Direct mapping of bending and torsional dynamics in individual nanostructures.

Investigating coherent acoustic vibrations in nanostructured materials provides fundamental insights into optomechanical responses and microscopic energy flow. Extensive measurements of vibrational dynamics have been performed for a wide variety of nanoparticles and nanoparticle assemblies. However, virtually all of them show that only the dilation modes are launched after laser excitations, and the acoustic bending and torsional motions, which are commonly observed in photoexcited chemical bonds, are absent. Unambiguous identification and refined characterization of these "missing" modes have been a long-standing issue. In this report, we investigated the acoustic vibrational dynamics of individual Au nanoprisms on free-standing graphene substrates using an ultrafast high-sensitivity dark-field imaging approach in four-dimensional transmission electron microscopy. Following optical excitations, we observed low-frequency multiple-mode oscillations and higher superposition amplitudes at nanoprism corners and edges on the subnanoparticle level. In combination with finite-element simulations, we determined that these vibrational modes correspond to out-of-plane bending and torsional motions, superimposed by an overall tilting effect of the nanoprisms. The launch and relaxation processes of these modes are highly pertinent to substrate effects and nanoparticle geometries. These findings contribute to the fundamental understanding about acoustic dynamics of individual nanostructures and their interaction with substrates.

Differential fluorescence microscopy by using a dynamic cylindrical-vector field.

Fluorescence emission difference (FED) microscopy, as an emerging super-resolution imaging modality, uses double-exposure and subtraction between double-exposed fluorescence images to achieve high spatial resolution beyond the diffraction limit. Here we report on a new FED imaging approach with a single-exposure scheme based on dynamic cylindrical-vector fields, where the fluorescence excitation beam can be switched between radial and azimuthal polarization states at a designated high radio frequency. Lateral spatial resolution of ${\sim} \lambda/4$ is achieved. Being able to integrate with lock-in amplifier detection, the proposed method will find promising applications for high-speed fluorescence imaging with improved signal-to-noise ratio.

In vivo label-free two-photon excitation autofluorescence microscopy of microvasculature using a 520 nm femtosecond fiber laser.

Observing microvasculature in its native environment provides invaluable information to understand the initiation and development of microcirculatory related diseases. However, the lack of a high-resolution three-dimensional (3D) imaging technique hinders in vivo investigation of the microvasculature. Recently, we found that the red blood cells can emit autofluorescence signals with short-wavelength two-photon excitation. In this study, we exploited this property and developed a time-resolved two-photon excitation microscopy system using a homemade 520 nm femtosecond fiber laser as the excitation source. Using this system, we could achieve intravital high-resolution 3D imaging of a microvascular network noninvasively. In a mouse tumor model, tumorous blood vessels could be observed and distinguished clearly from the normal vessels.

Ultrafast Broadband Charge Collection from Clean Graphene/CH3NH3PbI3 Interface.

Photocarrier generation in a material, transportation to the material surface, and collection at the electrode interface are of paramount importance in any optoelectronic and photovoltaic device. In the last collection process, ideal performance comprises ultrafast charge collection to enhance current conversion efficiency and broadband collection to enhance energy conversion efficiency. Here, for the first time, we demonstrate ultrafast broadband charge collection achieved simultaneously at the clean graphene/organic-inorganic halide perovskite interface. The clean interface is realized by directly growing perovskite on graphene surface without polymer contamination. The tunable two-color pump-probe spectroscopy, time-resolved photoluminescence spectroscopy, and time-dependent density functional theory all reveal that the clean-interfacial graphene collects band-edge photocarriers of perovskite in an ultrashort time of ∼100 fs, with a current collection efficiency close to 99%. In addition, graphene can extract deep-band hot carriers of perovskite within only ∼50 fs, several orders faster than hot carrier relaxation and cooling in perovskite itself, due to the unique Dirac linear band structure of graphene, indicating a potential high energy conversion efficiency exceeding the Shockley-Queisser limit. Adding other graphene superiority of good transparency, high carrier mobility, and extreme flexibility, clean-interfacial graphene provides an ideal charge collection layer and electrode candidate for future optoelectronic and photovoltaic applications in two dimensions.

