IR Pulses Research Articles

Understanding AFM-IR Signal Dependence on Sample Thickness and Laser Excitation: Experimental and Theoretical Insights.

Photothermal induced resonance (PTIR), also known as atomic force microscopy-infrared (AFM-IR), enables nanoscale IR absorption spectroscopy by transducing the local photothermal expansion and contraction of a sample with the tip of an atomic force microscope. PTIR spectra enable material identification at the nanoscale and can measure sample composition at depths >1 μm. However, implementation of quantitative, multivariate, nanoscale IR analysis requires an improved understanding of PTIR signal transduction and of the intensity dependence on sample characteristics and measurement parameters. Here, PTIR spectra measured on three-dimensional printed conical structures up to 2.5 μm tall elucidate the signal dependence on sample thickness for different IR laser repetition rates and pulse lengths. Additionally, we develop a model linking sample thermal expansion dynamics to cantilever excitation amplitudes that includes samples that do not fully thermalize between consecutive pulses. Remarkable qualitative agreement between experiments and theory demonstrates a monotonic increase in the PTIR signal intensity with thickness, with decreasing sensitivities at higher repetition rates, while signal intensity is nearly unaffected by laser pulse length. Although we observe slight deviations from linearity over the entire 2.5 μm thickness range, the signal's approximate linearity for bands of sample thicknesses up to ≈500 nm suggests that samples with comparably low topographic variations are most amenable to quantitative analysis. Importantly, we measure absorptive undistorted profiles in PTIR spectra for strongly absorbing modes, up to ≈1650 nm, and >2500 nm for other modes. These insights are foundational toward quantitative nanoscale PTIR analyses and material identification, furthering their impact across many applications.

Tip‐Enhanced Imaging and Control of Infrared Strong Light‐Matter Interaction

AbstractOptical antenna resonators enable control of light‐matter interactions on the nano‐scale via electron–photon hybrid states in strong coupling. Specifically, mid‐infrared (MIR) nano‐antennas coupled to saturable intersubband transitions in multi‐quantum‐well (MQW) semiconductor heterostructures allow for the coupling strength to be tuned through antenna resonance and field intensity. Here, tip‐enhanced nano‐scale variation of antenna‐MQW coupling across the antenna is demonstrated, with a spatially‐dependent coupling strength varying from 73 (strong coupling) to 24 (weak coupling). This behavior is modeled based on the spatially dependent local constructive and destructive interference between tip and antenna fields. Using a quantum‐mechanical density‐matrix model of the MQW system with its designed values of transition dipole moment, doping density, and population decay time, the picosecond IR pulse coupling to intersubband transitions and the associated tip induced strong‐field saturation effects are described. These results present a new regime of nonlinear IR light‐matter control based on the dynamic manipulation of quantum hybrid states on the nanoscale and in the infrared, with a perspective regarding extension to molecular vibrations.

Site-Specific Photochemistry along a Protonated Peptide Scaffold.

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S1 state and the vibrationally excited triplet state(s) formed by intersystem crossing, Tn(v). Photofragment mass spectra are recorded out of the S1 origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH3 loss mediated by electron transfer. In YGGFL, IR excitation in the S1 state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH3+ group leading to loss of neutral NH3. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH3+. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S1 and Tn(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S1 or Tn(v) states fragments the peptide backbone exclusively at amide(4), producing the b4 cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.

Characterization of XUV+IR ionization using the circular dichroic phase

A strong helicity dependence of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT) with circularly polarized XUV and IR pulses was reported by Han [ and ]. They attributed a circular dichroic phase in RABBITT to the helical structure of the photoelectron wave packets in the final state. We exploit this effect to determine the magnitude and phase of two-photon XUV+IR ionization amplitudes. In s-electron targets (H, He, Li), such a determination is fully and requires no further approximations. In heavier noble gases like Ar, characterization of two-photon ionization amplitudes can be made from the circular dichroic phase with minimal and very realistic assumptions. Published by the American Physical Society 2024

Quantum Interferometric Pathway Selectivity in Difference-Frequency-Generation Spectroscopy.

Even-order spectroscopies such as sum-frequency generation (SFG) and difference-frequency generation (DFG) can serve as direct probes of molecular chirality. Such signals are usually given by the sum of several interaction pathways that carry different information about matter. Here we focus on DFG, involving impulsive optical-optical-IR interactions, where the last IR pulse probes vibrational transitions in the ground or excited electronic state manifolds, depending on the interaction pathway. Spectroscopy with classical light can use phase matching to select the two pathways. In this theoretical study, we propose a novel quantum interferometric protocol that uses entangled photons to isolate individual pathways. This additional selectivity originates from engineering the state of light using a Zou-Wang-Mandel interferometer combined with coincidence detection.

