Diffraction imposes severe limitations on miniaturization in optical applications, such as high-density photonic integrated circuits or ultra-high-density data storage. To meet these challenges, non-traditional approaches to photonic sciences and technology must be applied. Such an approach is based on local electromagnetic interactions between nanoscale elements and optical near fields. This area, called near-field nanophotonics, is far from mature, and it opens up new avenues for research and innovations.
Our work comprises two major lines: (1) investigations of the fundamental properties of optical near fields, and (2) development of measurement methods and equipment, such as apertureless scanning near-field optical microscope (SNOM) and magnetic SNOM probes, and applications of near fields for optical studies of nanoscopic objects, including quantum dots, plasmonic and photonic structures, etc.
Optical near fields
Optical near fields are non-propagating waves that contain all the high-resolution details. These fields can be manipulated by manmade microscopic structures such as metamaterials. For example, light can be squeezed through a system of sub-wavelength holes in a metal sheet in quantities much larger than expected.
Optical near fields interacting with microscopic elements must be represented by Maxwell’s vector theory, giving rise to polarization effects and electromagnetic coherence in the presence of random fluctuations. The concepts of polarization, coherence, similarity, and entropy in fluctuating electromagnetic fields are under debate currently. Related notions in quantum statistical optics are purity and entanglement, and we work in collaboration with the quantum information group.
Electromagnetic coherence and polarization are of importance also in (high-NA) polarization microscopy and (microscale) polarization-modulating cavities. One of the goals in ADOPT is to clarify these issues and employ them in nanophotonic components, as well as to initiate experimental demonstrations for instance using tapered SNOM tips or molecular scattering.
The plasmonic tip-enhanced SNOM techniques have proven highly successful for high-resolution detection and spectroscopy. A metal-coated tip, illuminated with laser light, serves as a strong, highly localized source, which can be used for instance for Raman and absorption spectroscopy, second-harmonic imaging, two-photon excited luminescence, near-field optical trapping and spanning, and nanolithography. Similar imaging techniques with Terahertz radiation (millimeter waves) have recently gained increased attention, and we are looking into those in ADOPT as well.
Near field – matter interaction
Interaction of the optical near field with elementary excitations in nanostructures, such as semiconductor quantum dots, constitutes the basis of nanophotonics. Nano-sized light sources, detectors and switches, coupled by the optical near field, not only offer a possibility of ultra-dense integration of photonic integrated circuits, but also offer completely new functionalities. However, for realization of such nanodevices, the electronic and optical properties of their constituents, such as quantum dots, should be well understood.
Another crucial aspect for the development of nanophotonic devices is nanoparticle coupling, both electronic and through the near field. In ADOPT, we study the dynamics of the nanoparticle optical response and coupling with the help of time-resolved near-field spectroscopy. Combining a short pulse laser and a SNOM, we examine these effects with nanometer spatial and femtosecond temporal resolution. In addition, steady-state and dynamical SNOM measurements are used to study mode profile and pulse propagation in slow light and plasmonic structures, ultrafast switching in microcavities, as well as characterization of recombination properties in novel materials and devices. We expect that our results will have a significant impact on further development of nanophotonics.
Magnetic near-field probes
The ever-growing demand for digital storage has fuelled phenomenal progress in magnetic storage density, achieved via nanoscale magnetic media to store, Giant Magneto-Resistive (GMR) head to read, and separate magnetic head to write the information. At present, recording the information in the form magnetically stable nanosized domains is the bottle-neck of hard drive technology. Heat-assisted magnetic recording (HAMR) proposed recently enables efficient recording by temporarily heating high anisotropy media. We investigate in ADOPT fabrication and characterization of near-field optical heating and magnetic writing probes based on multilayered waveguides clad with transparent magnets.