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Forschungsarbeit

Towards novel photonic bio-sensors based on silicon nanostructures

von Dominic F. Dorfner

Silicon photonic nanostructures are of widespread interest for applications in integrated photonics. In particular, photonic crystal (PhC) resonators confine light to ultra small volumes and exhibit cavity modes with high optical finesse. We take advantage of these properties to investigate label-free bio-sensor applications based on the linear optical response of the system.

Label-free optical bio-sensing is one of the fastest growing research areas with potential applications ranging from medical and clinical diagnostics, pharmaceutical screening, and fundamental research to home health care for portable devices. The detection of specific bio-molecules such as DNA, proteins or antibody-antigen interaction in a complex analyte without the use of radioactive or fluorescent labels reduces the complexity in the screening process and allows time resolved measurements without affecting the intrinsic properties of the target molecules. The most well established technique is surface plasmon resonance. Other optical methods are based on waveguides, porous silicon or cantilevers. All these techniques require large sensing areas in the order of mm2 and are not easily adaptable for planar lab-on-chip devices. PhCs allow the control over light in very small dimensions and are well suited for bio-sensing applications on chips.

The photonic crystal is formed by a hexagonal lattice of air holes into the top layer of a SOI material system. (b) cavity formed by decreasing the size of one hole (c) corresponding localized E-field.[Bildunterschrift / Subline]: The photonic crystal is formed by a hexagonal lattice of air holes into the top layer of a SOI material system. (b) cavity formed by decreasing the size of one hole (c) corresponding localized E-field.

 

 

PhCs are typically micro-fabricated periodic nanostructures on a 2D planar silicon-on-insulator chip that can be designed to exhibit a photonic bandgap. The effective photon density of states is zero in this wavelength range and optical nanocavities are formed by systematically introducing defects within the perfect hexagonal lattice. In Fig. 1 we show an SEM image of a PhC nanocavity fabricated by electron beam lithography, reactive ion etching and hydrofluoric acid to establish a freestanding membrane. Fig.1(c) illustrates the localized E-Field at the cavity site obtained by calculations.

Emission spectrum of PhC-cavity[Bildunterschrift / Subline]: Emission spectrum of PhC-cavity
Increasing refractive index leading to a shift of the mode to higher wavelength.[Bildunterschrift / Subline]: Increasing refractive index leading to a shift of the mode to higher wavelength.

In order to characterize these structures, light from a tunable laser is focused on the sample edge and guided towards the PhC. If the wavelength matches the resonance of the cavity a signal can be detected via a microscope objective. A sample spectrum of a point defect cavity is shown in Fig.2.

To show the refractive index (RI) sensing potential of the structures, we changed the background RI material from air (n=1) to water (n=1.33) and isopropanol (n=1.377). This was established by a flow cell in the cavity area. An increased background refractive index leads to a reduction of the index contrast and, therefore, to a higher emission wavelength of the cavity. We illustrate this effect in Fig.3. A clear shift can be observed when changing the background from air (black) over water (red) to isapropanol (green). The dashed lines represent the calculations and show a good agreement of the expected trend.

In the meanwhile, the bio-sensing principle was successfully demonstrated by the mechanism of BSA adsorption on the sample surface and will be reported elsewhere soon. We are confident, that this approach will offer the possibility for label-free optical biosensing for future lab-on-chip systems.

 

 


Dominic Dorfner
* 1980

Stationen
  • 2001-2005
  • Dipl. Phys. at the Technical University of Munich (Germany)
  • 2004-2005
  • Master. Sc. in Physics at the University of Illinois (USA)
  • 2004-2006
  • Manage&More, Technical University of Munich (Germany)
  • May-Jun 2007
  • Guest Scientist at the Technical University of Copenhagen (Denmark)
  • Feb-May 2008
  • Guest Scientist at the University of Tokyo (Japan)
  • since 10/2005
  • in CompInt (Elitenetzwerk Bayern)
  • PhD program at the Walter Schottky Institute (TU Munich)

Veröffentlichungen
  • D. F. Dorfner, T. Hürlimann, G. Abstreiter, and J. J. Finley (2007), “Optical characterization of silicon on insulator photonic crystal nanocavities infiltrated with colloidal PbS quantum dots”, Applied Physical Letters, 91: 233111