Time-of-flight detection of femtosecond laser pulses for precise measurement of large microelectronic step height.

We propose and demonstrate a new method which employs time-of-flight detection of femtosecond laser pulses for precise height measurement of large steps. By using time-of-flight detection with fiber-loop optical-microwave phase detectors, precise measurement of large step height is realized. The proposed method shows uncertainties of 15 nm and 6.5 nm at sampling periods of 40 ms and 800 ms, respectively. This method employs only one free-running femtosecond mode-locked laser and requires no scanning of laser repetition rate, making it easier to operate. Precise measurements of 6 µm and 0.5 mm step heights have been demonstrated, which show good functionality of this method for measurement of step heights.

Single nanoparticle detection using split-mode microcavity Raman lasers.

Ultrasensitive nanoparticle detection holds great potential for early-stage diagnosis of human diseases and for environmental monitoring. In this work, we report for the first time, to our knowledge, single nanoparticle detection by monitoring the beat frequency of split-mode Raman lasers in high-Q optical microcavities. We first demonstrate this method by controllably transferring single 50-nm-radius nanoparticles to and from the cavity surface using a fiber taper. We then realize real-time detection of single nanoparticles in an aqueous environment, with a record low detection limit of 20 nm in radius, without using additional techniques for laser noise suppression. Because Raman scattering occurs in most materials under practically any pump wavelength, this Raman laser-based sensing method not only removes the need for doping the microcavity with a gain medium but also loosens the requirement of specific wavelength bands for the pump lasers, thus representing a significant step toward practical microlaser sensors.

Optical digital coherent detection technology enabled flexible and ultra-fast quantitative phase imaging.

Quantitative phase imaging has been an important labeling-free microscopy modality for many biomedical and material science applications. In which, ultra-fast quantitative phase imaging is indispensable for dynamic or transient characteristics analysis. Conventional wide field optical interferometry is a common scheme for quantitative phase imaging, while its data acquisition rate is usually hindered by the frame rate of arrayed detector. By utilizing novel balanced-photo-detector based digital optics coherent detection techniques, we report on a method of constructing ultra-fast quantitative phase microscopy at the line-scan rate of 100 MHz with ~2 µm spatial resolution.

Compressive coherent anti-Stokes Raman scattering holography.

Coherent anti-Stokes Raman scattering (CARS) holography captures both the amplitude and the phase of the anti-Stokes field generated from a sample and can thus perform single-shot, chemically selective three-dimensional imaging. We present compressive CARS holography, a numerical technique based on the concept of compressive sensing, to improve the quality of reconstructed images by leveraging sparsity in the source distribution and reducing the out-of-focus background noise. In particular, we use the two-step iterative shrinkage threshold (TwIST) algorithm with an l1 norm regularizer to iteratively retrieve images from an off axis CARS digital hologram. It is shown that the use of compressive CARS holography enhances the CARS holographic imaging technique by reducing noise and thereby effectively emulating a higher axial resolution using only a single shot hologram.

Background-free 3D chemical imaging based on polarization coherent Raman holography.

We report on a holographic coherent anti-Stokes Raman scattering imaging by using polarization discrimination for nonresonant background suppression. With reduced polarization scrambling effect under weakly focused excitation, nonresonant background-free coherent Raman imaging is demonstrated. A fast chemically selective imaging is achieved in a three-dimensional volume of 70 µm ×70 µm ×100 µm in 2 s.

Feed-forward digital phase compensation for long-distance precise frequency dissemination via fiber network.

