
- Photonics Research
- Vol. 2, Issue 2, 75 (2014)
Abstract
1. INTRODUCTION
Everybody is in some way or another acquainted with or affected by the enormous impact of integrated electronics and integrated circuits (ICs). They have shaped virtually all aspects of our social, professional, and cultural lives, and this development took its beginnings in the 1960s with the emergence of ICs, work for which the Nobel Prize was awarded in 2000. The material of choice was silicon, existing in abundance on Earth, and the development was hugely aided by the existence of a natural passivating oxide that protects the crystalline circuits.
At the same time in the ’60s and maybe inspired by the unfolding of integrated electronics, another concept saw the light—that of integrated optics (today named integrated photonics, describing photonics integrated circuits, PICs), at Bell Labs. Though superficially related, they were in fact very different, an essential feature being that we are physically dealing with fermions in electronics and bosons in photonics. Also, for integrated photonics, one was over the years working with several constituent materials and device structures, in contrast to electronic IC circuits.
Integrated photonics has over the decades developed at a considerably slower pace than integrated electronics, in integration density as well in total number of devices on a chip. As a matter of fact, it was jokingly said “Integrated photonics is the technology of the future and will remain the technology of the future.” However, this state of affairs has altogether been changed by progress in material technology in III–V compounds (GaAs, InP systems, etc.), ferroelectrics (
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Photonics in general has over the past decades developed into a key enabling technology, with inroads in information and communications technology (ICT), with the optical fiber as one of the landmarks, awarded the Nobel Prize in 2009, in biotechnology, for a wealth of sensors and in lighting and energy. Most of these fields are more or less amenable to integrated photonics. In addition, we have areas such as medicine (for therapy and diagnosis), manufacturing (e.g., high-power fiber lasers), and security and surveillance, with maybe less use of integrated photonics. This shows the tremendous versatility and impact of photonics and makes photonics a counterpart to integrated electronics, with different but complementary functions.
But there are also basic physical differences between electronics in the shape of electronics ICs and ancillary devices and photonics. In the former case we are, as noted, generally dealing with fermions (electrons), which obey Fermi–Dirac statistics, whereas in the latter case we are employing bosons (photons), obeying Bose–Einstein statistics. This has significant consequences in the sense that it appears all but impossible to create photonics devices that perform digital signal processing and RAM-type memory functions, operations where electronics excel. On the other hand, photonics is the technology of choice for transmission and routing of vast amounts of high-speed data, useful in a wide variety of applications and spanning distances from global dimensions to (near) future photonic interconnect network fabric on multicore architecture chips and maybe eventually intracore. This is in view of the dominating power dissipation being that of interconnects.
Below we focus on the fabric between the sources (primarily lasers) and detectors.
This paper is organized as follows. After a brief treatment of Moore’s law for integrated photonics, some general waveguide parameters and propagation properties are discussed. Silicon-on-insulator (SOI) and plasmonics-based integrated photonics are discussed, with several examples. The paper is concluded with views on challenges and prospects for integrated photonics in the 21st century.
2. MOORE’S LAW FOR PHOTONICS, FORMULATED IN TERMS OF INTEGRATION DENSITY
While the total number of devices in an integrated photonics chip is still of the order of hundreds depending on how one counts, that of the electronic IC is of the order of a billion transistors with an exponential growth in accordance with the famed Moore’s law [
Figure 1.Moore’s law for integration density in terms of equivalent number of elements per square micrometer of integrated photonics devices, showing a growth faster than the IC Moorés law, adapted from [3]. The figure covers, in time order, a lithium niobate
Some differences between the two types of integrated circuits might be worthwhile to point out: Moore’s law for electronic ICs pertains to circuits with generic elements (transistors, resistors, capacitors), some fraction of which are operative in the sense that they dissipate power. These elements are fabricated by standard processes, applicable to all elements, basically in A Moore’s law for photonics will have to take into account the fact that no generic elements like those in electronics exist; on the contrary, the elements are different, employ differing fabrication processes, and the materials are different (III–V semiconductors, silicon, ferroelectrics, polymers, etc). There is no or small power dissipation in the passive fabric case [such as the arrayed waveguide gratings (AWGs), switch arrays in ferroelectrics, etc.] and there is “high” power dissipation for active devices (lasers, optical amplifiers. etc.) and intermediate in high-speed modulators.
The exponential development in integration density (Fig.
