Special Issue "Highly Efficient UV and Visible Light Sources"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (30 September 2018)

Special Issue Editors

Guest Editor
Prof. Dr. Edik U. Rafailov

Optoelectronics and Biomedical Photonics Group, Aston University, Aston St, Birmingham B4 7ET, UK
Website | E-Mail
Interests: cutting-edge experimental and theoretical research on a wide variety of compact high-power CW and ultrashort-pulse laser sources; emitting in the visible, near- and mid-IR and THz spectral ranges; nanostructrures, nonlinear and integrated optics, and biophotonics
Guest Editor
Prof. Grigorii Sokolovskii

Ioffe Institute, 26 Polytechnicheskaya str, St Petersburg, 194021, Russia
E-Mail
Interests: nanophotonics, physics of semiconductors, laser diodes and LEDs
Guest Editor
Dr. Evgeny Zherebtsov

Aston Institute of Photonic Technologies (AIPT), Aston University, Aston Express Way, Birmingham B4 7ET, UK
Website | E-Mail
Interests: biophotonics; optical measurements in vivo, fluorescence spectroscopy; microcirculation measurements; hyperspectral imaging; optical biopsy and assessment of tissue vitality
Guest Editor
Dr. Amit Yadav

Aston Institute of Photonic Technologies (AIPT), Aston University, Birmingham B4 7ET, UK
Website | E-Mail
Interests: optoelectronics; LEDs; semiconductor lasers; applied optics; mode-locked lasers; VECSEL

Special Issue Information

Dear Colleagues,

The progress in the development of light sources for different regions of the electromagnetic spectrum has been non-uniform, primarily due to the introduction of appropriate materials and technological advances. The near-IR region has seen tremendous development, and, nowadays, has benefited the most efficient light sources. The progress in visible light sources seems to be less steep, despite the numerous achievements of the last few decades. However, countless applications of visible wavelengths, ranging from lighting to visible light communication and biomedical applications, will greatly benefit from the progress made in the development of efficient light sources.

This Special Issue, on “Highly-Efficient Visible Light Sources”, offers to bring together the most recent research focused on practical and theoretical approaches for the further improvement of the efficiency of visible light sources. The topics of this Special Issue include, but are not limited to, the following:

  • III-V semiconductor lasers and LEDs
  • LED efficiency droop: mechanisms and mitigation
  • UV light sources
  • New visible lasers and LEDs concepts and optimisation
  • Defects and their effects on emission efficiency
  • Visible light sources via frequency conversion and efficiency
  • Visible light for biophotonics and emerging applications

Prof. Dr. Edik U. Rafailov
Prof. Dr. Grigorii Sokolovskii
Dr. Evgeny Zherebtsov
Dr. Amit Yadav
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Applied Sciences is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1500 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Light-emitting diodes
  • Laser diodes
  • Quantum dots
  • III-V semiconductors
  • Nonlinear phenomena
  • Novel semiconductor-based lighting devices
  • Biophotonics and emerging applications

Published Papers (7 papers)

