Andrew J. Traverso

Background clean-up in Brillouin microspectroscopy of scattering medium

Brillouin spectroscopy is an emerging tool for microscopic optical imaging as it allows for non-contact, non-invasive, direct assessment of the elastic properties of materials. However, strong elastic scattering and stray light from various sources often contaminate the Brillouin spectrum. A molecular absorption cell was introduced into the virtually imaged phased array (VIPA) based Brillouin spectroscopy setup to absorb the Rayleigh component, which resulted in a substantial improvement of the Brillouin spectrum quality.

Dual Raman-Brillouin Microscope for Chemical and Mechanical Characterization and Imaging

We present a unique confocal microscope capable of measuring the Raman and Brillouin spectra simultaneously from a single spatial location. Raman and Brillouin scattering offer complementary information about a materials chemical and mechanical structure, respectively, and concurrent monitoring of both of these spectra would set a new standard for material characterization. We achieve this by applying recent innovations in Brillouin spectroscopy that reduce the necessary acquisition times to durations comparable to conventional Raman spectroscopy while attaining a high level of spectral accuracy. To demonstrate the potential of the system, we map the Raman and Brillouin spectra of a molded poly(ethylene glycol) diacrylate (PEGDA) hydrogel sample in cyclohexane to create two-dimensional images with high contrast at microscale resolutions. This powerful tool has the potential for very diverse analytical applications in basic science, industry, and medicine.

Seeing cells in a new light: a renaissance of Brillouin spectroscopy

Over the years, light scattering from acoustic waves has grown to be increasingly important in the fields of biology and medicine. This type of scattering, known as Brillouin scattering, has already seen a plethora of applications in fields such as physics. However, the potential for Brillouin scattering for medical imaging and diagnostics has only recently been considered. In this work, we summarize most of the applications of Brillouin scattering in biology to date, and some current work in our lab showing how Brillouin scattering is a worthy prospect for many emerging problems in biology and medical diagnostics.

Stimulated Brillouin Scattering Microscopic Imaging

Two-dimensional stimulated Brillouin scattering microscopy is demonstrated for the first time using low power continuous-wave lasers tunable around 780 nm. Spontaneous Brillouin spectroscopy has much potential for probing viscoelastic properties remotely and non-invasively on a microscopic scale. Nonlinear Brillouin scattering spectroscopy and microscopy may provide a way to tremendously accelerate the data aquisition and improve spatial resolution. This general imaging setup can be easily adapted for specific applications in biology and material science. The low power and optical wavelengths in the water transparency window used in this setup provide a powerful bioimaging technique for probing the mechanical properties of hard and soft tissue.

Coherence brightened laser source for atmospheric remote sensing

We have studied coherent emission from ambient air and demonstrated efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The generated optical gain is a result of two-photon photolysis of atmospheric O2, followed by two-photon excitation of atomic oxygen. We have analyzed the temporal shapes of the emitted pulses and have observed very short duration intensity spikes as well as a large Rabi frequency that corresponds to the emitted field. Our results suggest that the emission process exhibits nonadiabatic atomic coherence, which is similar in nature to Dicke superradiance where atomic coherence is large and can be contrasted with ordinary lasing where atomic coherence is negligible. This atomic coherence in oxygen adds insight to the optical emission physics and holds promise for remote sensing techniques employing nonlinear spectroscopy.

Brillouin spectroscopy as a new method of screening for increased CSF total protein during bacterial meningitis.

Bacterial meningitis is a disease of pronounced clinical significance, especially in the developing world. Immediate treatment with antibiotics is essential, and no single test can provide a conclusive diagnosis. It is well established that elevated total protein in cerebrospinal fluid (CSF) is associated with bacterial meningitis. Brillouin spectroscopy is a widely used optical technique for noninvasive determination of the elastic moduli of materials. We found that elevated protein levels in CSF alter the fluid elasticity sufficiently to be measurable by Brillouin spectroscopy, with model healthy and diseased fluids distinguishable to marked significance (P = 0.014), which increases with sample concentration by dialysis. Typical raw output of a 2-stage VIPA Brillouin spectrometer: inelastically scattered Brillouin peaks (arrows) and elastically scattered incident radiation (center cross).

Impulsive Brillouin microscopy

Brillouin scattering has been emerging as a viable tool for microscopy. However, most of the work done has been with the use of spontaneous Brillouin scattering, which has several hindrances to its use. In this work, we propose and demonstrate nonlinear Brillouin scattering as a solution to many of these hindrances. Here we demonstrate fast two-dimensional microscopic optical imaging of materials’ mechanical properties for the very first time (to our knowledge) using nonlinear Brillouin scattering. Impulsive stimulated Brillouin scattering (ISBS) was used in an optical configuration that is capable of providing accurate local assessment of viscoelastic properties faster than conventional Brillouin spectroscopy. This proof-of-principle imaging experiment has been demonstrated for materials of known properties and microfluidic devices. Applications to noninvasive biomedical imaging are discussed. The fast acquisition times and strong signal of ISBS coupled with the ability of Brillouin scattering to easily measure materials’ viscoelastic properties make this an attractive technique for biological use.

Femtosecond-laser-induced shockwaves in water generated at an air-water interface

We report generation of femtosecond-laser-induced shockwaves at an air-water interface by millijoule femtosecond laser pulses. We document and discuss the main processes accompanying this phenomenon, including light emission, development of the ablation plume in the air, formation of an ablation cavity, and, subsequently, a bubble developing in water. We also discuss the possibility of remotely controlling the characteristics of laser-induced sound waves in water through linear acoustic superposition of sound waves that results from millijoule femtosecond laser-pulse interaction with an air-water interface, thus opening up the possibility of remote acoustic applications in oceanic and riverine environments.

Amplitude concentration in a phase-modulated spectrum due to femtosecond filamentation

We present a method by which the spectral intensity of an ultrafast laser pulse can be accumulated at selected frequencies by a controllable amount. Using a 4-f pulse shaper we modulate the phase of the frequency components of a femtosecond laser. By inducing femtosecond filamentation with the modulated pulse, we can concentrate the spectral amplitude of the pulse at various frequencies. The phase mask applied by the pulse shaper determines the frequencies for which accumulation occurs, as well as the intensity of the spectral concentration. This technique provides a way to obtain pulses with adjustable amplitude using only phase modulation and the nonlinear response of a medium. This provides a means whereby information which is encoded into spectral phase jumps may be decoded into measurable spectral intensity spikes.

Directional coherent light via intensity-induced sideband emission

We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet (XUV) to the deep infrared. This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system. A nonlinear process of four-wave mixing then occurs across the split system, driving coherent emission at sidebands both red- and blue-detuned from the pump frequency, and propagates both forward and backward along the pump beam path. We observed this phenomenon in dense rubidium vapor along both the D1 and D2 transitions. The sideband emission exhibits a short pulse duration (<1 ns) with threshold-like behavior dependent on both the pump intensity and Rb vapor density. This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies.