- Email: [email protected]
High power unidirectional-emission micro-cavity lasers and their array
Accepted Manuscript Title: High power unidirectional-emission micro-cavity lasers and their array Author: Changling Yan Jianwei Shi Peng Li PII: DOI: Reference:
S0030-4026(16)31294-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.10.113 IJLEO 58385
To appear in: Received date: Accepted date:
31-8-2016 30-10-2016
Please cite this article as: Changling Yan, Jianwei Shi, Peng Li, High power unidirectional-emission micro-cavity lasers and their array, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.10.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High power unidirectional-emission micro-cavity lasers and their array
Changling Yan*, Jianwei Shi, and Peng Li
State Key Lab on High Power Semiconductor Lasers, Changchun University of Science and Technology, 7089 Weixing Road, Changchun 130022, People’s Republic of China
*To whom correspondence should be addressed: e-mail: [email protected]
1
Abstract: Unidirectional emission micro-cavity lasers and their array are reported. Quantum cascade laser material with an emission wavelength centered at about 10 μm has been shaped into compact, notched ellipse-shaped cavities to obtain unidirectional emission. Up to 13 mW of light output peak power at room temperature is obtained with a unidirectional emission at far-field divergence angle of about 6.5° at full width of half maximum from the micro-cavity laser with structure size of 120 m. Meanwhile, the micro-cavity lasers with different structure sizes are also fabricated. Their light output characteristics such as output power and threshold current density are compared and investigated in experiment. Finally the unidirectional emission micro-cavity laser array was also achieved to reach a higher power of 58mW.
Key words: micro-cavity lasers, directional emission, and laser array.
2
1. Introduction: The research on optical micro-cavity structure has been performed for many years [1][2]. Micro-cavity devices, such as micro-disk, micro-cylinder, and micro-cone resonators [3-5], in which the lights are confined with a small mode volume and high quality factor (Q-factor), have attracted lots of attention, and have been worldwide investigated not only for optoelectronic applications with densely integrated optical components but also for fundamental studies such as cavity quantum electrodynamics, chaotic ray dynamics and nonlinear optical processes. The micro-cavity lasers with whispering-gallery mode (WGM) support the ultra-low threshold and ultrahigh Q-factor by using total internal reflection mechanism, but obvious disadvantages still impede their further development, such as low optical power output and isotropic far-field profile pattern due to their circular symmetric device structures [6]. To overcome the intrinsic problems of traditional micro-cavity lasers, the micro-disk lasers with several types of deformed structures were proposed, including stadium-shaped lasers, peanut-shaped lasers, spiral-shaped lasers, a rounded isosceles triangular, and so on [7-10]. Even unidirectional emission is possible, which has been demonstrated for several shapes, e.g., the limaçon, the circle with a point scatterer, and the notched ellipse [11-13]. The notched ellipse shows a highly directional emission at far-field divergence angle of about 6 degree with light output peak power of about 5mW [13]. Besides directional emission feature, a higher light output power is also expected in the laser applications; therefore, in this paper, unidirectional emitting notched ellipse-shaped cavity lasers with large structure size, even up to 150 m, are investigated for improvement of the light output power. In experiment, mid-infrared InP based InGaAs/InAlAs quantum cascade laser material at about 10m wavelength was employed to fabricate these micro-cavity lasers. The micro-cavity lasers with different structure sizes were presented and demonstrated on their output emission characteristics such as light output power and threshold current density. Since the micro-cavity laser device is capable of achieving a highly unidirectional light output emission, there is an excellent potential for realizing an array device. Finally, a micro-cavity laser array has also been presented to reach a higher output 3
power level.
