S. N. Losev

High power Bessel beams from EP-VECSELs

We report on demonstration of non-diffracting (Bessel) beams from Electrically Pumped Vertical External Cavity Surface Emitting Lasers (EP-VECSELs), with output powers ranging up to hundreds of milliwatts and central lobe diameters of 10-100 μm with propagation lengths up to few tens of centimeters. To our knowledge, this is the best result for Bessel beams generated from semiconductor light sources and is comparable to that achievable from vibronic lasers.

Optical trapping with Bessel beams generated from semiconductor lasers

In this paper, we study generation of Bessel beams from semiconductor lasers with high beam propagation parameter M2 and their utilization for optical trapping and manipulation of microscopic particles including living cells. The demonstrated optical tweezing with diodegenerated Bessel beams paves the way to replace their vibronic-generated counterparts for a range of applications towards novel lab-on-a-chip configurations.

Non-diffracting beams from surface-emitting lasers

We present an overview of recent advances in generation of non-diffracting (Bessel) beams from surface-emitting lasers, such as electrically and optically pumped VECSELs, and discuss their applications in optical trapping/tweezing and manipulation of micromachines. Our experiments on VECSEL-generated watt power level Bessel beams with central lobe diameters of a few to tens micrometers suggest that the semiconductor surface-emitting lasers are the best candidates for replacement of gas and solid-state counterparts for power-demanding applications in optical manipulation.

Metamaterial for efficient second harmonic generation

A theoretical study is carried out of the possibility of effective second-harmonic generation in a metamaterial representing a structure of alternating layers of semiconductor material with intrinsic and metallic conductivity that can be grown by epitaxial methods.

Manipulation of Microparticles Using Bessel Beams from Semiconductor Lasers

Optical manipulation of microscopic objects (including living cells) using Bessel beams from semiconductor lasers has been demonstrated for the first time. In addition, it has been found in the experiments that a Bessel beam of sufficient power from a semiconductor laser makes it possible to manipulate simultaneously several microscopic objects captured into its central lobe and the first ring.

Superfocusing of high-M2 semiconductor laser beams:experimental demonstration

The focusing of multimode laser diode beams is probably the most significant problem that hinders the expansion of the high-power semiconductor lasers in many spatially-demanding applications. Generally, the ‘quality’ of laser beams is characterized by so-called ‘beam propagation parameter’ M2, which is defined as the ratio of the divergence of the laser beam to that of a diffraction-limited counterpart. Therefore, M2 determines the ratio of the beam focal-spot size to that of the ‘ideal’ Gaussian beam focused by the same optical system. Typically, M2 takes the value of 20-50 for high-power broad-stripe laser diodes thus making the focal-spot 1-2 orders of magnitude larger than the diffraction limit. The idea of ‘superfocusing’ for high-M2 beams relies on a technique developed for the generation of Bessel beams from laser diodes using a cone-shaped lens (axicon). With traditional focusing of multimode radiation, different curvatures of the wavefronts of the various constituent modes lead to a shift of their focal points along the optical axis that in turn implies larger focal-spot sizes with correspondingly increased values of M2. In contrast, the generation of a Bessel-type beam with an axicon relies on ‘self-interference’ of each mode thus eliminating the underlying reason for an increase in the focal-spot size. For an experimental demonstration of the proposed technique, we used a fiber-coupled laser diode with M2 below 20 and an emission wavelength in ~1μm range. Utilization of the axicons with apex angle of 140deg, made by direct laser writing on a fiber tip, enabled the demonstration of an order of magnitude decrease of the focal-spot size compared to that achievable using an ‘ideal’ lens of unity numerical aperture.

3D laser nano-printing on fibre paves the way for super-focusing of multimode laser radiation

Multimode high-power laser diodes suffer from inefficient beam focusing, leading to a focal spot 10–100 times greater than the diffraction limit. This inevitably restricts their wider use in ‘direct-diode’ applications in materials processing and biomedical photonics. We report here a ‘super-focusing’ characteristic for laser diodes, where the exploitation of self-interference of modes enables a significant reduction of the focal spot size. This is achieved by employing a conical microlens fabricated on the tip of a multimode optical fibre using 3D laser nano-printing (also known as multi-photon lithography). When refracted by the conical surface, the modes of the fibre-coupled laser beam self-interfere and form an elongated narrow focus, usually referred to as a ‘needle’ beam. The multiphoton lithography technique allows the realisation of almost any optical element on a fibre tip, thus providing the most suitable interface for free-space applications of multimode fibre-delivered laser beams. In addition, we demonstrate the optical trapping of microscopic objects with a super-focused multimode laser diode beam thus rising new opportunities within the applications sector where lab-on-chip configurations can be exploited. Most importantly, the demonstrated super-focusing approach opens up new avenues for the ‘direct-diode’ applications in material processing and 3D printing, where both high power and tight focusing is required.

Conical refraction of a high-M2 laser beam

We report on experiments with conical refraction of laser beams possessing a high beam propagation parameter M2. With beam propagation parameter values M2=3 and M2=5, unusual Lloyd’s distributions with correspondingly three and five dark rings were observed. In order to explain this phenomenon, we extend the dual-cone model of the conical refraction that describes it as a product of interference of two cones that converge and diverge behind the exit facet of the crystal. In the extended model, these converging/diverging cones are represented as the cone-shaped quasi-Gaussian beams possessing the M2 parameter of an original beam. In this formalism, a beam-waist of these cone-shaped beams is proportional to the M2 value and defines the area of their interference which is a width of the Lloyd’s ring. Therefore, the number of dark rings in the Lloyd distribution is defined by the M2 value and can be much greater than unity. The results of the numerical simulations within the extended dual-cone model are in excellent agreement with the experiment.

Conical refraction with low-coherent light sources

We report on conical refraction (CR) experiments with low-coherent light sources such as light-emitting diodes (LEDs) that demonstrated different CR patterns. The change of a pinhole size from 25 to 100 μm reduced the spatial coherence of the LED radiation and resulted in the disappearance of the dark Poggendorf ring in the Lloyds plane. This is attributed to the interference nature of the Lloyds distribution and is found to be in excellent agreement with the paraxial dual-cone model of CR.

Optical trapping with superfocused high-M2 laser diode beam

Many applications of high-power laser diodes demand tight focusing. This is often not possible due to the multimode nature of semiconductor laser radiation possessing beam propagation parameter M2 values in double-digits. We propose a method of ‘interference’ superfocusing of high-M2 diode laser beams with a technique developed for the generation of Bessel beams based on the employment of an axicon fabricated on the tip of a 100 μm diameter optical fiber with high-precision direct laser writing. Using axicons with apex angle 1400 and rounded tip area as small as ~10 μm diameter, we demonstrate 2-4 μm diameter focused laser ‘needle’ beams with approximately 20 μm propagation length generated from multimode diode laser with beam propagation parameter M2=18 and emission wavelength of 960 nm. This is a few-fold reduction compared to the minimal focal spot size of ~11 μm that could be achieved if focused by an ‘ideal’ lens of unity numerical aperture. The same technique using a 1600 axicon allowed us to demonstrate few-μm-wide laser ‘needle’ beams with nearly 100 μm propagation length with which to demonstrate optical trapping of 5-6 μm rat blood red cells in a water-heparin solution. Our results indicate the good potential of superfocused diode laser beams for applications relating to optical trapping and manipulation of microscopic objects including living biological objects with aspirations towards subsequent novel lab-on-chip configurations.