Understanding how light interacts with matter at the nanometre scale is a fundamental issue in optoelectronics and nanophotonics. In particular, many applications (such as bio-sensing, cancer therapy and all-optical signal processing) rely on surface-bound optical excitations in metallic nanoparticles. However, so far no experimental technique has been capable of imaging localized optical excitations with sufficient resolution to reveal their dramatic spatial variation over one single nanoparticle. Here, we present a novel method applied on silver nanotriangles, achieving such resolution by recording maps of plasmons in the near-infrared/visible/ultraviolet domain using electron beams instead of photons. This method relies on the detection of plasmons as resonance peaks in the energy-loss spectra of subnanometre electron beams rastered on nanoparticles of well-defined geometrical parameters. This represents a significant improvement in the spatial resolution with which plasmonic modes can be imaged, and provides a powerful tool in the development of nanometre-level optics.

Mapping surface plasmons on a single metallic nanoparticle

Theoretical analysis suggests that there exists an optical attractive force capable of “pulling” microparticles towards a light source. This backwards force is generated by using interference to optimize the scattering of light in the forwards direction.

Optical pulling force

In any optical system, distortions to a propagating wavefront reduce the spatial coherence of a light field, making it increasingly difficult to obtain the theoretical diffraction-limited spot size. Such aberrations are severely detrimental to optimal performance in imaging, nanosurgery, nanofabrication and micromanipulation, as well as other techniques within modern microscopy. We present a generic method based on complex modulation for true in situ wavefront correction that allows compensation of all aberrations along the entire optical train. The power of the method is demonstrated for the field of micromanipulation, which is very sensitive to wavefront distortions. We present direct trapping with optimally focused laser light carrying power of a fraction of a milliwatt as well as the first trapping through highly turbid and diffusive media. This opens up new perspectives for optical micromanipulation in colloidal and biological physics and may be useful for various forms of advanced imaging.

In situ wavefront correction and its application to micromanipulation

We present SLIM, a new optical method measuring optical pathlength changes of 0.3 nm spatially and 0.03nm temporally. SLIM combines two classic ideas in light imaging: Zernike’s phase contrast microscopyand Gabor’s holography.

Spatial light interference microscopy (SLIM)

Since the invention of optical tweezers, optical manipulation has advanced significantly in scientific areas such as atomic physics, optics and biological science. Especially in the past decade, numerous optical beams and nanoscale devices have been proposed to mechanically act on nanoparticles in increasingly precise, stable and flexible ways. Both the linear and angular momenta of light can be exploited to produce optical tractor beams, tweezers and optical torque from the microscale to the nanoscale. Research on optical forces helps to reveal the nature of light-matter interactions and to resolve the fundamental aspects, which require an appropriate description of momenta and the forces on objects in matter. In this review, starting from basic theories and computational approaches, we highlight the latest optical trapping configurations and their applications in bioscience, as well as recent advances down to the nanoscale. Finally, we discuss the future prospects of nanomanipulation, which has considerable potential applications in a variety of scientific fields and everyday life.

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Optical manipulation from the microscale to the nanoscale: fundamentals, advances and prospects.

We present the first theoretical and experimental study of dielectric sub-micron particle behaviour in an optical field generated by interference of co-propagating non-diffracting beams of different propagation constants. In such a field, there are periodic oscillations of the on-axial intensity maxima (self- imaging) that are frequently mentioned as useful for optical trapping. We show thatinthreedimensionsthisistrueonlyforverysmallparticlesandtheincreasing number of interfering beams does not enable confinement of substantially bigger particles under the studied conditions. Experimentally, we succeeded in optical confinement of beads radii from 100nm up to 300nm but only with the help of fluid flow against the beams propagation. We observed self-organization of the particles into the periodic 1D array with the interparticle distance equal to 7.68 µm. We observed how a bead jump from one trap to the neighbouring- occupied trap caused a domino effect propagating with constant velocity over the subsequent occupied traps. Phase shift in one beam induced controlled bi-directional shift of the whole structure over a maximal distance of 250 µm in two co-propagating Bessel beams.

https://iopscience.iop.org/article/10.1088/1367-2630/8/3/043/pdf

Sub-micron particle organization by self-imaging of non-diffracting beams

We present a simple method using an axicon and spatial light modulator to create multiple parallel Bessel beams and precisely control their individual positions in three dimensions. This technique is tested as an alternative to classical holographic beam shaping commonly used now in optical tweezers. Various applications of precise control of multiple Bessel beams are demonstrated within a single microscope giving rise to new methods for three-dimensional positional control of trapped particles or active sorting of micro-objects as well as “focus-free” photoporation of living cells. Overall this concept is termed a ‘biophotonics workstation’ where users may readily trap, sort and porate material using Bessel light modes in a microscope.

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Generation of multiple Bessel beams for a biophotonics workstation

Recently, composed vortex fields generated by means of a spatial light modulator working in a dynamical regime have been examined as prospective information carriers. In this paper, an advanced optical set-up utilizing vortices for transfer of information is proposed and experimentally verified. Its operation is based on a sophisticated design of phase-only masks enabling information encoding and decoding. In the proposed vortex communication channel, the photolithographically prepared masks are used for generation of a composed vortex field carrying information. Dynamics of information encoding and decoding is achieved by switching of an array of light sources illuminating the phase masks.

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Optical implementation of the vortex information channel

It is verified that the condition of nondiffracting propagation of an optical beam provides the degrees of freedom that are applicable to the controlled shaping of the beam's transverse intensity profile. The proposed method of beam shaping permits experimental realization of nondiffracting patterns and arrays with a predetermined spatial shape.

Controlled spatial shaping of nondiffracting patterns and arrays

We demonstrate a new imaging method enabling a selective edge contrast enhancement of three-dimensional amplitude objects with spatially incoherent light. The imaging process is achieved in a spiral modification of Fresnel incoherent correlation holography and uses a vortex impulse response function. The correlation recordings of the object are acquired in a one-way interferometer with the wavefront division carried out by a spatial light modulator. Two different methods based on applying a helical reference wave in the hologram recording and a digital spiral phase modulation in image reconstruction are proposed for edge enhancement of amplitude objects. Results of both isotropic and anisotropic spiral imaging are demonstrated in experiments using an LED as an incoherent source of light.

Zdeněk Bouchal

Papers

Sub-micron particle organization by self-imaging of non-diffracting beams

Generation of multiple Bessel beams for a biophotonics workstation

Optical implementation of the vortex information channel

Controlled spatial shaping of nondiffracting patterns and arrays

Selective edge enhancement in three-dimensional vortex imaging with incoherent light.