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Research

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Open AccessArticle Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties
Appl. Sci. 2018, 8(11), 2254; https://doi.org/10.3390/app8112254
Received: 22 October 2018 / Revised: 5 November 2018 / Accepted: 7 November 2018 / Published: 15 November 2018
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Abstract
This paper investigates the effect of two different welding methods, direct welding (DW) and vacuum furnace welding (VFW), on flip-chip light-emitting diode (FC-LED) filament properties. Shearing force, SEM, steady-state voltage, steady-state luminous flux, and change of photoelectric performance with aging time were employed [...] Read more.
This paper investigates the effect of two different welding methods, direct welding (DW) and vacuum furnace welding (VFW), on flip-chip light-emitting diode (FC-LED) filament properties. Shearing force, SEM, steady-state voltage, steady-state luminous flux, and change of photoelectric performance with aging time were employed to characterize the differences in filament properties between the two welding methods. The shearing test revealed that the average shearing force of the VFW group was higher than that of the DW group, but the two groups followed the standard. Furthermore, the microstructure of the VFW group fault was more smoother, and the voids were fewer and smaller based on the SEM test results. The steady-state voltage and luminous flux revealed that the VFW group had a more concentrated voltage and a higher luminous flux. The aging data revealed that the steady-state voltage change rate of both groups was not very different, and both luminous flux maintenance rates of the VFW group were higher than those of the DW group, but all were within the standard range. In conclusion, if there is a higher requirement for filament in a practical application, such as the filament is connected in series or in parallel and needs a higher luminous flux, it can be welded using vacuum furnace welding. If the focus is on production efficiency and the high performance of filaments is not required, direct welding can be used. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Open AccessArticle Di-Chromatic InGaN Based Color Tuneable Monolithic LED with High Color Rendering Index
Appl. Sci. 2018, 8(7), 1158; https://doi.org/10.3390/app8071158
Received: 23 May 2018 / Revised: 20 June 2018 / Accepted: 13 July 2018 / Published: 17 July 2018
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Abstract
We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over [...] Read more.
We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm2 to 12.9 A/cm2. It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Open AccessArticle Spectral Linewidth vs. Front Facet Reflectivity of 780 nm DFB Diode Lasers at High Optical Output Power
Appl. Sci. 2018, 8(7), 1104; https://doi.org/10.3390/app8071104
Received: 26 May 2018 / Revised: 19 June 2018 / Accepted: 29 June 2018 / Published: 9 July 2018
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Abstract
The influence of the front facet reflectivity on the spectral linewidth of high power DFB (distributed feedback) diode lasers emitting at 780 nm has been investigated theoretically and experimentally. Characterization of lasers at various front facet reflections showed substantial reduction of the linewidth. [...] Read more.
The influence of the front facet reflectivity on the spectral linewidth of high power DFB (distributed feedback) diode lasers emitting at 780 nm has been investigated theoretically and experimentally. Characterization of lasers at various front facet reflections showed substantial reduction of the linewidth. This behavior is in reasonable agreement with simulation results. A minimum linewidth of 8 kHz was achieved at an output power of 85 mW with the laser featuring a front facet reflectivity of 30%. The device with a front facet reflectivity of 5% reached the same linewidth value at an output power of 290 mW. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Open AccessFeature PaperArticle Effect of Carrier Localization on Recombination Processes and Efficiency of InGaN-Based LEDs Operating in the “Green Gap”
Appl. Sci. 2018, 8(5), 818; https://doi.org/10.3390/app8050818
Received: 16 March 2018 / Revised: 3 May 2018 / Accepted: 17 May 2018 / Published: 18 May 2018
Cited by 4 | PDF Full-text (1377 KB) | HTML Full-text | XML Full-text
Abstract
A semi-empirical model of carrier recombination accounting for hole localization by composition fluctuations in InGaN alloys is extended to polar and nonpolar quantum-well structures. The model provides quantitative agreement with available data on wavelength-dependent radiative and Auger recombination coefficients in polar LEDs. Comparison [...] Read more.
A semi-empirical model of carrier recombination accounting for hole localization by composition fluctuations in InGaN alloys is extended to polar and nonpolar quantum-well structures. The model provides quantitative agreement with available data on wavelength-dependent radiative and Auger recombination coefficients in polar LEDs. Comparison of calculated internal quantum efficiencies of polar and nonpolar LEDs enables an assessment of the roles of carrier localization, quantum-confined Stark effect, and native material properties for the efficiency decline in the “green gap”. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Open AccessArticle Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
Appl. Sci. 2018, 8(4), 610; https://doi.org/10.3390/app8040610
Received: 14 March 2018 / Revised: 2 April 2018 / Accepted: 2 April 2018 / Published: 12 April 2018
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Abstract
We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At [...] Read more.
We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e.g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h~384 h and 768 h~1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Review

Jump to: Research

Open AccessFeature PaperReview III-Nitride Short Period Superlattices for Deep UV Light Emitters
Appl. Sci. 2018, 8(12), 2362; https://doi.org/10.3390/app8122362
Received: 17 October 2018 / Revised: 12 November 2018 / Accepted: 20 November 2018 / Published: 23 November 2018
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Abstract
III-Nitride short period superlattices (SPSLs), whose period does not exceed ~2 nm (~8 monolayers), have a few unique properties allowing engineering of light-emitting devices emitting in deep UV range of wavelengths with significant reduction of dislocation density in the active layer. Such SPSLs [...] Read more.
III-Nitride short period superlattices (SPSLs), whose period does not exceed ~2 nm (~8 monolayers), have a few unique properties allowing engineering of light-emitting devices emitting in deep UV range of wavelengths with significant reduction of dislocation density in the active layer. Such SPSLs can be grown using both molecular beam epitaxy and metal organic chemical vapor deposition approaches. Of the two growth methods, the former is discussed in more detail in this review. The electrical and optical properties of such SPSLs, as well as the design and fabrication of deep UV light-emitting devices based on these materials, are described and discussed. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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Open AccessReview A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
Appl. Sci. 2018, 8(8), 1264; https://doi.org/10.3390/app8081264
Received: 15 June 2018 / Revised: 13 July 2018 / Accepted: 19 July 2018 / Published: 31 July 2018
Cited by 7 | PDF Full-text (9540 KB) | HTML Full-text | XML Full-text
Abstract
This paper reviews the progress of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs), mainly focusing in the work of the authors’ group. The background to the development of the current device structure on sapphire is described and the reason for using a (0001) [...] Read more.
This paper reviews the progress of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs), mainly focusing in the work of the authors’ group. The background to the development of the current device structure on sapphire is described and the reason for using a (0001) sapphire with a miscut angle of 1.0° relative to the m-axis is clarified. Our LEDs incorporate uneven quantum wells (QWs) grown on an AlN template with dense macrosteps. Due to the low threading dislocation density of AlGaN and AlN templates of about 5 × 108/cm2, the number of nonradiative recombination centers is decreased. In addition, the uneven QW show high external quantum efficiency (EQE) and wall-plug efficiency, which are considered to be boosted by the increased internal quantum efficiency (IQE) by enhancing carrier localization adjacent to macrosteps. The achieved LED performance is considered to be sufficient for practical applications. The advantage of the uneven QW is discussed in terms of the EQE and IQE. A DUV-LED die with an output of over 100 mW at 280–300 nm is considered feasible by applying techniques including the encapsulation. In addition, the fundamental achievements of various groups are reviewed for the future improvements of AlGaN-based DUV-LEDs. Finally, the applications of DUV-LEDs are described from an industrial viewpoint. The demonstrations of W/cm2-class irradiation modules are shown for UV curing. Full article
(This article belongs to the Special Issue Highly Efficient UV and Visible Light Sources)
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