Single photon emitters (SPE) are essential building blocks for proposed quantum technologies. An ideal SPE emits only one photon at a time and the photons are indistinguishable. CdSe core/shell quantum dots (QDs) provide a physical system that approximates a SPE. We discuss progress to improve the SPE properties of CdSe QDs. We present synthesis and spectroscopic analysis of wurtzite-CdSe/CdS, mixed shell CdSe/Cd_0.5Zn_0.5S, and continuously graded CdSe/Cd_xZn_1-xSe/ZnSe_yS_1-y QDs. We compare their behaviors at room and cryogenic temperatures using photoluminescence spectroscopy, Michelson interferometry, and Hanbury Brown and Twiss interferometry. We also address efforts to mitigate undesirable behaviors observed in QDs such as charging.
High harmonic generation (HHG) is readily achieved by focusing mid-infrared (mid-IR), femtosecond laser pulses into polycrystalline Zinc Selenide (p-ZnSe). In this high-power regime (>> 100 GW/cm2), the HHG harmonics are self-phase modulated into a continuum. In this talk, we explore mid-IR frequency conversion in p-ZnSe in a low-power regime (1 – 100 GW/cm2) using mid-IR, 30 picosecond pulses which results in spectrally isolated visible and near-IR harmonics. In this regime, harmonic intensities clearly decrease with harmonic order with conversion efficiencies of 10-4 to 10-12 for second to ninth harmonics while retaining a non-perturbative character consistent with frequency conversion between harmonics permitted via random quasi-phase matching.
Silicon optoelectronics devices have been well explored in the near-IR regime with emphasis on telecom applications. In the mid-IR regime, group IV optoelectronic devices (silicon and/or germanium based) could one day serve as waveguides, nonlinear media for χ(2) and χ(3) wave mixing, and highly adaptable platforms for low cost, lab-on-chip chemical and biological sensors. However, nonlinear optical absorption in these materials limit potential applications. In this report, we observe dramatic decreases in transmission in silicon and germanium at middle-infrared wavelengths when utilizing intense (~ 10 GW/cm2) 100 fs pulses. We suggest potential mechanisms to explain the observed nonlinear effects and describe future experiments to decouple high order multiphoton absorption, electron-hole pair generation and light-dopant interactions which might contribute to observed effects.
Hyper-Raman spectroscopy is a nonlinear optical variant of vibrational spectroscopy to acquire information about molecular structures. Hyper-Raman spectroscopy complements existing infrared and Raman spectroscopy techniques due to differences in the selection rules. Performing hyper-Raman spectroscopy at 532 nm results in the signal emission in the UV spectral range (266 nm – 296 nm), which benefits from near-resonant conditions for many biomolecules. Even operating in the electronic resonant enhancement regime, hyper-Raman spectroscopy requires high average and peak power, picosecond laser systems to achieve reasonable collection times (1 minute – 30 minutes). In this report, we explore applications of hyper-Raman spectroscopy to aromatic structures (L-phenylalanine and imidazole) that experience significant two-photon absorption and two-photon fluorescence which can obstruct measurements of the hyper-Raman spectra of these molecules. Since competing two-photon processes could significantly limit future UV hyper-Raman applications, we explore mitigating strategies to circumvent the fluorescence background of Lphenylalanine and imidazole by applying a quenching agent (hydrogen peroxide). We also outline a more general solution to alleviate two-photon absorption and fluorescence by proposing tailored laser configurations where the excitation wavelength could be tuned to avoid two-photon absorption resonances while remaining in the UV regime.
Hydrogen bonding plays an essential role in biological processes. In this report we apply hyper-Raman scattering spectroscopy to probe the effects of the alkyl groups on hydrogen bonding in mixtures of DMSO-methanol. We characterize the dependence of hyper-Raman spectra on concentration and observe suppression of the hyper-Raman responses of the methanol alkyl group at intermediate concentrations. In addition, small frequency shifts in the vibrational frequencies of DMSO and methanol were detected. These results provide new insights into the nature of the hydrogen bonding in solution and into the details of the hydrogen bond’s interaction with the alkyl groups.