2. Device structure and fabrication: Micro-cavity lasers with directional emision have been demonstrated with a wide range of laser materials showing many interesting results, such as InGaAsP/InP semiconductor lasers, quantum dot lasers, quantum cascade lasers, and even all-polymer lasers [14-16]. The quantum cascade laser material, however, is expected to have advantages in connection with the disk geometry since they are especially suited for the disk geometry due to their lack of surface recombination with the unipolar nature of carrier transport, inherently in-plane transverse-magnetic (TM) mode emission, and the flexibility fabrication of micro-cavity lasers with smooth sidewall quality [15]. The unidirectional emission notched ellipse-shaped cavity structure was fabricated on Ga0.47In0.53As/Al0.48In0.52As quantum cascade laser material at an emission wavelength of about 10 m, which was grown by molecular beam epitaxy (MBE) on top of an n-doped InP substrate. The laser material was formed with alternate stacks of an active region formed by coupled quantum wells and superlattice injector region that serves as an energy relaxation region for electrons exiting the previous stage. The active region was sandwiched between two n-doped (3×1016 cm−3) InGaAs layers, which were followed by 3.5 μm-thick n-doped (1×1017 cm−3) InP cladding layers. A 0.5 μm-thick n-doped (5×1018 cm−3) InP layer was deposited on top of the cladding layer, followed by a highly n-doped (1×1019 cm−3) InGaAs contact layer to obtain good ohmic contact. The contour of a micro-cavity structure was formed by using a standard photolithography process, and a schematic diagram of a notched ellipse-shaped cavity structure is shown in Fig. 1(a). The notch is located at the intersection of the short axis and the boundary of the ellipse resonator with the long-to-short aspect ratio Y/X=1.2. The notch scatters the light waves travelling around the ellipse resonator to make a unidirectional emission and the notch size should be comparable to the wavelength in the micro-cavity lasing material [13]. The notch size was chosen as W=4 m and D=2 m in our devices, where W and D are 4
the width and the depth of the notch, respectively. In order to obtain high light output power, the micro-cavity laser structure with large structure size was employed in our experimental device process, up to 120 m for the half short axis of the ellipse-shaped cavity structure. The cavity structures were etched through the gain medium with a depth of about 10 μm by using dry etching process. The top metal contacts Ti/Au (10 nm/200 nm) were deposited by using electron-beam evaporation, and the substrate was subsequently thinned down to about 160 m, then the bottom Ti-Au contact was deposited. A scanning electron microscope (SEM) image of the micro-cavity device is shown in Fig.1 (b). After the device processing, the micro-cavity laser devices were cleaved and bounded on copper heatsinks in epi-layer up configuration for further laser performance testing.
3. Device characterization: After sample fabrication, we measured the light emission characteristics of the unidirectional-emission micro-cavity lasers under the pulse mode operation condition with 200 ns current pulses and 10-kHz repetition frequency at room temperature. The experimental measurement of the light output power-injection current (L-I) characteristic was carried out by using a calibrated thermopile power meter, and the light output power was collected in the plane of the micro-cavity laser with a collection angle of about 100°. Fig. 2 shows the light output peak power of the micro-cavity laser as function of the electrical injection current with the thermopile power meter positioned around 0° line of the micro-cavity laser. The light output peak power of the micro-cavity laser with the structure size of 120 m is up to 13.0 mW, and the threshold current of the micro-cavity laser is about 1.15 A, corresponding to the threshold current density of 2.12 kA/cm2, which is lower than the threshold current density of 2.98 kA/cm2 , (Ith=1.61 A), for the traditional edge-emitting ridge F-P cavity quantum cascade laser fabricated from the same wafer material with a length of 3.0mm and 18m width as shown in the up-left inset Fig.2, due to the presence of WGMs in the micro-cavity laser structure, which is supported by the following spectral characteristic of the device. However, the light output power of the 5
