Nanophotonics group


The nanophotonics research group, lead by Prof. Tapio Niemi, conducts applied and theoretical research on nanophotonic structures and develops novel nanofabrication methods. Our research includes the fields of plasmonics, metamaterials, resonant nanostructures and nanoparticles.

Contents

 

Recent research areas

During the last years we have been involved with the following research topics.

Metallic nanocones by UV-nanoimprint lithography
 

Gold nanocones on glass, SEM image.
Figure 1. SEM image of gold nanocones, height 250 nm, on glass. Red light added for illustration.

Metallic nanostructures with sharp features are highly useful in plasmonics as they are very efficient in concentrating incident light, in a sense acting as optical antennas. A so-called lightning rod effect that also works in macroscale is mostly responsible for this, but particle plasmon resonances can also play a key role.

We have developed a cost-effective method for fabricating large arrays of metallic nanocones by nanoimprint lithography [Microelectr. Eng. 2010]. The method can be adapted to work with different materials and also more complex geometries are possible.

We have demonstrated that our gold cones can enhance second-harmonic generation by two orders of magnitude compared to an array of non-sharp reference cones [Opt. Lett. 2009]. We have also shown that a bridged double-cone structure was found to increase tip-enhanced Raman signal from crystal violet molecules by nearly an order of magnitude [Opt. Express 2010].

Contacts: Juha Kontio, Janne Simonen.

Resonant waveguide gratings
 

Resonant waveguide gratings (RWG) consist of a waveguide and a surface grating that diffracts light both into and out of the waveguide mode for a particular angle of incidence. Significantly, RWGs can also lead to strong local fields within the structure, enhancing light-matter interaction in processes such as two-photon fluorescence or second-harmonic generation (SHG).

Simulation of a resonant waveguide grating.
SiN resonant waveguide grating, SEM image.
Figure 2. Simulated magnetic field (left) of a fabricated SiN resonant waveguide grating (right).

We have demonstrated SHG enhancement by a factor of about 1000 from a purely dielectric sub-wavelength RWG compared to a thin-film of the same material of comparable thickness and in optimum geometry [to be presented in CLEO 2011].

Moreover, we have shown that a broadband mirror for the IR wavelength region comprising a subwavelength grating made of germanium, a dielectric material with a very high refractive index. The reflectivity of the mirror was over 95% for the wavelength range between 2245 and 3080nm [Opt. Lett. 2010].

This work is partly done in collaboration with the Nonlinear Optics Laboratory at TUT and the University of Eastern Finland in project, REDMETA, funded by the Academy of Finland.

Contacts: Outi Hyvärinen, Juha Kontio, Janne Simonen.

Fabrication of nanoparticles by pulsed laser ablation in liquids
 

TEM image of a TiO2 nanoparticle
Figure 3. TEM image of a TiO2 nanoparticle displaying its polycrystalline structure.

The ablation of a target either in vacuum or in liquids is used to form nanoparticles of various materials by pulsed laser ablation. The laser source is mainly a high repetition rate fiber laser producing picosecond pulses at a wavelength of ~1060 nm [Appl. Phys. A 2010].

The research is conducted on the physical processes involved in the formation of the particles, scaling up the production rate, investigating the optical properties of the particles and development of device applications. Fabrication of metal, oxide and semiconducting nanoparticles is possible merely by changing the ablation target material.

Possible application areas include sensors, coatings, electrodes, and fluorescent labeling. We can form to nanoparticles in pure solutions, such as water, or various solvents. Compared with chemical fabrication methods the resulting colloidal particle solutions are free from chemical residuals.

Nanoparticles in water.
Figure 4. Nanoparticles in water.

In our current European Regional Development Fund-funded project a steering group consisting of members from TUT and industrial partners is set up to support networking and seeking application possibilities for this technology.

Contacts: Turkka Salminen, Tapio Niemi.

Active plasmonics and low-loss metamaterials
 

A major disadvantage of using metals in plasmonics and metamaterials is their inherent absorption losses. Bringing the technology from the research labs to applications requires that the losses be reduced considerably. On the other hand, plasmonic nanostructures can be of considerable help in extracting light out of devices such as organic light-emitting diodes (OLEDs).

One route for loss-reduction is to introduce light amplifying media, i.e. gain into the components. Together with the Nanophotonics Laboratory at RIKEN, Japan, we have been involved in studies concerning plasmon-enhanced OLEDs [Appl. Phys. Lett. 2007] and plasmonic lasers [Opt. Comm. 2011].

On the other hand, the natural approach at ORC is to incorporate III-V semiconductor quantum wells and dots. This work is done in project ACEPLAN funded by ERA-NET NanoSci-E+ program. The partners are prof. Ortwin Hess from Imperial College London and Dr. Antonella Bogoni from the Integrated Research Center for Photonic Networks and Technologies, CNIT.

Contacts: Outi Hyvärinen, Janne Simonen.

Nanostructures fabricated by laser interference lithography
 

Laser interference lithography (LIL) is a maskless technique with high efficiency and low cost. It is ideal for the fabrication of periodic patterns on a large scale.  We have proved, both theoretically and experimentally, that LIL can also make graded structures [New J. Phys. 2008]. The focusing effect of a graded photonic crystal was analyzed theoretically [Opt. Comm., accepted].

