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postgraduate thesis: Manipulating optical emission from light-emitting diodes and their applications

TitleManipulating optical emission from light-emitting diodes and their applications
Authors
Advisors
Advisor(s):Choi, HWLai, PT
Issue Date2011
PublisherThe University of Hong Kong (Pokfulam, Hong Kong)
Citation
Zhu, L. [朱玲]. (2011). Manipulating optical emission from light-emitting diodes and their applications. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b4729746
AbstractMaterial properties, coupled with typical device structures of GaN-based light-emitting diode (LED) wafer give rise to Lambertian emission patterns with large beam divergence. However, this pattern may not be useful or beneficial to many applications. In some specific applications, such as spot lighting or light sources for fiber coupling, emission with narrow beam divergence is required, whereas in general lighting such as the street lamps and indoor lighting, a diffused light source rather than a point source is needed. By manipulating the optical emission of LEDs at the chip level, some performance metrics of LEDs can be enhanced and their applications can be extended into new fields rather than merely for lighting. Additionally, the need for external optics can be eliminated, thereby increasing the flexibility of design. In this thesis, five implementations are reported to achieve emission control, namely chip design, optics design, package design and system design, which are ordered according to the LED fabrication process flow. Manipulation of optical emission can be observed by comparing the proposed devices with the conventional devices, or the successful demonstration of a new application. By chip shaping via laser micromachining, a three-dimensional truncated-conic LED (TC-LED) is proposed to cut off efficiently lateral emissions from the LED sidewall, thus enhancing color uniformity from its top quantum-dot coated surface. The optical properties of TC-LED are investigated: the beam divergence is reduced by 32o and the power in the normal direction is enhanced by 21.7%. After applying quantum dots to achieve white-light emission, the top emission color uniformity is improved by 37%. By including optics on the chip level, beam divergence can be narrowed down. The hemispherical lens LED (HL-LED) with directional beam is proposed, achieving a 53.8% enhancement of fiber coupling efficiency. On top of a flip-chip-packaged TC-LED, a hemispherical BK-7 lens is capillary-bonded onto the sapphire surface. Compared with TC-LED, the divergence of HL-LED is significantly reduced by 50o. Vertically-mounted LED (vmLED) is proposed to broaden the emission pattern at the packaging level. By mounting the LED die upright to expose two large illumination surfaces instead of the traditional way of bonding the die flat down, the optical emission pattern is converted from Lambertian to a two-lobed pattern. Both the optical properties and thermal properties are investigated and it is found that there is a trade-off between the heat dissipation and light output. A sapphire-prism-mounted vmLED is further proposed to improve the heat sinking. In the last two chapters, micro-LED arrays with smaller illuminated active regions are introduced and the combination with external optics, including optical fibers and projection lens sets are used to demonstrate novel LED applications. By coupling a bi-linear micro-LED array into a fiber bundle, a portable microdisplay system is demonstrated and this comprehensive system can be used for image projection. Another application involved a linear UV-micro-LED array coupled with a projection lens set; this optical system has been demonstrated as a direct-write lithographic tool for the fabrication of polymer microlens arrays on InGaN LEDs.
DegreeDoctor of Philosophy
SubjectLight emitting diodes.
Dept/ProgramElectrical and Electronic Engineering

 

DC FieldValueLanguage
dc.contributor.advisorChoi, HW-
dc.contributor.advisorLai, PT-
dc.contributor.authorZhu, Ling-
dc.contributor.author朱玲-
dc.date.issued2011-
dc.identifier.citationZhu, L. [朱玲]. (2011). Manipulating optical emission from light-emitting diodes and their applications. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b4729746-
dc.description.abstractMaterial properties, coupled with typical device structures of GaN-based light-emitting diode (LED) wafer give rise to Lambertian emission patterns with large beam divergence. However, this pattern may not be useful or beneficial to many applications. In some specific applications, such as spot lighting or light sources for fiber coupling, emission with narrow beam divergence is required, whereas in general lighting such as the street lamps and indoor lighting, a diffused light source rather than a point source is needed. By manipulating the optical emission of LEDs at the chip level, some performance metrics of LEDs can be enhanced and their applications can be extended into new fields rather than merely for lighting. Additionally, the need for external optics can be eliminated, thereby increasing the flexibility of design. In this thesis, five implementations are reported to achieve emission control, namely chip design, optics design, package design and system design, which are ordered according to the LED fabrication process flow. Manipulation of optical emission can be observed by comparing the proposed devices with the conventional devices, or the successful demonstration of a new application. By chip shaping via laser micromachining, a three-dimensional truncated-conic LED (TC-LED) is proposed to cut off efficiently lateral emissions from the LED sidewall, thus enhancing color uniformity from its top quantum-dot coated surface. The optical properties of TC-LED are investigated: the beam divergence is reduced by 32o and the power in the normal direction is enhanced by 21.7%. After applying quantum dots to achieve white-light emission, the top emission color uniformity is improved by 37%. By including optics on the chip level, beam divergence can be narrowed down. The hemispherical lens LED (HL-LED) with directional beam is proposed, achieving a 53.8% enhancement of fiber coupling efficiency. On top of a flip-chip-packaged TC-LED, a hemispherical BK-7 lens is capillary-bonded onto the sapphire surface. Compared with TC-LED, the divergence of HL-LED is significantly reduced by 50o. Vertically-mounted LED (vmLED) is proposed to broaden the emission pattern at the packaging level. By mounting the LED die upright to expose two large illumination surfaces instead of the traditional way of bonding the die flat down, the optical emission pattern is converted from Lambertian to a two-lobed pattern. Both the optical properties and thermal properties are investigated and it is found that there is a trade-off between the heat dissipation and light output. A sapphire-prism-mounted vmLED is further proposed to improve the heat sinking. In the last two chapters, micro-LED arrays with smaller illuminated active regions are introduced and the combination with external optics, including optical fibers and projection lens sets are used to demonstrate novel LED applications. By coupling a bi-linear micro-LED array into a fiber bundle, a portable microdisplay system is demonstrated and this comprehensive system can be used for image projection. Another application involved a linear UV-micro-LED array coupled with a projection lens set; this optical system has been demonstrated as a direct-write lithographic tool for the fabrication of polymer microlens arrays on InGaN LEDs.-
dc.languageeng-
dc.publisherThe University of Hong Kong (Pokfulam, Hong Kong)-
dc.relation.ispartofHKU Theses Online (HKUTO)-
dc.rightsThe author retains all proprietary rights, (such as patent rights) and the right to use in future works.-
dc.rightsCreative Commons: Attribution 3.0 Hong Kong License-
dc.source.urihttp://hub.hku.hk/bib/B47297463-
dc.subject.lcshLight emitting diodes.-
dc.titleManipulating optical emission from light-emitting diodes and their applications-
dc.typePG_Thesis-
dc.identifier.hkulb4729746-
dc.description.thesisnameDoctor of Philosophy-
dc.description.thesislevelDoctoral-
dc.description.thesisdisciplineElectrical and Electronic Engineering-
dc.description.naturepublished_or_final_version-
dc.identifier.doi10.5353/th_b4729746-
dc.date.hkucongregation2011-

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