Incoherent Broadband Light Source Research Articles

Charge Dynamics of a CuO Thin Film on Picosecond to Microsecond Timescales Revealed by Transient Absorption Spectroscopy.

Understanding the mechanism of charge dynamics in photocatalysts is the key to design and optimize more efficient materials for renewable energy applications. In this study, the charge dynamics of a CuO thin film are unraveled via transient absorption spectroscopy (TAS) on the picosecond to microsecond timescale for three different excitation energies, i.e., above, near, and below the band gap, to explore the role of incoherent broadband light sources. The shape of the ps-TAS spectra changes with the delay time, while that of the ns-TAS spectra is invariant for all the excitation energies. Regardless of the excitations, three time constants, τ1 ∼ 0.34-0.59 ps, τ2 ∼ 162-175 ns, and τ3 ∼ 2.5-3.3 μs, are resolved, indicating the dominating charge dynamics at very different timescales. Based on these observations, the UV-vis absorption spectrum, and previous findings in the literature, a compelling transition energy diagram is proposed. Two conduction bands and two defect (deep and shallow) states dominate the initial photo-induced electron transitions, and a sub-valence band energy state is involved in the subsequent transient absorption. By solving the rate equations for the pump-induced population dynamics and implementing the assumed Lorentzian absorption spectral shape between two energy states, the TAS spectra are modeled which capture the main spectral and time-dependent features for t > 1 ps. By further considering the contributions from free-electron absorption during very early delay times, the modeled spectra reproduce the experimental spectra very well over the entire time range and under different excitation conditions.

Visualization of transparent particles based on optical spatial differentiation.

Optical analog computing operates on the amplitude, phase, polarization, and frequency distributions of the electromagnetic field through the interaction of light and matter. The differentiation operation is widely used in all-optical image processing technology, such as edge detection. Here, we propose a concise way to observe transparent particles, incorporating the optical differential operation that occurs on a single particle. The particle's scattering and cross-polarization components combine into our differentiator. We achieve high-contrast optical images of transparent liquid crystal molecules. The visualization of aleurone grains (the structures that store protein particles in plant cells) in maize seed was experimentally demonstrated with a broadband incoherent light source. Avoiding the interference of stains, our designed method provides the possibility to observe protein particles directly in complex biological tissues.

Implementation of an incoherent broadband cavity-enhanced absorption spectroscopy technique in an atmospheric simulation chamber for in situ NO<sub>3</sub> monitoring: characterization and validation for kinetic studies

Abstract. An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) technique has been developed for the in situ monitoring of NO3 radicals at the parts per trillion level in the CSA simulation chamber (at LISA). The technique couples an incoherent broadband light source centered at 662 nm with a high-finesse optical cavity made of two highly reflecting mirrors. The optical cavity which has an effective length of 82 cm allows for up to 3 km of effective absorption and a high sensitivity for NO3 detection (up to 6 ppt for an integration time of 10 s). This technique also allows for NO2 monitoring (up to 9 ppb for an integration time of 10 s). Here, we present the experimental setup as well as tests for its characterization and validation. The validation tests include an intercomparison with another independent technique (Fourier-transform infrared, FTIR) and the absolute rate determination for the reaction trans-2-butene + NO3, which is already well documented in the literature. The value of (4.13 ± 0.45) × 10−13 cm3 molecule−1 s−1 has been found, which is in good agreement with previous determinations. From these experiments, optimal operation conditions are proposed. The technique is now fully operational and can be used to determine rate constants for fast reactions involving complex volatile organic compounds (VOCs; with rate constants up to 10−10 cm3 molecule−1 s−1).

The water soluble axially disubstituted silicon phthalocyanines: photophysicochemical properties and in vitro studies.

Two novel silicon(IV) phthalocyanines bearing 1,3-bis[3(dimethylamino)phenoxy]propan-2-ol or 1,3-bis[3(diethylamino)phenoxy]propan-2-ol groups at their axial positions were synthesized. These phthalocyanines were converted into their water soluble derivatives by the quaternization reaction with methyl iodide. The quaternized phthalocyanines show excellent solubility aqueous solutions without any aggregation which makes them potential photosensitizers for use in photodynamic therapy (PDT). For this reason, the photophysical and photochemical properties such as fluorescence quantum yields, lifetimes, singlet oxygen generation and photodegradation of both non-ionic (3 and 5) and quaternized cationic silicon(IV) phthalocyanines were investigated. Furthermore, the cytotoxicity of PDT was determined by colorimetric proliferation assay against to hepatocellular carcinoma (HuH-7) cancer cells. In this study, the cells were incubated with a novel water soluble silicon(IV) phthalocyanine derivatives and thereafter the cells were illuminated using broad-band incoherent light source.

