Laser Excitation Source Research Articles

Invited Article: Coherent Raman and mid-IR microscopy using shaped pulses in a single-beam setup

The possibility to exploit shaped ultrabroadband laser pulses for molecular coherent control offers a unique implementation of numerous nonlinear spectroscopic methods in one setup. Nonlinear processes, such as coherent Raman microscopy, second harmonic generation, or two-photon fluorescence, which have been applied in many different applications in both the material and life sciences, can be selectively addressed and optimized just by changing the phase imprinted by a programmable pulse shaper. Here, the experimental realization of this concept for multimodal nonlinear microscopy is discussed and the successful implementation of adaptive spectral focussing schemes not only for nonlinear Raman but also for difference frequency generation based mid-infrared (Mid-IR) spectroscopy using a single broadband pulse from a Ti:sapphire laser is shown. Flexible pulse shaping enables tuning of the resonance frequency and the spectral width of the excitation. By variation of the instantaneous frequency difference and the amount of chirp, the experiment can be optimized to achieve high resolution spectroscopy reaching up to 20 cm−1. Matching the resolution of the experiment with the linewidths of the sample on the other hand optimizes the contrast for imaging at high signal levels. The combination and flexible switching between Raman or mid-IR excitation for spectroscopy and microscopy is demonstrated on alkynes, polymer films, and skin tissue. The simple addition of this complementary modality to an existing nonlinear microscope is a further step toward an all-purpose laser excitation source for multimodal microscopy.

Applications of Surface Enhanced Raman Scattering toward the Detection of the Bioactive Lipid 20-HETE

The detection and analysis of lipids in biological matrices for clinical applications poses many challenges, but rapid and reliable detection will prove invaluable for clinical diagnosis. Herein, we report the application of drop-cast triangular Ag nanoplatelets as surface enhanced Raman scattering (SERS) substrates for qualitative detection of 20-hydroxyeicosatetraenoic acid (20-HETE), which is a potential biomarker for diagnosis of hypertensive disorders. The substrates were modified to increase SERS signal enhancement of 20-HETE, both by itself and in combination with structurally similar lipids that are known to exist in significantly high concentrations in human blood, simulating an ethanol extract of plasma, and analyzed with a Raman system employing a 532 nm laser excitation source. Biomarker peaks of 20-HETE can be reliably detected and differentiated from those of the structurally similar lipids (arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid) commonly found in human blood, eve...

Stand-off deep-UV Raman spectroscopy

UV Raman spectra were measured using a novel experimental configuration. This configuration allows many of the difficulties associated with UV excitation and high-power pulsed laser sources to be mitigated. Large sample areas are imaged into the detection system allowing high power excitation sources to be used while simultaneously avoiding sample degradation and multi-photon absorption effects. Such large detection areas allow large numbers of molecular scatters to be probed even with minimal penetration depth. Alignment issues between sample and collection optics are also simplified. Several common solvents were studied using 213 nm light and their spectra reported.

A label-free cellulose SERS biosensor chip with improvement of nanoparticle-enhanced LSPR effects for early diagnosis of subarachnoid hemorrhage-induced complications

It is very difficult to predict some complications after subarachnoid hemorrhage (SAH), despite rapid advances in medical science. Herein, we introduce a label-free cellulose surface-enhanced Raman spectroscopy (SERS) biosensor chip with pH-functionalized, gold nanoparticle (AuNP)-enhanced localized surface plasmon resonance (LSPR) effects for identification of SAH-induced cerebral vasospasm and hydrocephalus caused by cerebrospinal fluid (CSF). The SERS biosensor chip was implemented by the synthesis reaction of the AuNPs, which were charged positively through pH level adjustment, onto a negatively-charged cellulose substrate with ξ = –30.7 mV. The zeta potential, nanostructural properties, nanocrystallinity, and computational calculation-based electric field distributions of the cellulose-originated AuNPs were optimized to maximize LSPR phenomena and then characterized. Additionally, the performance of the SERS biosensor was compared under two representative excitation laser sources in the visible region (532 nm) and near-infrared region (785 nm). The Raman activities of our SERS biosensor chip were evaluated by trace small molecules (crystal violet, 2 µL), and the biosensor achieved an enhancement factor of 3.29 × 109 for the analytic concept with an excellent reproducibility of 8.5% relative standard deviation and a detection limit of 0.74 pM. Furthermore, the experimental results revealed that the five proposed SERS-based biomarkers could provide important information for identifying and predicting SAH-induced cerebral vasospasm and hydrocephalus complications (91.1% reliability and 19.3% reproducibility). Therefore, this facile and effective principle of our SERS biosensor chip may inspire the basis and strategies for the development of sensing platforms to predict critical complications in various neurosurgical diagnoses.

