There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Optical tweezers (OT) have emerged as an essential tool for manipulating single biological cells and performing sophisticated biophysical/biomechanical characterizations. Distinct advantages of using tweezers for these characterizations include non-contact force for cell manipulation, force resolution as accurate as 100 aN and amiability to liquid medium environments. Their wide range of applications, such as transporting foreign materials into single cells, delivering cells to specific locations and sorting cells in microfluidic systems, are reviewed in this article. Recent developments of OT for nanomechanical characterization of various biological cells are discussed in terms of both their theoretical and experimental advancements. The future trends of employing OT in single cells, especially in stem cell delivery, tissue engineering and regenerative medicine, are prospected. More importantly, current limitations and future challenges of OT for these new paradigms are also highlighted in this review.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2008.0052

Optical tweezers for single cells

The separation or sorting of cellular and colloidal particles is currently a central topics of research In this chapter, we give an overview of the range of optical methods for cell sorting We begin with an overview of fluorescence and magnetically activated cell sorting We progress to describing methods at the microfluidic scale level particularly those exploiting optical forces We distinguish between what we term passive and active schemes for sorting Optical forces pertinent to the sorting schemes are described, notably the gradient force and the optical radiation pressure (or scattering force) We discuss some of the most recent advances This includes techniques without fluid flow where we have either stationary or moving light patterns to initiate separation Further methods have shown how using an externally driven flow either counter-propagating against a light field (optical chromatography) or over a periodic light pattern (an optical potential energy landscape) may result in the selection of particles and cells based on physical attributes such as size and refractive index We contrast these schemes with the field of dielectrophoresis where electric field gradients may separate cells and also briefly mention the upcoming area of light-induced dielectrophoresis which marries the reconfigurability of optical fields with the power of dielectrophoresis

Cellular and Colloidal Separation Using Optical Forces

The change in position of fluorescent beads captured inside the focal volume of optical tweezers is monitored using fluorescence emission induced by two‐photon absorption of a continuous‐wave Nd:YAG laser (λ=1064 nm). The displacement of a bead due to interactions with its environment leads to a fluorescence intensity variation that is used to design a novel spatial sensor. We determine changes in the axial position of a CY3‐labeled latex bead with a diameter of 1.03 μm to a precision better than 10 nm. At an intensity of 600 mW/ μm2 the two‐photon bleaching rate is lower than 50% per 2000 s.

High-resolution axial and lateral position sensing using two-photon excitation of fluorophores by a continuous-wave Nd:YAG laser

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.

https://www.mdpi.com/1424-8220/14/1/877/pdf

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

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.

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

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.

http://www.isibrno.cz/omitec/download/Siler_SPIE_05.PDF

Submicron particle localization using evanescent field

Several attempts have appeared recently to control optical trapping systems via touch tablets and cameras instead of a mouse and joystick. Our approach is based on a modern low-cost hardware combined with fingertips and speech recognition software. Positions of operator's hands or fingertips control the positions of trapping beams in holographic optical tweezers that provide optical manipulation with microobjects. We tested and adapted two systems for hands position detection and gestures recognition – Creative Interactive Gesture Camera and Leap Motion. We further enhanced the system of Holographic Raman tweezers (HRT) by voice commands controlling the micropositioning stage and acquisition of Raman spectra. Interface communicates with HRT either directly by which requires adaptation of HRT firmware, or indirectly by simulating mouse and keyboard messages. Its utilization in real experiments speeded up the operator’s communication with the system cca. Two times in comparison with the traditional control by the mouse and the keyboard.

/pdf/holographic-raman-tweezers-controlled-by-hand-gestures-and-1mjglqh0dy.pdf

Holographic Raman Tweezers Controlled by Hand Gestures and Voice Commands

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.

Mojmir Sery

Papers

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

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

Submicron particle localization using evanescent field

Holographic Raman Tweezers Controlled by Hand Gestures and Voice Commands

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