travelling-wave laser amplifiers with angled facets,” Electron. Lett.23(19), 990 (1987). [CrossRef]
  11. M. Troccoli, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Mid-infrared (λ≈7.4 µm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality,” Appl. Phys. Lett.80(22), 4103 (2002). [CrossRef]
  12. E. A. Zibik, W. H. Ng, D. G. Revin, L. R. Wilson, J. W. Cockburn, K. M. Groom, and M. Hopkinson, “Broadband 6 µm < λ < 8 µm superluminescent quantum cascade light-emitting diodes,” Appl. Phys. Lett.88(12), 121109 (2006). [CrossRef]
  13. Y. Bai, S. Slivken, Q. Y. Lu, N. Bandyopadhyay, and M. Razeghi, “Angled cavity broad area quantum cascade lasers,” Appl. Phys. Lett.101(8), 081106 (2012). [CrossRef]
  14. C. F. Lin, “Superluminescent diodes with angled facet etched by chemically assisted ion beam etching,” Electron. Lett.27(11), 968 (1991). [CrossRef]
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