We studied multi-shot femtosecond laser surface modification as a permanent tuning technique for obtaining the desired resonance wavelengths for silicon microring resonators. In this multi-shot tuning approach, each microring resonator was irradiated with 10 or 100 laser pulses in the same location of the waveguide with selected laser fluence. The laser beam has 800 nm wavelength, 130 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 13.1μm, and pulse energies of 12 nJ to 186 nJ to generate the applied range of laser fluences. The silicon microring resonators used in the study have a 15μm diameter and are coupled to a waveguide of 500 nm in width with a gap of 260 nm. We found that the multi-shot tuning curves (resonance wavelength shifts as a function of laser fluences) do not monotonically increase with laser fluences: they first increase, then plateau, and finally decrease. We observed that the laser energies required for the multi-shot case to obtain the onset of crystalline modifications and ablation are several times below the single-shot case because of the incubation effect. With proper control we can achieve both positive and negative resonance wavelength shifts with acceptable induced roundtrip waveguide losses.

We use terahertz time-domain spectroscopy to measure the complex dielectric function of long-wave infrared Hg1-xCdxTe films (x = 0.18, 0.20, 0.22) as a function of temperature in a noncontact manner. Using a Drude-Lorentz model fit to the measured complex transmission function combined with a Kane model description of the band structure, we obtain the temperature-dependent conduction band carrier density, effective mass, scattering time, and carrier mobility for all three Hg1-xCdxTe films. The optical properties of a bare substrate of Cd0.96Zn0.04Te were also measured in the terahertz region. The high quality of the Hg1-xCdxTe films is demonstrated by ultrahigh mobilities exceeding 10(5) cm(2)V(-1)s(-1) and ionized donor densities less than 3 x 10(15) cm(-3) at temperatures below 100 K.

Mercury cadmium telluride (Hg1−xCdxTe or MCT) is the premier material for infrared detection. However, despite its importance, studies exploring the ultrafast photoresponse in this semiconductor alloy are limited. Here, we use time-resolved terahertz spectroscopy to perform a detailed study of the picosecond charge carrier dynamics in long-wave infrared Hg1−xCdxTe (x ∼ 0.2) films, providing insight into ultrafast carrier cooling and temperature-dependent scattering mechanisms. Due to the multilayer photoexcited sample geometry, an elementary thin-film analysis leads to a negative photoconductivity artifact. We, therefore, derive a modified thin-film photoconductivity formula to accurately extract a Drude photoconductivity spectrum. In our analysis, we include the effects of carrier diffusion and the conduction band non-parabolicity in Hg1−xCdxTe. We extract ultrahigh electron mobilities as large as 6 × 105 cm2 V−1 s−1 at 25 K. At cryogenic temperatures, we find the photoexcited electron mobility is up to four times larger than the dark mobility, which we attribute to suppression of ionized impurity scattering due to hole capture by acceptor-type Hg vacancies. In addition, after photoexcitation, we observe a relatively slow rise in photoconductivity over a 10 ps timescale with a monotonically increasing carrier scattering time and a carrier effective mass that decays exponentially with a time constant of 1.9 ps, which we attribute to hot-carrier cooling dynamics in the non-parabolic conduction band.

The characterization of carrier dynamics in mercury cadmium telluride (HgCdTe) thin films is essential for novel applications in long-wave infrared detection. Here, we conduct a novel study of ultrahigh carrier mobilities in narrow-gap HgCdTe films as measured by both terahertz time-domain spectroscopy (THz-TDS) at equilibrium and time-resolved THz spectroscopy (TRTS) after ultrafast photoexcitation. The observed THz transmission through the photoexcited samples is accurately modelled by a three-layer thin-film formula. At low temperatures, the carrier mobility obtained by TRTS is significantly higher than that obtained by THz-TDS. We attribute this enhancement in the low-temperature photocarrier mobility to suppression of impurity scattering by photoexcited holes.

