LIGHT GUIDING LIGHT
Clifford Patterson Lecture, Royal Society, London
10 December 2001

I would like to begin by painting a picture. Imagine a transparent cube of material (like the one in this slide Fig 1) with a myriad of interconnections and components are created by light itself. Circuits that lie on top of each other. Circuits that are reconfigurable. Virtual Circuitry. This is our dream. Light directs and manipulates light without any intervening fabricated components.

Ladies and Gentleman, I am greatly honoured to share these ideas with you, especially on this particular day. Because, 10 December is a monumental date in history.

On this day, 100 years ago, Marconi transmitted the first wireless signal across the Atlantic. We all know what that historic day foreshadowed. But, what amazes me is that there are possibly hundreds of people alive today who are old enough to remember that celebrated event.

The technological advances that have transpired throughout their lifespan have been nothing short of extraordinary.

There have even been extraordinary advances in telecommunications over the last 35 years. That's when I first arrived here in London to research light transmission along optical fibres. Though, strange as it may seem, not from a background of optical communications, but rather from my interests on visual photoreceptors of animal eyes like the ones shown here.

Now today, optical fibre telecommunications is the worldwide standard. But, back in 1967 experts argued that it was an unnecessary technology and worse still, it was infeasible due to the insurmountable problem of glass absorption losses.

Yet only a few years later, previously unimaginable amounts of information were being transmitted across the Atlantic by optical fibres. In many ways, the situation regarding the feasibility of light guiding light technologies are analogous to the state of optical fibre telecommunications back in 1967.

Today, like in 1967 many experts proclaim that we don't need such a technology and that it is unlikely that we will ever surmount problems with materials. I want to emphasise up front that, unlike the experts of 1967, these present day sceptics may very well be right.

Nevertheless, the concepts of this futuristic technology are so beautiful and the conceivable devices so alluring, that I am compelled to hope that this technology will one day be a reality - devices and maybe even toys that implement light guiding light concepts.

But, I must stress from the outset that I will tonight discuss only the concepts behind the technology and not the technology itself nor the impediments to it. So let us begin this futuristic journey - a journey which could be the impetus for one of you to make this technology a reality.

As I have said, the building blocks for light guiding light are free-standing beams, known as spatial solitons. Unlike the usual light beams, which spread or diffract, solitons create their own channel as they travel in a uniform medium, remaining localized and preserving their shape. To produce a soliton, you need a material whose refractive index changes with the beam's brightness

Normally beams do not influence each other. But solitons can attract, repel and spiral around each other and this interaction can even be described by the classical force laws treating the beams as particles with mass.

Whereas linear waves always pass through one another, solitons can be dramatically altered by collisions. They can annihilate one another, fuse (Fig. 5) or give birth to multiple solitons. These phenomena turn out to be of fundamental importance to the emerging technology of light guiding light and light written circuitry.

I want to emphasise that the story of manipulating light with light to create virtual circuitry is not just about an emerging technology. Research into this field has also revealed new conceptual approaches for understanding how curious light beams, known as optical spatial solitons, can be made to remain localized in space while performing some rather amazing acrobatics.

Advances of late have been remarkably swift, coming from different groups across the globe:

• Spatial solitons, once the domain of high power lasers, can now be launched by an incandescent light bulb;

• Soliton dynamics, once the province of esoteric mathematics, is now accessible with undergraduate physics;

• Mere theoretical predictions of a few years ago, such as the possibility of one soliton being made to spiral about another, the fusion or the creation of solitons upon collision, and the transportation of a dim beam by a bright beam are now readily observable in the laboratory.

I have been describing what solitons can do. But advances in technology require an understanding of why they can do it. Clearly, we need to have a physical understanding of solitons. And, this at first posed a significant bottleneck, because it seemed to necessitate intractable mathematics of nonlinear physics.