Synthesis of highly stable silver nanorods and their application as SERS substrates

Abstract We report on the improved stability and yield of silver nanorods with well controlled aspect ratios synthesized using a modified seed mediated approach conducted at room temperature. It is found that the longitudinal surface plasmon resonance of these nanoparticles can be tuned in the spectral range 400–700 nm by varying the concentration of seed particles. The surface enhanced Raman scattering (SERS) activity of these nanorods with varying aspect ratios was tested with four dye molecules viz., crystal violet, malachite green, nile blue chloride and rhodamine-6G, using visible and near-infrared laser excitation sources viz., 514.4 and 784.8 nm, respectively. The mechanism of enhancement for the dye molecules adsorbed on these nanorods was investigated in detail. A maximum enhancement factor in the order of 108 was obtained when factors such as the peak wavelength corresponding to the plasmon of the nanorods, the absorption of dye and the excitation line were in close approximation. The linearity obtained in the calibration curves drawn for intense Raman peaks in the SERS spectra of different dye molecules indicated that these substrates are suitable for applications such as biosensing.

Polymer optical fiber sensor side-pumped with polymer clad doped lasing compounds

Optical fibers were produced by the system manufactured for this purpose and then, PMMA core of polymer optical fiber (POF) and PMMA doped Rhodamine B (RhB) claddings were studied and determine their UV–vis absorption and emission. The study adopted the mechanism of lateral pumping of the product polymer optical fiber by using laser with 404 nm excitation to study optical specifications of the factory fiber. It was noted that there were blue shift in maximum peak wavelength in absorption and fluorescence from the doped polymer before use it as clad. The obtained results by using the doping polymer with (RhB) for clad the amplified spontaneous emission ASE seems in fluorescence study. The side excitation shows that there were no an overlap between the excitation laser source and the RhB molecule fluorescence. The results indicate that POF can be a strong candidate for the development of optical fiber sensors.

Assessing performance of modern Brillouin spectrometers.

Brillouin spectroscopy and imaging has experienced a renaissance in recent years seeing vast improvements in methodology and increasing number of applications. With this resurgence has come the development of new spontaneous Brillouin instruments that often tout superior performance compared to established conventional systems such as tandem Fabry-Perot interferometers (TFPI). The performance of these new systems cannot always be thoroughly examined beyond the scope of the intended application, as applications often take precedence in reports. We therefore present evaluation of three modern Brillouin spectrometers: two VIPA-based spectrometers with wavelength-specific notch filters, and one scanning 6-pass TFPI. Performance analysis is presented along with a discussion about the dependence of measurements on excitation laser source and the various susceptibilities of each system.

Spectrally-Compressed Fiber Laser Source for Supercontinuum-Based Broadband-CARS Spectroscopy

Supercontinuum (SC)-based broadband coherent anti-stokes Raman scattering (BCARS) spectroscopy can identify a large spectral band of complex samples simultaneously, which has been widely employed in analytical biochemistry. On the excitation laser source, there is usually a tradeoff between spectral bandwidth and resolution in BCARS. Specifically, femtosecond lasers are preferred for SC generation (as stokes), while narrowband picosecond pulses (as pump) are more suitable for obtaining high spectral resolution. Here, we report a fiber laser source for SC-based BCARS spectroscopy based on a spectrally compressed broadband seed, which ensures broadband and high resolution simultaneously. The spectral compression is achieved by self-phase modulation suffered by negative-chirp pulses in optical fibers, the power efficiency of which is much higher than that of conventional spectral filtering. High-power narrowband picosecond pump pulses, with pulse energy of 16.7 nJ and linewidth of 5.8 cm−1, are obtained for BCARS, by spectral compression in a cascaded fiber amplifier with compression ratio of 16. Combining with SC pulses as stokes beam, we obtain BCARS spectra in 700–1900 cm−1 spectral range simultaneously. Moreover, the influences of spectrally-compressed pump on BCARS signal are investigated.