The application of machine learning (ML) has accelerated the development of laser-induced breakdown spectroscopy (LIBS) in soil analysis. However, analyzing remote LIBS data in real time using ML is challenging due to several factors. Firstly, building robust ML models requires extensive calibration datasets, which are not always possible with limited LIBS experimental data. Secondly, matrix effects can worsen LIBS performance, and changes in sample physical properties or the apparatus can impact the distribution and intensity of emission lines. These issues may lead to concept drift in real-time/online data streaming, causing the relationship between the input and the target spectra to change over time. Consequently, an ML model designed for one LIBS system may not apply to another. To conquer these challenges, we propose a framework based on transfer learning to use limited experimental data and adapt to the emission line variation in the LIBS streaming. A model is first pre-trained using a large labelled source dataset and then fine-tuned with new experimental measurements to classify soil samples. LIBS measurements are conducted with variations in sample properties and experimental parameters to simulate differences in remote LIBS sensors. The collected spectra are fed into the model by chunks, and data evolution is dynamically learned by self-balanced learning to self-adapt to the domain shift. The proposed framework is found effective in improving classification accuracy during data streaming by implementing transfer learning and supporting adaptation compared to the literature. The code of the proposed method is available in the GitHub at https://github.com/kelci2017/LIBS_streaming .

Understanding the nature of ultrafast charge-carrier dynamics in nanomaterials is important for applications in photocatalysis. We combine time-resolved terahertz spectroscopy (TRTS) and time-resolved photoluminescence (TRPL) measurements to reveal charge-carrier separation in CdS nanowires that occurs over timescales on the order of 150 ps. The charge separation time is observed to increase with photoexcitation fluence and is not significantly affected by the specific nanowire growth conditions. Hot carrier cooling times and surface recombination velocities are also extracted from simultaneous fits to the TRTS and TRPL decay dynamics.

We report on position-dependent measurements of photocarrier transport using time-resolved terahertz spectroscopy (TRTS) across a slot-die printed perovskite film of varying morphology. Using the Drude-Smith model for the photoconductivity spectra we report maximum intrinsic mobilities of $540 \pm 20$ ${\mathrm {cm}}^{2} {\mathrm {V}}^{-1} {\mathrm {s}}^{-1}$. In addition, we measure variation across the film in the extracted mobilities and the localization c-parameter, correlated with complementary measurements of the film morphology and optical properties. To the best of our knowledge this is the first TRTS study performed on perovskite films produced using a slot-die printing technique.

Multi-shot ablation thresholds, $$F_{\text {th}}(N)$$ for N number of shots, were investigated for polycrystalline copper and single crystalline silicon using a Near-Infrared femtosecond laser, with wavelength of 800nm and pulse duration of 130fs. Fluences, F, above and below the single-shot threshold $$F_1$$ were used in the study. To better understand the incubation effects, the results are compared to two existing incubation models. The first one is the widely used power law and the second one includes the effects of absorption change and critical fluence, $$F_\infty $$ . No ablation would result for a material even with infinite number of laser shots if $$F < F_\infty $$ . From the data generated by $$F>F_1$$ , $$F_{\text {th}}(N)$$ were determined. The single-shot ablation threshold, $$F_1$$ , for polycrystalline copper and single crystalline silicon were determined to be 0.87J/cm $$^2$$ and 0.34J/cm $$^2$$ respectively. From the data generated by $$F < F_1$$ , $$F_{\text {th}}(N)$$ were also determined and $$F_\infty $$ for polycrystalline copper and single crystal silicon were estimated to be 0.18J/cm $$^2$$ and 0.21J/cm $$^2$$ , respectively. For copper, $$F_{\text {th}}(N)$$ data from $$F > F_1$$ and $$F < F_1$$ are consistent with each other, and the power law fit the experimental data reasonably well until the $$F_\infty $$ effect sets in when $$N > 1000$$ . For silicon, we found values of $$F_{\text {th}}(N)$$ from $$F < F_1$$ are significantly higher than those of $$F > F_1$$ . This study provides important information for the femtosecond laser nanomilling technique when nanometer depth resolution can be made possible by using multiple pulses with $$F < F_1$$ .

Ultrafast electron microscopy techniques allow for the structural dynamics of materials to be studied, and are typically achieved by modifying a commercial electron microscope to grant optical access to both the cathode and sample regions. To enhance the temporal resolution of these techniques, the electron bunch probe can be compressed by intense terahertz pulses confined in a tapered waveguide structure. The necessary modifications for optical access and waveguide integration may not be viable on all commercial systems, and has the potential to compromise the column. NanoMi is an open-source, modular electron microscopy platform that is highly amenable to customization, making it ideal for establishing ultrafast functionality in addition to exploring terahertz-electron interactions.

