Wavelength-tunable Pulsed Laser Research Articles

Mechanism of transient photothermal inactivation of bacteria using a wavelength-tunable nanosecond pulsed laser

There is a great demand for novel disinfection technologies to inactivate various pathogenic viruses and bacteria. In this situation, ultraviolet (UVC) disinfection technologies seem to be promising because biocontaminated air and surfaces are the major media for disease transmission. However, UVC is strongly absorbed by human cells and protein components; therefore, there are concerns about damaging plasma components and causing dermatitis and skin cancer. To avoid these concerns, in this study, we demonstrate that the efficient inactivation of bacteria is achieved by visible pulsed light irradiation. The principle of inactivation is based on transient photothermal heating. First, we provide experimental confirmation that extremely high temperatures above 1000 K can be achieved by pulsed laser irradiation. Evidence of this high temperature is directly confirmed by melting gold nanoparticles (GNPs). Inorganic GNPs are used because of their well-established thermophysical properties. Second, we show inactivation behaviour by pulsed laser irradiation. This inactivation behaviour cannot be explained by a simple optical absorption effect. We experimentally and theoretically clarify this inactivation mechanism based on both optical absorption and scattering effects. We find that scattering and absorption play an important role in inactivation because the input irradiation is inherently scattered by the bacteria; therefore, the dose that bacteria feel is reduced. This scattering effect can be clearly shown by a technique that combines stained Escherichia coli and site selective irradiation obtained by a wavelength tunable pulsed laser. By measuring Live/Dead fluorescence microscopy images, we show that the inactivation attained by the transient photothermal heating is possible to instantaneously and selectively kill microorganisms such as Escherichia coli bacteria. Thus, this method is promising for the site selective inactivation of various pathogenic viruses and bacteria in a safe and simple manner.

Distributed temperature sensing system based on a densely spaced FBG array for small fire recognition

A distributed temperature sensing (DTS) system based on a densely spaced fiber Bragg grating (FBG) array is proposed in this work for small fire recognition. A large capacity identical FBG array with centimeter-scale grating spacing is employed to build a high-density sensor network, increasing the probability that the small fire will affect at least one FBG. To retrieve the sensing information, we exploit an FBG interrogation system based on optical time-domain reflectometry and swept laser techniques, together with a fiber section (FS) spectrum interrogation method. The sensing technique consists in scanning the sensing array by means of a low-coherence wavelength-tunable pulsed laser and in reconstructing and analyzing the returned spectra signals. The sensing performance of the proposed DTS system is studied by theoretical analysis and experimental demonstration, verifying the feasibility and superiority of the densely-spaced FBG array based DTS system for small fire recognition and location.

Radiative cooling dynamics of isolated N2O+ ions in a cryogenic electrostatic ion storage ring

We study radiative cooling processes of ${\mathrm{N}}_{2}{\mathrm{O}}^{+}$ ions isolated in vacuum by taking advantage of the extended storage times possible with a cryogenic electrostatic ion storage ring. The temporal evolution of the internal state populations of the ${\mathrm{N}}_{2}{\mathrm{O}}^{+}$ ions after being stored in the ring was state-selectively measured by action spectroscopy via a predissociation process using a wavelength-tunable pulsed laser. The rotational level population remained unchanged up to about 200 s after ion storage, which is consistent with theoretical predictions and the small permanent dipole moment. On the other hand, we found that depletion of the vibrational excited states in the NN-O stretching mode proceeded within 5 s, while a noticeable increase of the ground-state population was not observed. This implies that the initial populations of the vibrational excited states are relatively small. We discuss such behavior in the context of the initial population distribution of ions extracted from the ion source with a simulation based on simple cascade modeling.

Hybrid multi-wavelength nonlinear photoacoustic sensing and imaging.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this Letter, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on the combination of a single-wavelength pulsed laser source and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling the laser illumination sequence (pulse-CW-pulse) and extracting the PA signal difference before and after the heating of CW lasers, the optical absorption property of multi-wavelength light illumination could be obtained. Compared with conventional PA imaging, the proposed hPAI shows a much lower system cost due to the usage of single-wavelength pulsed lasers and multiple inexpensive CW lasers. As the preliminary results show in this Letter, hPAI imaging has the potential to provide another pathway for high spectroscopic optical absorption contrast in PA imaging.