Two-Dimensional Infrared Spectroscopy of Isolated Molecular Ions.

Two-dimensional infrared (2D IR) spectroscopy of mass-selected, cryogenically cooled molecular ions is presented. Nonlinear response pathways, encoded in the time-domain photodissociation action response of weakly bound N2 messenger tags, were isolated using pulse shaping techniques following excitation with four collinear ultrafast IR pulses. 2D IR spectra of Re(CO)3(CH3CN)3+ ions capture off-diagonal cross-peak bleach signals between the asymmetric and symmetric carbonyl stretching transitions. These cross peaks display intensity variations as a function of pump-probe delay time due to coherent coupling between the vibrational modes. Well-resolved 2D IR features in the congested fingerprint region of protonated caffeine (C8H10N4O2H+) are also reported. Importantly, intense cross-peak signals were observed at 3 ps waiting time, indicating that tag-loss dynamics are not competing with the measured nonlinear signals. These demonstrations pave the way for more precise studies of molecular interactions and dynamics that are not easily obtainable with current condensed-phase methodologies.

Twisting attosecond pulse trains by amplitude-polarization IR pulses

Twisting attosecond pulse trains by amplitude-polarization IR pulses

Dual-wavelength femtosecond laser-induced single-shot damage and ablation of silicon

An experimental and theoretical study of laser-induced damage and ablation of silicon by two individual femtosecond pulses of different wavelengths, 1030 and 515 nm, is performed to address the physical mechanisms of dual-wavelength ablation and reveal possibilities for increasing the ablation efficiency. The produced craters and damaged areas are analyzed as a function of laser fluence and time separation between the pulses and are compared with monochromatic irradiation. The order of pulses is demonstrated to be essential in bi-color ablation with higher material removal rates when a shorter-wavelength pulse arrives first at the surface. Simulations based on the two-temperature model show that the visible pulse is profitable for the generation of the electron-hole plasma while the delayed IR pulse is efficiently absorbed in the plasma enhancing energy coupling to the target. At long delays of 30–100 ps, the dual-wavelength ablation is found to be particularly strong with formation of deep smooth craters. This is explained by the expansion of a hot liquid layer produced by the first pulse with a drastic decrease in the surface reflectivity at this timescale. The results provide insight into the processes of dual-wavelength laser ablation offering a better control of the energy deposition into material.

Video-rate mid-infrared photothermal imaging by single-pulse photothermal detection per pixel.

By optically sensing absorption-induced photothermal effect, mid-infrared (IR) photothermal (MIP) microscope enables super-resolution IR imaging of biological systems in water. However, the speed of current sample-scanning MIP system is limited to milliseconds per pixel, which is insufficient for capturing living dynamics. By detecting the transient photothermal signal induced by a single IR pulse through fast digitization, we report a laser-scanning MIP microscope that increases the imaging speed by three orders of magnitude. To realize single-pulse photothermal detection, we use synchronized galvo scanning of both mid-IR and probe beams to achieve an imaging line rate of more than 2 kilohertz. With video-rate speed, we observed the dynamics of various biomolecules in living organisms at multiple scales. Furthermore, by using hyperspectral imaging, we chemically dissected the layered ultrastructure of fungal cell wall. Last, with a uniform field of view more than 200 by 200 square micrometer, we mapped fat storage in free-moving Caenorhabditis elegans and live embryos.

Child Safety Monitoring System Based on IoT

Nowadays human trafficking is threatening society more often around the world. Our main idea of this paper is to consider the safety parameters of children. This system is built with the Internet of Things. IoT has become a fast-growing technology, therefore we could even operate anything wireless via the internet. Due to the abuses, the emotional and mental stability of the children gets affected which in turn ruins their careers and future. The use high defined modules and costlier microcontrollers are used widely. This prototype will alert the parents regarding their child’s inabilities during their hard times. In our nation, children are the most precious resources as they are imminent of the country. The parents always look forward to having their children in a safeguarded place where they can make their time without any complication. Unluckily children are threatened. The violation has been growing increasingly. The security of graduate school-going youngsters is a noteworthy constituent motivated to proceed along with the assistance of innovative advancement. During children’s transit to and from school, there are many miserable cases observed in the media. In our nation, due to the absence of preventative considerations children seem to be nowhere to be found and later end up in the trouble. In this paper the main objective is to design a child safety system through smartphones that provide the possibility to trace a child's location as well as during emergency children can alert parents by saying child in an emergency via message. This paper explains the use of ESP 32 microcontroller, Global Positioning System module, Global system for mobile communication module, Heart Sensor, and Temperature Sensor. Key Words: ESP 32, IR Pulse Sensor, ESP8266, Accelerometer Sensor, Child Monitoring, journals

Time-dependent band position difference between vibrational sum and difference frequency generation: a phenomenon originating from dispersion in the visible pulse.

