Nonlinear spectroscopy, such as sum-frequency generation spectroscopy and coherent Raman spectroscopy, are powerful tools for analyzing transient molecular structural changes. Achieving fast and high-resolution spectroscopy with these methods requires bandwidth compression techniques to convert broadband femtosecond pulses into synchronized narrowband picosecond pulses. Here we present a novel single-pass narrowband SHG method based on a novel pulse-shaping scheme using dispersion-engineered optical filters. In our first verification experiment, we found that the SHG bandwidth after passing the filter was compressed to 1/6, and the wavelength conversion efficiency was improved by 18 times compared to the case without the filter. This result demonstrates that precision-engineered optical filters can be used as a pulse-shaping tool. This alignment-free, single-pass bandwidth compression method may be an important tool for promoting the use of nonlinear spectroscopy in a wider range of fields.
Plant disease outbreaks pose serious threats to global food security. A rigid methodology that accounts for rapid identification of the earliest point of infection caused by plant viruses is necessary. Raman spectroscopy that generates spectral signatures of cellular-level dynamics resonates the virus induced alteration in plants through moderations in spectral features. Here, we present a model study to identify the earliest point of infection. Measured spectra from healthy and virus infected Arabidopsis thaliana plants are applied to principal component analysis. We found a separation as early as 8 days post inoculation between healthy and virus infected plants.
In the inactivation of pathogens by ultraviolet light, the inactivation rate has been reported to decrease with increasing irradiation dose, commonly known as the tailing effect. In this study, we compared the inactivation effects between CW-LED and femtosecond pulsed laser for developing a new method to suppress such tailing effect. We found that femtosecond pulsed lasers suppressed the tailing effect and inactivated pathogens by more than one order of magnitude in comparison with CW-LED. The results of this study provide a new method for the practical application of bacterial inactivation by ultraviolet light.
Stimulated Raman scattering (SRS) microscopy offers label-free, high-speed measurements of small molecules in living cells and tissues, holding promise for potential biomedical applications such as drug screening and therapeutic drug monitoring. Nevertheless, with conventional SRS microscopy, fluctuations in the background Raman signal from cells and tissues limit quantitative measurements at drug concentrations below a few mM. Here we present novel SRS microscopy using temporal filtering combined with a phase-locking technique to break the detection limit due to the background Raman signals. We demonstrate that the new phase-locked system enables drug measurements at sub millimolar concentrations with linear calibration curves.
Terahertz (THz) light that resonates with the vibrational frequency of hydrated water molecules, can alter functional expression of proteins. Previous studies have shown that high-intensity THz light promotes actin fiber formation, but such mechanism is not comprehensively investigated. Here, we aim to clarify this mechanism by irradiating actin solution with THz pulsed light of lower average power and higher electric field intensity to suppress thermal effects. We compare the degree of fiber formation with and without irradiation. The result shows the contribution of electric field intensity in the accelerated actin fiber formation and clarifies the existence of non-thermal effects.
We analyze the tissue penetration of a steroid drug commonly used for the treatment of dermal and eye inflammations using phase-modulated stimulated Raman scattering (PM-SRS) microscopy. Depth-resolved imaging and penetration profile analysis reveal a clear difference in the drug penetration between the skin epithelial and the corneal epithelial tissue models due to the different barrier properties. The results suggest that PM-SRS imaging can be used to visualize the penetration pathway of topical drugs based on the spatial information of the cell and tissue structures, which should contribute to the design of drug functions.
Stimulated Raman scattering (SRS) offers high sensitivity to monitor low-concentration drugs in real time. However, the "needle" of a drug signal at millimolar concentrations is hidden in the "haystack" of tissue background Raman signals, which practically limits the specificity of the drug measurements. Here, we compare two promising solutions, namely frequency-modulated SRS (FM-SRS) and time-resolved phase-modulated SRS (PM-SRS). We show that the temporal response of FM-SRS is a simple sinusoidal function and its specificity must be compromised. However, the time-resolved PM-SRS with shaped probe pulses provides temporal high-pass filtering with steep cutoff characteristics, enabling higher specificity in the drug measurement.
In Raman-based diagnostic applications, principal component analysis (PCA) has often been used to distinguish different cell types or abnormalities. The performance of PCA greatly depends on the baseline adjustment of the measured spectra. Hence, the effect of erroneous baseline fitting on PCA requires to be addressed. Thus, we investigate the impact of baseline error for Raman spectra on PCA through the application of polynomial function with different orders in the fingerprint region (~600-1800cm-1). We found that the third order polynomial baseline fitting generated the fitted spectra closest to the mean spectrum and provided more precise PCA results.
