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Lasers & Sources
Bridging laser technology to silicon maturity
Silicon, today's mainstay in the electronics industry, is fast becoming the triggering material of a photonics-based computational era.
13 February 2013, SPIE Newsroom. DOI: 10.1117/2.1201302.004709
We are living in the exponential times of an increasingly aware, interconnected, global society. This mirrors the relentless growth of data traffic in today's telecommunications infrastructures, driving an increase in transmission rates and computing capabilities.1Such a data stream burst will soon challenge the intrinsic limit of copper-based, short-reach interconnects and microelectronic circuits in data centers and server architectures to offer enough modulation bandwidth at reasonable power dissipation. By contrast, optics-based telecommunications have the potential of light to carry information over large distances at very high data rates with minor power dissipation. Thus, the functional merge of photonics on electronics chips and cards could trigger the beginning of a photonics-based computational era.
Silicon is set to become the common bridging platform between the emerging opportunities for photonics and the maturity of electronics.2 But its indirect bandgap makes the material unattractive for efficient light emission. However, the heterogeneous integration of group III-V semiconductor alloys on silicon offers an approach where efficient light amplification is achieved within the III-V direct-bandgap layers, while optical functions are implemented in the passive silicon architectures.3 Within this mindset, laser sources can be fabricated on silicon and optical carriers can be generated for data transfer at very high bit rates.4, 5
A laser source is the combination of a light emitter medium—called gain medium—and a light-confining structure that retains the photons in close vicinity to the gain medium to promote stimulated photon emission. The essential challenge in our work, as well as common to the entire world of micro-nano-photonics, lies in achieving light confinement within the smallest possible space during the longest time duration to allow the production of highly compact photonic devices such as micro-lasers. This is indeed very challenging due to the natural propensity of light to escape freely because of its dual nature of tiny massless photon particles and electromagnetic waves at optical frequencies. However, as electron transport in solids is governed by the atomic scale architecture, in a similar way photonic crystals (PhCs)—as a periodic patterning of the optical medium at lightwave scale—can be used for controlling the light flow.6
The photonic crystals used in our work offer two concurring strategies for quasi-3D control of the lightwave in vertical-cavity surface-emitting micro-lasers (VCSELs): photon trapping and slow-down. The light-confining micro-structure of a VCSEL using a double set of photonic crystals, both structured in silicon, combines these two approaches (see Figure 1).7
A double photonic crystal, vertical-cavity surface-emitting micro-laser (VCSEL) for silicon photonics applications. Silicon-made photonic crystals (PhCs; gray bars) are used as ultra-compact, power-efficient mirrors for the laser cavity to enhance its optical features as well as the electromagnetic density overlap with the optical medium between the two PhCs. Two videos of the laser under operation are available online.11, 12
: Silicon dioxide.
First, light trapping can be easily pictured as the bouncing back and forth of photons by two PhCs. The photons are literally trapped, unable to move across the periodic silicon bar array because of their electromagnetic nature. The sub-wavelength period and silicon filling factor adjust to yield a diffraction-induced interference phenomenom that prevents them from fleeing. While they can't escape vertically, the cage remains open laterally, allowing the photons to escape. The light slow-down can then occur because of the hybrid characteristics of the laser cavity's lightwave. Photons are in fact endowed with the capability to spend their lifetime in the optical cavity, traveling between the two PhCs and being guided within their silicon membranes. It is specifically the waveguided component of the photons' double-life that we exploit to impede their lateral escape. The PhCs are designed to efficiently slow down the guided photons, retaining them long enough near the active light-emitting medium. Such strong slowing-down relies on the rather large optical index contrast between the silicon bars and the surrounding silica material, 3.5 compared to 1.5, when evaluated in the 1550nm spectral range, which is a specific characteristic of PhCs.8
Figure 2. (a) Cross-sectional scanning electron micrograph of the double PhC-VCSEL cavity measuring 3.3μm×25μm (thickness × width). The bottom photonic crystal mirror (PCM) is made of crystalline silicon (Si), whereas the top mirror is obtained from a deposited amorphous silicon layer tailored for minor absorption losses in the wavelength range of interest. (b) Close-up shots of a 2-inch-wide indium phosphide (InP)-based epitaxy bonded on a 200mm silicon-on-insulator (SOI) wafer after substrate removal, denoting bonding yields close to 100%. (c) Modal, thermal, and polarization features of double photonic crystal VCSELs under optical pumping excitation. Continuous wave (CW) lasing behavior of double PhC-VCSELs at different pumping powers and stage temperatures. Single-mode emission with 26dB of transverse side-mode suppression ratio (SMSR) is obtained up to a stage temperature of 43°C. The estimated thermal tuning coefficient is 0.06nm/K. (d) Light-in-light-out curves correspond to different stage temperatures. A.U.: Arbitrary units. InGaAsP: Indium gallium arsenide phosphide. Rel.: Relative.