Laser-induced plasmas in ambient air for incoherent broadband cavity-enhanced absorption spectroscopy.

The emission from a laser-induced plasma in ambient air, generated by a high power femtosecond laser, was utilized as pulsed incoherent broadband light source in the center of a quasi-confocal high finesse cavity. The time dependent spectra of the light leaking from the cavity was compared with those of the laser-induced plasma emission without the cavity. It was found that the light emission was sustained by the cavity despite the initially large optical losses of the laser-induced plasma in the cavity. The light sustained by the cavity was used to measure part of the S(1) ← S(0) absorption spectrum of gaseous azulene at its vapour pressure at room temperature in ambient air as well as the strongly forbidden γ-band in molecular oxygen: b(1)Σ(g)(+)(ν'=2)←X(3)Σ(g)(-)(ν''=0).

A broadband cavity ring-down spectrometer based on an incoherent near infrared light source

We report on the development of a cavity ring-down (CRD) spectrometer based on an incoherent broadband light source centered at 1,545 nm. One measurement spans over 46 nm. Due to the challenge of broadband CRD spectroscopy in the addressed spectral range, an extended measurement principle is developed. First successful measurements in the spectral range between 1,547 and 1,593 nm of a cavity filled with CO2 are shown. In order to compare the experimentally determined data to values of the HITRAN database, the calculation of the expected values is discussed under consideration of the spectrometer’s spectral resolution. Comparison with the data based on the HITRAN database shows good agreement.

RIN‐reduced light source for ultra‐low noise interferometric fibre optic gyroscopes

Incoherent broadband light sources are used in interferometric fibre optic gyroscopes (IFOGs) to mitigate the deleterious effects of Kerr and polarisation nonreciprocity, and backscattering effects in gyroscopes. However, incoherent broadband sources suffer from excess noise or relative intensity noise (RIN), caused by the beating of different spectral components on the photodetector. Unless RIN noise is suppressed, IFOG performance cannot be improved once light on the photodetector exceeds one photon per coherence time (∼μW's). Demonstrated is a low RIN light source capable of significantly improving angle random walk of an IFOG by 12 dB.

Theoretical Analysis for Sensitivity Enhancement in Broad-Band Thermal Lens Microscope

Thermal lens microscope (TLM) coupled to lab-on-chip chemistry in microfluidic systems is a powerful tool for analyzing a variety of compounds. However, the limited range of emission lines of laser-based TLM severely restricted the specificity of this technique and therefore limited the use of such TLM systems for practical applications. Here, a broad-band incoherent light source was introduced into the TLM to improve the specificity. A three-layer theoretical model was built. Through numerical simulation, it was found that at usual sample length of 100 μm in micro-volumes, the sensitivity can be increased by approximately one order of magnitude at low frequencies when the top and bottom layers are organic solvents. The sensitivity is even higher than that in laser excitation case.

Determination of spatio-spectral properties of individual modes within multimode waveguides using spectrally resolved near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is used to measure the relative group velocity and relative amplitude of modes in a multimode waveguide with an incoherent broadband light source. A spectrally resolving NSOM probe was used to observe localized spectral interference in the multimode waveguide. Fourier analysis was used to identify temporal delays. The modes can be further identified by their temporally discriminated spatial profile which is based on the product of their fields. This data provide the relative amplitude and phases of the modes.

White light Bessel-like beams generated by miniature all-fiber device

Micron-sized white light propagation invariant beams generated by a simple and compact fiber device are presented. The all-fiber device is fabricated by splicing a short piece of large-core multimode fiber onto a small-core single mode white light delivery fiber. Because this fiber device offers an inherent spatial coherence, nondiffracting white light beams can be created with a temporally incoherent broadband light source (a halogen bulb) and, most importantly, the surrounding fringes don't fade as the bandwidth of the light source increases because the underlying physics of this fiber device is different from that of the axicon. White light Bessel-like beams have been generated from multimode fibers with core diameters of 50 μm, 105 μm, and 200 μm. The distance of nondiffracting propagation of the white light Bessel beam increases with increasing core size of the multimode fiber. Propagation characteristics of red, green, and blue individual beams are also presented.

