Average Excitation Power Research Articles

Using Femtosecond Laser Light to Investigate the Concentration- and Size-Dependent Nonlinear Optical Properties of Laser-Ablated CuO Quantum Dots.

In this work, the nonlinear optical (NLO) properties of CuO nanoparticles (CuO NPs) were studied experimentally using the pulsed laser ablation (PLA) technique. A nanosecond Nd: YAG laser was employed as the ablation excitation source to create CuO NPs in distilled water. Various CuO NPs samples were prepared at ablation periods of 20, 30, and 40 min. Utilizing HR-TEM, the structure of the synthesized CuO NPs samples was verified. In addition, a UV-VIS spectrophotometer was used to investigate the linear features of the samples. The Z-scan technique was utilized to explore the NLO properties of CuO NPs samples, including the nonlinear absorption coefficient (β) and nonlinear refractive index (n2). An experimental study on the NLO features was conducted at a variety of excitation wavelengths (750-850 nm), average excitation powers (0.8-1.2 W), and CuO NPs sample concentrations and sizes. The reverse saturable absorption (RSA) behavior of all CuO NPs samples differed with the excitation wavelength and average excitation power. In addition, the CuO NPs samples demonstrated excellent optical limiters at various excitation wavelengths, with limitations dependent on the size and concentration of CuO NPs.

Low-power swept-source Raman spectroscopy.

'Molecular fingerprinting' with Raman spectroscopy can address important problems-from ensuring our food safety, detecting dangerous substances, to supporting disease diagnosis and management. However, the broad adoption of Raman spectroscopy demands low-cost, portable instruments that are sensitive and use lasers that are safe for human eye and skin. This is currently not possible with existing Raman spectroscopy approaches. Portability has been achieved with dispersive Raman spectrometers, however, fundamental entropic limits to light collection both limits sensitivity and demands high-power lasers and cooled expensive detectors. Here, we demonstrate a swept-source Raman spectrometer that improves light collection efficiency by up to 1000× compared to portable dispersive spectrometers. We demonstrate high detection sensitivity with only 1.5 mW average excitation power and an uncooled amplified silicon photodiode. The low optical power requirement allowed us to utilize miniature chip-scale MEMS-tunable lasers with close to eye-safe optical powers for excitation. We characterize the dynamic range and spectral characteristics of this Raman spectrometer in detail, and use it for fingerprinting of different molecular species consumed everyday including analgesic tablets, nutrients in vegetables, and contaminated alcohol. By moving the complexity of Raman spectroscopy from bulky spectrometers to chip-scale light sources, and by replacing expensive cooled detectors with low-cost uncooled alternatives, this swept-source Raman spectroscopy technique could make molecular fingerprinting more accessible.

Determination of the contributions of two simultaneous absorption orders using 2-beam action spectroscopy.

The concept of a 2-beam action (2-BA) spectroscopy was recently introduced as a method for determining the order of effective nonlinear absorption in multiphoton photoresists. Here we demonstrate that the 2-BA approach can be extended to any measureable observable generated by linear and/or nonlinear absorption. As an example, 2-beam constant-amplitude photocurrent spectroscopy is used to study absorption of a tightly focused, mode-locked or continuous-wave, 800 nm laser by a GaAsP photodiode. The effective order of the absorption process can be measured at any desired value of the photocurrent or photovoltage. A self-consistent framework is presented for using non-integral 2-BA exponents to determine the relative contributions of two absorption mechanisms of different order. The dependence of the ratio of the quadratic and linear contributions on the average excitation power is used to verify that these are the dominant orders of absorption in the photodiode with 800 nm excitation.

Stone/tissue differentiation for holmium laser lithotripsy using autofluorescence.

