Mojmir Sery

Raman microspectroscopy of algal lipid bodies: β-carotene as a volume sensor

Advanced optical instruments are useful for analysis and manipulation of individual living cells and their internal structures. We have employed Raman microspectroscopic analysis for assessment of algal lipid body (LB) volume in vivo. Some algae contain β-carotene in high amounts in their LBs, including strains which are considered useful in biotechnology for lipid and pigment production. We have detected proportionality between the Raman vibrations of β-carotene and the LB volume. This finding may allow fast acquisition of LB volume approximation valuable e.g. for Raman microspectroscopy assisted cell sorting. We combine optical manipulation and analysis on a microfluidic platform in order to achieve fast, effective, and non-invasive sorting based on spectroscopic features of the individual living cells. The resultant apparatus could find its use in demanding biotechnological applications such as selection of rare natural mutants or artificially modified cells resulting from genetic manipulations.

Submicron particle localization using evanescent field

Recently a non-contact organization of submicron colloidal particles on the surface attracted a great attention in connection with development of imaging techniques using total internal reflection. We focus here on the theoretical description of the forces acting on a submicron particle placed in an interference field created by two counter-propagating evanescent waves. Numerical results elucidate how these forces or trap depth depend on the particle size and angle of incidence of both beams. Experimental results proved these conclusions and several polystyrene particles of diameter 520 nm were confined in evanescent standing wave.

Short-Range Six-Axis Interferometer Controlled Positioning for Scanning Probe Microscopy

We present a design of a nanometrology measuring setup which is a part of the national standard instrumentation for nanometrology operated by the Czech Metrology Institute (CMI) in Brno, Czech Republic. The system employs a full six-axis interferometric position measurement of the sample holder consisting of six independent interferometers. Here we report on description of alignment issues and accurate adjustment of orthogonality of the measuring axes. Consequently, suppression of cosine errors and reduction of sensitivity to Abbe offset is achieved through full control in all six degrees of freedom. Due to the geometric configuration including a wide basis of the two units measuring in y-direction and the three measuring in z-direction the angle resolution of the whole setup is minimize to tens of nanoradians. Moreover, the servo-control of all six degrees of freedom allows to keep guidance errors below 100 nrad. This small range system is based on a commercial nanopositioning stage driven by piezoelectric transducers with the range (200 × 200 × 10) μm. Thermally compensated miniature interferometric units with fiber-optic light delivery and integrated homodyne detection system were developed especially for this system and serve as sensors for othogonality alignment.

Natural user interface as a supplement of the holographic Raman tweezers

Holographic Raman tweezers (HRT) manipulates with microobjects by controlling the positions of multiple optical traps via the mouse or joystick. Several attempts have appeared recently to exploit touch tablets, 2D cameras or Kinect game console instead. We proposed a multimodal “Natural User Interface” (NUI) approach integrating hands tracking, gestures recognition, eye tracking and speech recognition. For this purpose we exploited “Leap Motion” and “MyGaze” low-cost sensors and a simple speech recognition program “Tazti”. We developed own NUI software which processes signals from the sensors and sends the control commands to HRT which subsequently controls the positions of trapping beams, micropositioning stage and the acquisition system of Raman spectra. System allows various modes of operation proper for specific tasks. Virtual tools (called “pin” and “tweezers”) serving for the manipulation with particles are displayed on the transparent “overlay” window above the live camera image. Eye tracker identifies the position of the observed particle and uses it for the autofocus. Laser trap manipulation navigated by the dominant hand can be combined with the gestures recognition of the secondary hand. Speech commands recognition is useful if both hands are busy. Proposed methods make manual control of HRT more efficient and they are also a good platform for its future semi-automated and fully automated work.

Characterization of microorganisms using Raman tweezers

The ability to identify and characterize microorganisms (algae, bacteria, eukaryotic cells) from minute sample volumes in a rapid and reliable way is the crucial first step in their classification and characterization. In the light of this challenge related to microorganisms exploitation Raman spectroscopy can be used as a powerful tool for chemical analysis. Raman spectroscopy can elucidate fundamental questions about the metabolic processes and intercellular variability on a single cell level. Moreover, Raman spectroscopy can be combined with optical tweezers and with microfluidic chips to measure nutrient dynamics and metabolism in vivo, in real-time, and label free. We demonstrate the feasibility to employ Raman spectroscopy-based sensor to sort microorganisms (bacteria, algae) according to the Raman spectra. It is now quite feasible to sort algal cells according to the degree of unsaturation (iodine value) in lipid storage bodies.

Behavior of colloidal microparticles in a planar 3-beam interference field

We describe a general way how to calculate optical forces acting on Rayleigh particles or colloids placed into general interference field. In this paper we focus on a configuration with 3 beams laying in one plane and we present an analysis of the particles behaviour. We found that this arrangement can be used for sorting of particles having refractive index higher or lower comparing to the surrounding immersion medium and even for sorting of particles according to their size.

Behavior of nanoparticle and microparticle in the standing wave trap

The basic behavior of microparticles placed in the Gaussian standing wave is studied theoretically in this article. It is shown that the optical force depends periodically on the particle size and, as the consequence, the equilibrium object position is alternating between the standing wave antinodes and nodes. It is presented that the particle confinement is disabled for certain particle sizes. Simplified theoretical description giving analytical formulae for weak dielectric spherical objects of micrometer sizes is presented. Coincidence with the generalized Lorenz- Mie theory is studied here. Experimental confirmation of the theoretical results is briefly discussed.

How the size of a particle approaching dielectric interface influences its behavior

The influence of size of the trapped object on its position near the dielectric interface is studied experimentally. The trapping beam is reflected on a surface and creates weak standing wave component in resulting field distribution. This component causes unwanted jumps of the trapped particle, when the beam waist moves axially in the surface vicinity. Particles of different sizes are more and less influenced by the standing wave, respectively. The position of the trapped particle is measured with quadrant photodiode and photomultiplier tube at the same time.

Influence of weak reflections from dielectric interfaces on properties of optical trap

It is studied theoretically and experimentally how a wave reflected from the surface influences the distance of the trapped sphere from the beam waist. The reflected wave interferes with the incident one and they create a standing wave component in the final axial intensity distribution. This component modulates the trapping potential and creates several equilibrium positions for the trapped sphere. When the beam waist approaches the surface, the potential profiles changes and causes jumps of the trapped objects to deeper potential well. We proved that these unwanted jumps of the trapped objects between the neighboring equilibrium positions can be eliminated by proper size of the sphere.

The use of an optically trapped microprobe for scanning details of surface

We present two methods for surface profiles measurement using optically trapped probe in tightly focused laser beam (optical tweezers). The first method is based on a continuous contact of the probe with the surface (contact mode) and the second one employes the alternating contact (tapping mode). The probe deviations are detected by two-photon fluorescence excited by the trapping beam and emitted by the trapped dyed probe.