But, today we now can appreciate solitons from elementary physics. To set the stage, recall the physics of linear optical waveguides. Optical beams have an innate tendency to spread as they propagate in a homogeneous medium. However, this beam diffraction can be compensated for by the lens like action of beam refraction, if the refractive index is increased in the region of the beam.

The resulting optical waveguide provides a balance between diffraction and refraction. Spatial solitons can also be understood from this familiar perspective.

Conceptually speaking, nonlinear beams interact with matter to create their own waveguides (Fig 7). This occurs because the refractive index of a nonlinear medium depends on the physical properties of the light beam. We emphasise that these induced waveguides are composed of linear material and are of arbitrary shape, even twisted and contorted. Beams then propagate along their own induced waveguide according to the familiar physics of linear optics.

Of course, if you change the shape or form of the exciting beams then you change the shape and form of the induced waveguide. A simple self-consistency relationship exists between the soliton and the modes of its induced waveguide. A soliton is a mode or modes of the optical waveguide it induces.

Armed with this elementary perspective, we can appreciate that disparate types of solitons are actually the same animal. In the simplest case, a soliton is one mode of the waveguide it induces. This describes the classical optical soliton of Chaio, Garmire and Townes but it also predicted more esoteric solitons such as the so-called vortex solitons.

More generally, a soliton can be two or more modes of its induced waveguide. This elaboration predicted interesting soliton dynamics, it also explains incoherent solitons, it explains multi-humped solitons and the coexistence of different classes of solitons.

If a soliton can be composed of a number of modes, each travelling at a different speed, then it should be possible to decompose the soliton into its constituent modes in exact analogy to Newton's refraction of white light by a prism into its component colours. And this elementary physical picture foreshadows novelties such as symmetric soliton beams being transformed into asymmetric beams upon colliding with one another.

The fact that nonlinear propagation has a linear waveguide equivalent provides a powerful conceptual tool, one that guides us in a physical manner to the fundamental equations and to their solutions. It allows us to predict novel phenomena, motivate light written circuitry, and foreshadow the design of lossless waveguide components as we discuss below.

Put simply, all soliton dynamics have a optical fibre analogue, albeit some unusual shaped optical fibre system. Conversely all phenomenon on optical fibres has its soliton equivalent in some nonlinear medium. A self-consistency relation unites the linear and nonlinear equivalents.

I cannot over state the importance of this theoretical framework. Until its appearance, soliton science had a mystique because it appeared to be divorced from our intuition - an intuition that has been built upon linear phenomenon.

The linear perspective on solitons that I have presented smashes this misconception. And, it facilitates unforeseen generalisations and allows for numerous predictions, such as vortex solitons, dynamic spiralling solitons, fusing solitons.

For example, one major challenge is to find a simple analytical description of solitons and their interactions. To achieve this, it turns out that we need only borrow from the literature of the linear harmonic oscillator. This is because every optical fibre has a soliton equivalent. Therefore it is natural to first consider the simplest optical fibre possible - one whose refractive index falls off parabolically.

Light beams are described by the familiar harmonic oscillator in this medium - one of the most basic equations of physics. This elementary model tells us that Gaussian shaped beams remain Gaussian shaped as they propagate. In general the beams undulate periodically, undergoing periodic trajectories.

According to the above linear perspective on solitons, Gaussian shaped solitons must also exist in some homogeneous nonlinear medium with the same behaviour as beams in a parabolic index optical waveguide composed of linear material.

The particular nonlinear medium is found by using the self-consistency relation. This leads to a medium whose nonlinear induced refractive index change depends on the beam total power only. This arises, for example, when the medium has a nonlocal response with a correlation length that is much larger than the beam diameter.

In such a medium, Gaussian shaped soliton beams remain Gaussian and they are unaltered by colliding with one another. For a special beam radius and power, a Gaussian beam will propagate without change. Such a beam is called a stationary soliton. It induces a graded index optical fiber which can guide a signal beam. Apart from those beams with a special radius, all other beams "breathe" as they propagate with their radius oscillating periodically.