Instantaneous Microwave Frequency Measurement Based on Amplified Fiber-Optic Recirculating Delay Loop and BroadBand Incoherent Light Source

A novel approach to implementing instantaneous frequency measurement (IFM) based on an amplified fiber-optic recirculating delay loop and a broadband incoherent light source (ILS) is proposed, analyzed, and experimentally demonstrated. Since the semiconductor optical amplifier-based fiber-optic delay loop has an infinite impulse response that varies from a large positive value to negative infinity on a log scale, a unique relationship between the output power, and the frequency of the input continuous-wave (CW) microwave signal is established. Meanwhile, it is experimentally shown that the use of the ILS can greatly improve the stability of the proposed IFM system. When the input power of CW microwave signal is within the range of -7 dBm to -16 dBm, the measured errors remain within ±400 KHz over a frequency range of 6.94-6.958 GHz. The measurement error, the complexity and cost of the proposed IFM system can be considerably reduced by only using one ILS, one modulator, and one photodetector. Since the proposed IFM system has a capability of optical integration, it is theoretically estimated that the measurement range can be extended to 20 GHz with a measurement resolution of 1.36 dB/GHz.

Optical signal processors based on a time-spectrum convolution

We show that a fiber-optics architecture conventionally used for microwave photonic filtering can implement a time-spectrum convolution (TSC) process of general interest for a wide range of fundamental optical signal processing and analysis operations. This process is practically implemented by temporally modulating a specially filtered broadband incoherent light source followed by propagation through a suitable linear dispersive medium. The TSC concept allows the time-domain realization of fundamental analog processing operations over both temporal and spectral intensity waveforms. Three particular, relevant signal-processing operations are proposed and experimentally demonstrated here to illustrate the broad potential of application of the TSC concept, namely time-integration, spectrum-integration, and time-frequency correlation of discrete binary codes.

Water-soluble phthalocyanines mediated photodynamic effect on mesothelioma cells

The new peripherally 2-mercaptopyridine tetrasubstituted zinc phthalocyanine (2) and its quaternized derivative (3) have been synthesized and characterized by elemental analysis, IR, 1 H NMR spectroscopy, electronic spectroscopy and mass spectra. The quaternized compound (3) shows excellent solubility in water, which makes it a potential photosensitizer for use in photodynamic therapy (PDT) of cancer. Fluorescence and singlet oxygen quantum yield measurements were conducted on 2-mercaptopyridine appended zinc phthalocyanines in dimethylsulphoxide (DMSO) for both the non-ionic (2) and quaternized (3) derivatives, and in aqueous media for the water-soluble complex 3. General trends are described for fluorescence and singlet oxygen quantum yields of these compounds. In this study, the cells were incubated with a novel water-soluble zinc phthalocyanine derivative (3) and thereafter the cells were illuminated using broad-band incoherent light source of various energy levels. Cytotoxicity of PDT on two pleural malign mesothelioma cell lines was determined by colorimetric proliferation assay. In addition, after PDT treatment, determination of activity matrix metalloproteinases (MMPs) were evaluated using gelatine zymography.

Effects of In-Band Crosstalk in Wavelength-Locked Fabry–PÉrot Laser-Diode-Based WDM PONs

We investigated the impact of in-band crosstalk on the system's performance in wavelength-locked Fabry-Perot laser-diode (FP-LD)-based wavelength-division-multiplexed passive optical networks (WDM PONs). As expected, wavelength-locked FP-LD, which was injected by using an incoherent broadband light source, was more tolerant to in-band crosstalk than distributed-feedback (DFB) laser diode. A 1-dB power penalty in the wavelength-locked FP-LD-based PON system was observed when the crosstalk-to-signal ratio was ~-9 dB. We also compared the measured power penalties with the calculated power penalties. It has been shown that the in-band crosstalk-induced power penalty in wavelength-locked FP-LD-based WDM PON could be estimated properly by taking into account both effects of power addition and signal-crosstalk beat noise.

