MAX PLANCK INSTITUTE
FOR THE SCIENCE OF LIGHT
Vollmer Lab of Nanophotonics and Biosensing

### Featured Publications

Enhanced Raman Scattering of Graphene using Arrays of Split Ring Resonators Label-free Biosensing with Micro- and Nanoscale Optical Resonators

### Work in the Laboratory of Nanophotonics & Biosensing can involve...

Biology: establishing biosensing assays for clinically relevant biomarkers understanding conformational changes associated with adsorption of proteins developing single cell analysis techniques

Physics: implementing detection schemes based on linear and non-linear optics study of the forces and surface potentials experienced by a trapped nanoparticle optomechanics

Fluid dynamics: study of laser-induced microfluidic flow patterns, Rayleigh-Benard convection

Engineering: integration of optical microcavity sensors with microfluidics engineering of hand held point-of-care biosensing devices optofluidics

Photonics: computer-aided design and EM simulation of photonic micro- and nanostructures micro- and nanofabrication of photonic sensor circuits, sensor chips

## Biosensing with Nanoantenna Coupled to Optical Microcavities

Coupling of a whispering gallery mode (WGM) excited in a miniature glass microsphere to a metallic nanostructure such as a simple gold (Au) nanoparticle immobilized on the cavity surface can create giant EM field enhancement (hot spot) at the site of the Au particle. The plasmonic field enhancement can be used to improve sensitivity levels towards single molecule detection from frequency shifts of WGMs. The frequency shift upon binding of a single molecule is greatly enhanced if the molecule binds at the Au nanoparticle site: $$\frac{\Delta \omega}{\omega} = \frac{\Delta W_\text{particle}}{W_\text{total}} = \frac{-\alpha \vec{E}\left(\vec{r}_0\right)^2}{2\int \epsilon \vec{E}\left(\vec{r}_0\right)^2 \mathrm{d} V}$$

We demonstrate this new sensing concept for the first time in our recent APL cover paper.

## WGM-Carousel-Trap

This is a sped-up video (16x real time) of a single nanoparticle ($a = 375\,\mathrm{nm}$) being trapped and propelled by the WGM momentum flux. The fiber is coupled to the microsphere $R = 48\, \mu \mathrm{m}$ by contact slightly off the equator on the backside. The WGM has $Q = 1.5\times10^6$, and is driven with a power $P = 25 \,\mu \mathrm{W}$. Light travels in the fiber from right to left (WGM scatter can be seen on the left edge of the microsphere). The trapped particle is observed through elastic scattering as a bright spot in front and in back of the microsphere. The ring pattern around the bright spot is caused by diffraction by the microscope objective. The nanoparticle is trapped, and propelled for just over two revolutions with a period of $140\,\mathrm{s}$ before escaping. The particle appears to move faster on the backside due the transverse magnification in the microsphere image.

## Optical Microcavities Biosensing and Nanoparticle-Based Detection

Frequency changes in optical resonators (microcavities) are used to analyze molecular binding events, such as the binding of an antigen to a previously immobilized antibody. We are seeking to improve this label-free sensing technology towards the goal of achieving single molecule detection capability. Furthermore, forces generated by electromagnetic fields in optical resonators are utilized to simultaneously detect, trap and manipulate nanoparticles, virions and macromolecules.

## Biosensing - Bioinspired Photonics

We take inspiration from nature to design novel devices by:

1. Using a biomaterial to modify the functionality of an existing device as exemplified by our approach of using a bacteriorhodopsin coated microcavity to realize an all-optical switch.
2. Using a concept found in nature and apply this concept to the engineering of a novel device as exemplified by our approach of using variability as design element to engineer random microcavities in photonic crystal structures.

Future work will focus on developing programmable molecular systems, similar to DNA origami, to increase sensitivity and selectivity in biosensing applications, as well as on interfacing biomolecules with optical microcavities to realize smart optical devices.

## Reactive Sensing Mechanism

An optical resonator transduces the binding of molecules into a resonance frequency shift which can be detected with great precision due to the high Q-factor of the optical detection system. The shift occurs due to changes in the optical path length caused by the binding of molecules to the microcavity. The frequency shift $\Delta \omega$ of an optical resonance due to binding of molecules can be precisely calculated from first order perturbation theory. Assuming that bound molecules do not change the light field, then the fractional change in resonance wavelength $\Delta \omega/\omega$ is calculated as: $$\frac{\Delta \omega}{\omega} = \frac{\Delta W_\text{particle}}{W_\text{total}} = \frac{-\alpha \vec{E}\left(\vec{r}_0\right)^2}{2\int \epsilon \vec{E}\left(\vec{r}_0\right)^2 \mathrm{d} V}$$

This so called reactive sensing principle can be applied to any microcavity structure and to any biomolecule.

## Optical Resonators

We use optical microcavity biosensors to understand fundamental mechanisms of molecular recognition, conformational changes, affinity and avidity, protein and virus interactions with membranes, signal transduction, as well as effects of protein denaturation and renaturation.