Holmium laser lithotripsy is a safe and effective method to disintegrate urinary stones of all compositions in an endoscopic procedure. However, handling and safety could be improved by a real-time feedback system permanently monitoring the position of the treatment fiber. The laser is fired only when the fiber is identified as being placed in front of stone. This work evaluates the potential of fluorescence detection with an excitation wavelength of 532 nm for this purpose. A fiber-based fluorescence measurement was set-up to acquire autofluorescence signals from several human renal calculi, artificial stones, and porcine tissue samples (renal calix and ureter). Three different approaches were investigated. First, experiments were performed with a pulsed laser source with a wavelength of 532 nm, pulse energy 36.5 ± 1 μJ, pulse duration 1.2 ± 0.5 nanoseconds, and a repetition rate of 1 kHz with 15 urinary concretions. In the second step, a series of measurements on 42 human urinary calculi samples was carried out using low power continuous wave excitation of 0.4 ± 0.1 mW. Fluorescence was also measured simultaneously to stone fragmentation by holmium laser pulses (pulse energy 240 ± 50 mJ, repetition rate 10 Hz). Finally, a modulated excitation/detection scheme (lock-in technique) was implemented to render fluorescence detection insensitive to white background light. Unlike porcine renal calix, ureter, and artificial stone human urinary calculi show a strong fluorescence signal when excited with 532 nm. With pulsed excitation on urinary stone (20,000 ± 11,000) counts were registered at 587 nm with the CCD-array of a grating spectrometer in an integration time of 50 milliseconds. Tissue gave lower count rates of ≤(5,500 ± 1,100) even with longer integration times (500 milliseconds/1 second). With a cw excitation power of 0.4 mW (13,000 ± 11,000) counts were registered in an integration time of 200 milliseconds at 587 nm (porcine renal calix: (770 ± 340)). Modulated excitation (66 Hz) with an average power of 0.3 mW and detection with a photodiode resulted in a lock-in amplifier signal of 1.5-4.3V on stone (background and skin: <0.5V). With the lock-in technique, autofluorescence from stones can be detected with only the average excitation power of a green aiming beam overlaid to the Ho:YAG-laser beam (power ≤ 1 mW). Since tissue shows very little autofluorescence when excited with 532 nm, this fluorescence signal enables monitoring of the correct position of the treatment fiber during ureteroscopic procedures.

A flexible wide-field FLIM endoscope utilising blue excitation light for label-free contrast of tissue.

Fluorescence lifetime imaging (FLIM) has previously been shown to provide contrast between normal and diseased tissue. Here we present progress towards clinical and preclinical FLIM endoscopy of tissue autofluorescence, demonstrating a flexible wide‐field endoscope that utilised a low average power blue picosecond laser diode excitation source and was able to acquire ∼mm‐scale spatial maps of autofluorescence lifetimes from fresh ex vivo diseased human larynx biopsies in ∼8 seconds using an average excitation power of ∼0.5 mW at the specimen. To illustrate its potential for FLIM at higher acquisition rates, a higher power mode‐locked frequency doubled Ti:Sapphire laser was used to demonstrate FLIM of ex vivo mouse bowel at up to 2.5 Hz using 10 mW of average excitation power at the specimen. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Pulsed laser excitation power dependence of photoluminescence peak energies in bulk ZnO

Photoluminescence (PL) spectra of hydrothermal bulk ZnO were measured in the temperature range from 5 to 298 K. The sample was excited by means of the 266-nm line of an Nd3+: YAG Q-switched pulsed laser with numerous average excitation powers in the range from 0.33 to 7.50 mW. At constant temperatures, the most intense PL peak red-shifts with average excitation power, whereas positions of other near-band-edge peaks remain unchanged. It was experimentally proven that the red-shift is not due to local heating at the excited spot. Rather, it is due to relaxation of photoexcited carriers to lower energy transitions as the most intense transition is saturated by high excitation photon density. Furthermore, the temperature dependence of energy of the most intense PL peak was fitted with the Varshni equation. The Varshni coefficients α and β decrease with increasing pulsed laser excitation power.

10.1007/s11454-008-2015-0

Spontaneous emission sources in the VUV range based on one- and two-barrier pulsed-periodic gas (Xe) discharges are experimentally investigated. Volt-coulomb figures, as well as typical waveforms of current pulses and discharge voltage are presented. The time dependences of the voltage drop across the gas-discharge plasma, excitation power, and excitation energy are calculated. The radiation power versus xenon pressure, exciting pulse repetition rate, and average excitation power is determined. The radiation power average densities for one- and two-barrier excilamps are found to be 20–30 and ∼40 mW/cm2, respectively.

