Silicon Brains, Photonics, etc.

The semester is officially over, which is exciting: I finally get to get back into the swing of research (with the occasional GRE study break, of course). As such, blogging will tend to be lighter; but before I lock myself in the lab, here are a few things that came to pass while I was busy finishing up the semester…

Building Brains in Silicon
Among other things, I wrote a paper for my computational neuroscience class on – you guessed it – some really cool work coming out of Kwabena Boahen’s group (formerly here at Penn, now at Stanford) on silicon-based artifical neural systems. This is sometimes classed as ‘neuromorphic engineering’, a term (coined by Carver Mead in the 1980’s) which has come to refer to a relatively recent interdisciplinary paradigm dealing with the development and study of artificial neural systems, drawing on principles from such fields as physics, biology, and computer/electrical engineering to design electronic-based analogues of biological systems. A number of people are using this to try to design new VLSI-based systems based on biological systems.

Some others are trying to reverse this scenario: while ‘real’ neural systems are experimentally studied by neurobiologists while grossly simplified ones are modeled by computational neuroscientists, groups like Boahen’s are trying to bridge these modes of inquiry by exploiting similarities between electronic and neural circuits. Mahowald and Douglas wrote a seminal paper in 1991 describing the first ‘silicon neuron’, and a good deal of work has gone on since then. For example, a number of ‘thermodynamic’ models of ion channels have been developed, building on concepts like Hodgkin/Huxley-type models. Anyway, by exploiting the beautiful similarity between ion channels and metal-oxide-semiconductor (MOS) transistors as two-state systems (simplistically, ion channels are either open or closed, with the energy barrier – and hence the transition rate – between the two states being modulated via, for example, a voltage; on the other hand, a voltage applied across the source and the drain of a MOSFET causes charges to diffuse through the ‘conduction channel’, with the effective barrier to this diffusion being modulated by a gate voltage), Boahen and his graduate student Kai Hynna have recently taken an important step toward ‘building a brain in silicon’. Using an approach that combines the advantages of experiment and artificial modeling, they have developed a simple electronic circuit that replicates the nonlinear dynamics of the gating particles of voltage-dependent ion channels.

References:
- Hynna and Boahen’s recent paper: K. M. Hynna and K. Boahen, Neural Computation 19, 327 (2007).
- 1991 silicon neuron paper: M. Mahowald and R. Douglas, Nature 354, 515 (1991).
- Thermodynamic models of ion channels: A. Destexhe and J. R. Huguenard, J. Comput. Neurosci. 9, 259 (2000).

Update: I guess Tech Review thought this stuff is cool, too: the latest issue has an article on Boahen’s work. It takes a broader view of his work than I have above - I just focused on one particular aspect.

Quasicrystals and Complex Materials as 3D Photonic Structures
I wrote another paper for my modern optics class, based on this recent experimental paper by Man, Megens, Steinhardt and Chaikin on three-dimensional quasicrystals as complete photonic bandgap materials. Here’s the deal: since Schrödinger’s wave equation and the electromagnetic wave equation are formally similar (neglecting spin statistics), it isn’t all that surprising that a number of analogies exist between electronic waves and light. In particular, electromagnetic waves can propagate in structures of periodic dielectric constant, and interference due to multiple Bragg reflections from these interfaces leads to directional-dependent energy band gaps. A major goal is to try to develop artificial structures to act as complete, omnidirectional photonic bandgap (PBG) crystals with bandgaps in the visible regime (wavelength ~ 400-700nm), and a lot of effort has gone into this. Interestingly, recent innovations in materials science and the study of complex materials – such as quasicrystals (QC), liquid crystals (LCs), and colloidal self-assembly – have breathed new life into this quest.

References:
- Experimental confirmation of the almost-spherical effective Brillouin zone (and hence the potential of developing a 3D PBG structure) of a macroscopic 3D icosahedral photonic QC: W. Man, M. Megens, P. J. Steinhardt and P. M. Chaikin, Nature 436, 993 (2005).
- Experimental approach towards assembling 3D analogues of the QC structures studied by Man et al. on a smaller scale using holographic optical trapping: Y. Roichman and D. G. Grier, Opt. Exp. 13, 5434 (2005).
- Another experimental approach, using a novel 7-beam optical interference holography technique: W. Y. Tam, Appl. Phys. Lett. 89, 251111 (2006).
- Using nematic liquid crystals in ‘inverse opal’ structures as PBG materials (tuned by parameters such as an external electric field) – for example, since liquid crystals are birefringent, modulating their orientational order using a field can influence their optical properties (a principle on which liquid crystal displays are based): K. Busch and S. John, Phys. Rev. Lett. 83, 967 (1998).
- Recent computational work has indicated a feasible method of fabricating 3D visible PBG crystals with two different types of lattice structure using self-assembly of a mixture of colloidal spheres of two different sizes: A. P. Hynninen, J. H. Thijssen, E. C. Vermolen, M. Dijkstra, and A. van Blaaderen, Nature Mater. 6, 202 (2007).

