Periodic Reporting for period 2 - UV-LASE (Out of the blue: membrane-based microcavity lasers from the blue to the ultraviolet wavelength regime)
Reporting period: 2022-02-01 to 2023-07-31
Achieving ultraviolet (UV) emission has proven to be difficult, in particular for microcavity lasers due to high optical losses and high defect densities in aluminum gallium nitride (AlGaN), the material of choice for UV. The aim of UV-LASE is to develop the first electrically injected blue microcavity laser with good enough performance to be useful in real-world applications and project out of the blue and into the ultraviolet to realize the very first electrically injected UV microcavity laser.
Compared to edge-emitting lasers, microcavity lasers have a number of inherent advantages such as low threshold current, circular-symmetric low-divergent output beam, high modulation speed at low drive currents, ease to fabricate into two-dimensional arrays, and low-cost manufacturing due to on wafer-testing. Such devices would be of interest to numerous applications such as solid-state lighting, photolithography, biomedical applications, enhancing health-promoting substances in plants, gas sensing, UV curing and sterilization.
UV-LASE is based upon two recent breakthroughs by our group:
• The discovery of an overlooked loss mechanism in the AlGaN-laser cavity. Our proposed designs to circumvent this loss are now being implemented worldwide and have led to record-high optical output power in blue-emitting lasers of 15.7 mW in contrast to previous best results of ~1 mW.
• A unique membrane technique to enable microcavity lasers with highly reflective dielectric mirrors on both sides of the cavity. This is a device concept that up to this point was un-realizable for UV-lasers. The new method is based upon an electrochemical etching of the otherwise chemically inert AlGaN, and allows for lift-off of device membranes with record-smooth surfaces from the substrate and mirror-deposition on the bottom side.
The reach the goals, the membrane technology will be pushed to its limits to enable underetching of even higher Al containing materials, i.e. materials for UVC (<280 nm) emission. The underetching is based upon process where highly conductive layers are etched in front of lower conductive layers. It will thus be a challenge to make this technology compatible with doped device structures, ensuring that the electrically conductive device layers do not etch or porosify.
Compared to edge-emitting lasers, microcavity lasers have a number of inherent advantages such as low threshold current, circular-symmetric low-divergent output beam, high modulation speed at low drive currents, ease to fabricate into two-dimensional arrays, and low-cost manufacturing due to on wafer-testing. Such devices would be of interest to numerous applications such as solid-state lighting, photolithography, biomedical applications, enhancing health-promoting substances in plants, gas sensing, UV curing and sterilization.
UV-LASE is based upon two recent breakthroughs by our group:
• The discovery of an overlooked loss mechanism in the AlGaN-laser cavity. Our proposed designs to circumvent this loss are now being implemented worldwide and have led to record-high optical output power in blue-emitting lasers of 15.7 mW in contrast to previous best results of ~1 mW.
• A unique membrane technique to enable microcavity lasers with highly reflective dielectric mirrors on both sides of the cavity. This is a device concept that up to this point was un-realizable for UV-lasers. The new method is based upon an electrochemical etching of the otherwise chemically inert AlGaN, and allows for lift-off of device membranes with record-smooth surfaces from the substrate and mirror-deposition on the bottom side.
The reach the goals, the membrane technology will be pushed to its limits to enable underetching of even higher Al containing materials, i.e. materials for UVC (<280 nm) emission. The underetching is based upon process where highly conductive layers are etched in front of lower conductive layers. It will thus be a challenge to make this technology compatible with doped device structures, ensuring that the electrically conductive device layers do not etch or porosify.
The first half of UV-LASE has been focused on demonstrating and pushing the membrane technology to its limits and beyond for lasers and LEDs. We have accomplished four crucial milestones:
1. A new methodology based on electrochemical etching to underetch UVC emitters that otherwise are not be possible to underetch even when pushing electrochemical etching to its limits. We are now with experiments and simulations working on understanding and explaining the new methodology.
2. Underetching of electrically driven LED structures without porosification of doped device layers and the realization of thin-film flip-chip UVB LEDs.
3. Realization of resonant cavity LEDs (RCLEDs) with tunnel junctions without porosification of the device despite the high doping levels used. The only thing that differentiates a RCLED from a VCSEL is current confinement, which will be implemented next.
4. Realization of optically pumped UVC VCSELs, a first important step towards electrically driven UVC microcavity lasers.