What happens to two stationary solitons that are initially launched in parallel to each other?

In a homogeneous linear medium they would diffract as they travel in a straight trajectory. In this nonlinear medium they can attract and undergo periodic collisions with one another or, if launched skew to each other, spiral about each other as shown.

Finally, a distant "dim" beam can remain localized and be guided and steered by a "bright" soliton beam. Most experiments to date have involved comparatively narrow solitons launched by a coherent source. These beams induce waveguides that can propagate one or several modes.

At the other extreme it is possible to have comparatively large incoherent soliton beams. And, in a beautiful experiment, Mitchell and Segev have even launched them from an ordinary incandescent light bulb.

Such "big" incoherent solitons can be very neatly viewed as being composed of an enormous number of modes of the multimoded waveguide they induce. This is a straightforward application of the self-consistency principle.

But, diffuse light propagation along multimoded linear waveguides can be described by classical geometric optics.

So again invoking the self-consistency principle, incoherent solitons can also be viewed as bundles of rays, each ray obeying the paraxial ray equation, or equivalently as a bundle of classical (non-interacting) point particles, each particle obeying Newton's laws of motion. This really is extraordinary - solitons described by Newton's laws of motion rather than the esoteric nonlinear Schrodinger equation. This formulation leads to predictions that are unique to incoherent solitons.

For example, incoherent solitons can have any shape in their two-dimensional cross-section, and can even travel in parallel without interacting, unlike coherent beams in the same intensity dependent medium. Experiments have now confirmed these predictions. The crucial concept here is that all these phenomena can be appreciated by simple physics.

Now lets move on to discuss device and logic applications: especially switching light with light.

The dream of photonics is to have a completely optical technology. Here the traditional carriers of information, electrons, are envisaged to be replaced by photons for devices based on switching and logic. Spatial solitons offer one potential way to achieve this dream.

We have described the physics of how waveguides are induced by solitons. The challenge is to develop methods for controllable steering of these induced waveguides by light itself and to produce reconfigurable waveguides. But this is easy. Because as I have shown you, solitons can attract, repel and guide beams, light can be used to switch light for various device and logic applications which are presently being performed by electronics.

Now in one scheme the solitons can be considered as the information flow itself or the soliton merely induces optical waveguides which in turn carry the information. This information can take the form of a weak ("dim") probe beam at a different wavelength or different polarisation than the ("bright") soliton beam. In either scheme, intricate virtual circuitry can be written in bulk nonlinear media.

I have been describing the simplest form of spatial switching. Alternatively, it is possible to induce waveguide components by having two or more solitons interact. Here the solitons are used to guide dim signal beams. In this way structures can be created to tap a weak signal, re-route it and this process can be made both dynamic and reconfigurable. For example, colliding solitons can induce a waveguide coupler.

And, the amount of coupling can be controlled by changing the physical properties of one soliton. Finally, it is also possible for two colliding solitons to fuse, or to split into two or more solitons - creating additional dimensions of switching. If two solitons can fuse, it must be also possible to reverse the process.

A weak signal can be used to split a soliton beam into two or more beams, allowing for even greater versatility.

Depending on the photosensitive properties of the material, the circuitry can have a life span which allows for the possibility of self-reconfigurable circuits. Such plasticity opens the door to adaptive circuits that can be designed to transform themselves to the desired application. The speed required for switching depends on the application. It can be as slow as seconds for circuit reconfiguration in network application or as fast as picoseconds for optical computing.

As I said the steering of one soliton by another soliton or of a weak signal beam by a soliton forms the simplest case of spatial switching.

Structures can be created to tap a signal, make a copy or re-route it and these processes can be made dynamic. For example, the coupling ratio of a two soliton beam induced coupler can be adjusted by changing the properties of one of the solitons. It is also possible for two colliding solitons to fuse, or to split into two or three solitons and to control this splitting by a weak probe beam, thus creating additional forms of spatial switching.

