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Lasers & Sources

Megapixel laser chips for high-density emitter arrays

Photonic quantum ring hole emitters with operating currents lower than 1μA/pixel can be used in massive parallel interconnects, intelligent transportation systems technology, and mega-cell biotechnology.
30 January 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.0980

Megapixel lasers were recently fabricated on an 8x8mm2 chip, featuring individual pixels operating in continuous-wave mode with sub-μA currents. The chip is robust, with stable operating temperatures well above 140°C and minimal redshifts. It is expected to enable massive parallel interconnects, intelligent transportation systems technology, and mega cell biotechnology.

The past decade witnessed intensive development of microdisk semiconductor lasers with whispering gallery (WG) modes for low-power, high-density, photonic array applications. In recent years, these efforts have led to the demonstration of cylindrical vertical-cavity surface-emitting laser (VCSEL)-like diodes that exhibit WG modes with unusual photonic quantum ring (PQR) characteristics such as μA-range threshold currents and spectral dependence.1,2 In contrast to conventional 2D WG mode lasers, the resonance of the PQR laser results in 3D WG modes of helical standing waves with a hole-emitting geometry and a major polarization state that favors strong carrier-photon coupling with the recombinant carriers. This enables the generation of the PQR pattern within several picoseconds.3,4 The quantum ring behavior results in thermally stable spectra and low threshold currents (Jth = 47 A/cm2 or less), as recently demonstrated in a single mode PQR laser.5 Concave 3D WG mode-PQR microresonators are usually made of cylindrical mesas, or traditional micropillars with smoothly etched sidewalls, fabricated mostly from wafers consisting of a few Al-GaAs quantum-well active planes between p-type and n-type GaAs/AlAs Bragg reflectors for vertical optical confinement.3


Figure 1. Enlarged picture of a megapixel hole PQR laser chip. For dense arrays, 1-3μm PQR diameters with 5-10μm spacings are normal. Larger 5-20μm PQR diameters with 10-30μm spacings are also used for larger emitter arrays. Arrays for plasma display panels are also being developed.

In a recent development, the first “hole” PQR laser array was reported from semiconductor microcavities with 3D optical confinement and thresholds potentially reducible to micro- or nano-ampere regimes.3 Much easier to fabricate than dense mesa PQRs, the device can generate unusual ‘convex’ WG modes via gain guiding and photonic crystal effects. The megapixel laser chip is based on a hole PQR laser with a 1.5μm diameter that can generate 736nA/hole threshold currents. GaN-based PQR arrays to replace light-emitting diodes in high-end displays are also being developed with competitive fabrication costs.6

Such PQR chips (see Figure 1) are attractive for several applications. First, a high-density laser array represents the cornerstone of the next-generation 3D semiconductor strategy for massive inter-chip and intra-chip optical interconnect schemes. Next, the Vision Zero guidelines set by the Intelligent Transportation Society (ITS) of America7 to address traffic problems and fatalities could make use of a concept for safe locomotion based on eye-free escort photonics using the position-sensing capability of the 3D PQR laser combined with fiber communication systems. Finally, the genuine Laguerre-Gaussian beam patterns of the megapixel PQR chip are already working as massive biocell tweezers and sorters, as recently demonstrated with mouse myeloid leukaemia (M1) cells.8


O'Dae Kwon
Department of Electronic and Electrical Engineering
Pohang University of Science Technology
Pohang, Korea

O'Dae Kwon is a professor of electronic and electrical engineering at Pohang University and Head of the Photonic (00-05) National Research Lab. He received his PhD in 1978 from Rice University. After several years at Cornell University and the Dow Chemical Co., he joined the Pohang University faculty in 1986. He visited the AT&T Bell laboratories in 1993-1994 to work on the Miller SEED project and was also a member of the laboratory of E. Yablonovitch at UCLA in 2000-2001. O'Dae Kwon's group has recently fabricated the world's highest-density laser array.


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