One- and two-barrier excilamps on xenon dimers operating in the VUV range

Spontaneous emission sources in the VUV range based on one- and two-barrier pulsed-periodic gas (Xe) discharges are experimentally investigated. Volt-coulomb figures, as well as typical waveforms of current pulses and discharge voltage are presented. The time dependences of the voltage drop across the gas-discharge plasma, excitation power, and excitation energy are calculated. The radiation power versus xenon pressure, exciting pulse repetition rate, and average excitation power is determined. The radiation power average densities for one- and two-barrier excilamps are found to be 20–30 and ∼40 mW/cm2, respectively.

Independence of Maximum Single Molecule Fluorescence Count Rate on the Temporal and Spectral Laser Pulse Width in Two-Photon FCS

We investigate the fluorescence emission characteristics of standard dye tetramethylrhodamine (TMR) in two-photon fluorescence correlation spectroscopy for different temporal and spectral properties of the femtosecond excitation pulses. After determining the second-order dispersion of our setup, including the microscope objective, a pulse stretcher was employed to control the temporal width at the location of the specimen. As expected, the fluorescence per molecule and therefore the signal-to-noise ratio of an FCS-measurement can be improved at constant average excitation power by altering either the temporal or spectral width of the excitation pulses. We found however, that the maximum achievable molecular brightness is largely independent of the temporal and spectral width in the regime analyzed. This observation confirms the current working hypothesis for two-photon fluorescence correlation spectroscopy that bleaching and saturation, and thus, the inherent properties of the dye system, are the dominant effects limiting the quality of measurements. As a practical consequence, elaborate optimization of temporal and spectral laser pulse width, e.g. by introducing pulse stretchers in the beam path, is less critical than previously expected.

Effective emission of Xe2* and Kr2* excited by a pulsed corona discharge bounded by a dielectric barrier

Experimental time–amplitude and spectral emission curves from B(1Σ+u)−X (1Σ+g), A(3Σ+u)−X (1Σ+g) transitions in Xe2*(λ = 172 nm), Kr2* (λ = 146 nm), and Ar2* (λ =126 nm) molecules excited by a dielectric barrier bounded pulsed corona discharge are presented as a function of gas pressure, pulse repetition rate, and average excitation power. Experimentally, under optimal conditions, the Xe and Kr dimers emission efficiency were 45–50 and 25–28%, respectively.

Spectrally resolved fluorescence lifetime imaging microscopy (SFLIM)—an appropriate method for imaging single molecules in living cells

Ever since the first examination of cellular structures, scientists have been fascinated by investigations of single cells. Over the last decade, many methods (e.g., capillary electrophoresis, electrochemical detection, mass spectroscopy, and optical spectroscopy) have been proposed for analyzing single cells [1]. Many of these methods are appropriate for in vitro experiments, which provide only a snapshot view of cells. However, some optical methods, like fluorescence spectroscopy, are also advanced enough for investigations of living cells. This is due to their simple application without the need for fixation or lysis of the respective cell. A variety of light microscopy techniques for imaging living cells, such as widefield excitation, confocal scanning and total internal reflection excitation, have been reviewed by D. J. Stephens and V. J. Allan [2]. The fluorescently labeled components required (e.g., proteins or DNA) can be added to the cell culture medium, from where they are taken up by the cells if the components are able to pass through the cell membrane. Alternatively, they can be directly injected into living cells via micropipets. Due to their toxicity, the concentration of the fluorescent dye should be as low as possible. This can be achieved by using single-molecule detection techniques that enable the measurement of low concentrations (approximately 10 M) of chromophores in homogeneous solutions and of individual immobilized molecules. In 2001, Sauer at al. established spectrally resolved fluorescence lifetime imaging microscopy (SFLIM) [3], which uses a standard confocal microscope set-up (Fig. 1a). For this technique, a pulsed laser diode with a wavelength of usually 635 nm and a repetition rate of 40–80 MHz is used. The laser beam is coupled into an oil immersion objective and focused onto the sample surface on a scanning table. Generally, the average excitation power is adjusted to 0.25–5 kW/cm. The fluorescence light emitted by the sample is collected by the same objective and focused onto a pinhole with a diameter of 50–100 μm. Fluorescence light passing through the pinhole is spectrally separated by a dichroic beam splitter and detected by two avalanche photodiodes. To generate fluorescence lifetime images, the signals from the two avalanche photodiodes are recorded by a timecorrelated single-photon counting interface card and the data are analyzed by applying a maximum likelihood estimator (MLE) algorithm, which is an appropriate method of determining monoexponential fluorescence lifetimes from low photon count statistics. Figures 1b–d show images of an untreated living cell (3T3 mouse fibroblast) in RPMI 1640 medium recorded by SFLIM [4]. The first image (Fig. 1b) shows the overall fluorescence intensity. The cell culture medium exhibits a constant background with a count rate of approximately 4 kHz, which is due to autofluorescence of the fetal calf serum (which is in the medium), whereas the cell itself shows lower autofluorescence. Some more intense signals occur in the cytoplasm arising from the Anal Bioanal Chem (2007) 387:37–40 DOI 10.1007/s00216-006-0762-1