Lastly, let me change gears from light guiding light, to now discuss how to fabricate designer components using solitons. We have shown how soliton dynamics can be approached from the perspective of linear waveguides. Curiously the reverse is also true.

The phenomenon of soliton dynamics provides a method of actually fabricating ideal lossless waveguide components whose design had not even been foreshadowed from our knowledge of linear waveguides. The concept is elementary.

Solitons are allowed to interact in the appropriate photosensitive material so that the desired induced waveguide configurations are then permanently written.

For example, consider the optical device known as the X-junction. This is one of many building block components used for processing optical signals. It can be used to mix or split two signals in any desired proportion. From our knowledge on linear waveguides, we anticipate some reflection and radiation loss when light travels along one area but solitons can undergo lossless collision.

And we can pack many such elements on top of each other. Because, unless beams collide at grazing incidence, they always pass through one another without influencing each other. In this way it is possible to compress circuitry into a compact space with many circuits sharing the same physical location.

Furthermore, certain photosensitive materials offer the potential for erasing one light written device and replacing it by another. Hence, we have the building blocks for dense, reconfigurable, virtual circuitry.

We have unfolded the building blocks for virtual circuitry leading to a transparent cube with a myriad of interconnections created, maintained, and arranged by light itself - with truly awesome potential. I have told you some of the things that solitons can do but will this lead to viable technology?

I am not the person to answer this question. But I do know that some formidable challenges remain in crafting the requisite nonlinear and photosensitive materials. The present situation regarding light guiding light is reminiscent of the mid 1960's when optical fibre communication was first being mooted.

That was deemed a great idea, if only glass absorption could be dramatically reduced - and in a few years it was. We are definitely faced with a challenge, but if surmounted, great things are possible. Scientists have been trying to confine light to artificial boundaries. Here is the opportunity for light to be the master of its own destiny.

I spoke at the outset of the extraordinary achievements since Marconi's historic transmission exactly 100 years ago. What about the next 100 years? Breakthroughs involve leaps of the mind. If we could somehow accelerate the very process of such creative leaps, this in turn could accelerate the pace of technological breakthroughs. And, this possibility consumes any energies of late.

Now the greatest block to creativity is the simple fact that we are blinded by our knowledge.

We don't actually see what is out there - rather we can only interpret the world through our beliefs and knowledge - our mindsets - accumulated from past experiences. This is a brilliant strategy for manoeuvring rapidly in familiar situations, but it is an impediment when confronted with a novel idea.

Suppose we could bypass our mindsets and see the world as it is? Suppose we could become, more literal. Certain brain damaged people do just that. They are literal but they are so at a cost. They lack the necessary mindsets to appreciate what they do see.

For example, Nadia, a severely brain damaged three year old can draw in incredible detail. She did so spontaneously without training. Yet, Nadia could not recognize her mother from the nurse and she has no language ability. Nadia also has no idea how she does it. She just 'sees it'!

Suppose it were possible to artificially switch off a part of the brain in normal people, to switch off our mindsets and see the world as it is? In particular, to switch off that specific part of the brain which is damaged in Nadia.

In fact, it is possible. We do this by using transcranial magnetic stimulation - magnetic pulses used to induce artificial lesions. These magnetic pulses create electric currents via the Faraday effect which in turn shut down local regions of the brain.

As described by the recent BBC documentary "Fragments of Genius", we and other groups have been performing these experiments with very promising results. For example, people do seem to be able to draw better, more realistically.

We are right now developing EEG assisted feedback control techniques to enhance objectivity without the need for the magnetic pulse stimulation. So perhaps, the great technological achievement of the future will be that which help accelerate our objectivity.

And, curiously, this new frontier of, lets call it mind physics, actually involves electromagnetic technologies. Something that Marconi himself would have appreciated, and something that Marconi's genius could well have made accessible to everyone.