In our previous study, we found that femtosecond pulsed laser can inactivate bacteria more effectively than continuous wave light with UV-C irradiation. We hypothesized that the mechanism of enhanced bacterial inactivation by femtosecond pulsed laser is caused by a nonlinear optical effect due to the high peak intensity. In this experiment, we performed an experiment to compare the bacterial inactivation as the irradiation intensity was varied. This comparative experiment revealed, in the case of UV-C irradiation, that bacterial inactivation effect was the same regardless of the peak intensity and a nonlinear effect of bacterial inactivation does not be confirmed.
Mastitis is a common disease in dairy cows and is considered to be one of the most intractable diseases in the world. We analyzed Raman spectra of milk samples from cows that have been treated for mastitis and have recovered, and those from cows that have no history of mastitis. The result shows that signal components derived from carotenoids and unsaturated fatty acids can be used as markers to predict mastitis history. Our new method based on simple and direct milk measurements will be a powerful tool to determine the prevalence and severity of mastitis in future field diagnosis applications.
Therapeutic drug monitoring (TDM) is required for an optimal treatment plan to control the dosage of high-risk drugs by monitoring their blood concentration. Currently, a combination of liquid chromatography and tandem mass spectrometry is used for TDM. However, this method requires expertise and skills in sample pretreatment and is not available in all hospitals. Raman spectroscopy allows us to quantify substances in biological samples with only simple pretreatment. Two methods have been reported for detecting low-concentration substances using Raman spectroscopy: surface-enhanced Raman spectroscopy and drop coating deposition Raman spectroscopy. However, it is difficult to quantify the concentration by these methods because the Raman spectra are measured in a dry state. Here, we present a new method to quantify low-concentration pharmaceutical analytes using droplet evaporation Raman spectroscopy. Methotrexate (MTX), one of the immunosuppressive drugs, is reported to cause adverse effects above 10 μM of its blood concentration 24 hours after administration. To quantify low-concentration MTX, we drop the solution onto a superhydrophobic substrate, and simultaneously measure the Raman spectra and the volume of the droplets before completely dried. In particular, we control the evaporation rate of the droplets in a humidified environment, allowing Raman measurements with sufficient exposure time. The initial concentration of the solution is determined from the measured drug concentration during evaporation and the concentration ratio obtained from the volume measurement. Using the new method, we can quantify the concentration of MTX at 50 μM, which is in the order of magnitude required for clinical use.
Graphite has been often used as an electrode for batteries due to its resistance to corrosion and good electrical conductivity. Graphite electrodes was obtained by modifying the surface of a synthetic diamond substrate, but the uniformity of the graphitized area on the surface should be monitored. Hall measurement was generally used to evaluate the performance of electrodes, but it can only provide the average conductivity between the electrodes. In this study, we performed Raman scattering micro-spectroscopy to evaluate the conductivity of graphite with non-destructive, noncontact measurements and the high spatial resolution. The samples in this study was graphitized from CVD-grown diamond by thermal treatment with deposition of the catalytic metal Nickel. Raman spectra were measured in the Ni-deposited and non-Ni-deposited regions, and it can be seen that only the catalytically reacted area was graphitized from diamond. The sample with graphite-derived peaks in Raman spectrum also showed symmetrical voltamogram in electrolysis measurements. However, for the samples with insufficient modification to graphite, the electrolysis voltamogram were asymmetric. It was possible to predict the properties of the redox reaction between the electrode and the solution in electrochemistry from the Raman spectra. These results verify the production of graphite-diamond hybrid electrodes.
We propose a new method for therapeutic drug monitoring using Raman spectroscopy to quantify the concentration of analytes during droplet evaporation. The proposed method is demonstrated for quantitative measurement of methotrexate down to 0.5 mM
We compared the bacterial inactivating effect of continuous light and pulsed light irradiation to develop a novel technology for safety transfusion. This experiment demonstrated that pulsed light achieved higher inactivation than continuous light.
Quantitative measurements of a small amount of chemicals in label-free tissue imaging by conventional SRS microscopy remain challenging because of background signals. We present a time-resolved, phase-modulated (PM) SRS microscopy method with increased signal contrast. In addition to removing background signals generated via amplitude modulation, PM-SRS can reduce intrinsic tissue background signals by temporally separating the excitation and detection processes. Furthermore, polarization artifacts in tissues can also be removed by rapidly modulating the relative delay between the pump and probe pulses. This new technology enables robust quantitative measurements in tissue and extends the potential of SRS imaging in biomedical applications.
