Degenerate two-photon absorption (2PA) of a series of organic fluorophores is measured using femtosecond fluorescence excitation method in the wavelength range, λ
= 680-1050 nm, and ~100 MHz pulse ...repetition rate. The function of relative 2PA spectral shape is obtained with estimated accuracy 5%, and the absolute 2PA cross section is measured at selected wavelengths with the accuracy 8%. Significant improvement of the accuracy is achieved by means of rigorous evaluation of the quadratic dependence of the fluorescence signal on the incident photon flux in the whole wavelength range, by comparing results obtained from two independent experiments, as well as due to meticulous evaluation of critical experimental parameters, including the excitation spatial- and temporal pulse shape, laser power and sample geometry. Application of the reference standards in nonlinear transmittance measurements is discussed.
We examine properties of an ultrashort laser pulse propagating through an artificial Saturable Absorber based on Nonlinear Polarization Evolution device which has been realized with Polarization ...Maintaining fibers only (PM NPE). We study and compare in-line and Faraday Mirror geometries showing that the latter is immune to errors in the PM NPE construction. Experimental results for the transmission measurements of the PM NPE setup for different input linear polarization angles and various input pulse powers are presented. We show that PM NPE topology is of crucial importance for controlling the properties of the output pulse as it rules the contribution of cross-phase modulation to an overall nonlinear phase change. We also demonstrate an excellent agreement between the numerical model and experimental results.
We report the implementation of a self-referenced optical frequency comb generated by a passively mode-locked all polarization maintaining (PM) Yb fiber laser based on a nonlinear amplifying loop ...mirror (NALM). After spectral broadening the optical spectrum spans from 650 nm to 1400 nm, allowing for the generation of an optical octave and carrier envelope offset frequency (f
) stabilization through a conventional f-2f interferometer. We demonstrate for the first time the stabilization of the f
of such a PM Yb system with an in-loop fractional frequency stability scaled to an optical frequency of low 10
at 1 second averaging time, offering a great potential for applications in optical atomic clock metrology.
Chirped fiber Bragg gratings in Yb-doped fiber lasers allow tuning the net cavity dispersion, thus enabling access to pulse dynamics beyond the typical all-normal-dispersion dissipative soliton ...regime. This Letter demonstrates an ultrafast dispersion-managed all-polarization-maintaining fiber oscillator mode-locked with a nonlinear optical loop mirror. Using a chirped fiber Bragg grating inside the oscillator, it can work at net cavity dispersion in the range from <inline-formula> <tex-math notation="LaTeX">-0.098\,\,\mathrm {ps^{2}} </tex-math></inline-formula> up to <inline-formula> <tex-math notation="LaTeX">+ 0.067\,\,\mathrm {ps^{2}} </tex-math></inline-formula> and above at the wavelength of <inline-formula> <tex-math notation="LaTeX">1~\mu \text{m} </tex-math></inline-formula>. Depending on the configuration, the system can deliver stable pulses with energies up to 2.3 nJ and pulse durations as short as 98 fs.
Over the past few years we have witnessed growing interest in ultrafast laser micromachining of bioresorbable polymers for fabrication of medical implants and surface modification. In this paper we ...show that surface structuring of poly(L-lactide) with 300 fs laser pulses at 515 and 1030 nm wavelength leads to formation of defects inside the polymer as a result of laser beam filamentation. Filament-induced channels have diameter around 1 μm and length of hundreds of micrometers. SEM images of microchannels cross-sections are presented. The influence of wavelength and pulse spacing on bulk modification extent was investigated and parameters limiting filamentation were determined. We show that filamentation can be used for controlling properties of PLLA. The presence of filament-induced modifications such as empty microchannels and pressure wave-induced stress lead to increased ability of polymer to crystallize at lower temperature. Crystallization behaviour and crystal morphology after laser treatment was investigated in details using different analytical techniques such as WAXD, DSC and FTIR/ATR. Hydrolytic degradation experiment was performed. Presented method can be applied for controllable, spatially distributed modification of polymer crystallinity, crystalline phase structure and hydrolytic degradation profile.
Surface-enhanced Raman spectroscopy (SERS) is a research method in which a lack of cost-effective, versatile platforms with high enhancement factor (EF) is still a major obstacle to its widespread ...use. The platforms should be also easy to manufacture, stable in time (for weeks or even for months) and manufactured with a highly reproducible method.