micro-cavity laser is still lower than that of the traditional edge-emitting ridge F-P cavity laser. Therefore it is necessary to implore another way to further improve the light output of the micro-cavity lasers. The down-right inset Fig. 2 shows the lasing spectrum of the micro-cavity laser, which was obtained by employing a high-resolution Fourier transform infrared spectrometer. The lasing peak wavelength is at 10.03 μm (wave number: 997.39 cm-1) with multimode lasing performance, and the average mode spacing is approximately 3.8 cm−1, which mainly agrees with the calculated value 3.76 cm−1 for WGMs, given by 1/(Lneff), where L is the perimeter of the structure and neff is the effective refraction index of material. The deviation could be attributed to small uncertainties in the fabricated micro-cavity structure size, effective refractive index, or the simplicity of the model as well. To obtain the far-field profile in the plane perpendicular to the QCL growth direction of the micro-cavity laser, a setup based on a motorized rotation stage was employed, and the micro-cavity laser was mounted at the centre of a rotation stage and a mid-infrared mercury-cadmium telluride detector was placed about 20 cm away to measure light output emission of the micro-cavity laser, and the measurement would be carried out over 360 scanning range with a rotating step of 0.5. The measured parallel far-field pattern of the micro-cavity laser is shown in the left inset Fig. 3, and a highly unidirectional emission lobe was observed at about 0 line in the polar coordinate system. The zoom-in far-field emission pattern around the unidirectional emission lobe is also shown in the Fig.3 in the Cartesian coordinate system, and the far-field divergence angle at full width of half maximum (FWHM) of the highly unidirectional emission lobe at about 0 line is about 6.5°. The notched ellipse micro-cavity laser shows a highly unidirectional emission characteristic; however a lower light output power is still its weakness for its future practical applications. In order to improve the light output power, a larger structure size of up to 150 m was also used into the device fabrication with the same material as above, and the same wavelength-scale notch size of W=4 m and D=2 m was chose during the device procedure. The micro-cavity lasers with different structure 6
sizes were pumped under the identical operation pulse mode of 200 ns and 10 kHz rate at room temperature, the light output peak power and threshold current density of the micro-cavity lasers with different structure sizes were characterized and compared in experiment, and the experimental comparison results are shown in Fig. 4. From Fig.4, it is obvious that the light output peak power is about 15 mW for the micro-cavity laser with structure size of 150m, which also shows a good unidirectional emission characteristic, while that is only 2.0 mW for the laser with structure size of 60 m. Therefore, a higher output power was practically obtained by increasing the micro-cavity active region volume through enlarging the device structure size. Meanwhile, the threshold current density of the micro-cavity laser with different structure sizes was also compared, and the threshold current density as a function of structure size was shown in left inset Fig. 4. It is very clear that the threshold current density of the micro-cavity disk laser rapidly goes up with increasing the micro-cavity structure size, and the ascent of threshold current density with structure size could be attributed to the poor injection current pumping efficiency in the micro-cavity laser with large structure size owing to the WGM waves travelling along with the boundary of the cavity. Therefore simply employing larger cavity structure size may not be a good choice to obtain higher light output power. As the notched ellipse-shaped micro-cavity laser shows a highly unidirectional emission feature as shown in Fig.3, an excellent potential is revealed to obtain total high light output power through employing a laser array technique. Therefore, the unidirectional-emitting micro-cavity laser array devices were fabricated with 120 m-size micro-cavity laser elements separated by about 300 m distance from the same material, whose emission directions were paralleled straight forwards. Fig. 5 shows the measured light output peak power for the micro-cavity laser arrays, and as expected, the array technique offers a high total light output power. The light output peak power of the micro-cavity laser array is about 33 mW for 31 array, as shown in Fig. 5, and up to 58mW for 51 array device as shown in left inset Fig.5, respectively, which is slightly lower than 5 times the value of the single micro-cavity laser as shown in Fig.2. The slight lower performance was most likely due to the thermal 7
interaction between elements in the laser array device. Further study work on laser array technique will be performed in the future such as the optimization of the inter-element width of the micro-cavity laser array and thermal characteristics, and the light output power reported here will be significantly improved after more elements are integrated into a laser array.