A Low-Cost Time-Resolved Spectrometer for the Study of Ruby Emission

A low-cost time-resolved emission spectrometer optimized for ruby emission is presented. The use of a Class II diode laser module as the excitation source reduces costs and hazards. The design presented here can facilitate the inclusion of time-resolved emission spectroscopy with laser excitation sources in the undergraduate laboratory curriculum. The ruby decay data show evidence of radiation trapping, an interesting optical phenomenon discussed in the research literature.

Indistinguishable and efficient single photons from a quantum dot in a planar nanobeam waveguide

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94+-1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

Single Molecule Upconversion-Linked Immunosorbent Assay with Extended Dynamic Range for the Sensitive Detection of Diagnostic Biomarkers.

The ability to detect disease markers at the single molecule level promises the ultimate sensitivity in clinical diagnosis. Fluorescence-based single-molecule analysis, however, is limited by matrix interference and can only probe a very small detection volume, which is typically not suitable for real world analytical applications. We have developed a microtiter plate immunoassay for counting single molecules of the cancer marker prostate specific antigen (PSA) using photon-upconversion nanoparticles (UCNPs) as labels that can be detected without background fluorescence. Individual sandwich immunocomplexes consisting of (1) an anti-PSA antibody immobilized to the surface of a microtiter well, (2) PSA, and (3) an anti-PSA antibody-UCNP conjugate were counted under a wide-field epifluorescence microscope equipped with a 980 nm laser excitation source. The single-molecule (digital) upconversion-linked immunosorbent assay (ULISA) reaches a limit of detection of 1.2 pg mL-1 (42 fM) PSA in 25% blood serum, which is about ten times more sensitive than commercial ELISAs, and covers a dynamic range of three orders of magnitude. This upconversion detection mode has the potential to pave the way for a new generation of digital immunoassays.

Ultrafast Microscopy: Imaging Light with Photoelectrons on the Nano-Femto Scale.

Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron-based imaging or rely on super-resolution optical techniques to enable probing of matter on the nano-femto scale. Among several actively developed methods, ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source and it is immediately capable of probing of coherent phenomena in solid-state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope.

Photoacoustic thermal flowmetry with a single light source.

We report a photoacoustic thermal flowmetry based on optical-resolution photoacoustic microscopy (OR-PAM) using a single laser source for both thermal tagging and photoacoustic excitation. When an optically absorbing medium is flowing across the optical focal zone of OR-PAM, a small volume of the medium within the optical focus is repeatedly illuminated and heated by a train of laser pulses with a high repetition rate. The average temperature of the heated volume at each laser pulse is indicated by the photoacoustic signal excited by the same laser pulse due to the well-established linear relationship between the Grueneisen coefficient and the local temperature. The thermal dynamics of the heated medium volume, which are closely related to the flow speed, can therefore be measured from the time course of the detected photoacoustic signals. Here, we have developed a lumped mathematical model to describe the time course of the photoacoustic signals as a function of the medium's flow speed. We conclude that the rising time constant of the photoacoustic signals is linearly dependent on the flow speed. Thus, the flow speed can be quantified by fitting the measured photoacoustic signals using the derived mathematical model. We first performed proof-of-concept experiments using defibrinated bovine blood flowing in a plastic tube. The experiment results have demonstrated that the proposed method has high accuracy (∼±6%) and a wide range of measurable flow speeds. We further validated the method by measuring the blood flow speeds of the microvasculature in a mouse ear in vivo.

Optical coherence tomography and non-linear microscopy for paintings - a study of the complementary capabilities and laser degradation effects.