Vibrational spectroscopy is significant for identifying chemical specification. Here, the spectral band frequencies corresponding to the same molecular vibration in sum frequency generation (SFG) and difference frequency generation (DFG) spectra present delay-dependent deviation. Through numerical analysis of time resolved SFG and DFG spectra with a frequency marker in the incident IR pulse, the frequency ambiguity was not caused by any structure and dynamic variation on the surface, but from the dispersion in the incident visible pulse. Our results provide a helpful method to correct the vibrational frequency deviation and improve the assignment accuracy for SFG and DFG spectroscopies.

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor-Acceptor Systems.

Charge transfer (CT) in molecular electron donor-acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor‐Acceptor Systems

AbstractCharge transfer (CT) in molecular electron donor‐acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time‐resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

RoboDoc: Smart Robot Design Dealing with Contagious Patients for Essential Vitals Amid COVID-19 Pandemic

The COVID-19 pandemic took valuable lives all around the world. The virus was so contagious and lethal that some of the doctors who worked with COVID-19 patients either were seriously infected or died, even after using personal protective equipment. Therefore, the challenge was not only to help communities recover from the pandemic, but also to protect the healthcare staff/professionals. In this regard, this paper presents a comprehensive design of a customized pseudo-humanoid robot to specifically deal with contagious patients by taking basic vitals through a healthcare staff member from a remote location amid the COVID-19 pandemic. The proposed design consists of two portions: (1) a complete design of mechanical, electrical/electronic, mechatronic, control, and communication parts along with complete assembly to make a complete multitask-performing robot that interacts with patients to take vitals, termed as RoboDoc, and (2) the design of the healthcare staff side (master/operator side) control of a joystick mechanism with haptic feedback. The proposed RoboDoc design can be majorly divided into three parts: (1) the locomotion part is composed of two-wheeled DC motors on a rover base and two omni wheels to support the movements of the robot; (2) the interaction part consists of a single degree-of-freedom (s-DOF) neck to have communication with different heights of patients and (3) two anthropomorphic arms with three degrees-of-freedom (3-DOF). These parts help RoboDoc to reach to patient’s location and take all of the vitals using relevant devices such as an IR temperature thermometer, pulse oximeter, and electronic stethoscope for taking live auscultations from the lungs and heart of the patient. The mechanical design was created using solid works, and the electronic control design was made via proteus 8.9. For haptic teleoperation, an XBOX 360 controller based on wireless communication is used at the master/operator side. For the convenience of the healthcare staff (operator), an interactive desktop-based GUI was developed for live monitoring of all the vital signs of patients. For the remote conversation between the healthcare staff and the patient, a tablet is mounted (that also serves as the robot’s face), and that tablet is controlled via a mobile application. For visual aid, a DSLR camera is integrated and controlled remotely, which helps the doctor monitor the patient’s location as well as examine the patient’s throat. Finally, successful experimental results of basic vitals of the remote patient such as temperature sensing, pulse oximeter, and heart rate (using haptic feedback) were obtained to show the significance of the proposed cost-effective RoboDoc design.

An Increase in the Signal-To-Noise Ratio of the Detected Terahertz Field in the CdTe Crystal With a Change in the Relative Angle between THz and IR Pulse Polarizations

An Increase in the Signal-To-Noise Ratio of the Detected Terahertz Field in the CdTe Crystal With a Change in the Relative Angle between THz and IR Pulse Polarizations

Few-Femtosecond C2H4+ Internal Relaxation Dynamics Accessed by Selective Excitation.

Dissociation of the ethylene cation is a prototypical multistep pathway in which the exact mechanisms leading to internal energy conversions are not fully known. For example, it is still unclear how the energy is exactly redistributed among the internal modes and which step is rate-determining. Here we use few-femtosecond extreme-ultraviolet pulses of tunable energy to excite a different superposition of the four lowest states of C2H4+ and probe the subsequent fast relaxation with a short infrared pulse. Our results demonstrate that the infrared pulse photoexcites the cationic ground state (GS) to higher excited states, producing a hot GS upon relaxation, which enhances the fragmentation yield. As the photoexcitation probability of the GS strongly depends on the molecular geometry, the probing by the IR pulse provides information about the ultrafast excited-state dynamics and the type of conical intersection (planar or twisted) involved in the first 20 fs of the nonradiative relaxation.

Ultrafast Continuum IR Generation and Its Application in IR Spectroscopy

The spectral range of femtosecond time-resolved infrared spectroscopy is limited by the bandwidth of mid-IR pulses (100~400 cm-1) generated from the combination of Ti:Sapphire amplifier, Optical Parametric Amplifier (OPA), and Difference Frequency Generation (DFG). To overcome this limitation, we implement a compact continuum mid-IR source producing ultrafast pulses that span the frequency range from 1000 to 4200 cm-1 (from 10 to 2.4 μm), which utilize the mixing of fundamental, second-harmonic, and third-harmonic of 800 nm pulse in the air. After building an IR spectrometer with continuum IR and a monochromator, we found that the distortion of the measured IR spectrum originated from the contamination of higher-order diffraction. We used bandpass filters to eliminate the higher-order contributions and correct the measured IR spectrum. We further characterized the spectral properties of fundamental, second-harmonic, and third-harmonic fields after the plasmonic filamentation process, which helps to improve the efficiency of the continuum IR generation. Using the generated continuum IR pulses, we measured the IR absorption spectrum of a water-benzonitrile mixture, which was found to be consistent with the spectrum obtained with a commercial FT-IR spectrometer. The present work will be useful for the efficient generation of continuum IR pulses for IR pump-probe and two-dimensional IR spectroscopy experiments in the future.

Mapping the direction of electron ionization to phase delay between VUV and IR laser pulses

We theoretically demonstrate a one-to-one mapping between the direction of electron ionization and the phase delay between a linearly polarized VUV and a circular IR laser pulse. To achieve this, we use an ultrashort VUV pulse that defines the moment in time and space when an above threshold electron is released in the IR pulse. The electron can then be accelerated to high velocities escaping in a direction completely determined by the phase delay between the two pulses. The dipole matrix element to transition from an initial bound state of the N$_2$ molecule, considered in this work, to the continuum is obtained using quantum mechanical techniques that involve computing accurate continuum molecular states. Following release of the electron in the IR pulse, we evolve classical trajectories, neglecting the Coulomb potential and accounting for quantum interference, to compute the distribution of the direction and magnitude of the final electron momentum. The concept we theoretically develop can be implemented to produce nanoscale ring currents that generate large magnetic fields.

Lorentz-force shifts in strong-field ionization with mid-infrared laser fields

In the past, the ionization of atoms and molecules by strong, mid-infrared (IR) laser fields has attracted recurrent interest. Measurements with different IR pulses have demonstrated the crucial role of the magnetic field on the electron dynamics, classically known as the Lorentz force ${\mathbf{F}}_{L}=q\phantom{\rule{0.16em}{0ex}}(\mathsc{E}+\mathbf{v}\ifmmode\times\else\texttimes\fi{}\mathsc{B})$, that acts upon all particles with charge $q$ in motion. These measurements also require the advancement of theory beyond the presently applied methods. In particular, the strong-field approximation (SFA) is typically based on the dipole approximation alone and neglects both the magnetic field and the spatial dependence of the driving electric field. Here we show and discuss that several, if not most, observations from strong-field ionization experiments with mid-IR fields can be quantitatively explained within the framework of SFA, if the Lorentz force is taken into account by nondipole Volkov states in the formalism. The details of such a treatment are analyzed for the (peak) shifts of the polar-angle distribution of above-threshold ionization photoelectrons along the laser propagation, the steering of electron momenta by two not quite collinear laser beams, or the enhanced momentum transfer to photoelectrons in standing-light fields. Moreover, the same formalism promises to explain the generation of high harmonics and other strong-field rescattering phenomena when driven by mid-IR laser fields. All these results show how strong-field processes can be understood on equal footings within the SFA, if one goes beyond the commonly applied dipole approximation.

Fast Simultaneous CO2 Gas Temperature and Concentration Measurements by Quantum Cascade Laser Absorption Spectroscopy

A quantum cascade laser-based sensing technique is presented which allows for in situ high-precision temperature and/or CO2 concentration measurements of gases in the room temperature regime with sampling rates up to about 40 kHz. The method is based on Boltzmann-like thermally populated fundamental and hot-band rovibrational transitions of CO2 with opposite temperature dependence. Single absorption spectra at about 2350 to 2352 cm−1 are recorded by a nanosecond frequency down chirped IR pulse of a pulsed distributed feedback quantum cascade laser (intrapulse mode). The statistical uncertainty (1σ) in the temperature measurement within one laser pulse is about 1 K and can be further reduced down to about 0.1 K by time averaging over 100 ms. Online temperature and CO2 concentration measurements on a breath simulator controlled gas flow were performed to demonstrate response-time and sensitivity for an application-driven test system.