4. Conclusion: In summary, unidirectional-emission micro-cavity lasers on quantum cascade laser material of about 10m wavelength have been fabricated and investigated. The light output power of the micro-cavity laser is increased to 13mW with a unidirectional emission of about 6.5°, for the laser with structure size of 120m, and even up to 15mW for the laser with structure size of 150m; however simply employing larger cavity structure size may not be a good choice to obtain higher light output power, since the device with large structure size suffers higher threshold current density and the poor injection current pumping efficiency. In order to reach a higher output power level, a simple micro-cavity laser array model has been developed because of the unidirectional emission feature of the micro-cavity laser, and the light output peak power of 58mW was achieved with the micro-cavity laser 51 array.
Acknowledgments: This work was partially supported by the National Natural Science Foundation of China under Grant No. 61376045, the Jilin Provincial Science & Technology Department under Grant No. 20160101254JC, and the Changchun University of Science and Technology under Grant No. XJJLG-2015-10.
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References [1] W. Wang, T. Chu, E. Little, A. Pasquazi, Y. S. Wang, L. R. Wang, W. F. Zhang, L. Wang, X. H. Hu, and G. X. Wang, Dual-pump Kerr Micro-cavity Optical Frequency Comb with varying FSR spacing, Scientific Reports 6 (2016) 28501. [2] D. Pluchon, N. Huby, L. Frein, A. Moreac, P. Panizza, and B. Beche, Flexible beam-waist technique for whispering gallery modes excitation in polymeric 3D micro-resonators, OPTIK 124 (2016) 2085-2088. [3] Y. Y. Zhang, Z. T. Ma, X. H. Zhang, T. Wang, and H. W. Choi, Optically pumped whispering-gallery mode lasing from 2-um GaN micro-disks pivoted on Si, Appl. Phys. Lett. 104 (2014) 221106. [4] Z. M. Yin, X. Y. Liu, H. C. Yao, Y. Z. Wu, X. P. Hao, M. Han, and X. G. Xu, Light Extraction Enhancement of GaN LEDs by Hybrid ZnO Micro-Cylinders and Nanorods Array, IEEE Photon. Technol. Lett. 25 (2013) 1989-1992. [5] Y. Zuta, I. Goykhman, B. Desiatov, and U. Levy, On-chip switching of a silicon nitride micro-ring resonator based on digital microfluidics platform, Opt. Express 18 (2010) 24762-24769. [6] J.Wiersig and M. Hentschel, Combining Directional Light Output and Ultralow Loss in Deformed Microdisks, Phys. Rev. Lett., 100 (2008) 033901. [7] L. Y. Liu, L. Shangm and L. Xu, Proceedings of 2008 10th anniversary international conference on transparent optical networks 4 (2008) 217-217. [8] M. Kneissl, M. Teepe, N. Miyashita, N. Johnson, G. Chern and R. Chang, Current-injection spiral-shaped microcavity disk laser diodes with unidirectional emission, Appl. Phys. Lett. 84 (2004) 2485-2487. [9] C. Gmachl, F. Capasso, E. Narimanov, Jens U. Nöckel, A. Douglas Stone, Jérôme Faist, Deborah L. Sivco, and Alfred Y. Cho, High-Power Directional Emission from Microlasers with Chaotic Resonators, Science 280 (1998) 1556–1564. [10] M. S. Kurdoglyan, S. Lee, S. Rim, and C. Kim, Unidirectional lasing from a microcavity with a rounded isosceles triangle shape, Opt. Lett. 29 (2004) 2758–2760. [11] Q. J. Wang, C. Yan, L. Diehl, M. Hentschel, J. Wiersig, N. F. Yu, C. Pflugl, M. A. Belkin, T. Edamura, M. Yamanishi, H. Kan, and F. Capasso, Deformed microcavity 9
quantum cascade lasers with directional emission, New J. Phys. 11(2009) 125018. [12] C. P. Dettmann, G. V. Morozov, M. Sieber, and H. Waalkens, Unidirectional emission from circular dielectric microresonators with a point scatterer, Phys. Rev. A 80 (2009) 063813. [13] Q. J. Wang, C. L. Yan, N. F. Yu, J. Unterhinninghofen, J. Wiersig, C. Pflugl, L. Diehl, T. Edamura, M. Yamanishi, H. Kan, and F. Capasso, Whispering gallery mode resonators for highly unidirectional laser action, P. Natl. Acad. Sci. USA 107 (2010) 22407–22412. [14] M. Raj, N. Serizawa, J. Wiedmann, and S. Arai, Theoretical analysis of GaInAsP/InP multiple micro-cavity laser, Japanese Journal of Applied Physics, 39 (2000) 5847-5854. [15] J. Faist, C. Gmachl, M. Striccolim C. Sirtori, F. Capasso, D. Sivco, and A. Cho, Quantum cascade disk lasers, Appl. Phys. Lett. 69 (1996) 2456-2458. [16] G. Canazza, F. Scotognella, G. Lanzani, S. D. Silvestri, M. Zavelani-Rossi and D. Comoretto, Lasing from all-polymer microcavities, Laser Phys. Lett., 11 (2014) 035804.
