174
5 Optical Measurement Techniques
source, thereby generating and detecting ps-short THz pulses in the 1 THz spectral
region. To achieve the desired performance of THz antennae, the type and density
of defects is usually optimized in low-temperature-grown GaAs (LT-GaAs) THz
antennae, but to further enhance the performance after growth, one can decorate
their surface in the metal-antenna gap region with nanoparticles, as demonstrated
independently in [165, 166].
5.6.3 Laser-Induced Plasma/Breakdown Spectroscopy
Laser-Induced Plasma/Breakdown Spectroscopy (LIBS/LIPS) is an element spectroscopy technique, which is based on the analysis of the spectral emission from
laser-induced plasmas produced by high-power ultrashort laser pulses applied to the
surface of a target material. An optically induced plasma is formed on the surface of
or in the sample when the laser power irradiance exceeds the breakdown threshold of
the medium. When a plasma is ignited, a plume of ionised matter emits radiation on
different time scales on the way to its normal configuration, with the plasma cooling
and electrons consecutively relaxing into bound element energy states. Typically,
characteristic element lines can be spectrally resolved for low-repetition-rate pulsed
laser-induced breakdown after μs delay times using a gated intensified-CCD (iCCD)
camera. The laser fluence is commonly in the range of J cm
−2 .
Fortunately, LIPS is minimally destructive because the amount of sample consumed is very small (nanograms). Thus, this method is suitable where only small
amounts of material are available. Although the technique is usually qualitative or
at best semi-quantitative, it opens up a unique pathway to the characterisation or
identification of materials, or even the contamination in environmental or biomedical samples by, for instance, metals and plastics. However, the abundance of carbon
in organic material can impose strong constraints when it comes to the analysis of
carbon-dominated materials in a carbon-rich environment. With a spatial resolving
power and spot sizes of the order of 100 µm, micro-regions can be analysed. This
limit can be pushed considerably down by implementing a microscope-based LIPS
technique that offers a rapid and simple analysis. Multiple elements in a sample
can be determined simultaneously, giving it an advantage over many other analysis techniques, which often even require extensive sample preparation. Particularly,
spatial profiling is the main advantage of such microscope-based technique. This
enables imaging of samples with microscopic resolution of the distribution of contaminants. Versatile sampling of all sorts of media including biomaterials/organic
samples promises successful application of LIPS for the analysis of contamination
levels, but also the discovery of micro-plastics in environmentally-relevant samples.
In an exploratory effort driven by the desire to identify pollutants in certain tissue
types and polymer-based debris in soil, the μLIPS technique (Fig. 5.18) was explored
by the author and co-workers in conjunction with machine-learning approaches to
identify different types of carbon-rich materials (such as by a comparison of different plastics among each other). In addition, the microscale localisation of debris
5 Optical Measurement Techniques
source, thereby generating and detecting ps-short THz pulses in the 1 THz spectral
region. To achieve the desired performance of THz antennae, the type and density
of defects is usually optimized in low-temperature-grown GaAs (LT-GaAs) THz
antennae, but to further enhance the performance after growth, one can decorate
their surface in the metal-antenna gap region with nanoparticles, as demonstrated
independently in [165, 166].
5.6.3 Laser-Induced Plasma/Breakdown Spectroscopy
Laser-Induced Plasma/Breakdown Spectroscopy (LIBS/LIPS) is an element spectroscopy technique, which is based on the analysis of the spectral emission from
laser-induced plasmas produced by high-power ultrashort laser pulses applied to the
surface of a target material. An optically induced plasma is formed on the surface of
or in the sample when the laser power irradiance exceeds the breakdown threshold of
the medium. When a plasma is ignited, a plume of ionised matter emits radiation on
different time scales on the way to its normal configuration, with the plasma cooling
and electrons consecutively relaxing into bound element energy states. Typically,
characteristic element lines can be spectrally resolved for low-repetition-rate pulsed
laser-induced breakdown after μs delay times using a gated intensified-CCD (iCCD)
camera. The laser fluence is commonly in the range of J cm
−2 .
Fortunately, LIPS is minimally destructive because the amount of sample consumed is very small (nanograms). Thus, this method is suitable where only small
amounts of material are available. Although the technique is usually qualitative or
at best semi-quantitative, it opens up a unique pathway to the characterisation or
identification of materials, or even the contamination in environmental or biomedical samples by, for instance, metals and plastics. However, the abundance of carbon
in organic material can impose strong constraints when it comes to the analysis of
carbon-dominated materials in a carbon-rich environment. With a spatial resolving
power and spot sizes of the order of 100 µm, micro-regions can be analysed. This
limit can be pushed considerably down by implementing a microscope-based LIPS
technique that offers a rapid and simple analysis. Multiple elements in a sample
can be determined simultaneously, giving it an advantage over many other analysis techniques, which often even require extensive sample preparation. Particularly,
spatial profiling is the main advantage of such microscope-based technique. This
enables imaging of samples with microscopic resolution of the distribution of contaminants. Versatile sampling of all sorts of media including biomaterials/organic
samples promises successful application of LIPS for the analysis of contamination
levels, but also the discovery of micro-plastics in environmentally-relevant samples.
In an exploratory effort driven by the desire to identify pollutants in certain tissue
types and polymer-based debris in soil, the μLIPS technique (Fig. 5.18) was explored
by the author and co-workers in conjunction with machine-learning approaches to
identify different types of carbon-rich materials (such as by a comparison of different plastics among each other). In addition, the microscale localisation of debris