3. WAVEGUIDE PARAMETERS AND PROPAGATION CHARACTERISTICS OF DIFFERENT KINDS OF WAVEGUIDES
One can classify optical waveguides depending on geometry and light confinement, as well as on materials and guiding principles. Planar or slab dielectric waveguides are built of layers of high and low refractive index materials providing confinement only in vertical direction. Nonplanar waveguides that can have different cross sections, such as circular (fibers), ridge, rib, stripe-loaded, or buried; and slot structures can have different forms of guiding core surrounded by cladding material. Channel dielectric waveguides, similarly to optical fibers, utilize total internal reflection, guiding light in a higher refractive index core surrounded by lower index cladding material. There also exist other methods for guiding electromagnetic waves, such as photonic crystal waveguides, where light is confined by the periodicity of the structure in one or more directions, as well as surface plasmon polariton waveguides that use coupling of the electromagnetic field to the oscillation of electron plasma of a conductor (metal) in a dielectric–conductor interface, where electromagnetic surface waves are excited and propagate along the interface. These surface waves are evanescently confined in the perpendicular direction due to very shallow penetration of the electromagnetic field into the metal, opening possibilities for subwavelength light confinement.
A. SOI Integrated Photonics
Channel dielectric waveguides and waveguide devices based on a silica-on-silicon material structure became a widely used technology for telecom applications and showed the ability to keep high performance even for devices with high levels of integration. The main drawback of this technology is the overall large size of the components, mostly due to the large bending radii of waveguides. This limitation is dictated by a low refractive index difference between the core and the cladding of the device, giving scant light confinement, which generally gives a single multichannel AWG a size of several square centimeters, and the integration of more complex structures can be difficult on a single wafer.
Technologies based on high index contrast recently became the subject of active research. Much higher light confinement allows for very small core sizes and sharp bends, leading to the shrinkage of component sizes by several orders of magnitude.
Waveguide parameters and propagation characteristics for waveguides fabricated with different material compositions are presented in Table
Column | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
Characteristics | III/V | SOI | ||||
Index difference | 0.3 | 0.45 | 0.75 | 3.3 | 7.0 (46) | 41 (46) |
Core size (μm) | ||||||
Loss (dB/cm) | 0.02 | 0.04 | 0.1 | 2.5–3.5 | 1.8–2.0 | |
Coupling loss (dB/point) | 0.1 | 0.4 | 3.7 (2) | 5 | 6.8 (0.8) | |
Bending radius (mm) | 25 | 15 | 5 | 0.8 | 0.25 (0.005) | 0.002–0.005 |
Table 1. Waveguide Parameters for Different Materials
The first three columns collect characteristics for silica-on-silicon technology with germanium-doped silica core [
Silicon has recently attracted a great deal of attention as a material for highly integrated photonics. It has low losses for the optical communication window, high refractive index of 3.5 at telecommunication wavelengths, and offers an opportunity for low-cost optoelectronic solutions for applications ranging from long distance down to chip-to-chip interconnects. Additionally, silicon photonic devices can be fabricated using standard silicon processing technology. Compatibility with CMOS techniques allows cheap mass production of monolithically integrated optoelectronic structures. Strong light confinement in a silicon core on a silica buffer (SOI) allows for very sharp bends and submicrometer core sizes (see column 6 in Table
Figure 2.Electric field distribution of TE mode in a silicon channel dielectric waveguide. The yellow and red curves express the amplitude distribution in the x and y directions, respectively; the substrate material is
B. Plasmonics and Hybrid Plasmonics: Selected Reported Results
Many large electronic companies started research and development programs devoted to silicon photonics, identified as a very promising solution for allowing movement of computer interconnects to the optical domain with considerably increased bandwidth and reduced energy consumption. The efforts that have been made still do not cover the mismatch of size between CMOS electronics with a feature size of 20 nm and very compact, but 1 order of magnitude larger, with diffraction limited modal field, silicon photonics. To further increase integration density and compactness of photonic structures for intercore and intracore applications, plasmonic waveguiding has been proposed as it allows breaking the diffraction limit of light. For example, in slot plasmonic waveguides, where a very thin (30–50 nm) dielectric film is situated between two metal layers, light is concentrated in the slot allowing for true nanoscale confinement. Many different geometries of plasmonic waveguide have been investigated in recent years, including strip lines, slot lines [
Hybrid plasmonic structures, which were proposed for the first time in 2007 [
A very thin low refractive index dielectric material, a slot layer (here
In our laboratory, a number of different passive components based on hybrid plasmonic waveguides have been simulated and fabricated, including waveguide couplers and splitters [
Figure 3.Ultrasmall subwavelength hybrid plasmonic microdisk. (a) Schematic diagram and (b) SEM image of the fabricated device with radius around 525 nm. At this radius the cavity has a resonance at about 1550 nm and the intrinsic quality factor
Figure 4.(a) Schematic diagram of the hybrid plasmonic microring modulator. (b) Cross-sectional view along the x–y plane of the Ez field distributions of a resonant mode at 1550 nm with an azimuthal number of 6. The modulator consists of an EOP ring with radius R and a width W sandwiched between a silver ring and a silicon ring with the same radii and widths. A microwave field is applied between the Ag cap and the bottom Si layer, and the refractive index of the EOP can be changed using the ultrafast EO (Pockels) effect; correspondingly, the cavity can be switched between on- and off-resonance modes at a given frequency, resulting in the modulation of transmission power if an access waveguide is placed aside.
