May 26, 2023

A new milestone on the way to Green light sources for HUD, Augmented and Mixed Reality applications!

In a recent article published by AIP, researchers from EXALOS have showcased cutting-edge technology for superluminescent light emitting diodes (SLEDs) that emit green light, based on InAlN n-type claddings. The performance of these devices, including electro-optical characteristics, optical confinement, and substrate leakage, has significantly improved when compared to traditional devices based on AlGaN claddings.

Despite substantial research focused on developing bright, efficient, low-power consumption, and compact RGB light engines for high-quality imaging in head-up displays (HUDs), mixed reality (MR), and augmented reality (AR) display architectures, there are still several technological challenges that need to be addressed to meet consumer demands. The main challenges involve the low efficiency of green light emitting devices and the temporal coherence of laser diodes (LDs). LDs, while being spatially coherent sources with high coupling efficiency to optical elements and waveguides (WGs), suffer from speckle noise and unwanted interference artifacts due to their temporal coherence.

Superluminescent light emitting diodes (SLEDs) present a promising alternative to LDs. SLEDs, which adopt a ridge-waveguide architecture similar to LDs, suppress optical feedback by tilting the waveguide and applying anti-reflection coatings to the exit facet. As a result, SLED devices emit spatially coherent but temporally incoherent light, relying on broadband amplified spontaneous emission (ASE) while preventing lasing through their design. However, the relatively low modal gains in the green spectral region have limited the wall plug efficiency of SLEDs and LDs developed thus far.

In their recent study published in Applied Physics Letters (AIP Volume 122, Issue 20, 5 May 2023), EXALOS researchers demonstrate the use of n-InAlN claddings in state-of-the-art SLEDs and LDs that emit in the green spectral range. The findings also reveal a significant reduction in operating current along with a substantial improvement in the emission profile quality, without any substrate leakage.

InAlN with an 18% indium composition is perfectly matched to GaN and exhibits a four-fold increase in refractive index contrast compared to Al0.06Ga0.94N, making it an ideal candidate for cladding layers in LDs and SLEDs. Despite being proposed two decades ago, the adoption of InAlN has been hindered by challenges in crystal growth, accurate control of indium composition (which is highly temperature-dependent), strong piezoelectric fields at the interfaces with GaN affecting vertical carrier conductivity, and extremely challenging p-type doping. Recent successful implementations of InAlN in vertically emitting and edge-emitting devices have been reported, but have so far been limited to laser diodes in the blue spectral range. EXALOS researchers have now achieved the successful implementation of InAlN in state-of-the-art 512 nm SLEDs, resulting in improved optical confinement and electro-optical performance.
These results are particularly important in the green spectral region where high material quality and low internal losses are crucial. The experimental outcomes align with theoretical predictions that indicate an increase in optical confinement and modal gain when using InAlN layers.

Considering the obtained crystal quality, InAlN claddings hold promise as a suitable option for LDs and SLEDs devices with longer emission wavelengths, such as deep-green emitters.

The results of the research will be presented on June 29 by Exalos´ Dr. Malinverni at CLEO/Europe, this year held in the occasion of Laser World of Photonics, in Munich.

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