| 91. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Thermal lensing effects on lateral leakage in GaN-based vertical-cavity surface-emitting laser cavities
- 2017
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Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 25:9, s. 9556-9568
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Tidskriftsartikel (refereegranskat)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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| 92. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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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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Ingår i: Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics. - : American Vacuum Society. - 2166-2746 .- 2166-2754. ; 33:5
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Tidskriftsartikel (refereegranskat)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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| 93. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Triggering of guiding and antiguiding effects in GaN-based VCSELs
- 2014
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9001
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Konferensbidrag (refereegranskat)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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| 94. |
- Healy, S. B., et al.
(författare)
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Active Region Design for High-Speed 850-nm VCSELs
- 2010
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Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 46:4, s. 506-512
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Tidskriftsartikel (refereegranskat)abstract
- Higher speed short-wavelength (850 nm) VCSELs are required for future high-capacity, short-reach data communication links. The modulation bandwidth of such devices is intrinsically limited by the differential gain of the quantum wells (QWs) used in the active region. We present gain calculations using an 8-band k.p Hamiltonian which show that the incorporation of 10% In in an InGaAs/AlGaAs QW structure can approximately double the differential gain compared to a GaAs/AlGaAs QW structure, with little additional improvement achieved by further increasing the In composition in the QW. This improvement is confirmed by extracting the differential gain value from measurements of the modulation response of VCSELs with optimized InGaAs/AlGaAs QW and conventional GaAs/AlGaAs QW active regions. Excellent agreement is obtained between the theoretically and experimentally determined values of the differential gain, confirming the benefits of strained InGaAs QW structures for high-speed 850-nm VCSEL applications.
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| 95. |
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| 96. |
- Hjort, Filip, 1991, et al.
(författare)
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A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser
- 2021
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 8:1, s. 135-141
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Tidskriftsartikel (refereegranskat)abstract
- Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280-320 nm) at room temperature, having an Al0.60Ga0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320-400 nm) to UVC (<280 nm).
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| 97. |
- Hjort, Filip, 1991, et al.
(författare)
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Advances in ultraviolet-emitting vertical-cavity surface-emitting lasers
- 2021
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 11686
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Konferensbidrag (refereegranskat)abstract
- We will give an overview of the progress in ultraviolet-emitting vertical-cavity surface-emitting lasers (VCSELs) and their potential applications in areas such as disinfection and medical therapy. This includes our demonstration of the shortest wavelength VCSEL, emitting at 310 nm under optical pumping, and a detailed analysis of its filamentary lasing characteristics. The UVB-emitting AlGaN-based VCSEL was realized by substrate removal using electrochemical etching, enabling the use of two high-reflectivity dielectric distributed Bragg reflectors. The potential of using this or alternative methods to push the emission to shorter wavelengths will be examined as well as concepts to realize electrically injected devices.
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| 98. |
- Hjort, Filip, 1991, et al.
(författare)
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Electrically conductive ZnO/GaN distributed Bragg reflectors grown by hybrid plasma-assisted molecular beam epitaxy
- 2017
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781510606494 ; 10104, s. 1010413-1
-
Konferensbidrag (refereegranskat)abstract
- III-nitride-based vertical-cavity surface-emitting lasers have so far used intracavity contacting schemes since electrically conductive distributed Bragg reflectors (DBRs) have been difficult to achieve. A promising material combination for conductive DBRs is ZnO/GaN due to the small conduction band offset and ease of n-type doping. In addition, this combination offers a small lattice mismatch and high refractive index contrast, which could yield a mirror with a broad stopband and a high peak reflectivity using less than 20 DBR-pairs. A crack-free ZnO/GaN DBR was grown by hybrid plasma-assisted molecular beam epitaxy. The ZnO layers were approximately 20 nm thick and had an electron concentration of 1×1019 cm-3, while the GaN layers were 80-110 nm thick with an electron concentration of 1.8×1018 cm-3. In order to measure the resistance, mesa structures were formed by dry etching through the top 3 DBR-pairs and depositing non-annealed Al contacts on the GaN-layers at the top and next to the mesas. The measured specific series resistance was dominated by the lateral and contact contributions and gave an upper limit of ~10-3Ωcm2 for the vertical resistance. Simulations show that the ZnO electron concentration and the cancellation of piezoelectric and spontaneous polarization in strained ZnO have a large impact on the vertical resistance and that it could be orders of magnitudes lower than what was measured. This is the first report on electrically conductive ZnO/GaN DBRs and the upper limit of the resistance reported here is close to the lowest values reported for III-nitride-based DBRs.
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| 99. |
- Hjort, Filip, 1991, et al.
(författare)
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Optical microprism cavities based on dislocation-free GaN
- 2020
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 117:23
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Tidskriftsartikel (refereegranskat)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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| 100. |
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