Moreover, 'direct writing' was realized on GaAs-related wafers [Nanotechnology 2009] and Si wafers when a high-power pulse laser was used. As no resist is needed in the process the processing time is reduced.

This work is conducted within the Marie Curie IRSES project Laser Nanoscale Manufacturing (LaserNaMi) that focuses on staff exchange between partners in EU and China.

Contacts: Chunlei Tan, Tapio Niemi.

LIL-processed holes on GaAs wafer
LIL direct writing on Si wafer.
Figure 5. Graded holes on GaAs wafer (left) and periodic structures on Si wafer, fabricated by ‘direct writing’ (right).

Projects

Here is a list of our projects. See also the project list of ORC.

  • ACEPLAN (EU ERA-NET NanoSci-E+): Active plasmonics and lossless metamaterials.
  • LaserNano (Pirkanmaan liitto): Laser based fabrication and modification of nanomaterials.
  • NEREUS (Academy of Finland): Negative refraction in semiconductor and photonics crystal metamaterials.
  • Produla (Tekes): Photonics production platform.
  • REDMETA (Academy of Finland): Resonance-domain metamaterials for subwavelength optics.


Publications

Here is the publication list of the Nanophotonics group. Also see the publication list of ORC.

  1. G. Bautista et al. Second-harmonic generation imaging of metal nano-objects with cylindrical vector beams. Accepted for publication in Nano Letters (2012).
  2. T. Ning et al. Strong second-harmonic generation in silicon nitride films. Appl. Phys. Lett. 100, 161902–161902–4 (2012).
  3. Y. Bai et al. Outcoupling of trapped optical modes in organic light-emitting devices with one-step fabricated periodic corrugation by laser ablation. Organic Electronics 12, 1927–1935 (2011).
  4. Tan, C., Niemi, T., Peng, C. & Pessa, M. Focusing effect of a graded index photonic crystal lens. Opt. Commun. 284, 3140–3143 (2011).
  5. H'Dhili, F., Okamoto, T., Simonen, J. & Kawata, S. Improving the emission efficiency of periodic plasmonic structures for lasing applications. Opt. Commun. 284, 561-566 (2011).
  6. Rao, S. et al. Tip-enhanced Raman scattering from bridged nanocones. Opt. Express 18, 23790-23795 (2010).
  7. Kontio, J.M., Simonen, J., Tommila, J. & Pessa, M. Arrays of metallic nanocones fabricated by UV-nanoimprint lithography. Microelectr. Eng. 87, 1711-1715 (2010).
  8. Tommila, J. et al. Nanostructured broadband antireflection coatings on AlInP fabricated by nanoimprint lithography. Solar Energy Materials and Solar Cells 94, 1845-1848 (2010).
  9. Salminen, T., Hahtala, M., Seppälä, I., Laukkanen, P. & Niemi, T. Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates. Appl. Phys. A 101, 735-738 (2010).
  10. Kontio, J.M., Simonen, J., Leinonen, K., Kuittinen, M. & Niemi, T. Broadband infrared mirror using guided-mode resonance in a subwavelength germanium grating. Opt. Lett. 35, 2564-2566 (2010).
  11. Kontio, J.M. et al. Nanoimprint fabrication of gold nanocones with ~10 nm tips for enhanced optical interactions. Opt. Lett. 34, 1979-1981 (2009).
  12. Okamoto, T., Simonen, J. & Kawata, S. Plasmonic crystal for efficient energy transfer from fluorescent molecules to long-range surface plasmons. Opt. Express 17, 8294-8301 (2009).
  13. Salminen, T., Hahtala, M., Seppälä, I., Niemi, T. & Pessa, M. Pulsed laser deposition of yttria-stabilized zirconium dioxide with a high repetition rate picosecond fiber laser. Appl. Phys. A 98, 487-490 (2009).
  14. Tan, C. et al. Ordered nanostructures written directly by laser interference. Nanotechnology 20, 125303 (2009).
  15. Okamoto, T., Simonen, J. & Kawata, S. Plasmonic band gaps of structured metallic thin films evaluated for a surface plasmon laser using the coupled-wave approach. Phys. Rev. B 77, 115425-8 (2008).
  16. Tan, C. et al. Line defects in two-dimensional four-beam interference patterns. New J. Phys. 10, 023023 (2008).
  17. Canfield, B.K. et al. Inhomogeneities in the nonlinear tensorial responses of arrays of gold nanodots. New J. Phys. 10, 013001 (2008).
  18. Feng, J., Okamoto, T., Simonen, J. & Kawata, S. Color-tunable electroluminescence from white organic light-emitting devices through coupled surface plasmons. Appl. Phys. Lett. 90, 081106-3 (2007).
  19. Viheriala, J., Niemi, T., Kontio, J., Rytkonen, T. & Pessa, M. Fabrication of surface reliefs on facets of singlemode optical fibres using nanoimprint lithography. Electronics Letters 43, 150-151 (2007).
  20. Tukiainen, A. et al. Selective growth experiments on gallium arsenide (1 0 0) surfaces patterned using UV-nanoimprint lithography. Microelectronics Journal 37, 1477-1480 (2006).
     

Contact

For more information of our research contact Prof. Tapio Niemi or the researchers for the topics described above.


Updated by: Simonen Janne, 16.05.2012 10:11.
Content owner: Niemi Tapio
Keywords: science and research, orc, nanophotonics, plasmonics, metamaterials, pulse laser deposition, pld, nanocones
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