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A list of captions for figures: Fig. 1 (a) Schematic diagram of a notched ellipse-shaped micro-cavity structure; (b) A scanning electronic microscope image of a notched ellipse-shaped micro-cavity structure.
Fig.2 Light output peak power as a function of injection current for the unidirectional-emission micro-cavity laser; Up-left inset: L-I characteristic of the traditional edge-emitting ridge F-P cavity laser fabricated from the same wafer material with a length of 3.0mm and 18um width; Down-right inset: Light spectrum of the micro-cavity laser.
Fig.3 Parallel far-field pattern of the unidirectional-emitting micro-cavity laser in Cartesian coordinate system; Left inset: parallel far-field pattern of the micro-cavity laser in polar coordinate system.
Fig.4 Light output peak power of micro-cavity lasers as a function of the laser structure size; Left inset: threshold current density of the micro-cavity lasers as a function of the laser structure size.
Fig. 5 Light output peak power as a function of injection current for the micro-cavity laser 31 array; Left inset: light output power for the micro-cavity laser 51array.
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Fig. 1
900 Y W X
D
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(b)
12
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Fig. 3
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Fig. 4
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Fig. 5
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S0030-4026(16)31294-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.10.113 IJLEO 58385
To appear in: Received date: Accepted date:
31-8-2016 30-10-2016
Please cite this article as: Changling Yan, Jianwei Shi, Peng Li, High power unidirectional-emission micro-cavity lasers and their array, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.10.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High power unidirectional-emission micro-cavity lasers and their array
Changling Yan*, Jianwei Shi, and Peng Li
State Key Lab on High Power Semiconductor Lasers, Changchun University of Science and Technology, 7089 Weixing Road, Changchun 130022, People’s Republic of China
*To whom correspondence should be addressed: e-mail: [email protected]
1
Abstract: Unidirectional emission micro-cavity lasers and their array are reported. Quantum cascade laser material with an emission wavelength centered at about 10 μm has been shaped into compact, notched ellipse-shaped cavities to obtain unidirectional emission. Up to 13 mW of light output peak power at room temperature is obtained with a unidirectional emission at far-field divergence angle of about 6.5° at full width of half maximum from the micro-cavity laser with structure size of 120 m. Meanwhile, the micro-cavity lasers with different structure sizes are also fabricated. Their light output characteristics such as output power and threshold current density are compared and investigated in experiment. Finally the unidirectional emission micro-cavity laser array was also achieved to reach a higher power of 58mW.
Key words: micro-cavity lasers, directional emission, and laser array.
