Photonic switches based on phase change materials that are nonvolatile in nature and consume lesser power during switching process while having ultra-low footprint are emerging fast to address the challenges faced by modern interconnects. In addition to optical interconnects and optical communication at 1.55 μm wavelength, such devices are likely to be in great demand for emerging optical communication window around 2 μm wavelength. The switching in phase change materials can be triggered by electrical, optical or thermal means. One such material Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> is technologically mature, cost effective and compatible with CMOS fabrication technology. It can exist in amorphous as well as crystalline phase and remains stable in both the phases. It can be switched rapidly and repeatedly for realizing photonic switching devices around wavelengths 1.55 μm and 2.0 μm. By integrating Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> on silicon-on-insulator platform, the switching functionalities with high performance can be achieved. Here, we present various types of switches based on different hybrid Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub>-Silicon waveguide. Different geometries will be discussed for operational wavelengths of 1.55 μm and 2.0 μm. Different design strategies that lead to realization of high performance photonic switches in terms of extinction ratio, insertion loss, switching energy and re-configurability using ultra-compact Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> embedded within the silicon waveguide and having indium tin oxide electrodes are described in detail.
A new window of optical communication is emerging around 2 μm. It is important to design and experimentally demonstrate the photonic devices and components that can support the optical communication in this spectral region by providing the functionalities of switching and routing. The silicon photonics platform for realizing the photonic devices and components will be preferred around 2 μm, like other optical communication windows of 1310 nm and 1550 nm, due to availability of cost effective and high yield CMOS fabrication technology. Photonic switches that are non-volatile in nature and consume lesser power while having ultra-low footprint are likely to be in great demand for future optical communication around 2 μm. Here, we report an ultra-compact 1×1 photonic switch operating at 2.1 μm using nonvolatile phase change material Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> embedded in silicon-on-insulator platform. Embedding of Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub>in silicon-on-insulator waveguide is done in two different ways to evaluate and compare the switching performance. The emphasis has been on optimization of position and dimensions of Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> in partially and fully etched silicon waveguide. We obtained an extinction ratio of 34.04 dB with low insertion loss of 0.49 dB in ON state with Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> of volume 920 nm× 240 nm × 800 nm (length × height × width) embedded into partially etched silicon waveguide. When Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> is embedded in fully etched silicon waveguide, maximum extinction ratio of ~14dB at the expense of insertion loss of 1.36 dB with Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> of volume 1020 nm× 240 nm × 800 nm.
We review recent progress of an effort led by the Stojanović (UC Berkeley), Ram (MIT) and Popović (CU Boulder) research groups to enable the design of photonic devices, and complete on-chip electro-optic systems and interfaces, directly in standard microelectronics CMOS processes in a microprocessor foundry, with no in-foundry process modifications. This approach allows tight and large-scale monolithic integration of silicon photonics with state-of-the-art (sub-100nm-node) microelectronics, here a 45nm SOI CMOS process. It enables natural scale-up to manufacturing, and rapid advances in device design due to process repeatability. The initial driver application was addressing the processor-to-memory communication energy bottleneck. Device results include 5Gbps modulators based on an interleaved junction that take advantage of the high resolution of the sub-100nm CMOS process. We demonstrate operation at 5fJ/bit with 1.5dB insertion loss and 8dB extinction ratio. We also demonstrate the first infrared detectors in a zero-change CMOS process, using absorption in transistor source/drain SiGe stressors. Subsystems described include the first monolithically integrated electronic-photonic transmitter on chip (modulator+driver) with 20-70fJ/bit wall plug energy/bit (2-3.5Gbps), to our knowledge the lowest transmitter energy demonstrated to date. We also demonstrate native-process infrared receivers at 220fJ/bit (5Gbps). These are encouraging signs for the prospects of monolithic electronics-photonics integration. Beyond processor-to-memory interconnects, our approach to photonics as a “More-than- Moore” technology inside advanced CMOS promises to enable VLSI electronic-photonic chip platforms tailored to a vast array of emerging applications, from optical and acoustic sensing, high-speed signal processing, RF and optical metrology and clocks, through to analog computation and quantum technology.
Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and
optical logic elements. Microdisk lasers, with a diameter below 10μm and fabricated in InP membranes with a high
index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous
integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the
active elements with compact, high-index contrast passive elements. The lasing mode in such microdisk lasers is a
whispering gallery mode, which can be either in the clockwise (CW) or counter clockwise direction (CCW) or in both.
The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be
achieved by terminating the SOI wires at one end with Bragg gratings. For all-optical flip-flops, the laser operation must
be switchable between CW and CCW, using short optical pulses. Unidirectional operation in either direction is only
possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if
the gain suppression is sufficiently large, requiring large internal power levels. All-optical flip-flops based on microdisk
lasers with diameter of 7.5μm have been demonstrated. They operate with a CW power consumption of a few mW and
switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The
surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good
HISTORIC aims to develop and test complex photonic integrated circuits containing a relatively large number
of digital photonic elements for use in e.g. all-optical packet switching. These photonic digital units are alloptical
flip-flops based on ultra compact laser diodes, such as microdisk lasers and photonic crystal lasers.
These lasers are fabricated making use of the heterogeneous integration of InP membranes on top of silicon
on insulator (SOI) passive optical circuits. The very small dimensions of the lasers are, at least for some
approaches, possible because of the high index contrast of the InP membranes and by making use of the
extreme accuracy of CMOS processing.
All-optical flip-flops based on heterogeneously integrated microdisk lasers with diameter of 7.5μm have
already been demonstrated. They operate with a CW power consumption of a few mW and can switch in 60ps
with switching energies as low as 1.8 fJ. Their operation as all-optical gate has also been demonstrated.
Work is also on-going to fabricate heterogeneously integrated photonic crystal lasers and all-optical flip-flops
based on such lasers. A lot of attention is given to the electrical pumping of the membrane InP-based photonic
crystal lasers and to the coupling to SOI wire waveguides. Optically pumped photonic crystal lasers coupled
to SOI wires have been demonstrated already.
The all-optical flip-flops and gates will be combined into more complex photonic integrated circuits,
implementing all-optical shift registers, D flip-flops, and other all-optical switching building blocks.
The possibility to integrate a large number of photonic digital units together, but also to integrate them with
compact passive optical routers such as AWGs, opens new perspectives for the design of integrated optical
processors or optical buffers. The project therefore also focuses on designing new architectures for such
optical processing or buffer chips.