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.
The optical activity of Raman scattering provides insight into the absolute configuration and conformation of chiral molecules. Applications of Raman optical activity (ROA) are limited by long integration times due to a relatively low sensitivity of the scattered light to chirality (typically 10-3 to 10-5). We apply ROA techniques to hyper-Raman scattering using incident circularly polarized light and a right-angle scattering geometry. We explore the sensitivity of hyper- Raman scattering to chirality as compared to spontaneous Raman optical activity. Using the excitation wavelength at around 532 nm, the photobleaching is minimized, while the hyper-Raman scattering benefits from the electronic resonant enhancement. For S/R-2-butanol and L/D-tartaric acid, we were unable to detect the hyper-Raman optical activity at the sensitivity level of 1%. We also explored parasitic thermal effects which can be mitigating by varying the repetition rate of the laser source used for excitation of hyper-Raman scattering.
SU-8 photoresist has been applied to three-dimensional (3D) patterning of photonics, micro/nanoelectromechanical systems and microfluidics. SU-8 photoresist can be patterned by absorption of ultraviolet radiation using a photomask; however, diffraction effects in the bulk resist and the use of a 2D mask limits complex 3D structure design. Direct laser writing (DLW) using multiphoton absorption can produce complex, sub-diffraction limited structures in the resist, but controlling DLW in SU-8 is complicated by many competing processes. Using 100 fs pulses at 1.7 μm, we reliably develop SU-8 photoresist via femtosecond optical curing on glass substrate and avoid competition from two, three, and four photon absorption processes. We verify optical curing of the resist by developing the resist without a post-bake.
Dimethyl sulfoxide (DMSO) is a biologically important solvent in part due to its dual miscibility with hydrophilic and hydrophobic molecules. Binary solutions of DMSO-water display non-ideal thermodynamics properties such as high viscosity and low freezing point due to hydrogen bonding. The unusual properties of DMSO-water solutions have been exploited to disrupt the formation of secondary structures of proteins during polymerase chain reaction assays and to act as a cryoprotectant for tissues. The exact coordination of the DMSO and water molecules remains unknown. Hyper- Raman scattering was employed for the first time to investigate binary systems of DMSO with water (H2O). As a part of this study, hyper-Raman and Raman spectra of pure solutions were first acquired and compared against existing Raman and IR spectroscopic data. Then the corresponding measurements were taken with deuterated DMSO-d6 and heavy water (D2O) to validate the analysis and to isolate overlapping spectral features. The permissive selection rules of hyper- Raman scattering provide new insight into disruptions of the self-hydrogen bonded networks of DMSO and water and the establishment of hydrogen bonded networks.
Raman spectroscopy is an essential optical tool for molecular fingerprints. The vibrational modes of biologically important molecules including proteins, nucleic acids and lipids have been studied to provide insight into their structure as well as insight into the metabolic processes and biomarker expression of cells. To explore hyper-Raman scattering as a complementary technique to Raman scattering, we build a laser system that can perform Raman and hyper-Raman scattering studies using a single setup. Using three amplification stages we are able to generate 8 ps, 1064 nm pulses at repetition rates up to 30 MHz. Converting the 1064 nm source laser to 532 nm, we achieve fast hyper-Raman detection and collect our spectrum with a commercial spectrometer and CCD. Using a single optical setup, we collect and compare Raman spectra at 532 nm to hyper-Raman spectra at 266 nm for water, ethanol and L-tartaric acid. Furthermore, we observe changes in the hyper-Raman peak intensities of an aqueous L-tartaric acid solution when selecting different laser repetition rates highlighting the need to control laser power and repetition rate to identify and mitigate thermal effects in biomolecules.