2
1. Introduction: The research on optical micro-cavity structure has been performed for many years [1][2]. Micro-cavity devices, such as micro-disk, micro-cylinder, and micro-cone resonators [3-5], in which the lights are confined with a small mode volume and high quality factor (Q-factor), have attracted lots of attention, and have been worldwide investigated not only for optoelectronic applications with densely integrated optical components but also for fundamental studies such as cavity quantum electrodynamics, chaotic ray dynamics and nonlinear optical processes. The micro-cavity lasers with whispering-gallery mode (WGM) support the ultra-low threshold and ultrahigh Q-factor by using total internal reflection mechanism, but obvious disadvantages still impede their further development, such as low optical power output and isotropic far-field profile pattern due to their circular symmetric device structures [6]. To overcome the intrinsic problems of traditional micro-cavity lasers, the micro-disk lasers with several types of deformed structures were proposed, including stadium-shaped lasers, peanut-shaped lasers, spiral-shaped lasers, a rounded isosceles triangular, and so on [7-10]. Even unidirectional emission is possible, which has been demonstrated for several shapes, e.g., the limaçon, the circle with a point scatterer, and the notched ellipse [11-13]. The notched ellipse shows a highly directional emission at far-field divergence angle of about 6 degree with light output peak power of about 5mW [13]. Besides directional emission feature, a higher light output power is also expected in the laser applications; therefore, in this paper, unidirectional emitting notched ellipse-shaped cavity lasers with large structure size, even up to 150 m, are investigated for improvement of the light output power. In experiment, mid-infrared InP based InGaAs/InAlAs quantum cascade laser material at about 10m wavelength was employed to fabricate these micro-cavity lasers. The micro-cavity lasers with different structure sizes were presented and demonstrated on their output emission characteristics such as light output power and threshold current density. Since the micro-cavity laser device is capable of achieving a highly unidirectional light output emission, there is an excellent potential for realizing an array device. Finally, a micro-cavity laser array has also been presented to reach a higher output 3
power level.
2. Device structure and fabrication: Micro-cavity lasers with directional emision have been demonstrated with a wide range of laser materials showing many interesting results, such as InGaAsP/InP semiconductor lasers, quantum dot lasers, quantum cascade lasers, and even all-polymer lasers [14-16]. The quantum cascade laser material, however, is expected to have advantages in connection with the disk geometry since they are especially suited for the disk geometry due to their lack of surface recombination with the unipolar nature of carrier transport, inherently in-plane transverse-magnetic (TM) mode emission, and the flexibility fabrication of micro-cavity lasers with smooth sidewall quality [15]. The unidirectional emission notched ellipse-shaped cavity structure was fabricated on Ga0.47In0.53As/Al0.48In0.52As quantum cascade laser material at an emission wavelength of about 10 m, which was grown by molecular beam epitaxy (MBE) on top of an n-doped InP substrate. The laser material was formed with alternate stacks of an active region formed by coupled quantum wells and superlattice injector region that serves as an energy relaxation region for electrons exiting the previous stage. The active region was sandwiched between two n-doped (3×1016 cm−3) InGaAs layers, which were followed by 3.5 μm-thick n-doped (1×1017 cm−3) InP cladding layers. A 0.5 μm-thick n-doped (5×1018 cm−3) InP layer was deposited on top of the cladding layer, followed by a highly n-doped (1×1019 cm−3) InGaAs contact layer to obtain good ohmic contact. The contour of a micro-cavity structure was formed by using a standard photolithography process, and a schematic diagram of a notched ellipse-shaped cavity structure is shown in Fig. 1(a). The notch is located at the intersection of the short axis and the boundary of the ellipse resonator with the long-to-short aspect ratio Y/X=1.2. The notch scatters the light waves travelling around the ellipse resonator to make a unidirectional emission and the notch size should be comparable to the wavelength in the micro-cavity lasing material [13]. The notch size was chosen as W=4 m and D=2 m in our devices, where W and D are 4
the width and the depth of the notch, respectively. In order to obtain high light output power, the micro-cavity laser structure with large structure size was employed in our experimental device process, up to 120 m for the half short axis of the ellipse-shaped cavity structure. The cavity structures were etched through the gain medium with a depth of about 10 μm by using dry etching process. The top metal contacts Ti/Au (10 nm/200 nm) were deposited by using electron-beam evaporation, and the substrate was subsequently thinned down to about 160 m, then the bottom Ti-Au contact was deposited. A scanning electron microscope (SEM) image of the micro-cavity device is shown in Fig.1 (b). After the device processing, the micro-cavity laser devices were cleaved and bounded on copper heatsinks in epi-layer up configuration for further laser performance testing.
