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Erbium Doped GaN Lasers by Optical Pumping

Studying ER:GaN materials under 980 nm resonant excitation could guide future crystal growth.

The main objective of this research was to construct an optical pump system that would allow the study of Er:GaN materials under 980 nm resonant excitation to be carried out. The results obtained from the optically pumped studies could then be utilized to guide crystal growth and laser design.

Experimental setup for optically pumped studies under 980 nm laser excitation.

High energy and high power solid-state lasers have enabled a variety of applications which have had, and will continue to have, profound and far-reaching impacts on emerging technologies. The optical gain medium is the heart of a high energy laser (HEL) system.

Compared to the presently dominant gain material for HEL, which is Nd doped yttrium aluminum garnet (Nd:YAG), the most outstanding property of GaN as a host material for HEL application is its outstanding thermal properties. The thermal conductivity of GaN is very high (κ = 230 W/m·K) and is more than one order of magnitude higher than YAG (κ = 14 W/m·K), while its thermal expansion coefficient (α ≈ 4 × 10-6 °C-1) is about 2 times smaller than that of YAG (κ ≈ 8 × 10-6 °C-1). These together make Er doped GaN (Er:GaN) an excellent gain material with a potential to outperform Nd:YAG lasers by a factor of about 60 – 120.

Another important advantage of GaN for HEL application is its significantly higher fracture toughness figure compared to YAG. Moreover, the 1.54 μm emission resulting from the intra-4f transition from the first excited manifold (4|13/2) to the ground state (4|15/2) in Er 3+ ions is a relatively eye-safe wavelength, in that the upper limit of eye-safe laser exposure at 1.5 μm is more than 4 orders of magnitude higher than that of the wavelength range below or close to 1 μm.

To realize the full potential of Er:GaN as a gain medium for HEL, however, Er:GaN bulk crystals in large wafer sizes are required to enable the fabrication of gain media in disk, rod or slab geometry to provide high energy and high power operation. Furthermore, a resonant excitation (e.g. 980 nm) is more desirable than a non-resonant excitation as resonant excitation involves direct transition between the ground state to a higher-lying inner 4f manifold in Er3+ ions without invoking a non-radiative energy transfer, hence generating a much smaller amount of heat than a non-resonant excitation. Additionally, 980 nm pump appears to be a preferred pump wavelength in terms of providing the best trade-off between the optical absorption length, minimizing the quantum defect and managing the constraints in hydride vapor phase epitaxy (HVPE) growth for obtaining Er:GaN crystals with a reasonable thickness. Furthermore, the absorption cross section of Yb3+ at 980 nm is about an order of magnitude larger than that of Er3+. Under 980 nm pump, in an Er and Yb co-doped GaN, from Yb 3+ the energy can be transferred resonantly to the 4|11/2 state of Er3+. Therefore, under 980 nm pump, Yb and Er co-doping can enhance the effective excitation cross section by at least one order of magnitude.

This work was done by Jingyu Lin and Hongxing Jiang of Texas Technical University for the Army Research Laboratory. ARL-0203

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Conductivity Engine components Engine mechanical components Engines Lasers Lasers & Laser Systems Manifolds Materials Materials properties Optics Parts Photonics Powertrains Pumps
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Erbium Doped GaN Lasers by Optical Pumping

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    Aerospace & Defense Technology Magazine

    This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

    Read more articles from this issue here.

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    Overview

    The document is a final report on a project focused on the development of erbium-doped gallium nitride (Er:GaN) lasers through optical pumping. The primary objective of the project was to construct an optical pump system capable of conducting studies on the optical and lasing properties of Er:GaN materials under 980 nm resonant excitation. This research is significant as it aims to enhance the understanding and application of Er:GaN as a gain medium in laser technology.

    Key accomplishments during the project included the successful installation of a high-power 980 nm laser, which is essential for the optical studies of Er:GaN crystals. The report highlights that an erbium doping level of 1.4 × 10^20 atoms/cm³ was confirmed through secondary ion mass spectrometry. The optical studies were conducted using this laser, allowing researchers to explore the photoluminescence (PL) emission spectra of the Er:GaN samples.

    The document provides detailed descriptions of the experimental setup and the materials used, including a freestanding 2-inch Er:GaN wafer with a thickness of 1.2 mm, which was obtained through hydride vapor phase epitaxy (HVPE) followed by laser lift-off (LLO) processing. After polishing, the wafer's thickness was reduced to 1.0 mm. The report includes optical images and cross-sectional views of the samples, illustrating the quality and structure of the Er:GaN wafers.

    Additionally, the report discusses the challenges faced during the project, particularly regarding heat management during optical excitation for larger samples. The authors are actively working on improving both the material quality and the cooling mechanisms to enhance the performance of the optical pump system.

    The findings from the optical studies are expected to provide valuable insights that will guide future crystal growth and laser design efforts. The report concludes with a reference to a publication submitted to "Applied Physics Letters," which discusses the potential of Er:GaN bulk crystals as a gain medium.

    Overall, this report encapsulates the progress made in the field of erbium-doped GaN lasers, emphasizing the importance of optical pumping studies in advancing laser technology and material science.

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