| 11. |
- Chang, Tsu Chi, et al.
(författare)
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Lasing Action in GaN-Based VCSELs with top High-Contrast Grating Reflectors
- 2018
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Ingår i: Conference Digest - IEEE International Semiconductor Laser Conference. - 0899-9406. ; 2018-September, s. 191-192
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Konferensbidrag (refereegranskat)abstract
- We report a GaN-based VCSEL with a high-contrast grating (HCG) as the top mirror. The HCG consisted of TiO2 and rested directly on the n-GaN without an airgap or the use of any DBR layers to boost the reflectivity. The full VCSEL structure was optically pumped at room temperature and showed a lasing threshold of approximately 0.99 MW/cm2 and a lasing wavelength at 369.8 nm. The demonstration of HCG GaN-based VCSEL opens up the possibility to explore all the potential benefits of HCGs in the blue and ultraviolet spectral regime.
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| 12. |
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| 13. |
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| 14. |
- Haglund, Åsa, 1976, et al.
(författare)
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Blue and ultraviolet vertical-cavity surface-emitting lasers
- 2019
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Ingår i: Optics InfoBase Conference Papers. - 2162-2701.
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Konferensbidrag (refereegranskat)abstract
- We will summarize state-of-the-art results in III-nitride-based vertical-cavity surface-emitting lasers (VCSELs) for blue and ultraviolet emission, including our schemes for optically guided devices and our approach for UV-VCSELs with double dielectric distributed Bragg reflectors.
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| 15. |
- Haglund, Åsa, 1976, et al.
(författare)
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GaN-based VCSELs
- 2015
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Ingår i: VI Workshop on Physics and Technology of Semiconductor Lasers.
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Konferensbidrag (refereegranskat)abstract
- The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, bio-medical and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs have shown to be challenging, and so far only a handful research groups have demonstrated lasing from such electrically pumped devices [1-6]. The presented performance is typically orders of magnitudes lower compared to that from electrically driven GaAs-based VCSELs. Some of the challenges are to achieve efficient transverse current spreading, transverse optical mode confinement, high-reflectivity mirrors and resonator length control. This talk will summarize the different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing [7]. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely a TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span, which is in excellent agreement to the reflectivity spectrum predicted by numerical simulations assuming an ideal HCG geometry [8]. References [1] T.-C. Lu, et al., Applied Physics Letters, 92, 14, (2008).[2] Y. Higuchi, et al., Applied physics express, 1, 12, 121102, (2008).[3] G. Cosendey, et al., Applied Physics Letters, 101, 15, (2012).[4] C. Holder, et al., Applied Physics Express, 5, 092104, (2012).[5] T. Onishi, et al., IEEE J. of Quantum Electronics, 48, 9,1107–1112, (2012).[6] W.-J. Liu, et al., Applied Physics Letters, 104, 251116 (2014).[7] E. Hashemi, et al., Optics Express, vol. 22 1, p. 411-426, (2014).[8] E. Hashemi, et al., Proceedings of SPIE, (0277-786X), vol. 9372 (2015).
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| 16. |
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| 17. |
- Haglund, Åsa, 1976, et al.
(författare)
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Progress and challenges in electrically pumped GaN-based VCSELs
- 2016
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9892
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Konferensbidrag (refereegranskat)abstract
- The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span.
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| 18. |
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| 19. |
- Haglund, Åsa, 1976, et al.
(författare)
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Should we strive to follow Nature’s checklist on how to claim lasing?
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Concerns have been raised by the laser community that sufficient evidence to support a claim of lasing, in particular in high-b cavities, is not always provided in manuscripts sent in for peer-review or even in published journal papers. Therefore, Nature Photonics compiled and launched a checklist for manuscripts with a claim of lasing, about a year ago.1 Authors are asked to complete it prior to peer-review, and the checklist will be published with the paper as supplementary information. Is this checklist really necessary, and if so, is it complete? To answer the first question, we have checked which of the points in the checklist that have been addressed in papers published on electrically injected GaN-VCSELs. The scrutinized papers are from eight different groups claiming lasing, and all except two were published before Nature’s checklist. Table 1 shows the summary; none of the papers addresses all points, most papers only address a few, and one does not provide evidence for any point. Table. 1. Checklist fulfillment by GaN-VCSEL papers Proving lasing in novel devices and materials can be very hard, in particular in high-β emitters. It is thus challenging to provide input to all the points in the checklist. Assuming the checklist can be completed, is it enough? Interestingly, a cavity-enhanced LED could tick almost all the boxes in the laser checklist, while a thresholdless laser would per definition not satisfy box 1. We therefore claim that determining the change in photon statistics from thermal light below threshold to coherent light above threshold via second-order photon autocorrelation measurements is crucial. This method has largely been ignored by the applied laser community, but is regularly used by the quantum optics community to study photon statistics of emission. Its importance to provide the means to distinguish between a cavity-enhanced LED and a laser is illustrated in Fig. 1.2 The intensity and linewidth dependence on excitation power are very similar for the two devices B and C. However, the equal-time second-order photon autocorrelation function, g(2)(0), is notably different with a clear reduction from 2 to 1 for the laser, i.e. showing a transition from spontaneous to coherent emission. In summary, we think the laser checklist is much needed and a good initiative by Nature Photonics, that hopefully will improve the quality of published papers, improve the repeatability of results and allow for a better comparison of devices. Even though it may not always be feasible to provide answers to all the questions in the checklist, we should do our best in providing the strongest evidence possible - including excitation power dependent g(2)(0) measurements for high-b lasers - for our claims. Fig. 1 Performance characteristics for an LED (A), a cavity-enhanced LED (B) and a high-b microlaser (C) in terms of a) optical output intensity, b) linewidth and c) second-order photon autocorrelation as a function of pump power, and intracvity photon number, respectively. References [1] https://www.nature.com/authors/policies/laserchecklist.pdf [2] S. Kreinberg et al., Light: Science & Applications, 6, e17030, (2017).
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| 20. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Air-suspended TiO2-based HCG reflectors for visible spectral range
- 2015
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9372
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Konferensbidrag (refereegranskat)abstract
- For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO2 grating with the help of a SiO2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial polymethyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO2/air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found.
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