3. Device characterization: After sample fabrication, we measured the light emission characteristics of the unidirectional-emission micro-cavity lasers under the pulse mode operation condition with 200 ns current pulses and 10-kHz repetition frequency at room temperature. The experimental measurement of the light output power-injection current (L-I) characteristic was carried out by using a calibrated thermopile power meter, and the light output power was collected in the plane of the micro-cavity laser with a collection angle of about 100°. Fig. 2 shows the light output peak power of the micro-cavity laser as function of the electrical injection current with the thermopile power meter positioned around 0° line of the micro-cavity laser. The light output peak power of the micro-cavity laser with the structure size of 120 m is up to 13.0 mW, and the threshold current of the micro-cavity laser is about 1.15 A, corresponding to the threshold current density of 2.12 kA/cm2, which is lower than the threshold current density of 2.98 kA/cm2 , (Ith=1.61 A), for the traditional edge-emitting ridge F-P cavity quantum cascade laser fabricated from the same wafer material with a length of 3.0mm and 18m width as shown in the up-left inset Fig.2, due to the presence of WGMs in the micro-cavity laser structure, which is supported by the following spectral characteristic of the device. However, the light output power of the 5
micro-cavity laser is still lower than that of the traditional edge-emitting ridge F-P cavity laser. Therefore it is necessary to implore another way to further improve the light output of the micro-cavity lasers. The down-right inset Fig. 2 shows the lasing spectrum of the micro-cavity laser, which was obtained by employing a high-resolution Fourier transform infrared spectrometer. The lasing peak wavelength is at 10.03 μm (wave number: 997.39 cm-1) with multimode lasing performance, and the average mode spacing is approximately 3.8 cm−1, which mainly agrees with the calculated value 3.76 cm−1 for WGMs, given by 1/(Lneff), where L is the perimeter of the structure and neff is the effective refraction index of material. The deviation could be attributed to small uncertainties in the fabricated micro-cavity structure size, effective refractive index, or the simplicity of the model as well. To obtain the far-field profile in the plane perpendicular to the QCL growth direction of the micro-cavity laser, a setup based on a motorized rotation stage was employed, and the micro-cavity laser was mounted at the centre of a rotation stage and a mid-infrared mercury-cadmium telluride detector was placed about 20 cm away to measure light output emission of the micro-cavity laser, and the measurement would be carried out over 360 scanning range with a rotating step of 0.5. The measured parallel far-field pattern of the micro-cavity laser is shown in the left inset Fig. 3, and a highly unidirectional emission lobe was observed at about 0 line in the polar coordinate system. The zoom-in far-field emission pattern around the unidirectional emission lobe is also shown in the Fig.3 in the Cartesian coordinate system, and the far-field divergence angle at full width of half maximum (FWHM) of the highly unidirectional emission lobe at about 0 line is about 6.5°. The notched ellipse micro-cavity laser shows a highly unidirectional emission characteristic; however a lower light output power is still its weakness for its future practical applications. In order to improve the light output power, a larger structure size of up to 150 m was also used into the device fabrication with the same material as above, and the same wavelength-scale notch size of W=4 m and D=2 m was chose during the device procedure. The micro-cavity lasers with different structure 6
sizes were pumped under the identical operation pulse mode of 200 ns and 10 kHz rate at room temperature, the light output peak power and threshold current density of the micro-cavity lasers with different structure sizes were characterized and compared in experiment, and the experimental comparison results are shown in Fig. 4. From Fig.4, it is obvious that the light output peak power is about 15 mW for the micro-cavity laser with structure size of 150m, which also shows a good unidirectional emission characteristic, while that is only 2.0 mW for the laser with structure size of 60 m. Therefore, a higher output power was practically obtained by increasing the micro-cavity active region volume through enlarging the device structure