UV-radiation in nitrogen excited by an electrodeless discharge

The purpose of the present study is to investigate optical media emitting in the wavelength range from 320 to 400 nm. An optical medium emitting in the above-indicated wavelength range on the second positive system of nitrogen consists of a gas with low reactivity. Previously, nitrogen was not used for fabrication of unsealed electrodeless lamps because of its low emission efficiency. In the present study, discharges initiated in N2 and N2–Ar mixtures filling a cylindrical tube 15 cm long with an inner diameter of 3 cm are investigated. Ring electrodes 2 cm wide surrounded the external tube wall. An alternating voltage was applied to the electrodes. A generator of unipolar impulses with tunable frequency and variable voltage that allowed the average excitation power to be changed was used in our experiments. A ТDS-334 oscillograph with a bandwidth of 0.3 GHz and a sampling rate of 1.25 GS/s (5 points every 2 ns) was used to register signals from a capacitive divider and current shunt. The radiation intensity was measured with a calibrated FEC-22 SPU photodiode using the procedure described in [4]. The energy deposited into the medium was estimated by the method described in [5]. The lamp emission efficiency was determined from the ratio of the emission power to the power deposited into the medium. A digital camera was used to photograph the glow of a discharge. In our experiments, we obtained the following results. Lines of the CПu–BПg transition of the nitrogen molecule, including lines at λ = 316 nm of the (1–0) band, λ = 337.1 nm of the (0–0) band, λ = 358 nm of the (0–1) band, λ = 381 nm of the (0–2) band, and λ = 406 nm of the (0–3) band were the most intensive emission lines of excited nitrogen. The average emission power of excited nitrogen from the external lamp surface was 2 mW/cm in the wavelength range 310–410 nm, and the emission efficiency was 0.7% of deposited energy. The best result was obtained for nitrogen excited at a frequency of 30 kHz (at a pressure of ~18 Torr and an excitation power of ~1 W/cm). The lamp operation mode was optimized by varying the frequency of exciting radiation from 10 to 100 kHz, input power from 5 to 30 W, and pressure in range 1– 30 Torr. A decrease in the exciting pulse repetition rate relative to the optimum frequency at the given voltage hindered a breakdown and resulted in unstable lamp operation. The discharge was not ignited with further decrease of the exciting pulse repetition rate. The decreased efficiency of spontaneous emission on the CПu–BПg nitrogen transition with increase in the exciting pulse repetition rate was due to a decrease in the gap breakdown voltage. A decrease in the pressure relative to the optimum one decreased the emission power and efficiency. We also investigated an optical medium based on a nitrogen and argon mixture for a wide range of conditions. It is well known that the output laser energy generated on the second positive system of nitrogen increases when the argon and nitrogen mixture is excited with an electron beam [7]. However, an admixture of argon to nitrogen for a nitrogen laser pumped by a self-sustained discharge resulted in a decrease of the energy and lasing efficiency [8]. In the present investigation, the best results were obtained with N2: Ar = 1: 70 (1.5 Torr N2 and 105 Torr Ar). A double mixture with a comparatively small nitrogen content allowed the average output power and efficiency of the lamp to be increased several folds. The results of optimization of the frequency and excitation power are shown in Fig. 1. It illustrates the dependence of the specific emission power I and efficiency ν on the specific excitation power Р at frequencies of 30 and 50 kHz. From the dependence obtained it follows that for the same excitation power, the emission power and efficiency for a frequency of 30 kHz are twice greater than for 50 kHz. A decrease in the pulse repetition rate, as in the case of pure N2, hindered a breakdown, and a decrease in the pressure at the same excitation power decreased the emission power and efficiency. The typical emission spectrum of the argon and nitrogen mixture is shown in Fig. 2. The maximum intensity has the line with λ = 337.1 nm.