This paper examines for the first time the potential complementary imaging capabilities of Optical coherence tomography (OCT) and non-linear microscopy (NLM) for multi-modal 3D examination of paintings following the successful application of OCT to the in situ, non-invasive examination of varnish and paint stratigraphy of historic paintings and the promising initial studies of NLM of varnish samples. OCT provides image contrast through the optical scattering and absorption properties of materials, while NLM provides molecular information through multi-photon fluorescence and higher harmonics generation (second and third harmonic generation). OCT is well-established in the in situ non-invasive imaging of the stratigraphy of varnish and paint layers. While NLM examination of transparent samples such as fresh varnish and some transparent paints showed promising results, the ultimate use of NLM on paintings is limited owing to the laser degradation effects caused by the high peak intensity of the laser source necessary for the generation of non-linear phenomena. The high intensity normally employed in NLM is found to be damaging to all non-transparent painting materials from slightly scattering degraded varnish to slightly absorbing paint at the wavelength of the laser excitation source. The results of this paper are potentially applicable to a wide range of materials given the diversity of the materials encountered in paintings (e.g. minerals, plants, insects, oil, egg, synthetic and natural varnish).

Demonstration of a highly sensitive photoacoustic spectrometer based on a miniaturized all-optical detecting sensor.

We report on the development of a highly sensitive photoacoustic (PA) spectrometer based on a miniaturized all-optical detecting sensor. The sensor has a cell volume of less than 6 μL and relies on a cantilever-based acoustic transducer, which is equipped with an optical fiber interferometric readout. The spectrometer reaches a noise equivalent concentration of 15 ppb (300 ms time constant) for acetylene detection using a 23 mW excitation laser source, which corresponds to a normalized noise equivalent absorption coefficient of 7.7 × 10-10 W cm-1 Hz-1/2. The performance offered by this PA spectrometer is thus comparable to those reported for bulkier PA analyzers. Furthermore, because both the excitation and detection signals are brought to the PA cell via optical fibers, our spectrometer can be used in harsh environments, where electronic devices are prone to failure, and it is specially suitable for multiplexed remote detection applications. We believe that our study paves the way for the development of PA spectrometers that allow in-situ gas detection in space-limited circumstances.

Real-time intravascular photoacoustic-ultrasound imaging of lipid-laden plaque in human coronary artery at 16 frames per second

Intravascular photoacoustic-ultrasound (IVPA-US) imaging is an emerging hybrid modality for the detection of lipid-laden plaques, as it provides simultaneous morphological and lipid-specific chemical information of an artery wall. Real-time imaging and display at video-rate speed are critical for clinical utility of the IVPA-US imaging technology. Here, we demonstrate a portable IVPA-US system capable of imaging at up to 25 frames per second in real-time display mode. This unprecedented imaging speed was achieved by concurrent innovations in excitation laser source, rotary joint assembly, 1 mm IVPA-US catheter size, differentiated A-line strategy, and real-time image processing and display algorithms. Spatial resolution, chemical specificity, and capability for imaging highly dynamic objects were evaluated by phantoms to characterize system performance. An imaging speed of 16 frames per second was determined to be adequate to suppress motion artifacts from cardiac pulsation for in vivo applications. The translational capability of this system for the detection of lipid-laden plaques was validated by ex vivo imaging of an atherosclerotic human coronary artery at 16 frames per second, which showed strong correlation to gold-standard histopathology. Thus, this high-speed IVPA-US imaging system presents significant advances in the translational intravascular and other endoscopic applications.

Development of Far-Red and Ultraviolet Digital Frequency-Domain Flow Cytometry Systems

Flow cytometry is a powerful single cell analysis technique that uses fluorescence and light scatter to characterize differences (i.e., phenotype, genotype, morphology, etc.) in large cell populations. Digital excited state lifetime measurements with flow cytometry have been largely underutilized; the measurement of fluorescence decay rates by molecules in or on the surface of cells is challenging to accurately measure when the samples are moving rapidly through the path of a laser excitation source. In past work, we have shown that time-resolved cytometry can provide accurate measurements of the fluorescence lifetime of organic fluorophores at standard cytometric throughputs (∼1000 cells/s). Herein, we advance our digital laser modulation and signal processing approaches to develop cytometry systems that operate across a broader spectral range. We optimized a far-red and ultraviolet (UV) cytometry instruments and present fluorescence microsphere results to determine detection sensitivity. The expansion of time-resolved systems across a wider color spectrum is important for many new applications including the development of near-infrared fluorescent proteins and the detection of UV-excitable autofluorescence in single cells.

