Principle of Laser-Induced Plasma Spectroscopy

Laser-induced plasma has been used for different diagnostic and technological applications as detection, thin film deposition, and elemental identification. Laser-Induced Plasma Spectroscopy ( LIPS), or sometimes called Laser-Induced Breakdown (LIBS) or Laser Spark Spectroscopy (LSS) is a powerful tool for rapid in-situ analyses of solid, liquid or gaseous samples. Some examples include on-line monitoring in the steel industry, control of materials and leakages in power plants, environmental analyses (soils, waters), analyses of objects related to Cultural Heritage, explosive identification in the protection against terrorism and planetary exploration (surface characterization).

Pulsed laser deposition is a thin film deposition (specifically a PVD) technique, where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the deposit material. Conceptually and experimentally, pulsed laser ablation is an extremely simple technique; probably the simplest among all thin-film growth techniques.

With progress in short-pulse laser techniques, newly generated plasma applications appear. Extreme ultraviolet (EUV) sources based on laser-produced plasmas (LPP) emitting at a wavelength of tens of nm are currently being developed with tremendous effort for the next generation of semiconductor microlithography. Besides high-power sources for high volume manufacturing, compact EUV sources of lower average power are also needed for metrology purposes, for example, for mask inspection, actinic material testing, or optics and sensor characterization. Moreover, tabletop LPP sources are also employed to generate soft x-ray (SXR) radiation for microscopy or absorption spectroscopy in the water window spectral range (2.2–4.4 nm).

The availability of synchrotron x-ray and laser-plasma x-ray sources have revolutionized the capabilities of time-resolved x-ray diffraction investigations, the development of streak camera x-ray detectors, static and streak mode charge-coupled device (CCD) x-ray area detector techniques, and specialized scintillation detection schemes.

Principle of Laser Flash Photolysis

Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule. Photolysis plays an important role in photosynthesis, during which it produces energy by splitting water molecules into gaseous oxygen and hydrogen ions. This part of photosynthesis occurs in the granum of a chloroplast where light is absorbed by chlorophyll. This reacts with water and splits the oxygen and hydrogen molecules apart.

Laser Flash Photolysis is a technique for studying transient chemical and biological species generated by the short intense light pulse from a nanosecond/picosecond/femtosecond pulsed laser source (pump pulse). This intense light pulse creates short lived photo-excited intermediates such as excited states, radicals and ions. All these intermediates are generated in concentrations large enough for chemical and physical interaction to occur and for direct observation of the associated temporally changing absorption characteristics. Typically the absorption of light by the sample is recorded within short time intervals (by a so-called test or probe pulses) to monitor relaxation or reaction processes initiated by the pump pulse. Usage of Optical Parametric Oscillators (OPO) opens new possibilities in spectroscopic experiments.

Principle of Sum Frequency Generation Vibrational Spectroscopy

Sum frequency generation vibrational spectroscopy (SFG-VS) is used to characterize vibrational bonds of molecules at surfaces or interfaces. SFG spectroscopy is particularly attractive because of molecular specificity and intrinsic interfacial sensitivity. Surface sensitivity of the technique arises from the fact that within the electric dipole approximation the nonlinear generation of the sum-frequency (SF) signal from the overlapped visible and infrared beams is forbidden in the media of randomly oriented molecules or in the centrosymmetric media but is allowed at the interface where inversion symmetry is broken. Molecular specificity emerges from the ability to record vibrational spectrum.

In SFG-VS measurements, carried out by employing SFG spectromer, a pulsed tunable infrared IR (ω_IR) laser beam is mixed with a visible VIS (ω_VIS) beam to produce an output at the sum frequency (ω_SFG = ω_IR + ω_VIS). SFG signal is generated in visible spectral range, so it can be efficiently measured using sensitive detectors (PMT or CCD).

Principle of Second Harmonic Generation (SHG) Spectroscopy

Second harmonic generation (SHG) is a second order nonlinear optical effect where two photons of frequency ω are converted to one photon of frequency 2ω. SHG is allowed only in media without inversion symmetry. This optical method it is non﹊nvasive, can be applied in situ, and can provide real time resolution.

