All-fiber single-cavity dual-comb for coherent anti-Stokes Raman scattering spectroscopy based on spectral focusing.

We report an all-fiber free-running bidirectional dual-comb laser system for coherent anti-Stokes Raman scattering spectroscopy based on spectral focusing. The mode-locked oscillator is a bidirectional ring-cavity erbium fiber laser running at a repetition rate of ∼114MHz. One output of the bidirectional laser is wavelength-shifted from 1560 to 1060 nm via supercontinuum generation for use as the pump source. We have been able to record the Raman spectra of various samples such as polystyrene, olive oil, polymethyl methacrylate (PMMA), and polyethylene in the C-H stretching window. We believe that this all-fiber laser design has promising potential for coherent Raman spectroscopy and also label-free imaging for a variety of practical applications.

Feasibility of multimodal multiphoton microscopy to facilitate surgical margin assessment in pancreatic cancer.

Pancreatic cancer is a common cancer with poor odds of survival for the patient, with surgical resection offering the only hope of cure. Current surgical practice is time-consuming and, due to time constraints, does not sample the whole cut surface sufficiently to check for remaining cancer. Although microscopy with hematoxylin and eosin (H&E) stain is the gold standard for microscopic evaluation, multiphoton microscopy (MPM) has emerged as an alternative tool for imaging tissue architecture and cellular morphology without labels. We explored the use of multimodal MPM for the label-free identification of normal and cancerous tissue of the pancreas in a mouse model by comparing the images to H&E microscopy. Our early studies indicate that MPM using second-harmonic generation, third-harmonic generation, and multiphoton excitation of endogenous fluorescent proteins can each contribute to the label-free analysis of the pancreatic surgical margin.

Characterization of multiphoton microscopes by the nonlinear knife-edge technique.

Imaging submicron fluorescent microspheres are the standard method for measuring resolution in multiphoton microscopy. However, when using high-energy pulsed lasers, photobleaching and heating of the solution medium may deteriorate the images, resulting in an inaccurate resolution measurement. Moreover, due to the weak higher-order response of fluorescent microspheres, measuring three-photon resolution using three-photon fluorescence (3PEF) and third-harmonic generation (THG) signals is more difficult. In this report, we demonstrate a methodology for complete characterization of multiphoton microscopes based on second- and third-harmonic generation signals from the sharp edge of GaAs wafers. This simple methodology, which we call the nonlinear knife-edge technique, provides fast and consistent lateral and axial resolution measurement with negligible photobleaching effect on semiconductor wafers. In addition, this technique provides information on the field curvature of the imaging system, and perhaps other distortions of the imaging system, adding greater capability compared to existing techniques.

All-fiber high-power 1700 nm femtosecond laser based on optical parametric chirped-pulse amplification.

We present the design and construction of an all-fiber high-power optical parametric chirped-pulse amplifier working at 1700 nm, an important wavelength for bio-photonics and medical treatments. The laser delivers 1.42 W of output average power at 1700 nm, which corresponds to ∼40 nJ pulse energy. The pulse can be de-chirped with a conventional grating pair compressor to ∼450 fs. Furthermore, the laser has a stable performance with relative intensity noise typically below the -130 dBc/Hz level for the idler pulses at 1700 nm from 10kHz to 16.95 MHz, half of the laser repetition rate f/2.

Watt-level all-fiber optical parametric chirped-pulse amplifier working at 1300 nm.

We report watt-level average output power near 1300 nm from an all-fiber ultrafast optical parametric chirped-pulse amplifier. A compressed output pulse duration of ∼300 fs is achieved. Multiphoton imaging of a variety of samples carried out with this light source shows a good signal-to-noise ratio. With the demonstrated imaging capability, we believe that this high-power ultrafast laser source addresses a key need in deep tissue multiphoton microscopy.

Multiphoton Microscopy of π-Conjugated Copolymers and Copolymer/Fullerene Blends for Organic Photovoltaic Applications.

Organic photovoltaic (OPV) cells based on π-conjugated copolymer/fullerene blends are devices with the highest power conversion efficiencies within the class of organic semiconductors. Although a number of image microscopies have been applied to films of π-conjugated copolymers and their fullerene blends, seldom have they been able to detect microscopic defects in the blend films. We have applied multiphoton microscopy (MPM) using a 65 fs laser at 1.56 µm for spectroscopy and mapping of films of various π-conjugated copolymers and their fullerene blends. All pristine copolymer films have shown third harmonic generation (THG) and two-photon or three-photon photoluminescence that could be used for mapping the films with micrometer spatial resolution. Since the fullerenes have much weaker THG efficiency than those of the copolymers, we could readily map the copolymer/fullerene blend films that showed interpenetrating micron-sized grains of the two constituents. In addition, we also found second harmonic generation from various micron-sized defects in the films that are formed during film deposition or light illumination at ambient conditions, which do not possess inversion symmetry. The MPM method is therefore beneficial for organic films and devices for investigating the properties and growth of copolymer/fullerene blends for OPV applications.

Pancreatic cancer cell detection by targeted lipid microbubbles and multiphoton imaging.

Surgical resection of pancreatic cancer represents the only chance of cure and long-term survival in this common disease. Unfortunately, determination of a cancer-free margin at surgery is based on one or two tiny frozen section biopsies, which is far from ideal. Not surprisingly, cancer is usually left behind and is responsible for metastatic disease. We demonstrate a method of receptor-targeted imaging using peptide ligands, lipid microbubbles, and multiphoton microscopy that could lead to a fast and accurate way of examining the entire cut surface during surgery. Using a plectin-targeted microbubble, we performed a blinded in-vitro study to demonstrate avid binding of targeted microbubbles to pancreatic cancer cells but not noncancerous cell lines. Further work should lead to a much-needed point-of-care diagnostic test for determining clean margins in oncologic surgery.

All-reflective multiphoton microscope.

We present the design, construction, and characterization of a multiphoton microscope that uses reflective elements for beam shaping and steering. This compact all reflective design removes the adverse effects of dispersion on laser pulse broadening as well as chromatic aberration in the focusing of broadband and multicolored laser sources. The design of this system is discussed in detail, including aberrations analysis via ray-tracing simulation and opto-mechanical design. The resolution of this mirror based all-reflective microscope is characterized using fluorescent microbeads. The performance of the system at multiple wavelengths is investigated along with some potential multiphoton imaging and writing applications.