Three-Photon Absorption Induced Photoluminescence in Organo-Lead Mixed Halide Perovskites

Organo-lead mixed halide perovskites have been showing remarkable performance for applications in solar cells and are very promising for numerous applications in optoelectronics and nonlinear optics. In this study, we report a room-temperature photoluminescence study of this material by using pulsed excitation laser sources at 1064 nm wavelength. Under our experimental conditions, strong photoluminescence was observed only for bromine-containing perovskites, CH3NH3Pb(I1−xBrx)3, thus suggesting an important role of bromine for photoluminescence of halide perovskites. The experimental results also showed that the photoluminescence peak was blue-shifted from 727 nm to 574 nm when x increased from 1/3 to 1. In particular, the photoluminescence peak featured a third-order dependence on the laser intensity. This direct observation of three-photon absorption-induced photoluminescence of organo-lead mixed halide perovskite materials thus opens up interesting applications in the field of optoelectronics and nonlinear optics.

Radiation Hardened Architecture of a Single-Ended Raman-Based Distributed Temperature Sensor

Raman-based Distributed Temperature Sensors (RDTS) allow performing spatially resolved (1 m) reliable temperature measurements over several km long Optical Fibers (OFs). These systems are based on the temperature dependence of the intensities of both the Stokes and anti-Stokes components of the Raman back-scattered signal. One of the specific issues associated with RDTS technology in radiation environments is the differential Radiation Induced Attenuation (RIA) between the two components that induces huge errors in the temperature evaluation. Such problem is particularly evident for commercially available single-ended DTS using one laser source. Double-ended configuration could be used to correct for the differential attenuation but are limited by RIA in terms of sensing range. In the present work, we show how a Radiation-Hardened-by-Design DTS (RHD-DTS) overcomes the observed radiation issues keeping the single-ended interrogation scheme. In the tested RHD-DTS two infrared excitation laser sources (~1550 nm and ~1650 nm) are employed: the wavelength of the Stokes component due to the first excitation source coincides with the wavelength of the second excitation; vice versa, the wavelength of the anti-Stokes component due to the second excitation source coincides with the wavelength of the first excitation. The overall result is that the two signal intensities are automatically corrected for the differential RIA all along the OF sensor length and the temperature measurements becomes robust against radiation effects. This study demonstrates the potential of such a sensor by reporting preliminary experimental results obtained with a prototype developed by Viavi Solutions exploiting radiation-sensitive or radiation-hardened optical fibers.

Evaluation of fluorescence emitting characteristics of a microparticle by illumination angle scanning utilizing a resin-based monolithic TAS chip

An optical sensor, such as a light waveguide implemented total analysis system (TAS), is one of the functional components that will be needed to realize a “ubiquitous human healthcare system” in the near future. We have already proposed the fundamental structure for a light waveguide capable of irradiating a living cell or particle running along a microfluidic channel, and of detecting fluorescence even from the extremely weak power of a minute particulate body. In order to foster advanced functions for obtaining information concerning the internal structure of living cells or particles conveniently, an angular scanning method that sequentially changes the direction in which the minute cell or particle is irradiated may be crucial. In this paper, we investigate fluorescence detection from resin particles attached to a fluorescent substance by switching irradiation direction of the exciting laser source power using a newly fabricated TAS chip with a unique intersection structure. To construct the experimental system cost effectively, we adopted a special light waveguide structure in which the intersection to a fluidic channel was arranged radially, with laser power inlet/outlet portions arranged in parallel. In addition, we incorporated a forced vibration mechanism on an optical fiber apex to construct a simple light-switching system. Preliminary experiments combining this unique TAS chip and the optical scanning mechanism revealed that the function of the radially arranged light waveguide switching worked adequately, and variation of transmittance of each light waveguide was accurately evaluated. Results were successfully applied to compensation of detected angular scanning signals. Finally, we performed an angular scanning experiment for a fluorescent substance attached to a microparticle under conditions of higher intersection-passing velocity of several 10s mm/s (single-shot scanning) and lower intersection-passing velocity of several mm/s (multiple-shot scanning). Based on the obtained scanning data, we successfully constructed directivities of emitting fluorescence from particles, and clarified that fluorescence emission indicated its peak at the forward and orthogonal directions of the incident light beam.