The development of a technology that allows for analyzing microscopic spatial distribution and dynamics of small gaseous molecules such as inhalational anesthetics and odors would advance our understanding of its biological activities in living cells. However, direct observation of such small molecules by optical microscopy is still challenging. We propose a new pump–probe stimulated Raman scattering (SRS) microscopy method for studying the localization, transport and metabolism of gaseous molecules in a living organism in a label-free manner. A technical challenge is how to detect the Raman signal of a small amount of drug molecules that is typically overwhelmed by unwanted nonlinear background, including nonresonant background and coherent Raman scattering of surrounding cells and tissues. In particular, the latter Raman-induced background is essentially inevitable in most standard coherent anti-Stokes Raman scattering (CARS) and SRS systems. We show that these background issues can be overcome by introducing a new pump–probe, time-resolved SRS detection approach coupled with a pair of spectrally-focused, asymmetrically shaped probe pulses (T. Ito et al. APL Photonics (2018)). In the pump–probe scheme, a long-lived vibration of the targeted molecules can be efficiently probed after short-lived vibrations of other background molecules such as water and fatty acids become silent. This unique lifetime-selective signal detection provides a significantly enhanced vibrational signal contrast. As a proof-of-concept experiment, we demonstrate that the passive transport of inhalational anesthetic molecules from aqueous solution to adipose cells can be monitored by time-lapse SRS imaging.
Single-pulse heterodyne CARS (coherent anti-Stokes Raman scattering) detection scheme using shaped femtosecond
pulses is one of the most sophisticated approach for managing the problem of non-resonant background
disturbance in CARS measurement. However, with the signal processing method conducted in the original
report,1-3 we found that background suppression and resonant peak extraction were sometimes difficult and
incomplete. We discuss the reason of this unsuccessful signal processing and propose an improved method for
signal extraction realizing the better quality of extracted spectra.
A novel interferometer, a Sagnac interferometer (SI), is presented for the measurement of the difference phase and amplitude spectra induced by photoexcitation with a femtosecond time resolution. The SI has a remarkable advantage of high stability owing to the common-path configuration. In order to separate the phase and amplitude changes, the optical path difference is scanned between the probe and reference pulses making the best use of polarization. This improved polarization-division Sagnac interferometer (PSI) provides a nearly sinusoidal fringe. To demonstrate PSI we examined the nonlinear phase and amplitude changes in CS2 and a GaAs/AlGaAs quantum well (QW) structure. The sepctral feature of the nonlinear dispersion relation in QW can be explained by the blue-shift of the excitonic resonance. The time dependence of these changes is observed to be determined by the exciton lifetime. These experimental results are consistently explained from neutralization of the built-in potential inside the QW sample.
Quantum wave packet engineering is demonstrated using a phase-programmable femtosecond optical source. This paper describes development of a programmable phase modulator and coherent control of quantum wave packets. Wave packet motion in a cyanine dye molecule is observed to be dependent on the chirp direction and rate of excitation pulses. Strong reduction in excited state population is efficient for negatively chirped pulses in the cyanine dye molecule, which is explained in terms of a pump-dump process. We discuss a possibility of mutual conversion between the optical and electronic phase information by means of nonlinear light-matter interaction.
Highly efficient Nd:YAG ceramic laser at 1064 nm was demonstrated. Using 1 W LD end-pumping scheme, with 883 mW pumping, 499 mW 1064 nm CW laser output has been obtained corresponding to an optical to optical efficiency of 56.5%. High power CW Nd :YAG ceramic rod laser was also demonstrated. The maximum output power of 62.5 W with was obtained at 1064 nm. The optical to optical conversion efficiency is 21.6%.
Time-resolved difference absorption spectra of J-aggregates in ethyleneglycol/water glass were measured by femtosecond pump- probe spectroscopy. the induced absorption near the J-band at 20 K was assigned to the transitions from n-exciton states to (n + 1)-exciton states (n >= 1). The decay time of the n(>= 2)-exciton states is determined to be about 200 fs. For applications of the J-aggregates as nonlinear optical deices, we present a new fabrication method, called vertical spin-coating, to prepare highly oriented 1D J-aggregates dispersed in polymer films. The films are stable even at room temperature. Linear dichroic spectra of the oriented J-aggregate of 1,1'-diethyl- 2,2'-quinocyanine bromide were measured. The dichroic ratio at the peak of J-band was 5 to 10, depending on the preparation conditions. Precise measurement of the dichroism at the J-band revealed that the J-band is composed of two bands with transition dipole moment perpendicular to each other. Large change in a static dipole moment upon electronic excitation was unexpectedly observed in the oriented J-aggregates by electromodulation spectroscopy. The difference absorption spectra due to the Kerr effect were induced by changes in a static dipole moment and a polarizability. The change in the static dipole moment associated with the transition from the ground state to the exciton state were measured with the applied AC field of Hz and the polarization parallel and perpendicular to the 1D axis of the oriented J-aggregates.
The planar niicrolens (PML) is a 2-D integrated niicrolens array fabricated by
the ion-exchange technique. This paper demonstrates light coupling between LD and
optcal fiber using the planar inicrolens. In oder to accept the light power from LD
effctively, two classes of High NA planar inicrolens are prepared. A coupled planar
inicrolens and new planar inicrolens with swelled structure are evaluated. The minimum
coulping loss between LD and single mode fiber was - 5.3 dB using planar muicrolens
with the swelled stuctre. (Including 0. 71 dB of Fresnel loss)
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