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Introduction — Follow the Light

Undoubtedly, light plays a paramount role in our day-to-day  lives. It helps us with orientation, enables us to perceive our  environment, and contributes significantly to our circadian  rhythm. The color and intensity of light may even influence  our mental state and emotions. Furthermore, it is key not only  to photosynthesis in plants—one of the main mechanisms rendering possible life on Planet Earth—but also to the mating  behavior of fireflies and to the navigation of moths.  The attentive observer will not have missed the fact that the  light surrounding us usually features more degrees of freedom  than just color and brightness. On the one hand, light can also  be polarized because it is a wave and thus the field forming the  light wave can be made to oscillate in a predefined plane or,  more generally speaking, this oscillation can be controlled. On  the other hand, these parameters may also feature a spatially or  temporally varying structure. An intuitive example is shown  in Figure 1. There, the structure of light’s intensity is a direct  consequence of the interaction with structured matter (clouds),  creating stunning patterns. In fact, we are immersed in a world  consisting of light and shade taking intricate forms. But there  is more to it than meets the eye. Not only can the intensity of  light be tailored spatially, but also other parameters such as the  aforementioned polarization, the phase, temporal parameters  (pulse shape), etc. may be sculpted as desired. Furthermore,  shaping one of those parameters may also influence others mutually, as we will see later on. 

Figure 1. Sunlight interacting with water droplets in the air re sulting in spatially structured intensity distributions across the  clouds and around them. .

Sculpting the Light Field — Shape It ‘Til You Make It

The possibilities of shaping light at will, as outlined above, seem to be virtually unlimited. In many research areas, ranging from  microscopy, imaging, and communications to nano-optics, plasmonics, quantum optics, etc., structured light fields of various  kinds are utilized [1]. In the majority of cases, however, collimated light beams from lasers propagating through the air are  transformed to feature a structure by transmitting them across  or reflecting them at passive or active optical elements, which interact with the incoming light beam in a position-dependent  manner, hence locally modifying the phase, polarization or amplitude of the field [1], [2]. In this manner, bespoke propagating  light beams can be created. A very prominent class of structured  vectorial light beams are so-called cylindrical vector beams, featuring a doughnut-like intensity distribution with the polarization direction varying across the beam as well. They can  be generated, for instance, by locally rotating the polarization of an incoming homogeneously polarized beam of light. An example, which takes this spatial variation of light’s parameters to the next level, is shown in Figure 2, where besides the intensity  and polarization direction, also the type of polarization and the  phase varies across the beam. 

Our electrodynamic intuition reminds us that light is a  transverse electromagnetic wave, and the field is therefore restricted to live and oscillate in a plane orthogonal to the propagation direction. This is true for planar waves or the collimated  beams we have discussed so far. When leaving this regime of collimated light propagation, for instance by tightly focusing  a light beam or by simply refraining from corresponding paraxial approximations, we enter a world that is even richer when  it comes to structure and structural features of electromagnetic fields. In the above example of tight focusing, complex focal  field landscapes can be created. In a global reference frame,  the field is not purely transverse anymore; strong field components oscillating along the mean propagation direction appear,  resulting in 3D field patterns varying on a sub-wavelength  scale [1]–[5]. If a small object—an atom, molecule or nanoparticle—is placed in such a complex field, the interaction will sensitively depend on the relative position in the field (see  Figure 3). This position-dependent interaction enables the  study of the field itself with the particle acting as a field probe, the investigation of the optical properties of the object, and the local manipulation of the field via this object.  

Figure 2: Cross-section of a paraxial light beam, featuring a complex spatial polarization, phase and intensity distribution (the local polarization states are indicated for selected positions along the horizontal and vertical axes). The beam is assumed to exhibit only transverse field components. If tightly focused (not shown here; see [1]), the corresponding focal field patterns feature sophisticatedly structured amplitude and phase distributions of the individual field components. This process of strong spatial confinement creates 3D field distributions in the focal plane and volume.

But the concept of field shaping is not at all limited to light  beams propagating in free-space. In fact, near- and evanescent fields inherently feature a textured nature owing to their spatial confinement to interfaces, giving rise to intriguing polarization properties embedded in the fine-structure of the field  [3]. This statement holds true for a vast variety of different  schemes—for instance, for the fields in close vicinity of photonic waveguides [3], [6], or, as a very intuitive example, the  near-field distribution around a dipole emitter or nanostructure. Controlling these fields at sub-wavelength dimensions  opens up new pathways for steering, manipulating and processing light at small length-scales. For forging light fields in the nanoworld, different routes can be taken. For instance, the shape, size and material of nano-objects an incoming light  beam interacts with can be chosen. This is routinely used to  create field hotspots or chiral near-fields [7], [8], e.g., for optical sensors. In addition, the excitation field can be tailored such  that desired modes are excited in a nanoscopic system [5]. In  essence, the spatial structure of these two ingredients of nanoscale light-matter interactions plays a crucial role.  