size. Meanwhile, the threshold current density of the micro-cavity laser with different structure sizes was also compared, and the threshold current density as a function of structure size was shown in left inset Fig. 4. It is very clear that the threshold current density of the micro-cavity disk laser rapidly goes up with increasing the micro-cavity structure size, and the ascent of threshold current density with structure size could be attributed to the poor injection current pumping efficiency in the micro-cavity laser with large structure size owing to the WGM waves travelling along with the boundary of the cavity. Therefore simply employing larger cavity structure size may not be a good choice to obtain higher light output power. As the notched ellipse-shaped micro-cavity laser shows a highly unidirectional emission feature as shown in Fig.3, an excellent potential is revealed to obtain total high light output power through employing a laser array technique. Therefore, the unidirectional-emitting micro-cavity laser array devices were fabricated with 120 m-size micro-cavity laser elements separated by about 300 m distance from the same material, whose emission directions were paralleled straight forwards. Fig. 5 shows the measured light output peak power for the micro-cavity laser arrays, and as expected, the array technique offers a high total light output power. The light output peak power of the micro-cavity laser array is about 33 mW for 31 array, as shown in Fig. 5, and up to 58mW for 51 array device as shown in left inset Fig.5, respectively, which is slightly lower than 5 times the value of the single micro-cavity laser as shown in Fig.2. The slight lower performance was most likely due to the thermal 7
interaction between elements in the laser array device. Further study work on laser array technique will be performed in the future such as the optimization of the inter-element width of the micro-cavity laser array and thermal characteristics, and the light output power reported here will be significantly improved after more elements are integrated into a laser array.
4. Conclusion: In summary, unidirectional-emission micro-cavity lasers on quantum cascade laser material of about 10m wavelength have been fabricated and investigated. The light output power of the micro-cavity laser is increased to 13mW with a unidirectional emission of about 6.5°, for the laser with structure size of 120m, and even up to 15mW for the laser with structure size of 150m; however simply employing larger cavity structure size may not be a good choice to obtain higher light output power, since the device with large structure size suffers higher threshold current density and the poor injection current pumping efficiency. In order to reach a higher output power level, a simple micro-cavity laser array model has been developed because of the unidirectional emission feature of the micro-cavity laser, and the light output peak power of 58mW was achieved with the micro-cavity laser 51 array.
Acknowledgments: This work was partially supported by the National Natural Science Foundation of China under Grant No. 61376045, the Jilin Provincial Science & Technology Department under Grant No. 20160101254JC, and the Changchun University of Science and Technology under Grant No. XJJLG-2015-10.
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A list of captions for figures: Fig. 1 (a) Schematic diagram of a notched ellipse-shaped micro-cavity structure; (b) A scanning electronic microscope image of a notched ellipse-shaped micro-cavity structure.
Fig.2 Light output peak power as a function of injection current for the unidirectional-emission micro-cavity laser; Up-left inset: L-I characteristic of the traditional edge-emitting ridge F-P cavity laser fabricated from the same wafer material with a length of 3.0mm and 18um width; Down-right inset: Light spectrum of the micro-cavity laser.
Fig.3 Parallel far-field pattern of the unidirectional-emitting micro-cavity laser in Cartesian coordinate system; Left inset: parallel far-field pattern of the micro-cavity laser in polar coordinate system.
Fig.4 Light output peak power of micro-cavity lasers as a function of the laser structure size; Left inset: threshold current density of the micro-cavity lasers as a function of the laser structure size.
Fig. 5 Light output peak power as a function of injection current for the micro-cavity laser 31 array; Left inset: light output power for the micro-cavity laser 51array.
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