SHG measurements provide information about: surface coverage, molecular orientation, adsorbtion-desorbtion processes, and reactions at interfaces. SHG has the ability to detect low concentrations of analytes, such as proteins, peptides, and small molecules, due to its high sensitivity, and the second harmonic response can be enhanced through the use of target molecules that are resonant with the incident (ω) and/or second harmonic (2ω) frequencies.

SHG microscopy allows for selective probing of a non-centrosymmetric area of sample. This type of nonlinear optical microscope was first used to observe ferroelectric domains and has been applied to various specimens including the biological samples to date. Imaging of the endogenous SHG of biological tissue can be utilized for the selective observation of filament systems in tissues such as collagen, myosin, and microtubules, which exhibit a polar structure. It has been reported that, by imaging exogenous SHG of the membrane, sensitive detection of membrane damage could be realized using the SHG microscope.

Principle of Pump-Probe Spectroscopy

Ultrafast spectroscopy is based on the use of light pulses that have shorter temporal duration compared to the underlying dynamics of the system. Shorter pulse- better time resolution, longer pulse- better spectral resolution. Two pulses separated by time delay are used. First pulse excites (electronic and/or vibrational levels, ionises, make Coulombic explosions, creates plasma, etc.) sample, second delayed pulse probes- checks what happened at the certain time moment. Dynamics of investigated system is recorded by changing delay between pulses. Various experimental techniques are employed to record time-resolved signals: for example transient absorption, four-wave mixing, sum frequency generation. Pump and probe spectral range can vary from UV or even X-ray to far infrared.

The advantage of pump-probe spectroscopy is direct investigation of dynamics. For example: excitation relaxation, energy transfer, photochemical reactions dynamics and movement of particles, structural changes. Tunability of pump and probe pulses opens two dimensional pump-probe spectroscopy where is possible to obtain temporally resolved energy map of system which shows separated and coupled states.

Principle of Solid-phase Photoluminescence Spectroscopy

Time-resolved Photoluminescence Spectroscopy (TRPL) is a contactless method to characterize recombination and transport in solid materials. Measuring TRPL requires exciting luminescence from a sample with a pulsed light source and then measuring the subsequent decay in photoluminescence as a function of time. Most experiments excite the sample with a pulsed laser source and detect the PL with a photodiode, streak camera, or photomultiplier tube set up for upconversion or single-photon counting. The system response time, wavelength range and sensitivity vary widely for each configuration.

It is possible to apply the general methodology of time-resolved photoluminescence for lifetime imaging of the charge carrier dynamics. Nonradiative surface recombination at the boundaries of a semiconductor device can be a major factor limiting the efficiency in light-emitting and laser diodes (LEDs and LDs), photovoltaic cells, and photodetectors. Therefore, the effective lifetime is a crucial parameter to obtain solar cells with a high conversion ratio.

Photoluminescence microscopy also is a powerful optical method for the study of crystal defects in semiconductors and organometallic complexes, with important applications in the manufacturing process of nanostructures, optoelectronic devices and solar cell systems.

Principle of Gas-phase Ion Luminescence Spectroscopy

Gas-phase ion luminescence spectroscopy is used to determine intrinsic electronic transition energies in the absence of a disturbing environment. Large ions produced by electrospray ionization are stored and mass-selected in a cylindrical Paul trap. Here they are irradiated by light from a 20-Hz pulsed tunable wavelelength laser from EKSPLA followed by detection of the emitted photons. The 20-Hz repetition rate of the tunable laser allows for mass selection in between every irradiation event, which implies that there is no fluorescence contribution from ion impurities. The tunability of the laser makes it possible to photo-excite a variety of dyes that absorb at different wavelengths. The figure shows spectra of oxazine dye cations that display emissions in a range from 500 nm to 750 nm. The technique can also be used to study three-dimensional structures of peptides and nucleic acids in the gas phase based on Förster Resonance Energy Transfer (FRET). The biomolecules are labeled with donor-acceptor dye pairs such as rhodamines 575 and 640. Structure information is provided as the efficiency of energy transfer depends on the distance between the donor and acceptor to the inverse power of six.

Principle of Supercontinuum Generation

Supercontinuum “White Light Lasers” have become a well-established turn-key fiber-laser technology addressing a wide range of applications from biomedical imaging to optical device characterization. They add value due to their unique combination of optical parameters, including an extremely wide spectral coverage from 400 nm to 2400 nm, several W of optical output power, and focus down to the diffraction limit of a perfect Gaussian beam.