Crafting Functional Nanomaterials — Crafts(wo)manship in the Nanoworld

The immense developments of nanofabrication technology boosted by miniaturization of electronics and corresponding  devices have also led to an unprecedented evolution of fabrication capabilities in the fields of optics and photonics. Count less methods can be used nowadays for fabricating sophisticated nanostructures [5], particle ensembles [9] or large-area  metasurfaces (structured material layers; see Figure 4) [7], [8],  [10]. These bespoke nanomaterials can be crafted from a vast  variety of different materials and can exhibit a wide range of  different geometries and shapes. Modern nanofab methods are  also not restricted to the fabrication of flat or quasi-2D structures. Also 3D nano-objects [5] or artificial multi-layered materials can be fabricated. Whole scientific communities now  specialize in manufacturing artificial materials, surfaces or  individual nanostructures with versatile functionalities and intriguing properties.  

In the context of the previous section, nanopatterned material layers can also be utilized to shape the light field in a very intuitive manner. The individual building blocks’ size, shape, and orientation can be chosen so that the local interaction with an impinging light beam results in the desired action on its polarization, phase, or amplitude. A change of at least  one of these parameters in dependence on the position in the  nanostructure array eventually results in a position-dependent  manipulation of the light field across the beam. Nowadays, advanced nanomanipulation also allows for creating heterogeneous systems with materials and functionalities tunable at the  smallest length-scales [9], [11]. This approach also makes possible the interfacing of individual scatterers or emitters with integrated waveguides [6], [12], eventually also paving the way for on-chip photonic light steering, processing and manipulation. However, these are just a few of the many examples  of tailored light-matter interactions.  

Figure 3. A sub-wavelength nanoparticle is scanned across a 3D field distribution (indicated by the red landscape) resulting from  spatial confinement. The interaction between particle and electromagnetic field depends strongly on the relative position.

A Plethora of Applications — From Juggling Particles and Traffic Control at  the Nanoscale to Quantum Light-Matter  Interfaces 

After the above discussion it does not come as a surprise any more that structured light and structured matter are not just  academic concepts stimulating our scientific curiosity, but they also can act as the main building-blocks for real-world applications and devices—from the most prominent examples of stimulated emission depletion or structured illumination microscopy, with light sculpted in intensity playing the key role, to optical trapping and machining, where focal forces and  interactions can be fine-controlled. Furthermore, the utilization of structured light is also explored in the context of optical communications taking advantage of the spatial structure of  light for monitoring turbulence or for encoding information. It also has been shown that tightly focused light fields interacting with individual nanoparticles are the enabling ingredients to novel ultra-precise localization, stabilization and light-routing techniques in the field of nanometrology [12]. This idea of polarization-dependent light routing has also been transferred already to the quantum regime by coupling the single photons emitted by an excited atom to a waveguide [6]. 

Figure 4. Artistic representation of a simple metasurface consisting of disc-shaped nanostructures on a substrate. The incoming  light is interacting with the individual sub-wavelength nanostructures, giving rise also to collective effects such as lattice modes.  Depending on the shape, material, size, and lattice parameters, the interaction with light can be controlled selectively and position dependently.

At the same time, metasurfaces have been demonstrated to act as small-footprint devices for beam shaping, routing and (de-)multiplexing while nanoparticles and ensembles thereof already find applications in sensing and environmental monitoring. In particular, nanostructured dielectric materials are a  promising route for reducing losses and opening up new pathways in integrated photonics. 

Outlook — The Future is Now 

Structured light and structured matter have already proven to be  powerful, versatile and game-changing platforms both for studying fundamental phenomena and for developing exciting applications. With this short article, we only scratch the surface of what  is possible. The possibilities are endless with the inherently structured nature of confined electromagnetic fields and the precisely  tunable interaction with matter in the nanoworld. Modern nano fabrication and beam-shaping technology pave the way for new  and exciting fields of application in medicine, biology or material sciences by developing novel imaging modalities, sensors, and light sources. The future of structured light and matter based devices and applications has begun already, and it is a ‘bright’ future. 

References

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