High-resolution Fourier-transform cavity-enhanced absorption spectroscopy in the near-infrared using an incoherent broad-band light source

An incoherent broad-band cavity-enhanced absorption (IBB-CEA) set-up was used in combination with a Fourier-transform (FT) spectrometer in order to explore the potential of this technique for high-resolution molecular spectroscopy in the near-infrared region. Absorption spectra of overtone bands of CO2, OCS, and HD18O were measured between 5800 and 7000 cm(-1) using a small sampling volume (1100 cm3, based on a 90 cm cavity length). The quality of the spectra in this study is comparable to that obtained with Fourier transform spectrometers employing standard multi-pass reflection cells, which require substantially larger sampling volumes. High-resolution methods such as FT-IBB-CEAS also provide an elegant way to determine effective mirror reflectivities (R(eff), i.e. a measure of the inherent overall cavity loss) by using a calibration gas with well-known line strengths. For narrow absorption features and non-congested spectra this approach does not even require a zero-absorption measurement with the empty cavity. Absolute cross-sections or line strengths of a target species can also be determined in one single measurement, if gas mixtures with known partial pressures are used. This feature of FT-IBB-CEAS reduces systematic errors significantly; it is illustrated based on CO2 as calibration gas.

Simple technique for the generation of white light Young's fringes with a cyclic optical configuration

A new (to our knowledge) technique for the generation of white light Young's type fringes using a cyclic path optical configuration is presented. The effect of the size of an incoherent broadband light source on the fringe visibility is discussed. Applications and advantages of the technique are described.

Incoherent broadband cavity-enhanced total internal reflection spectroscopy of surface-adsorbed metallo-porphyrins

An improvement of conventional attenuated total reflection (ATR) spectroscopy is demonstrated by applying an incoherent broadband light source (short-arc Xe-lamp) in a cavity-enhanced evanescent-wave absorption method. With this novel approach the absorption spectra of several metallo-octaethyl porphyrins (palladium (PdOEP), platinum (PtOEP) and zinc (ZnOEP)) in thin acetone solution layers and on a fused silica (FS) surface are studied between approximately 390 and 625 nm. The time dependence of the evaporation process of the solution on the FS surface is described. The maximum sensitivity of the setup is estimated at approximately 2 x 10(-5) per pass, which translates into a minimal detectable surface density of less than 2 x 10(-3) monolayers for the porphyrins studied (based on the strong absorption in the Soret bands). Changes of surface and solution spectra are characterised and discussed on the basis of observed band broadenings and spectral shifts. For Pd- and PtOEP the changes of spectral feature can be interpreted with respect to J-aggregate formation supported by polarization dependent measurements. The reason for an observed blue-shift of ca. 10 nm for the Soret band in ZnOEP in combination with a large red-shift of the Q-bands (11 nm for Q1 and 18 nm for Q2) is discussed.

Experimental demonstration of security-improved OCDMA scheme based on incoherent broadband light source and bipolar coding

Experimental demonstration of security-improved optical code-division-multiplexed-access (CDMA) scheme based on spectrally encoded incoherent broadband light source with bipolar coding is presented. Incoherent bipolar encoding is achieved with amplified spontaneous emission (ASE) noise of erbium-doped fiber amplifier. We show details of coding scheme and investigate security issues by measuring eye-diagrams and bit-error-rates for various cases. In addition, we present analytical and numerical simulation results for secure transmission of spectrally encoded incoherent optical CDMA signal.

Fourier-transform cavity-enhanced absorption spectroscopy using an incoherent broadband light source

A cavity-enhanced absorption setup employing an incoherent broadband light source was used in combination with a Fourier-transform spectrometer to measure the spin-forbidden B-band of gaseous oxygen at approximately 688 nm and several weak absorption transitions of water vapor in the same spectral region at room temperature in ambient air. The experiments demonstrate that the sensitivity of a Fourier-transform spectrometer can be significantly improved by increasing the effective path length, while retaining a rather small sample volume. In comparison with a single-pass absorption measurement, we report a path-length enhancement factor of 200 and an improvement of the signal-to-noise ratio of approximately 6 in the present cavity-enhanced absorption experiment. The practical advantages and limitations of this novel approach are outlined and potential applications are briefly discussed.

White light propagation invariant beams

Propagation invariant light fields such as Bessel light beams are of interest in a variety of current areas such as micromanipulation of atoms and mesoscopic particles, laser plasmas, and the study of optical angular momentum. Considering the optical fields as a superposition of conical waves, we discuss how the coherence properties of light play a key role in their formation. As an example, we show that Bessel beams can be created from temporally incoherent broadband light sources including a halogen bulb. By using a supercontinuum source we elucidate how the beam behaves as a function of bandwidth of the incident light field.