Understanding femtosecond pulse propagation in biological media has become increasingly relevant with the widespread use of femtosecond laser systems for imaging and diagnostic applications. Intense, femtosecond pulses are prone to undergo nonlinear effects in media. The ability to accurately simulate the nonlinear processes femtosecond pulses undergo in biological media is critical for designing new diagnostic techniques and determining maximum permissible exposure (MPE) limits for laser safety standards such as ANSI Z136.1. The combination of strong absorption, broad bandwidth, and dispersive effects makes standard nonlinear simulation methods based on the slowly varying envelope approximation unsuitable for the study of near-infrared (near-IR) pulses in water and biological tissues. Building off an existing linear ultrafast pulse propagation model we present preliminary work simulating supercontinuum broadening in water without using an envelope approximation. Using a one-dimensional simulation of self-phase modulation, we explain the infrared continuum broadening observed in a previous experiment in water using 35 fs near-IR pulses, but fail to explain the visible continuum, suggesting that the continuum is further broadened by self-focusing. We then extend the model to simulate the propagation of near-IR pulses in the human eye at the 100 fs ANSI MPE limit for pulse durations from 10 fs to 1 ps. Using this simulation, we explore the implications of supercontinuum generation on the ANSI MPE limits.
Understanding the optical properties of water is critical to both laser-tissue interactions as well as setting ocular laser safety standards. The nonlinear properties of water are responsible for supercontinuum generation; however, these effects are poorly understood for wavelengths longer than 1064 nm. A previous study suggested that the supercontinuum generation may convert retinal-safe femtosecond near-infrared pulses with wavelengths longer than 1064 nm into visible wavelength pulses that are above the maximum permissible exposure limit as defined by ANSI Z136.1-2014. To address this knowledge gap, we extend the Z-scan technique in distilled water to wavelengths between 1150 nm to 1400 nm, where linear absorption is strong. Utilizing wavelength tunable, nominally 100 fs laser pulses, we observe wavelength dependence of the nonlinear optical properties of water. The nonlinear refractive index at 1150 nm was consistent with measurements taken at 532 nm in previous studies, and was observed to increase at longer wavelengths. The nonlinear absorption was positive for wavelengths between 1150 nm and 1350 nm and reversed to saturable absorption at 1400 nm. Saturable absorption poses a previously unanticipated eye safety risk as current ocular laser safety standards assume strong absorption at 1400 nm. These results expand our current understanding of the nonlinear optical properties of water to wavelengths in the 1150 nm to 1400 nm region, and inform efforts to revise national and international exposure limits to account for retinal hazards due to nonlinear effects.
Filamentation in air is a profound effect caused by high energy photons. While it has been studied in a wide-range of laser systems, there still exist wavelength regimes where filamentation hasn’t been created, due to lack of sources. Using a tunable near-infrared femtosecond laser, we generated filamentation in air by wavelengths from 1.2 to 2.5 µm. The observed filaments produced harmonic and continuum generation well into the visible spectrum; a rainbow of colors.
Zinc Selenide (ZnSe) has long been recognized as a nonlinear optical material and is used in many optoelectronic devices such as light emitting diodes. ZnSe is known for its remarkably wide transmission range for infrared radiation leading to its use in infrared laser applications. In this report, we discuss higher order harmonic generation when exposing ZnSe to tunable femtosecond mid-IR laser pulses with wavelengths ranging from 2.7 μm to 8.0 μm and pulse energies between 3 and 17 μJ. Higher order harmonic generation was in some instances strong enough to be directly seen by the unaided eye. We also compare these results with measurements taken for other optical materials.
Calcium fluoride, BK7 and fused silica are common optical materials used in lenses and windows. In this report, we discuss supercontinuum generation using tunable femtosecond mid-IR laser pulses with wavelengths ranging from 2.7 μm to 7.0 μm and pulse energies between 3 and 18 microjoules. We observed harmonic generation in fused silica and BK7, but not supercontinuum generation. Other borosilicate targets generated supercontinuum in both visible and near infrared regions of the spectrum. The visible supercontinuum was, in some instances, strong enough to be observed directly by the human eye. These results contribute to ongoing work being done to refine eye safety standards for femtosecond lasers.
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