Effective suppression of fluorescence light in Raman measurements using ultrafast time gated charge coupled device camera

A high level of fluorescence background signal rejection was achieved for solid and powder samples by using a combination of simple low-resolution spectrograph and ultrafast gated charge coupled device (CCD) camera. The unique timing characteristics of the CCD camera match exceptionally well to characteristics of a Ti:sapphire oscillator allowing fast gated light detection at a repetition rate of up to 110 MHz, making this approach superior in terms of the duty cycle in comparison with other time-resolved Raman techniques. The achieved temporal resolution was about 150 ps under 785 nm Ti: sapphire laser excitation. At an average excitation power up to 300 mW there was no noticeable sample damage observed. Hence, the demonstrated approach extends the capabilities of Raman spectroscopy regarding the investigation of samples with a short fluorescence lifetime. The combination of a spectrometer and a gated CCD camera allows simultaneous study of spectral and temporal characteristics of emitted light. This capability opens an exciting possibility to build a universal instrument for solving multitask problems in applied laser spectroscopy.

Image contrast enhancement for two-photon fluorescence microscopy in a turbid medium

Image contrast enhancement is investigated for two-photon excitation fluorescence images of a microscopic sample that is buried underneath a turbid medium. The image contrast, which deteriorates rapidly with sample depth because of scattering loss, is enhanced by an increase in the average excitation power of the focused Gaussian (the TEM(00) mode) beam according to a compensation relation that has been derived by use of a Monte Carlo analysis of the scattering problem. A correct increase in the excitation power results in a detected fluorescence signal that remains invariant with sample depth. The scheme is demonstrated on images of DAPI-stained nuclei cells viewed underneath a suspension of 0.105-mum-diameter polystyrene spheres.

Capillary electrophoresis with laser-induced native fluorescence detection for profiling body fluids

Laser-induced native fluorescence detection with a KrF excimer laser ( λ=248 nm) was used to investigate the capillary electrophoretic (CE) profiles of human urine, saliva and serum without the need for sample derivatization. All separations were carried out in sodium phosphate and/or sodium tetraborate buffers at alkaline pH in a 50-μm I.D. capillary. Sodium dodecyl sulfate was added to the buffer for micellar electrokinetic chromatography (MEKC) analysis of human urine. Although inherently a pulsed source, the KrF excimer laser was operated at a high pulse repetition rate of 553, 1001 or 2009 Hz to simulate a continuous wave excitation source. Detection limits were found to vary with pulse rate, as expected, in proportion to average excitation power. The following detection limits (3σ) were determined in free solution CE: tryptophan, 4 n M; conalbumin, 10 n M; α-lactalbumin, 30 n M. Detection limits for indole-based compounds and catecholamine urinary metabolites under MEKC separation conditions were in the range 7–170 n M.

A practical high-power excimer lamp excited by a microwave discharge

A compact, low costs and high-power excimer lamp system has been developed for industrial applications. 80 W of the maximum continuous radiation power has been achieved for a XeCl lamp of 308 nm wavelength when it was excited by a 2.45 GHz and 10 kW microwave discharge. The irradiation intensity was 200 mW / cm 2 in the focused area of 200 × 10 mm 2 with the lamp lifetime of 160 h while it was excited with 1.5 kW excitation power. The maximum average excitation power was 5 W, in the case of a KrCl lamp of 222 and 258 nm. When it was excited with 35 kW and 1.2 μs pulse, the radiation energies were 1 mJ for the XeCl lamp and 0.28 mJ for the KrCl lamp. Lamp tubes are sealed fused quartz containing a mixture of Xe/Kr, Cl 2 and He/Ne gases and the total pressure is 50 Torr. Further improvement of the radiation power as well as the tube life time is under progress.

Picosecond phase fluorometry by mode-locked cw lasers

A new picosecond to nanosecond phase fluorometer is reported for measurements of fluorescence lifetime τ f and rotational diffusion time constants τ rot which uses a mode-locked cw laser as excitation source. Fourier component signals greater than the signal in a 100% sine-modulated phase fluorometer with the same average excitation power, a frequency range 125 MHz to 1000 MHz and a time resolution ⩾ 3 ps for τ rot have been achieved.