Research

Lasers inside single cells

We have for the first time demonstrated operation of a laser inside a live human cell. The lasers were made out of solid polystyrene beads ten times smaller than the diameter of a human hair. We fed these laser beads to live cells in culture, which eat the lasers within a few hours. The lasers can act as very sensitive sensors, enabling us to better understand cellular processes. For example, we measured the change in the refractive index which is directly related to the concentration of chemical constituents within the cells, such as DNA, proteins and lipids. Further, lasers can be used for cell tagging. Each laser within a cell emits light with a slightly different fingerprint that can be easily detected and used as a bar code to tag the cell. With careful laser design, up to a trillion cells (1,000,000,000,000) could be uniquely tagged. This would enable to uniquely tag every single cell in the human body. This will enable the study of cell migration including cancer metastasis. Further by using a micro pipette, we injected a tiny drop of oil containing fluorescent dyes into a cell. In contrast to the solid bead, forces acting inside the cells can deform the droplets. By analysing the light emitted by a droplet laser, we can measure that deformation and calculate the tiny forces acting within a cell. Finally, we realized that fat cells already contain lipid droplets that can work as natural lasers. That means each of us already has millions of lasers inside our fat tissue that are just waiting to be activated to produce laser light. The cell lasers could be used to locate target tissue, such as a cancerous tumour, and active pre-loaded, light-sensitive drugs only in that area.

Biodegradable optical waveguides

Advances in photonics have stimulated significant progress in diagnostics, surgery and therapeutics, with many techniques now in routine clinical use. However, the finite depth of light penetration, which is typically less than a few mm’s in tissue, is a serious limitation constraining clinical utility. To address this overriding problem, we have developed implantable light-delivery devices made of polymers that are bio-derived or biocompatible, and biodegradable. In contrast to conventional glass or plastic optical fibers, which must be removed from the body soon after use, the biodegradable and biocompatible waveguides may be used for long-term light delivery and need not be removed as they are gradually resorbed by the tissue. As proof of concept, we demonstrate this paradigm-shifting approach for photochemical tissue bonding for wound closure. The developed fibers have also great potential for biomedical applications, such as in vivo optical sensing and phototherapy.

Whispering-gallery modes

Spherical microparticles or microdroplets can capture light inside by total internal reflection, if their refraction index is larger than the outside medium. The light is circulating inside the particle by multiple total internal reflections and if it comes back to the same point in the same phase, the resonant condition is met. These kind of resonances are called whispering-gallery-modes (WGMs). The name comes from Whispering Gallery - the dome of St. Catherine’s cathedral in London where L. Rayleigh observed and analyzed sound bouncing of the dome walls. Because of small size of WGM resonators and their high Q-factors they are useful in basics research as well as in applications, especially in telecommunications. One of major challenges in development of applications is to be able to tune the WGMs.

Tunable whispering-gallery-mode microresonators in liquid-crystal droplets

Because of birefringence and large response to external stimuli, liquid crystals are well known for applications in tunable optical devices. For example, LC droplets embedded in a polymer matrix are used in PDLC displays and switchable windows. These droplets can also be used as whispering gallery mode (WGM) microresonators. WGMs are optical resonances of light captured inside transparent spherical micro object by means of total internal reflection on the interface.

The droplets were prepared by mechanical mixing fluorescent dye doped nematic liquid crystal with PDMS polymer or water. The droplets were illuminated by an argon laser and the spectrum of emitted light was measured with a spectrometer. A series of well resolved peaks were observed in the spectrum of the fluorescent light, corresponding to WGMs. The measured Q-factor of the resonances was approximately 104, corresponding to the linewidth of 0.05 nm. By exciting the droplets with a pulsed laser we also achieved low threshold lasing. With electric field we achieved WGM shift of the order of 20 nm at 2.6 V/µm in 17 µm diameter droplets. This value is one to two orders of magnitude larger compared to already published values for electrical tuning. The results were recently published in Nature Photonics. On the other hand we have achieved a shift of 33 nm by changing the temperature from 25°C to 38°C. In both cases the spectral shift exceeds the free spectral range, meaning that the resonator frequencies can be shifted to any value. The tuning is also almost linear with voltage or temperature and without hysteresis.

Liquid crystal droplets were for the first time used as WGM microcavities. Electrically and temperature tunable liquid crystal optical microresonators could be used as active optical microcomponents such as tunable laser sources, active filters, switches and sensors.

Electric field induced size change of 2D nematic colloidal crystals

2D nematic colloidal crystals are interesting for application in field-controlled Bragg diffraction gratings. We have shown that diffraction of visible light from 2D dipolar nematic colloidal crystals can be tuned electrically. When the external electric field of 1V/um is applied in a direction perpendicular to the plane of the 2D colloidal crystal, the induced strain in the liquid crystal is highly anisotropic, and the inter-colloidal spacing changes for as much as 20% along one direction and 2% along the perpendicular one. In the transverse direction, the response is non-monotonic with increasing electric field and is the result of two competing mechanisms: below the Freedericksz transition the field pushes the chains of colloidal particles apart, whereas above the Freedericksz transition, the surrounding liquid crystal tends to compress the colloidal structure. The time of the response is of the order of ten seconds and is an intrinsic property, resulting from the flow of the liquid crystal during the restructuring of the crystal.

This this novel tuning mechanism could be interesting for photonic applications, where the time response is not critical, but the range of change of crystal spacing should be as large as several tens of percents.

Measurement of elastic properties of TiO2-derived nanoribbons

By using an atomic force microscope (AFM) the Young’s modulus of TiO2-derived nanoribbons was investigated. Nanoribbons were deposited on a flat surface containing holes. Occasionally we found individual nanoribbons laying over the holes. The Young’s modulus was measured by using a three-point bending method. Thin nanoribbons (cross-section dimension 30 nm) have an average Young’s modulus of 260 GPa. For thicker nanoribbons, which are composed of several thinner nanostrips, the bending modulus rapidly decreases with increasing cross-section. In this case shear deformations become relevant. The estimated shear modulus is 0.07–0.4 GPa. The results demonstrate good elastic properties of titania-derived nanoribbons.