We have deliberately not published the methodology to underetch UVC emitters yet to give us an advantage to our competitors while we realize UVC VCSELs and LEDs. The project has so far resulted in seven peer-reviewed journal publications such as ACS Photonics, Photonics Research and one likely to be published in Laser and Photonics Reviews. The PI has received numerous invitations to prestigious conferences such as the International Workshop on Nitride semiconductors (2022), SPIE Photonics West (2021, 2023), and CLEO Europe (2023). Thanks to the recent breakthroughs, the PI just received an invitation to give a plenary talk at the International Conference on Nitride Semiconductors 2023, one of the two most prestigious conferences in the field that attract about 800 researchers.
The recent results have also caught the attention of industry and the PI was invited to write a review article for Compound Semiconductor magazine and present the results at CS International 2022. Many different companies have approached the PI directly to discuss how the membrane technology can be used for their future products. Discussions are now ongoing with one of the companies to hand in an ERC proof-of-concept application.
1. A new methodology based on electrochemical etching to underetch UVC emitters that otherwise are not be possible to underetch even when pushing electrochemical etching to its limits. We are now with experiments and simulations working on understanding and explaining the new methodology.
2. Underetching of electrically driven LED structures without porosification of doped device layers and the realization of thin-film flip-chip UVB LEDs.
3. Realization of resonant cavity LEDs (RCLEDs) with tunnel junctions without porosification of the device despite the high doping levels used. The only thing that differentiates a RCLED from a VCSEL is current confinement, which will be implemented next.
4. Realization of optically pumped UVC VCSELs, a first important step towards electrically driven UVC microcavity lasers.
We have deliberately not published the methodology to underetch UVC emitters yet to give us an advantage to our competitors while we realize UVC VCSELs and LEDs. The project has so far resulted in seven peer-reviewed journal publications such as ACS Photonics, Photonics Research and one likely to be published in Laser and Photonics Reviews. The PI has received numerous invitations to prestigious conferences such as the International Workshop on Nitride semiconductors (2022), SPIE Photonics West (2021, 2023), and CLEO Europe (2023). Thanks to the recent breakthroughs, the PI just received an invitation to give a plenary talk at the International Conference on Nitride Semiconductors 2023, one of the two most prestigious conferences in the field that attract about 800 researchers.
The recent results have also caught the attention of industry and the PI was invited to write a review article for Compound Semiconductor magazine and present the results at CS International 2022. Many different companies have approached the PI directly to discuss how the membrane technology can be used for their future products. Discussions are now ongoing with one of the companies to hand in an ERC proof-of-concept application.
We have developed a new methodology to underetch higher Aluminium content materials that is required to make membrane-based UVC emitters. The method is based upon electrochemical etching, but a special trick is applied to be able to fully etch the sacrificial layers. This led to the very recent demonstration of the world’s shortest emission wavelength from a VCSEL in the UVC, and even more importantly to a method to realize such lasers with an accurate cavity length control. Our method results in a cavity length that is controlled within a few nm, while the competing technology results in cavity length variations of 550 nm. Our methodology and device layout have been developed further and it is now possible to underetch even electrically driven UVC LEDs without porosification of the important device layers.
By using VCSELs with all dielectric-based mirrors allows us to incorporate other materials in the cavity. This opens a range of new possibilities that for example can be used to shift the resonance in the laser and thereby produce a VCSEL array where nearby devices have different lasing wavelengths. Even more interesting, it allowed for the demonstration of a new device concept that led to an athermalized lasing wavelength from a VCSEL, i.e. the most temperature a stable emission wavelength ever seen in a VCSEL. This concept can also be applied to any VCSELs in any material system if they allow for the incorporation of dielectric layer with special optical properties.
The second half of UV-LASE will be dedicated to realizing electrically driven lasers, i.e. the first electrically injected blue microcavity laser with good enough performance to be useful in real-world applications and project out of the blue and into the ultraviolet to realize the very first electrically injected UV microcavity laser.
By using VCSELs with all dielectric-based mirrors allows us to incorporate other materials in the cavity. This opens a range of new possibilities that for example can be used to shift the resonance in the laser and thereby produce a VCSEL array where nearby devices have different lasing wavelengths. Even more interesting, it allowed for the demonstration of a new device concept that led to an athermalized lasing wavelength from a VCSEL, i.e. the most temperature a stable emission wavelength ever seen in a VCSEL. This concept can also be applied to any VCSELs in any material system if they allow for the incorporation of dielectric layer with special optical properties.
The second half of UV-LASE will be dedicated to realizing electrically driven lasers, i.e. the first electrically injected blue microcavity laser with good enough performance to be useful in real-world applications and project out of the blue and into the ultraviolet to realize the very first electrically injected UV microcavity laser.