GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates.

https://iopscience.iop.org/article/10.1088/0034-4885/79/5/056501/pdf

GaN-based light-emitting diodes on various substrates: a critical review.

We review recent advances in subwavelength high-index-contrast gratings (HCGs) and a variety of applications in optoelectronic devices, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, high-Q optical resonators, and low-loss hollow-core waveguides (HWs). HCGs can serve as broadband (Delta lambda/lambda ~ 35%), high-reflectivity (>99%) mirrors for surface-normal incident light, which is useful to replace conventional distributed Bragg reflectors in optical devices. HCGs can also be designed as high-Q resonators with output coupling in the surface-normal direction. Finally, we discuss a novel design of HCG as shallow angle reflectors and HWs.

High-Index-Contrast Grating (HCG) and Its Applications in Optoelectronic Devices

A new concept of dielectric subwavelength grating has emerged. This grating leverages a high contrast in refractive indices for the grating medium and its surroundings. We will discuss how high-index-contrast grating (HCG) can manipulate light to achieve various extraordinary properties. We will discuss various designs to yield broadband, high-reflectivity mirrors for light incident in surface-normal direction and at a glancing angle, ultra high-Q resonators with surface-normal output, planar high focusing power reflectors and lenses, and ultralow loss hollow-core waveguides. The HCG will be a new, promising platform for integrated optics with applications for lasers, filters, waveguides, sensors and detectors.

https://iopscience.iop.org/article/10.1088/0268-1242/26/1/014043/pdf

High-contrast gratings as a new platform for integrated optoelectronics

We propose a novel ultra-low loss single-mode hollow-core waveguide using subwavelength high-contrast grating (HCG). We analyzed and simulated the propagation loss of the waveguide and show it can be as low as 0.006dB/m, three orders of magnitude lower than the lowest loss of the state-of-art chip-scale hollow waveguides. This novel HCG hollow-core waveguide design will serve as a basic building block in many chip-scale integrated photonic circuits enabling system-level applications including optical interconnects, optical delay lines, and optical sensors.

A novel ultra-low loss hollow-core waveguide using subwavelength high-contrast gratings

The Bragg reflection waveguide (BRW), or one-dimensional photonic crystal waveguide, has recently been proposed for a wide spectrum of applications ranging from particle acceleration to nonlinear frequency conversion. Here, we conduct a thorough analytical investigation of the quarter-wave BRW, in which the layers of the resonant cladding have a thickness corresponding to one quarter of the transverse wavelength of a desired guided mode. An analytical solution to the mode dispersion equation is derived, and it is shown that the quarter-wave BRW is polarization degenerate, although the TE and TM mode profiles differ significantly as the external Brewster's angle condition in the cladding is approached. Analytical expressions for waveguide properties such as the modal normalization constants, propagation loss, and overlap factors between the mode and each waveguide layer are derived, as are dispersion and tuning curves.

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Properties of the quarter-wave Bragg reflection waveguide: theory

We propose the control of group delay and chromatic dispersion in a hollow waveguide with a variable air core and highly reflective mirrors. Both group delay and chromatic dispersion in a hollow optical waveguide increase markedly with decreasing air core. A wide tunability of group velocity and chromatic dispersion is predicted by reducing the air core to nearly half of wavelength. We discuss the potential applications of the proposed hollow waveguide for compact photonic integrated circuits including tunable optical delay lines, interferometers, chromatic dispersion compensators and dispersion slope compensators.

https://iopscience.iop.org/article/10.1143/JJAP.43.5828/pdf

Control of Group Delay and Chromatic Dispersion in Tunable Hollow Waveguide with Highly Reflective Mirrors

We demonstrate a tunable hollow waveguide distributed Bragg reflector consisting of a grating loaded slab hollow waveguide with a variable air-core. The modeling shows that a change in an air-core thickness enables a large shift of several tens of nanometers in Bragg wavelength due to a change of several percents in a propagation constant. We fabricated a slab hollow waveguide Bragg reflector with 620 mum long and, 190 nm deep 1st-order circular grating composed of SiO2, exhibiting strong Bragg reflection at 1558 nm with an air-core thickness of 10 mum for TM mode. The peak reflectivity is 65% including fiber coupling losses, the 3-dB bandwidth is 2.8 nm and the grating-induced loss is less than 0.5 dB. We demonstrate a 3 nm wavelength tuning of the fabricated hollow waveguide Bragg reflector by changing an air-core thickness from 10 mum to 7.9 mum.

Tunable hollow waveguide distributed Bragg reflectors with variable air core

We propose a tunable stop-band hollow waveguide Bragg reflector with a variable tapered air core for an adjustable dispersion-compensation device. The tapered air-core structure gives us chirped Bragg reflection. The precise control of tapered air-core thickness and angle enables us to achieve the dynamic tuning of both stop-band width and center wavelength of Bragg reflection. We demonstrate center-wavelength tuning of 20.1nm corresponding to 1.3% of propagation constant change and stop-band expansion up to 5nm. Also, we demonstrate dispersion tuning operation either in negative or positive dispersion ranges with delay-time difference of about 10ps.

Tunable stop-band hollow waveguide Bragg reflectors with tapered air core for adaptive dispersion-compensation

We demonstrate a tunable hollow waveguide Bragg grating with low-temperature dependence. We fabricated a distributed Bragg reflector consisting of a grating loaded slab semiconductor hollow waveguide with a variable air-core. A change in an air-core thickness enables us to achieve a tunable propagation constant of several percents resulting in a large shift of several tens of nanometers in Bragg wavelength. We demonstrate 10nm continuous wavelength tuning of a peak reflectivity. This value corresponds to a propagation constant change of 0.64%, which is larger than that of thermo-optic effects or electro-optic effects. The measured temperature sensitivity of the peak wavelength is as low as 0.016nm∕K, which is seven times smaller than that of conventional semiconductor waveguide devices.

Tunable hollow waveguide Bragg grating with low-temperature dependence

We present the core thickness dependence of the propagation loss and the polarization dependence loss (PDL) of hollow waveguides of various air core thicknesses. The addition of phase-control layers to GaAs/AlAs multilayer mirrors enables us to reduce the PDL by one order of magnitude. The propagation loss and the PDL of a 5 µm air core hollow waveguide are 2.8 dB/cm and 1.7 dB/cm, respectively, which are currently limited by the reflectivity (0.998) of the GaAs/AlAs multilayer mirrors.

Yasuki Sakurai

Papers

Control of Group Delay and Chromatic Dispersion in Tunable Hollow Waveguide with Highly Reflective Mirrors

Tunable hollow waveguide distributed Bragg reflectors with variable air core

Tunable stop-band hollow waveguide Bragg reflectors with tapered air core for adaptive dispersion-compensation

Tunable hollow waveguide Bragg grating with low-temperature dependence

Air Core Thickness Dependence of Propagation Loss of Slab Hollow Waveguide