Optical Phase Conjugation

I just posted about superconducting effects in nanoscale systems, and in particular, the phenomenon of Andreev reflection, and I forgot to mention something cool I came across a while back that I recently re-read: this paper by Carlo Beenakker (although it’s listed on arxiv, the pdf doesn’t seem to be of the actual paper; I read it in chapter 4 of this excellent book on mesoscopic physics, although some googling brings up a full pdf version here, which may or may not last.) Beenakker uses “the analogy between Andreev reflection and optical phase-conjugation to answer the question: why does a metal-superconductor junction have a resistance?” Apart from being a very clear and interesting way of looking at this process, the paper’s particularly relevant to me since we recently covered optical phase conjugation (by ‘degenerate’ four-wave mixing) in my modern optics class.

Simplistically, phase conjugation is a nonlinear process by which an electromagnetic wave is reflected as (or alternatively , which is why it is often referred to as being a time-reversal process). This is analogous to Andreev reflection for a number of reasons (the ‘pump’ photons in the four-wave mixing process are like Cooper pairs, the pump frequency is like the Fermi energy, and the excitation energy corresponds to the frequency difference between the pump beams and the incident ‘probe’ beam). If the analogy did fully hold, one would expect the normal metal to be disorder/resistance-free, just as a disordered medium appears transparent when back by a phase-conjugated medium - the phase-conjugated light gets rid of aberrations due to inhomogeneities. The point is that the analogy fails because of the extra phase shifts involved in Andreev reflection processes, which explains in a sense why normal metal-superconductor junctions aren’t fully transparent. I’m not sure if there’s any more explanatory power that can be extracted from this analogy, but it’s definitely a cool way of tying together these two processes.

References:
- Where I first learned about optical phase conjugation: section 7.2 of R. W. Boyd, Nonlinear Optics 2nd ed. (Elsevier, 2003).
- The original four-wave mixing phase conjugation paper: A. Yariv and D. M. Pepper, “Amplified reflection, phase conjugation, and oscillation in degenerate four-wave mixing“, Optics Letters 1 16 (1977).

Quick Online Roundup

Intel Speeds Up Silicon Photonics: Who Says Silicon Isn’t Good For Photonics?

Silicon is ubiquitous in the world of microelectronics, and yet, silicon photonics isn’t nearly as well-developed as other materials. There are a number of fundamental reasons for this, the main ones being the fact that silicon has an indirect bandgap, a relatively large bandgap, and is difficult to make electro-optic modulators out of because of its electric susceptibility.

It would be nice, though, to realize useful silicon-based photonic integrated circuits e.g. for optical-based means of communication and data transmission. This necessitates a post in itself, and has been duly added to the list of things that will be blogged about at some indefinite point in the future.

The point is, though, that a number of very smart people have been able to do interesting things with silicon in the context of photonics; for example, a lot of progress has been made recently in developing silicon lasers (see, for example, this or this). Well, here’s another breakthrough:

Researchers at Intel have announced the world’s fastest silicon modulator–an advance that could cut bandwidth costs and make computers run faster and cooler.

MIT’s Tech Review has an easy-to-read article on this; the paper itself is available here.

Scientists Turned Filmmakers

Interestingly (and perhaps, somewhat amusingly), the Pentagon is funding screenwriting classes for scientists-turned-budding-filmmakers, in an effort to enhance the US’s strength in science and technology. As many have previously pointed out, the US appears to be suffering from a dearth of young scientists and engineers - it turns out that rock stars and i-bankers have more sex appeal than scientists, mathematicians and engineers. (What?!) Taking its cues from (among other things) the popular TV-series CSI - which apparently helped boost careers in forensics - the Pentagon’s decided that “by writing and producing movies that have more scientific themes - and more authentic and appealing science protagonists… the US could encourage more young people to pursue careers in plasma physics, molecular biology, and other fields.“

“If I want to watch sports, I can turn on any one of four to 12 channels, 24 hours a day, seven days a week,” says Alvin Chin, a professor of pediatrics at the University of Pennsylvania in Philadelphia… “Imagine if science programming had that kind of presence.”

The article is here. 

This Week’s Science Roundup

This week, there have been some interesting papers dealing with new magnetic materials; using thin-films in new and interesting ways (such as in transistor memory devices and gate dielectrics in carbon nanotube transistors); nanoscale photonics using nanowires and nanotubes; exploring the possibility of creating quantum dots in graphene using electrostatic potential barriers; using scanning tunneling microscopy to look at the Kondo effect in molecules and carrier dynamics in p-n junctions while they’re being operated; figuring out what part of the brain is responsible for our wandering minds; and two interesting applications of quantum mechanics in biology - theoretically considering phonon-assisted tunneling of electrons in elucidating how we smell, and using computational quantum mechanical calculations to study protein splicing. Whew.

Nanoscale/Condensed Matter-Related:
- Hybrid metal-organic materials that are magnetic at room temperature
- Thin-film ferromagnetic devices whose magnetization is modulated via an applied electric field
- A new organic (pentacene) thin-film field-effect transistor (FET) as a possible non-volatile memory device
- Using self-assembled monolayers (SAMs) as the gate dielectric in carbon nanotube FETs
- Nanoscale photonics: nanowire LEDs and nanotube coaxial cables
- Creating quantum dots electrostatically in graphene
- Manipulating the Kondo effect in molecular systems using STM
- Using STM to study carrier dynamics in a p-n junction

Bio-related:
- Why your brain wanders when you’re bored
- Could Humans Recognize Odor by Phonon Assisted Tunneling?
- Studying Protein Cleavage Using Quantum Mechanical Calculations