Charles W. Ballmann

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.

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.

Enhanced coupling of light into a turbid medium through microscopic interface engineering

Significance Optical scattering severely limits the range and sensitivity of detection techniques within/through turbid media, such as biological samples and security-related materials. In this article, we demonstrate a method to enhance the coupling of light into a highly scattering medium. This enhanced coupling results in higher optical sensitivity to particles within the medium, as well as enhanced optical transmission through the medium. These results pave the way for enhanced spectroscopy in biological and other turbid media. There are many optical detection and sensing methods used today that provide powerful ways to diagnose, characterize, and study materials. For example, the measurement of spontaneous Raman scattering allows for remote detection and identification of chemicals. Many other optical techniques provide unique solutions to learn about biological, chemical, and even structural systems. However, when these systems exist in a highly scattering or turbid medium, the optical scattering effects reduce the effectiveness of these methods. In this article, we demonstrate a method to engineer the geometry of the optical interface of a turbid medium, thereby drastically enhancing the coupling efficiency of light into the material. This enhanced optical coupling means that light incident on the material will penetrate deeper into (and through) the medium. It also means that light thus injected into the material will have an enhanced interaction time with particles contained within the material. These results show that, by using the multiple scattering of light in a turbid medium, enhanced light–matter interaction can be achieved; this has a direct impact on spectroscopic methods such as Raman scattering and fluorescence detection in highly scattering regimes. Furthermore, the enhanced penetration depth achieved by this method will directly impact optical techniques that have previously been limited by the inability to deposit sufficient amounts of optical energy below or through highly scattering layers.

Simple approach to high-fidelity tunable narrow-band pulse generation

Tunable narrow-band short-pulse coherent emission can be generated by the optical parametric amplification of a seeded continuous wave (CW) laser. However, the residual CW pedestal can affect the accuracy of the optical measurements and the exact interpretation of the experimental data. We demonstrate a simple approach to removing the residual CW seed in a frequency tunable, seeded parametric amplification setup in the nanosecond regime by adding an additional parametric amplification stage which is seeded by an idler wave from the first stage. We validate this method by using a pump-probe experiment in an atomic vapor. Our results show the elimination of an atomic vapor hyperfine pumping signal after the CW pedestal has been removed.

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.

Pulsed cooperative backward emissions from non-degenerate atomic transitions in sodium

We study backward cooperative emissions from a dense sodium atomic vapor. Ultrashort pulses produced from a conventional amplified femtosecond laser system with an optical parametric amplifier are used to excite sodium atoms resonantly on the two-photon 3S?4S transition. Backward superfluorescent emissions (BSFEs), both on the 4S?3P and 4S?3P transitions, are observed. The picosecond temporal characteristics of the BSFE are observed using an ultrafast streak camera. The power laws for the dependencies of the average time delay and the intensity of the BSFEs on input power are analyzed in the sense of cooperative emission from nonidentical atomic species. As a result, an absolute (rather than relative) time delay and its fluctuations (free of any possible external noise) are determined experimentally. The possibility of a backward swept-gain superfluorescence as an artificial laser guide star in the sodium layer in the mesosphere is also discussed.

Dual-tip-enhanced ultrafast CARS nanoscopy

Coherent anti-Stokes Raman scattering (CARS) and, in particular, femtosecond adaptive spectroscopic techniques (FAST CARS) have been successfully used for molecular spectroscopy and microscopic imaging. Recent progress in ultrafast nano-optics provides flexibility in generation and control of optical near fields, and holds promise to extend CARS techniques to the nanoscale. In this theoretical study, we demonstrate ultrafast subwavelentgh control of coherent Raman spectra of molecules in the vicinity of a plasmonic nanostructure excited by ultrashort laser pulses. The simulated nanostructure design provides localized excitation sources for CARS by focusing incident laser pulses into subwavelength hot spots via two self-similar nanolens antennas connected by a waveguide. Hot-spot-selective dual-tip-enhanced CARS (2TECARS) nanospectra of DNA nucleobases are obtained by simulating optimized pump, Stokes and probe near fields using tips, laser polarization- and pulse-shaping. This technique may be used to explore ultrafast energy and electron transfer dynamics in real space with nanometre resolution.

Generation of tunable high-repetition rate middle infrared transform-limited picosecond pulses

Tunable middle infrared generation is now affordable through optical parametric generation and amplification in a number of infrared nonlinear crystals. However, maintaining narrow bandwidth, while achieving high conversion efficiency, remains a challenge. In this report, we propose and experimentally demonstrate a relatively simple setup, which utilizes a single-wavelength diode laser as a seed laser for an optical parametric amplifier.

BISTRO measurement also means better measurement (Conference Presentation)

Brillouin microspectroscopy is an emerging technique in optical elastography. It allows measuring local high-frequency viscoelastic modulus in cells and tissues in a matter of seconds or hundreds of milliseconds. BISTRO (Brillouin Imaging and Sensing via Time-Resolved Optical) measurements relies on impulsive stimulated Brillouin scattering to increase the signal strength and, hence, the speed of imaging, which can be as high as 1,000,000 pixels per second. However, there are several other untapped advantages of BISTRO measurements, and a particular intriguing one is the accuracy of Brillouin shift and linewidth assessments, which are substantially improved through BISTRO assessment. This allows high quality measurements not only to be done fast and more accurately, but also allows highly reproducible measurements over an extended period of time.