The majority of commercial supercontinuum lasers are fully fiber-based systems consisting of a modelocked fiber oscillator as the master seed laser, providing ps pulses at ∼1064 nm and repetition rates in the tens of MHz regime. Injected into a fiber amplifier giving rise to high peak power, and finally a few meters of a specially designed index-guiding PCF with suitable dispersion landscape. The supercontinuum source, when combined with a tunable spectral filter, transforms into a widely tunable laser, making it a versatile laser tool for a wide range of applications.

Principle of Time-Resolved Photoconductivity

Time-resolved photoconductivity (TRMC) are key techniques used to perform the contactless determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal-insulator composites, which find use in conductive inks, thin-film transistors, light-emitting diodes, photocatalysts, and photovoltaics.

The behavior of photoconductivity with photon energy, light intensity and temperature, and its time evolution and frequency dependence, can reveal a great deal about carrier generation, transport and recombination processes. Many of these processes now have a sound theoretical basis and so it is possible, with due caution, to use photoconductivity as a diagnostic tool in the study of new electronic materials and devices.

LIDAR operating principle

LIDAR is an acronym for “LIght Detection And Ranging”. LIDAR sends out short laser pulses into the atmosphere, where all along its path, the light is scattered by small particles, aerosols, and molecules of the air and is collected by telescope for analysis. Due to the constant velocity of light, time is related to the scatter’s distance, therefore, the spatial information is retrieved along the beam path. LIDAR uses ultraviolet, visible, or near infrared light to image objects. It can target a wide range of materials, including non-metallic objects, rocks, rain, chemical compounds, aerosols, pollutants, clouds, and even single molecules. LIDAR especially helps in those cases where access with conventional methods is troublesome.

Z-scan operating principle

In nonlinear optics, z-scan technique is used to measure the non-linear index n₂ (Kerr nonlinearity) and the non-linear absorption coefficient Δα via the “closed” and “open” methods to measure both real and imaginary components of the nonlinear refractive index.

For measuring the real part of the nonlinear refractive index, the z-scan setup is used in its closed-aperture form. The sample is typically placed in the focal plane of the lens, and then moved along the z axis, defined by the Rayleigh length. In this form, since the nonlinear material reacts like a weak z-dependent lens, the far-field aperture makes it possible to detect small beam distortions in the original beam. Since the focusing power of this weak nonlinear lens depends on the nonlinear refractive index, it is possible to extract its value by analyzing the z-dependent data acquired by the detector and by interpreting them using an appropriate theory.

For measurements of the imaginary part of the nonlinear refractive index, or the nonlinear absorption coefficient, the z-scan setup is used in its open-aperture form. In open-aperture measurements, the far-field aperture is removed and the whole signal is measured by the detector. By measuring the whole signal, the beam small distortions become insignificant and the z-dependent signal variation is due to the nonlinear absorption entirely.

The main cause of non-linear absorption is two-photon absorption. Due to high pulse intensity and cost effectiveness,picosecond high energy lasers are the most appropriate choice for z-scan measurements.

LIBS operating principle

Laser-induced breakdown spectroscopy (LIBS) utilizes a high intensity short laser pulse to convert a very small amount of target material to plasma for optical analysis of the spectra. LIBS can be used on solid, liquid, or gas samples, and, depending on the spectrograph and detector, can detect all elements. LIBS is non-contact, so it can be used in a wide variety of environments, including remote analysis and micro-sampling.

When coupled with appropriate optics and stages, elemental maps of a surface can be created. Multiple LIBS scans can effectively resolve material composition throughout the volume, building a full three dimensional elemental map.

Principle of Terahertz Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) probes inter-molecular interactions within solid materials. THz-TDS covers the spectral region of 0.1-10 THz which is a low energy and non-ionizing region of the electromagnetic spectrum. THz-TDS is a powerful technique for material characterization and process control and has several distinct advantages for use in spectroscopy. It can give the amplitude and phase information of the sample simultaneously. Many materials are transparent at terahertz wavelengths, and this radiation is safe for biological tissue being non-ionizing (as opposed to X-rays). It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications.