Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

By alloying GaN with AlN the emission of AlGaN light-emitting diodes can be tuned to cover almost the entire ultraviolet spectral range (210–400 nm), making ultraviolet light-emitting diodes perfectly suited to applications across a wide number of fields, whether biological, environmental, industrial or medical. However, technical developments notwithstanding, deep-ultraviolet light-emitting diodes still exhibit relatively low external quantum efficiencies because of properties intrinsic to aluminium-rich group III nitride materials. Here, we review recent progress in the development of AlGaN-based deep-ultraviolet light-emitting devices. We also describe the key obstacles to enhancing their efficiency and how to improve their performance in terms of defect density, carrier-injection efficiency, light extraction efficiency and heat dissipation. This Review covers recent progress in AlGaN-based deep-ultraviolet light-emitting devices. The key technologies of how to improve their performance, carrier-injection efficiency, light extraction efficiency and heat dissipation are discussed.

The emergence and prospects of deep-ultraviolet light-emitting diode technologies

Photoluminescence is a radiative recombination process of electron-hole pairs. Self-trapped excitons (STEs), occurring in a material with soft lattice and strong electron-phonon coupling, emit photons with broad spectrum and large Stokes shift. Recently, series halide perovskites with efficient STE emission have been reported and showed promise for solid-state lighting. In this Perspective, we present an overview of various photoluminescence phenomena with the emphasis on the mechanism and characteristics of emission derived from STEs. This is followed by the introduction of STE emission in hybrid halide perovskites. We then introduce all-inorganic STE emitters and focus in particular on the mechanism of STEs in double-perovskite Cs2AgInCl6 and strategies for efficiency improvement. Finally, we summarize the current photoluminescence and electroluminescence applications of STE emitters as well as the potential in luminescent solar concentrators and provide an overview of future research opportunities.

Self-Trapped Excitons in All-Inorganic Halide Perovskites: Fundamentals, Status, and Potential Applications

AlGaN-based materials own direct transition energy bands and wide bandgap and thus can be used in high-efficiency ultraviolet (UV) emitters and detectors. Over the past two decades, AlGaN-based materials and devices experienced rapid development. Deep ultraviolet AlGaN-based light-emitting diodes (LEDs) with improved efficiency of 20.3% (at 275 nm) have been produced. An electron beam (EB)-pumped AlGaN-based UV light source at 238 nm, output power of 100 mW, and power conversion efficiency (PCE) of 40% has also been fabricated. UV stimulated emission from AlGaN multiple-quantum-wells laser diodes (LDs) using electrical pumping at room temperature has also been achieved at a wavelength of 336 nm. Compared with GaN-based blue and green LEDs and LDs, the efficiency of AlGaN-based UV LEDs and LDs is lower. Further optimization and improvements in both structure and fabrication are required to realize high-performance devices. In AlGaN-based UV photodetectors (PDs), gain as high as 104 orders of magnitude has been reported using the separated absorption and multiplication region avalanche photodiode structure but is still far from detecting the weak signal, and thus UV single-photon detectors with high detectivity is challenging. Recently, there has been extensive work in the nonlinear optical properties of AlGaN and AlGaN-based passive devices, such as waveguides and resonators. However, how to minimize the scattering and defect-related absorption needs to be further studied. In this review, first, approaches used to grow an AlGaN epilayer and p-type doping are introduced. Second, progress in AlGaN-based UV LEDs, EB-pumped light sources, LDs, PDs, passive devices, and the nonlinear optical properties are presented. Finally, an overview of potential future trends in AlGaN-based materials and UV devices is given.

AlGaN photonics: recent advances in materials and ultraviolet devices

This chapter provides a brief introduction to group III-nitride ultraviolet light emitting diode (LED) technologies and an overview of a number of key application areas for UV-LEDs. It covers the state of the art of UV-LEDs as well a survey of novel approaches for the development of high performance UV light emitters.

III-Nitride Ultraviolet Emitters

III-nitride ultraviolet emitters : technology and applications

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0  0) and on semipolar (1 0  2), (1 1  2), (1 0  1) as well as (2 0  1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0  1) < (1 1  2) = (0 0 0 1) < (2 0  1) < (1 0  0) = (1 0  2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0  0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0  0) and the semipolar (1 0  2) and (2 0  1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1  2) and (1 0  1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

https://iopscience.iop.org/article/10.1088/0268-1242/27/2/024014/pdf

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal-organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm-2. We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire.

Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates

The design and Mg-doping profile of AlN/Al0.7Ga0.3N electron blocking heterostructures (EBH) for AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) emitting below 250 nm was investigated. By inserting an AlN electron blocking layer (EBL) into the EBH, we were able to increase the quantum well emission power and significantly reduce long wavelength parasitic luminescence. Furthermore, electron leakage was suppressed by optimizing the thickness of the AlN EBL while still maintaining sufficient hole injection. Ultraviolet (UV)-C LEDs with very low parasitic luminescence (7% of total emission power) and external quantum efficiencies of 0.19% at 246 nm have been realized. This concept was applied to AlGaN MQW LEDs emitting between 235 nm and 263 nm with external quantum efficiencies ranging from 0.002% to 0.93%. After processing, we were able to demonstrate an UV-C LED emitting at 234 nm with 14.5 μW integrated optical output power and an external quantum efficiency of 0.012% at 18.2 A/cm2.

Jens Rass

Papers

III-Nitride Ultraviolet Emitters

III-nitride ultraviolet emitters : technology and applications

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates

Efficient charge carrier injection into sub-250 nm AlGaN multiple quantum well light emitting diodes