We demonstrate SERS platforms based on silicon modified on the surface by laser ablation and covered with SERS-active metal. The substrates were fabricated by a femtosecond laser, thus the method is simple, very fast and creates highly uniform SERS platforms in a large number. The platform was tested with para-mercaptobenzoic acid (p-MBA) in terms of sensitivity and reproducibility. The calculated EF was at the level of 108 and the standard deviation (SD) gives 7% for 10−6 M solution of p-MBA based on the intensity of the band at 1073 cm−1. Optimized SERS substrate also exhibits excellent stability for up to six months.
We also give the proof-of-concept of using our platform and, for the first time, the SERS analysis of the most important human opportunistic fungal pathogen Candida spp. (Candida glabrata, Candida albicans SN148 and C. albicans BWP17). Finally, the chemometric analysis in the form of Principal Component Analysis (PCA) allowed to strain differentiation of Candida spp., and to distinguish the studied Candida species from Gram-positive bacterial samples with Staphylococcus aureus. Our results demonstrate that the proposed SERS platform is a perfect substrate for detection, identification and differentiation between fungal and bacterial pathogens using SERS technique.
In this paper we demonstrate the first, to the best of our knowledge, all-fiber self-starting laser oscillator consisting entirely of polarization-maintaining large mode area (PLMA) fibers ...mode-locked with nonlinear optical loop mirror (NOLM). The system works in a Raman-free dissipative soliton regime and operates at a central wavelength of 1030 nm. It delivers stable ultrashort pulses of high energy of 12 nJ at a 7.56 MHz repetition rate which can be compressed down to the Fourier transform limit of ∼250 fs. Higher energies were limited by the formation of multiple pulses in the cavity.
The accurate identification of microorganisms belonging to vaginal microflora is crucial for establishing which microorganisms are responsible for microbial shifting from beneficial symbiotic to ...pathogenic bacteria and understanding pathogenesis leading to vaginosis and vaginal infections. In this study, we involved the surface-enhanced Raman spectroscopy (SERS) technique to compile the spectral signatures of the most significant microorganisms being part of the natural vaginal microbiota and some vaginal pathogens. Obtained data will supply our still developing spectral SERS database of microorganisms. The SERS results were assisted by Partial Least Squares Regression (PLSR), which visually discloses some dependencies between spectral images and hence their biochemical compositions of the outer structure. In our work, we focused on the most common and typical of the reproductive system microorganisms (Lactobacillus spp. and Bifidobacterium spp.) and vaginal pathogens: bacteria (e.g., Gardnerella vaginalis, Prevotella bivia, Atopobium vaginae), fungi (e.g., Candida albicans, Candida glabrata), and protozoa (Trichomonas vaginalis). The obtained results proved that each microorganism has its unique spectral fingerprint that differentiates it from the rest. Moreover, the discrimination was obtained at a high level of explained information by subsequent factors, e.g., in the inter-species distinction of Candida spp. the first three factors explain 98% of the variance in block Y with 95% of data within the X matrix, while in differentiation between Lactobacillus spp. and Bifidobacterium spp. (natural flora) and pathogen (e.g., Candida glabrata) the information is explained at the level of 45% of the Y matrix with 94% of original data. PLSR gave us insight into discriminating variables based on which the marker bands representing specific compounds in the outer structure of microorganisms were found: for Lactobacillus spp. 1400 cm−1, for fungi 905 and 1209 cm−1, and for protozoa 805, 890, 1062, 1185, 1300, 1555, and 1610 cm−1. Then, they can be used as significant marker bands in the analysis of clinical subjects, e.g., vaginal swabs.
► New setup for tracing temporal evolution of entire emission spectrum and fluorescence anisotropy. ► Unrelaxed emission in tetraphenylporphyrin occurs from S2. ► Relaxation model proposed, assuming ...reversible internal conversion between S2 and S1 states.
Femtosecond studies of fluorescence of free-base tetraphenylporphyrin in solution were carried out using a homebuilt spectrometer which allows tracing the temporal evolution of the entire emission spectrum with the resolution of 150fs, as well as the measurement of time-resolved fluorescence anisotropy. The S1–S0 (Qx) fluorescence was found to appear immediately after excitation, independent of the excitation wavelength. In addition, a shorter wavelength fluorescence was observed, decaying on a picosecond time scale. It was assigned to the population of the S2 (Qy) state. A model which can explain both the very fast rise of S1 emission and the picosecond decay of S2 was proposed: it assumes a reversible internal conversion between S2 and S1 states, modulated by relaxation from the higher vibronic levels of the latter.
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