Diffractive lenses are optical elements commonly used in many applications owing to their simple and compact design. A multifocal diffractive lens enables the simultaneous focusing of an incident laser beam on several positions along the optical axis. The distances between the focal points and the energy densities in the foci can be changed by varying grating parameters, i.e., the modulation depth and the grating period. This requires grating structures that have to be fabricated for each targeted optical arrangement. Even though some methods for designing diffractive lenses with variable focal positions are available, they do not provide real-time control over the energy distribution and focal locations of the foci. We aimed to develop coaxial or multiaxial multifocus diffractive lenses with computer-generated holograms. The focal distances and energy densities in the foci can be dynamically controlled by programmable holographic codes. The generated holographic lenses can be used for imaging with dynamic repositioning of created images fast and easily. Such a holographic diffractive lens can withstand a highly intense laser radiation, so it can be used in nonlinear optics experiments and can be employed for high harmonic generation, pump-probe experiments, optical tweezers, filamentation, and other applications.
We present our current efforts to create pure transversal modes in ultrashort and intense pulses of 800 nm radiation
(Ti:sapphire). Our longer-term goal is to investigate if and how optical orbital angular momentum affects intense-field
excitation and ionization processes. For this purpose we need strong pulses in pure modes. Optical orbital angular
momentum is present in Laguerre-Gaussian modes; in the past, we established a technique to create spatial-chirp free
pure Laguerre-Gaussian modes using a pair of holographic gratings on film [Opt. Express 13, 7599-7608 (2005)].
However, this technique is unsuitable for high intensity beams, and we are currently exploring the possibilities of a
liquid-crystal spatial light modulator (SLM). After careful testing, we have found that our SLM, which we operate as a
phase plate, withstands without any problem the full 1010 W/cm2 peak intensity of an amplified pulsed Ti:sapphire beam
(< 50 fs pulse duration, repetition rate 1 kHz). Using the SLM, we have recently created a beam of ultrashort and intense
pulses with a Hermite-Gaussian(1,0) profile and started to analyze this beam using optical and ionization methods.