| 71. |
- Hashemi, Seyed Ehsan, 1986, et al.
(author)
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Engineering the transverse optical guiding in GaN-based VCSELs to avoid detrimental optical loss
- 2012
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In: International Workshop on Nitride Semiconductors 2012, October 14-19, 2012, Sapporo, Japan, p. ThP-OD-33.
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Conference paper (other academic/artistic)abstract
- In order to improve the current injection in GaN-based blue vertical cavity surface emitting lasers (VCSELs) a dielectric aperture is generally used in combination with an indium-tin-oxide (ITO) layer on the top intracavity p-contact layer. The most straightforward way to realize this introduces a depression of the structure near the optical axis and we show, by using a 2D effective index method and a 3D coupled-cavity beam propagation method, that this typically results in optically anti-guided structures with associated high optical losses and thus very high threshold gains. Remarkably, the threshold gain reduces with increased negative guiding, which is due to improved lateral confinement and reduction of lateral leakage. Still, moderately positively guided designs should be preferred to avoid the detrimental effect of lateral leakage and high diffraction loss. To ensure positive index guiding, we propose to planarize the structure or introduce an elevation near the optical axis by additional processing, with an associated reduction in threshold material gain from 6000 cm-1 to 2000 cm-1 for the studied structures.
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| 72. |
- Hashemi, Seyed Ehsan, 1986, et al.
(author)
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Thermal lensing effects on lateral leakage in GaN-based vertical-cavity surface-emitting laser cavities
- 2017
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In: Optics Express. - 1094-4087 .- 1094-4087. ; 25:9, s. 9556-9568
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Journal article (peer-reviewed)abstract
- Lateral leakage of light has been identified as a detrimental loss source in many suggested and experimentally realized GaN-based VCSELs. In the present work we include thermal effects to realistically account for the substantial Joule heating in these devices. In contrast to what could be expected from the previous results, the induced thermal lensing does not make antiguided cavities more positively guided, so that they approach the unguided regime with extremely high lateral leakage. Rather, thermal lensing strongly suppresses lateral leakage for both antiguided and guided cavities. This is explained in terms of lowered launch of power from the central part of the cavity and/or lower total internal reflection in the peripheral part; the former effect is active in all cavities whereas the latter only contributes to the very strongly reduced leakage in weakly antiguided cavities. Thermal lensing suppresses lateral leakage both for the fundamental and the first higher order mode, but a strong modal discrimination is still achieved for the antiguided cavities. Thus, strongly antiguided cavities could be used to achieve single-mode devices, but at the cost of slightly higher threshold gain and stronger temperature dependent performance characteristics.
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| 73. |
- Hashemi, Seyed Ehsan, 1986, et al.
(author)
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TiO2 membrane high-contrast grating reflectors for vertical-cavity light-emitters in the visible wavelength regime
- 2015
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In: Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics. - : American Vacuum Society. - 2166-2746 .- 2166-2754. ; 33:5
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Journal article (peer-reviewed)abstract
- © 2015 American Vacuum Society. In this work, the authors describe a novel route to achieve a high reflectivity, wide bandwidth feedback mirror for GaN-based vertical-cavity light emitters; using air-suspended high contrast gratings in TiO2, with SiO2 as a sacrificial layer. The TiO2 film deposition and the etching processes are developed to yield grating bars without bending, and with near-ideal rectangular cross-sections. Measured optical reflectivity spectra of the fabricated high contrast gratings show very good agreement with simulations, with a high reflectivity of >95% over a 25 nm wavelength span centered around 435 nm for the transverse-magnetic polarization.
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| 74. |
- Hashemi, Seyed Ehsan, 1986, et al.
(author)
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Triggering of guiding and antiguiding effects in GaN-based VCSELs
- 2014
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In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9001
-
Conference paper (peer-reviewed)abstract
- We show numerically that many recently proposed GaN-based VCSEL cavities, with DBR mirrors deposited onto the current aperture, balance dangerously close to the border between the guided and antiguided regime. A guided cavity is often preferred because of its lower optical loss, but a strongly antiguided cavity offers built-in modal discrimination favoring single fundamental mode operation. We show that very small changes in the VCSEL structure are sufficient to strongly change the guiding character of the VCSEL cavity, and that thermal lensing caused by device self-heating under operation can dramatically reduce the optical loss but not the modal discrimination in the antiguided cavities.
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| 75. |
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| 76. |
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| 77. |
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| 78. |
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| 79. |
- Hjort, Filip, 1991, et al.
(author)
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Optical microprism cavities based on dislocation-free GaN
- 2020
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 117:23
-
Journal article (peer-reviewed)abstract
- Three-dimensional growth of nanostructures can be used to reduce the threading dislocation density that degrades III-nitride laser performance. Here, nanowire-based hexagonal GaN microprisms with flat top and bottom c-facets are embedded between two dielectric distributed Bragg reflectors to create dislocation-free vertical optical cavities. The cavities are electron beam pumped, and the quality (Q) factor is deduced from the cavity-filtered yellow luminescence. The Q factor is similar to 500 for a 1000nm wide prism cavity and only similar to 60 for a 600nm wide cavity, showing the strong decrease in Q factor when diffraction losses become dominant. Measured Q factors are in good agreement with those obtained from quasi-3D finite element frequency-domain method and 3D beam propagation method simulations. Simulations further predict that a prism cavity with a 1000nm width will have a Q factor of around 2000 in the blue spectral regime, which would be the target regime for real devices. These results demonstrate the potential of GaN prisms as a scalable platform for realizing small footprint lasers with low threshold currents.
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| 80. |
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