This silicon-based photonic building block has been conceived within a large-scale CMOS-compatible processing technology context. The silicon patterning of 1D photonic crystal mirrors (PCMs)—see Figure 2(a)—on 200mm-wide semiconductor-on-insulator (SOI) wafers aims at both an efficient light harnessing and optimal optical confinement. The III-V epitaxial layers providing light amplification are wafer-bonded to SOI by state-of-the-art molecular bonding—see Figure 2(b)—to ensure laser performance, CMOS-compatibility, and large-scale cost-effective fabrication.
VCSELs for CMOS-compatible integration using a double set of silicon-made photonic crystal mirrors are not just a promising solution as compact, power-efficient, next-generation emitters for silicon photonics applications, but also represent an elegant answer to many open questions inherent in the wider field of VCSEL photonics. The originality of the laser structure does not merely reside in its compactness9 or optical performances: see Figure 2(c) and (d).10 The novelty comes with the use of CMOS-compatible materials and processing, which aim at the cost-effective scaling of top-notch optoelectronics on the mature silicon footprint using standard pilot lines for microelectronics fabrication.
In conclusion, the scope of our work cannot be simply confined to one out of many milestones achieved along the evolving path of VCSEL photonics, but should be considered as an authentic turning point in the ongoing effort of bridging laser technology to silicon maturity. Our work now focuses on the integration of state-of-the-art VCSEL optoelectronics on silicon, including the on-chip optical routing of light at room temperature generated from the VCSEL sources.
The authors acknowledge funding from the European Commission within the framework of the European project Helios.
National Center for Scientific Research
Lyon Institute of Nanotechnology
Corrado Sciancalepore received an MSc in physical engineering from Polytechnic University of Turin, Italy, in 2009, and a PhD in optical and electrical engineering from Ecole Centrale de Lyon, France, in 2012. His research interests include the physics of optoelectronic devices, photonic crystals, and VCSEL and silicon photonics.
Badhise Ben Bakir
French Atomic Energy Commission
Photonics on CMOS Labs
Badhise Ben Bakir received a PhD degree in optical and electrical engineering from Ecole Centrale de Lyon in 2007. His research interests include the physics of optoelectronic devices and nanostructures as well as micronano-fabrication related to silicon and III-V based materials for optical integrated circuits.
Xavier Letartre, Pierre Viktorovitch
National Center for Scientific Research (CNRS)
Lyon Institute of Nanotechnology
Xavier Letartre received a PhD in material sciences from the Universit des Sciences de Lille, France, in 1992. He is currently research director at CNRS, Ecole Centrale de Lyon. His areas of interest include the physics of optoelectronic devices, photonic crystals, and nanostructures for optical devices and circuits.
Pierre Viktorovitch is Emeritus Research Director at CNRS. His research interests concern micro-opto-electromechanical systems (MOEMS) based on III-V compound semiconductors, micro-nano-photonics and III-V on silicon heterogeneous integration. He is the author and co-author of about 300 articles in international journals and conferences proceedings, six book chapters, 70 invited conferences, and eight patents.
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