MoTe2-Based Broadband Wavelength Tunable Eye-Safe Pulsed Laser Source at <inline-formula> <tex-math notation="LaTeX">$1.9~\mu$ </tex-math> </inline-formula>m

In this letter, molybdenum ditelluride (MoTe2), a new member of the transition metal dichalcogenides family, has been characterized and fabricated as an optical modulator for the development of broadband wavelength tunable laser using a Tm3+-doped CaYAlO4 crystal as a laser gain medium. The maximum average output power reaches 0.75 W with the shortest pulse width of about $0.69~\mu \text{s}$ and repetition rate of 70.9 kHz, which leads to a pulse energy of about $10.58~\mu \text{J}$ and peak power of 15.3 W. Keeping the MoTe2 optical modulator inside the laser resonator and at the same time introducing an undoped YAG for wavelength tuning, wavelength tunable passively Q-switched laser operation with a wavelength range of about 112 nm has been demonstrated. This letter experimentally verifies the potential of the novel 2D material MoTe2 as an optical modulator. At the same time, this letter paves the way toward the realizations of simple and compact wavelength tunable pulsed laser sources based on broadband 2D materials saturable absorbers.

Photoacoustic imaging of muscle oxygenation during exercise

Monitoring muscle hemodynamics and oxygenation is important for studying muscle function and fatigue. Current state-of-the-art for noninvasive oximetry is near-infrared spectroscopy (NIRS), which provides relative oxygen saturation (SO2) with good sensitivity but has poor spatial resolution and sensing depth, and does not provide anatomical context. In this study, we demonstrated the utility of a dual-modality imaging system that could generate co-registered ultrasound (US) and photoacoustic (PA) images for real-time, functional imaging of human muscle. The system consisted of a wavelength-tunable pulsed laser (Opotek) integrated with a research ultrasound system (Verasonics). US images provided anatomical context and the PA images acquired at 690 nm and 830 nm were processed to estimate SO2 during a sustained isometric contraction and return to rest using a protocol approved by our Institutional Review Board. PA-based SO2 was compared to measurements acquired from a commercially available NIRS oximeter under the same conditions. Preliminary results showed good qualitative agreement between SO2 dynamics observed via PA-based approach and NIRS, with contraction coinciding with an exponential decay in SO2, and relaxation with a return to original levels. This in vivo study demonstrated the feasibility of PA imaging for measuring temporally and spatially resolved muscle oxygenation during functional tasks.

Wavelength dependence of UV laser selective Al xGa 1− xAs growth via adlayer stimulation in OMVPE

UV-stimulated organometallic vapor phase epitaxy (OMVPE) growth of AlGaAs from trimethylgallium, trimethylaluminum and arsine at 500°C was studied using a wavelength-tunable pulsed laser in the range 235–255 nm. We demonstrate that the high growth contrast of 3.2 : 1 is due to the photostimulation of adlayer reactions at these wavelengths. Under our growth conditions, the adlayer stimulation exhibits a wavelength edge at 252 nm, above which the enhanced growth rate drops abruptly to zero. The s-polarized laser beam induces a pronounced periodic surface structure in all cases, which is quantitatively examined by atomic force and scanning electron microscopy. The structural and optical qualities of the selectively grown layers are determined by Raman and photoluminescence spectroscopy.

Submicron selective organometallic vapor phase epitaxy growth using tunable deep UV excitation

We report on selective area growth of AlGaAs from trimethylgallium, trimethylaluminum, and arsine by UV laser stimulated organometallic vapor phase epitaxy. Using a wavelength tunable pulsed laser system, we investigated the stimulation mechanism and wavelength dependence of the growth enhancement in the range from 235 to 255 nm. Adlayer photolysis constitutes the dominant stimulation mechanism at these wavelengths, since the observation of laser-induced, submicron periodic surface ripples restricts the growth enhancement to a surface process and laser enhanced pyrolysis can be excluded by temperature simulations. We observe an abrupt wavelength edge at 252 nm, below which the stimulation rate is constant and above which the growth enhancement ceases. The growth contrast under our experimental conditions is 3.2:1 and the selectively grown AlGaAs is of optical quality.