We investigate the effect of laser wavelength on laser-induced breakdown spectroscopy (LIBS) on the measurement of carbon in agricultural soils. Two laser wavelengths, 1064 nm and 532 nm, were used to determine soil carbon concentration. No chemical pretreatment, grinding, or pelletization was performed on soil samples to simulate in-field conditions. A multivariate calibration model with outlier filtering and optimized parameters in partial least squared regression (PLSR) was established and validated. The calibration model estimated carbon content in soils with an average prediction error of 4.7% at a laser wavelength of 1064 nm and 2.7% at 532 nm. The limit of detection (LOD) range for 532 nm was 0.34-0.5 w/w%, approximately half of the LOD range for 1064 nm laser wavelength. The improvement in prediction error and LOD of LIBS measurements is attributed to the increase in plasma density achieved at 532 nm.

Laser-induced breakdown spectroscopy (LIBS) has become a promising technology for determining the chemical composition of soil samples. The application of machine learning (ML) has accelerated the development of LIBS in soil analysis. However, ML for LIBS is challenging because 1) building robust ML models requires large calibration datasets while LIBS experimental data is typically limited, further limiting data streaming, 2) matrix effects deteriorate LIBS performance, and LIBS data is sensitive to changes in the apparatus, causing emission lines distribution highly variable. These issues may cause concept drift in LIBS streaming and make the relation between the input and the target spectra variable over time, leading to an ML model constructed for one LIBS system becomes less applicable to a different LIBS system. We propose transfer learning to conquer the challenges of limited data. We then use domain adaptation with self-learning to self-adapt to the domain shift in LIBS streaming to alleviate the matrix effects and improve the model generalization. To test the efficiency of the proposed method, we conduct experiments on the same soil samples but with different experiment parameters, such as wavelength and laser energy. The collected spectra are fed into our model in chunks. The EMSLIBS dataset used in the 2019 EMSLIBS competition is utilized to construct the transfer-learning model, which serves as the foundation for the model developed using our experimental data. Following this, self-learning is undertaken for each chunk by repeatedly predicting the current chunk using the model trained by previous chunks and then taking the confident predictions as pseudo-labels for co-training the model. It is shown that the average accuracy is improved by 9% with transfer learning and up to 15% better with transfer learning and self-learning during data streaming compared to an ML model that does not implement transfer learning and support adaption.

Non-thermal biological effects of terahertz (THz) radiation have been reported in cellular- and tissue-level systems, however the fundamental interactions with biomolecular structures underlying these effects remains unclear. Microtubules (MTs) are cytoskeletal protein complexes that regulate the mitotic and morphologic characteristics of cells, and are a promising potential target underlying THz-induced functional changes observed at higher levels of biological organization. It is shown that extended exposure to intense THz pulses induces significant structural modifications to polymerized MTs in vitro. Results suggest potential therapeutic applications of THz radiation by inhibiting mitotic activity of diseased tissue.

Mercury cadmium telluride (MCT) is a well-established semiconductor alloy with a widely tunable band gap in the infrared. However, there have been no comprehensive studies on its ultrafast THz photoconductivity dynamics. We report on ultra-high carrier mobilities ($6 \times 10^{\mathbf{5}} \mathrm{cm}^{\mathrm{2}}$/Vs) and long lifetimes in long-wave infrared MCT films as measured by time-resolved terahertz spectroscopy (TRTS). A crossover to negative terahertz photoconductivity at higher frequencies is also observed. Our results suggest that TRTS could find industrial application as a non-contact probe of MCT films for infrared photodetectors and imaging arrays.

Inorganic semiconductor nanowires and quantum dots made of chalcogenides, III-V semiconductors and halide perovskites offer exciting potential for optoelectronic devices. The orthogonalization possible in nanowires between the normally competing processes of charge generation and charge separation have been used to project very high operating performance exceeding that of thin films and single crystals in light harvesting devices such as solar cells, photodetectors, photocatalysts and photoelectrolyzers. Likewise, inorganic quantum dots with size-tunable absorption and emission spectra are excellent candidates for light emitting devices as well as light harvesting devices. However, the practical performance of inorganic nanowire based optoelectronic devices has significantly lagged theoretical predictions. A major reason for the discrepancy between theory and experiment is the presence of surface traps and defects in nanowires and quantum dots, which exhibit a large surface area to volume ratio. Several chemical treatments and annealing regimens have been employed to heal surface defects in nanowires and quantum dots. One popular strategy involves wrapping nanowires and/or quantum dots with a thin coating of a molecular monolayer (e.g. alkanethiols) or an atomic layer deposited conformal oxide. While such core-shell architectures are frequently effective in reducing surface defects, the surface passivation is invariably accompanied by a deterioration in optoelectronic properties due to the difficulty experienced by charge carriers in tunneling through the thin shell layer. The resulting trade-off between surface passivation and carrier extraction limits performance improvements in light harvesting devices. Thus there is a strong need for passivating layers that do not negatively impact carrier extraction. Herein, we show that graphitic carbon nitride coatings are highly effective in passivating the surfaces of inorganic nanowires and quantum dots while preserving excellent carrier transport and extraction. Three illustrative examples are provided together with in-depth spectroscopic and electrical characterization: (1) Cesium lead bromide (CsPbBr3) quantum dots passivated by g-C3N4 nanosheets and performing spectacularly as CO2 photoreduction catalysts and water-splitting photoanodes (2) The double helical ternary semiconductor SnIP passivated by g-C3N4 nanosheets which experienced a remarkable improvement in photoelectrochemical performance (3) Cadmium sulfide (CdS) nanowires passivated by C3N5 nanosheets resulting in a superior photocatalytic performance

Terahertz radiation (THz) technology is fast-growing, with applications in sensing, security, monitoring and pharmaceutical industries. Since it is non-invasive, THz has been used as a diagnostic and therapeutic tool in medicine but its specific effects on biological systems is still largely under-studied. THz has been used to image tissues and cells mainly because it allows for identification of morphological features without the need for fluorescent or radioactive labels, but the potential effects of high intensities of THz radiation are currently not well understood. One of the hypotheses that has been proposed for possible effects of THz on living cells, is that it disrupts the cell membrane and induces increased permeability. To test this hypothesis we exposed a rat basophilic leukemia cell line (RBL-2H3) to non-thermal intense THz radiation (duration, dose, etc) and observed the internalization of propidium iodide, a fluorescent intercalating agent that binds to DNA. We did not observe any changes in RBL-2H3 fluorescence following exposure to these intense THz pulses suggesting that exposure of RBL-2H3 to THz radiation may not increase their membrane permeability. These experiments were preliminary and further optimization and analysis is required before we can make definitive conclusions. However, our preliminary observations have set a baseline of RBL-2H3 internalization of propidium iodide, show that it is possible to expose RBL-2H3 cells to THz radiation using our configuration, and set the stage for future experiments.

We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V-1 s-1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V-1 s-1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J-V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core-shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.

Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, our understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, we use time-resolved terahertz (THz) spectroscopy to probe the transient photoconductivity of SnIP nanowire films and, with insight into the highly anisotropic electronic structure from quantum chemical calculations, measure an electron mobility as high as 280 $cm^2V^{-1}s^{-1}$. Additionally, the THz vibrational spectrum reveals a photoexcitation-induced charge redistribution that reduces the amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, we show that the carrier lifetime and mobility are limited by a trap density greater than $10^{18}\,cm^{-3}$. Our results provide insight into the optical excitation and relaxation pathways of SnIP and demonstrate a remarkably high carrier mobility for such a soft and flexible material.

Terahertz (THz) radiation has shown unique advantages in biomedical applications for novel diagnostic technologies due to the high sensitivity to molecular structure and chemical concentration. However, emerging evidence shows that intense pulses of THz radiation can induce significant non-thermal biological effects that must be characterized. In human skin exposed to intense THz pulses, relatively large responses characterized by differential gene expression profiles are observed. These data are analyzed by signaling pathway perturbation analysis to predict phenotypic endpoints and dysregulatory effects on cancer-related processes. The activities of several important pathways that drive the initiation, development, and progression of many human cancers are predicted to be suppressed, and this effect is intensity-dependent. Some affected pathways are targets for current and emerging anti-cancer therapies. In particular, the activity of the Ras signaling and Calcium signaling pathways is predicted to be significantly inhibited. These results indicate the possibility of an additional therapeutic mechanism of intense THz pulses, due to the potential for targeted suppression of pro-mitotic activity in diseased tissue.

In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.