4.2 Coherent Light Sources
107
the sub-microcavity’s longitudinal confinement factor
10 but little affected by optical
pumping. Furthermore, in these experiments, a typical third-order nonlinearity was
determined through variations of the optical probe power [101, 105]. It is worth
noting that the same technique in transmission geometry had been recently used to
characterise nonlinear refraction and absorption of a single-crystalline perovskite
material [106].
Type-II Heterostructure Gain Chips
In the past decade, also the concept of type-II heterostructures in the active region
of VECSEL chips—previously introduced in and known from the domain of edge
emitters—had been explored [107, 108]. Recent demonstrations showed that with
an appropriate design of W-shaped quantum-well structures with two separate confinement layers for electrons around a central confinement layer for holes can indeed
deliver sufficient optical gain at 1.2 µm to extract output powers in excess of 4 W
from such SDL chip structure under optical pumping [107]. At low output powers
in the hundreds of mW, even a very good beam profile can be obtained [109].
In such quantum-well configuration with electron double-well (electronic confinement in a molecule-like situation), the wave-function for electrons can deliver an
up to approximately 60% overlap integral (wave-function overlap) due to the high
probability to have electrons present spatially on top of the hole’s wave-function
maximum, resulting in remarkable optical transition probabilities for such tailored
band alignments. Moreover, the dynamics in such a type-II (W-type) VECSEL chip
with regard to gain build-up were studied using ultrafast optical pump–probe spectroscopy in reflection geometry. This was performed in direct comparison to a type-I
chip designed for the same wavelength region, showing a clear delay in gain availability for the W-type chip under pulsed excitation [110].
Laser Pulse Applications
Pulsed lasers (not only the mode-locked ones, but also Q-switched devices) are
attractive for numerous applications, such as optical spectroscopy (not only for timeresolved experiments), optical sensing, optical tweezers, two-photon polymerisation
in nano-size 3D printing schemes, fluorescence or super-resolution or multi-photon
microscopy, and also quantum imaging or quantum light generation (see also the
briefly discussed subjects related to entangled photon pair or single-photon generation), to name but a few.
One of the uses in spectroscopic material characterisation employs high-energy
pulses focused onto samples to create tiny plasma plumes, which contain evaporated
sample material and irradiate characteristic element lines during the cooling process
after the ionised matter enters the electronic relaxation processes towards its initial state. This technique referred to as laser-induced plasma spectroscopy (LIPS) or
breakdown spectroscopy (LIBS) can be for instance used to detect different materials
in samples contactless, real-time and nearly-nondestructively [111–114], not only in
vacuum or air, but also under water, in a microscope system with strong spatial selectivity or robot-mounted and remotely controlled on another planet. Some attempts
10 Denoting spectral and spatial resonance with respect to the gain medium, here quantum wells.
107
the sub-microcavity’s longitudinal confinement factor
10 but little affected by optical
pumping. Furthermore, in these experiments, a typical third-order nonlinearity was
determined through variations of the optical probe power [101, 105]. It is worth
noting that the same technique in transmission geometry had been recently used to
characterise nonlinear refraction and absorption of a single-crystalline perovskite
material [106].
Type-II Heterostructure Gain Chips
In the past decade, also the concept of type-II heterostructures in the active region
of VECSEL chips—previously introduced in and known from the domain of edge
emitters—had been explored [107, 108]. Recent demonstrations showed that with
an appropriate design of W-shaped quantum-well structures with two separate confinement layers for electrons around a central confinement layer for holes can indeed
deliver sufficient optical gain at 1.2 µm to extract output powers in excess of 4 W
from such SDL chip structure under optical pumping [107]. At low output powers
in the hundreds of mW, even a very good beam profile can be obtained [109].
In such quantum-well configuration with electron double-well (electronic confinement in a molecule-like situation), the wave-function for electrons can deliver an
up to approximately 60% overlap integral (wave-function overlap) due to the high
probability to have electrons present spatially on top of the hole’s wave-function
maximum, resulting in remarkable optical transition probabilities for such tailored
band alignments. Moreover, the dynamics in such a type-II (W-type) VECSEL chip
with regard to gain build-up were studied using ultrafast optical pump–probe spectroscopy in reflection geometry. This was performed in direct comparison to a type-I
chip designed for the same wavelength region, showing a clear delay in gain availability for the W-type chip under pulsed excitation [110].
Laser Pulse Applications
Pulsed lasers (not only the mode-locked ones, but also Q-switched devices) are
attractive for numerous applications, such as optical spectroscopy (not only for timeresolved experiments), optical sensing, optical tweezers, two-photon polymerisation
in nano-size 3D printing schemes, fluorescence or super-resolution or multi-photon
microscopy, and also quantum imaging or quantum light generation (see also the
briefly discussed subjects related to entangled photon pair or single-photon generation), to name but a few.
One of the uses in spectroscopic material characterisation employs high-energy
pulses focused onto samples to create tiny plasma plumes, which contain evaporated
sample material and irradiate characteristic element lines during the cooling process
after the ionised matter enters the electronic relaxation processes towards its initial state. This technique referred to as laser-induced plasma spectroscopy (LIPS) or
breakdown spectroscopy (LIBS) can be for instance used to detect different materials
in samples contactless, real-time and nearly-nondestructively [111–114], not only in
vacuum or air, but also under water, in a microscope system with strong spatial selectivity or robot-mounted and remotely controlled on another planet. Some attempts
10 Denoting spectral and spatial resonance with respect to the gain medium, here quantum wells.