Hyper-Raman spectroscopy of biomolecules.

The hyper-Raman scattering (HRS) spectra of biologically significant molecules (D-glucose, L-alanine, L-arabinose, L-tartaric acid) in aqueous solutions are reported. The HRS spectra were measured using a picosecond laser at 532 nm operating at a MHz repetition rate. High signal to noise spectra were collected with a commercial spectrometer and CCD without resonant, nanoparticle, or surface enhancement. The HRS peak frequencies, relative intensities, band assignments, and depolarization ratios are examined. By comparing HRS to Raman scattering (RS) and infrared absorption spectra we verify that the IR-active vibrational modes of the target molecules are observed in HRS spectra but come with substantially different peak intensities. The HRS of the biomolecules as well as water, dimethyl sulfoxide, methanol, and ethanol were deposited into a data repository to support the development of theoretical descriptions of HRS for these molecules. Depositing the spectra in a repository also supports future dual detection RS, HRS microscopes which permit simultaneous high-spatial-resolution vibrational spectroscopy of IR-active and Raman-active vibrational modes.

New insights into a hydrogen bond: hyper-Raman spectroscopy of DMSO-water solution.

Hydrogen bonding plays an essential role in biological processes by stabilizing proteins and lipid structures as well as controlling the speed of enzyme catalyzed reactions. Dimethyl sulfoxide-water (DMSO-H2O) solution serves as a classical model system by which the direct and indirect effects of hydrogen bonding between water hydrogens and the sulfoxide functional group can be explored. The complex transition from self-bonding to heterogeneous bonding is important, and multiple spectroscopic approaches are needed to provide a detailed assessment of those interactions. In this report, for the first time, hyper-Raman scattering was successfully employed to investigate molecular interactions in DMSO-H2O system. We measured the improper blueshift of the C-S and C-H stretching modes of DMSO caused by partial charge transfer and enhanced bond polarization. By detecting differences in the frequency shifts of C-S and C-H modes for low DMSO concentrations (<33 mol%) we find evidence of the intermolecular bonds between water and the DMSO methyl groups. We exploit the high sensitivity of hyper-Raman scattering to the low frequency librations of H2O to observe a change in librational mode population providing insight into existing questions about the coordination of H2O around DMSO molecules and the formation of the H2O shell around DMSO molecules proposed in prior simulation studies. These results demonstrate that hyper-Raman spectroscopy can be a practical spectroscopic technique to study the intermolecular bonding of model systems and test claims about model system bonding generated by theoretical calculations.

Eye safety implications of high harmonic generation in zinc selenide.

Polycrystalline zinc selenide (ZnSe) has been the subject of many nonlinear optics studies for wavelengths under 4.0 µm including sum/difference frequency generation, harmonic generation, and filamentation. In this report, the conversion efficiency of high harmonic generation (HHG) in ZnSe is quantified for mid-infrared wavelengths ranging from 2.7 µm to 8.0 µm. By increasing the fundamental wavelength, we demonstrate that HHG in thick ZnSe targets is limited by the band gap. The high conversion efficiency of mid-infrared to near-infrared light in ZnSe raises concerns of a nonlinear retinal hazard. We contrast the HHG behavior of ZnSe against the observed harmonic generation of calcium fluoride, BK7, and fused silica over the same wavelengths.

Z-scan measurements of water from 1150 to 1400 nm.

Understanding the nonlinear properties of water is essential for laser surgery applications, as well as understanding supercontinuum generation in water. Unfortunately, the nonlinear properties of water for wavelengths longer than 1064 nm are poorly understood. We extend the application of the Z-scan technique in water to determine its nonlinear refractive index (n2) and nonlinear absorption (ß) for wavelengths in the 1150-1400 nm range, where linear absorption is also significant. We observe the wavelength-dependent variation of the nonlinear properties of water around the water absorption band.

Extreme nonlinear optics in a long pulse regime: High harmonic generation of picosecond mid-IR pulses in polycrystalline zinc selenide.

High harmonic generation (HHG) in semiconductors has been extensively studied recently in the high-intensity limit using middle infrared (mid-IR) femtosecond laser pulses resulting in emission spectra of self-phase modulated harmonics resting on top of a broadband continuum. In this report, a different approach to HHG in polycrystalline zinc selenide (poly-ZnSe) was explored utilizing a relatively low power regime (1-40 GW/cm2) and much longer (30 ps) mid-IR laser pulses. Through a combination of low power, picosecond excitation, and narrowband (<10 nm full width at half maximum) mid-IR excitation, the nonlinear optical effects in poly-ZnSe could be isolated and studied independently. From the clearly distinguishable HHG peaks, harmonic conversion efficiencies of 10-4-10-12 for second to ninth harmonic in poly-ZnSe were measured, and the relationship between the Nth harmonic intensity and excitation intensity (I0) was found to follow a power law, I0x with x ≤ N/2, as a result of the random quasi-phase matching process.

Biomedical optics applications of advanced lasers and nonlinear optics.

SIGNIFICANCE: 2019 SPIE Photonics West conference hosted over 5000 presentations. Some important presentations in the Industrial Laser, Laser Source and Application (LASE) and Optoelectronics, Photonic Materials and Devices (OPTO) sections of the SPIE Photonics West conference have a risk of being overlooked by the biomedical community despite their implications for the field of biophotonics. We review some recent advances in the area of development coherent radiation sources in the infrared (IR), ultraviolet (UV), and terahertz (THz) regimes. AIM: Recent advances in coherent radiation sources in the IR, deep UV, and THz regimes were outlined, and the importance of each presentation to one or more promising biomedical applications was assessed. APPROACH: Presentations and proceedings from the LASE and OPTO sections were reviewed for inclusion. Emphasis was placed on talks from the Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII conference, and the Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XII conference. Conference sections that directly focused on biomedical applications were excluded. RESULTS: Enhanced IR supercontinuum generation with compact supercontinuum sources may allow for real-time biomarker detection and create new opportunities for imaging tissues using the third biological window (1600 to 1850 nm). Efficient methods to generate deep UV (200 to 260 nm) radiation allow for the study of biologically important molecules through techniques such as resonance Raman spectroscopy while avoiding fluorescence overlap. Likewise, novel and improved THz generation methods seek to bridge the "THz gap" that has previously limited biomedical applications. CONCLUSIONS: Advances in coherent radiation sources in the IR, UV, and THz regimes have created new opportunities for biomedical optics research.

Filamentation in Atmospheric Air with Tunable 1100-2400 nm Near-Infrared Femtosecond Laser Source.

Intense femtosecond pulse filamentation in open-air has been utilized for long distance optical communication and remote sensing, but it results in nonlinear-effect driven eye hazards which are not addressed by current eye safety standards. A systematic study of filamentation in atmospheric air was performed using a tunable 100 fs near-infrared laser (1100 nm-2400 nm). While undergoing filamentation, each source wavelength was spectrally broadened resulting in supercontinuum and third harmonic generation in the visible and near-IR spectrum. We record the spectra at the center and fringes of the supercontinuum as it is imaged onto a planar surface. In a full beam collection regime, we report the energy of the sub-1000 nm light generation for source wavelengths from 1100 nm to 1600 nm and compare the energy density to the maximum permissible exposure values under the ANSI Z136.1 laser safety standard.