Comparing hybrid plasmonic structures with the similar silicon slot waveguides, where on both sides of a thin low refractive index material there are silicon layers [
.
4. CURRENT CHALLENGES FOR INTEGRATED PHOTONICS
As is obvious from the photonics Moore’s law above, the smallest devices with reasonably low loss can be made in hybrid plasmonics, but even then, one cannot really use these for extended waveguides and large-scale concatenation of devices. The key problem is here the inherently high losses of available metals at room temperature, with electron scattering rates of the order of tens of femtoseconds. Even SOI waveguides have fairly large propagation losses in comparison to optical fibers or planar waveguide components based on well-established silica-on-silicon technology.
For ICT type applications, we could formulate requirements for PICs in photonics fabrics, e.g., as Longitudinal size < or Transverse size < or Energy dissipation Insertion loss Bandwidth of transmission or switching control > or Transmitted signal power: (Polarization insensitivity); Low cost.
Table
Device and Wavelength | ||||||||
---|---|---|---|---|---|---|---|---|
1 | P, Layered metal/chalcogenide waveguide [ | 0.66 | 0.33 | 2 | 7 (3.5) | 0.003 (0.01) | Chalcogenide thickness 4 nm, index change 0.1 | |
2 | P, Array of Ag nanoparticles in EOP matrix, | 3 | 15 | 0.2 | 2.4 (12) | (Very approxi-mate) 2 (0.01) | 200 nm electrode separation. Very rough approximation, real values probably much better. Trading lower voltage for length impeded by loss | |
3 | P, Slotline Si/EOP/Si, | 160 | 2 | 80 | 0.1 (0.001) | 33 (8) | Doped Si serves as electrodes. 100 nm EOP | |
4 | A, Silicon microring resonant modulator, | 41 | 1 | 41 | Depletion mode modulator | |||
5 | A, III–V Electroabsorption QCSE [ | 400 | 2 | 200 active 500 total | 3–5 | Traveling-wave type EAM, 50 Ω transmission line |
Table 2. Comparison of Performance of Some Electronically Controlled Modulators
It can be seen that no existing technology meets the requirements stated above, although the array of silver nanoparticles in an EOP matrix comes close. It is also seen that a (much) lower loss plasmonic medium would meet and surpass the requirements. Thus continued progress in the integration density in Fig.
Another technology that holds promise and that is not being widely researched at the moment is that based on near field Förster resonant energy transfer (FRET)-coupled quantum dots (QDs) [
Intel’s state-of-the-art 22 nm feature size gives subpicosecond gate delay time and
Scant efforts have, surprisingly enough, been made to create lower loss negative-epsilon materials than those available, in spite of the huge possible rewards. In fact, the loss properties of a plasmonic material operating in a near-resonance configuration to achieve high field confinement (for low field confinement, one can always have arbitrary low optical losses) can be assessed by employing a quality factor
This equation, rather than the somewhat arbitrary quotient between real and imaginary parts of metal epsilon should be used to assess the loss properties of plasmonic media when operating at or close to resonance conditions.
5. CONCLUSIONS AND PROSPECTS
So what can we expect of integrated photonics in the 21st century? The development up to now has certainly been impressive, as shown in Fig.
This focus on material technology does not mean that we will not see new and unique device structures with existing materials, which was what happened, e.g., with AWGs. Another issue that has been very much debated is the relative merits of all-optical integrated photonics switches versus electronically controlled ones. This is treated in some detail in a recent paper [
An interesting issue concerns the relevance of the Moore’s law of Fig.
Figure
The table of requirements for PICs in photonics fabrics summarizes issues from Fig.
References
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