V.V. Dudelev

Nonvanishing turn-on delay in quantum dot lasers

A turn-on of a quantum dot semiconductor laser is analyzed in detail both theoretically and experimentally. We show that quantum dot lasers have a nonlinear damping rate which strongly affects laser turn-on dynamics due to the non-instantaneous capture of carriers to a dot. It results in nonvanishing turn-on delay even at very high pumping in good agreement with experiment.

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.

Dropout dynamics in pulsed quantum dot lasers due to mode jumping

We examine the response of a pulse pumped quantum dot laser both experimentally and numerically. As the maximum of the pump pulse comes closer to the excited-state threshold, the output pulse shape becomes unstable and leads to dropouts. We conjecture that these instabilities result from an increase of the linewidth enhancement factor α as the pump parameter comes close to the excitated state threshold. In order to analyze the dynamical mechanism of the dropout, we consider two cases for which the laser exhibits either a jump to a different single mode or a jump to fast intensity oscillations. The origin of these two instabilities is clarified by a combined analytical and numerical bifurcation diagram of the steady state intensity modes.

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.

AlGaInP red-emitting light emitting diode under extremely high pulsed pumping

Efficiency of commercial 620 nm AlGaInP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm2 and pulse duration from microsecond down to sub-nanosecond range. To understand the nature of LED efficiency decrease with current, pulse width variation is used. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major factor controlling the LED efficiency reduction at CW and sub-microsecond pumping. The overheating can be effectively avoided by the use of sub-nanosecond current pulses. A direct correlation between the onset of the efficiency decrease and LED overheating is demonstrated.

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.