We demonstrate precise microwave frequency dissemination of a hydrogen maser synchronized frequency comb over a 120 km commercial fiber link. The phase noise was compensated by a feed-forward digital compensation scheme, where the value of locally detected phase noise was first transmitted to the remote user end via a wavelength division multiplexing channel in the same fiber link and then compensated directly at the user end. The fractional frequency instability was measured to be at 5.28×10(-16)/s.

Bessel Beam Coherent Anti-Stokes Raman Scattering Spectroscopy for Turbulent Flow Diagnosis.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy plays an important role in chemical analysis for transient flow dynamics. Due to the turbulent ambient conditions, the CARS spectrum often suffers from a poor signal-to-noise ratio (SNR) and cannot provide a convincing measurement. Here, we report on a CARS spectroscopic method using a Bessel beam to enhance the spectral fidelity and SNR in a quasi-turbulent environment. Compared with traditional CARS, the measurement accuracy is significantly improved by taking advantage of the anti-scattering and self-healing characteristics of the Bessel beam. Our preliminary results indicate that Bessel beam CARS could be a promising method for high precision turbulent flow measurement fields.

Short-wavelength excitation two-photon intravital microscopy of endogenous fluorophores.

The noninvasive two-photon excitation autofluorescence imaging of cellular and subcellular structure and dynamics in live tissue could provide critical in vivo information for biomedical studies. However, the two-photon microscopy of short-wavelength endogenous fluorophores, such as tryptophan and hemoglobin, is extremely limited due to the lack of suitable imaging techniques. In this study, we developed a short-wavelength excitation time- and spectrum-resolved two-photon microscopy system. A 520-nm femtosecond fiber laser was used as the excitation source, and a time-correlated single-photon counting module connected with a spectrograph was used to provide time- and spectrum-resolved detection capability. The system was specially designed for measuring ultraviolet and violet-blue fluorescence signals and thus was very suitable for imaging short-wavelength endogenous fluorophores. Using the system, we systematically compared the fluorescence spectra and fluorescence lifetimes of short-wavelength endogenous fluorophores, including the fluorescent molecules tyrosine, tryptophan, serotonin (5-HT), niacin (vitamin B3), pyridoxine (vitamin B6), and NADH and the protein group (keratin, elastin, and hemoglobin). Then, high-resolution three-dimensional (3D) label-free imaging of different biological tissues, including rat esophageal tissue, rat oral cheek tissue, and mouse ear skin, was performed in vivo or ex vivo. Finally, we conducted time-lapse imaging of leukocyte migration in the lipopolysaccharide injection immunization model and a mechanical trauma immunization model. The results indicate that the system can specifically characterize short-wavelength endogenous fluorophores and provide noninvasive label-free 3D visualization of fine structures and dynamics in biological systems. The microscopy system developed here can empower more flexible imaging of endogenous fluorophores and provide a novel method for the 3D monitoring of biological events in their native environment.

New soliton dynamics revealed in the normal dispersion region.

Birefringence-involved phase matching is demonstrated to be a novel mechanism to generate transform limited solitary pulses in an ultrafast mode-locking fiber laser cavity with normal dispersion.

A protocol for single-source dual-pulse stimulated emission depletion setup with Bessel modulation.

STimulated Emission Depletion (STED) microscopy attains super-resolution in biological imaging beyond the diffraction limit. Here, we give a concise protocol to construct a dual-pulse STED setup with one super-continuum laser. Moreover, a flexible and dismountable Bessel modulation module is introduced for potential 2D-stack STED imaging. Experiments and notices are introduced in detail, with discussion on some important check-points for STED, such as detector saturation. Finally, the results validate the system working.

Single-Lens Light-Sheet Fluorescence Microscopy Based on Micro-Mirror Array.

Conventional light sheet fluorescence microscopy (LSFM) utilizes two perpendicularly arranged objective lenses for optical excitation and detection, respectively. Such a configuration often limits the use of high-numerical-aperture (NA) objectives or requires specially designed long-working-distance objectives. Here, a LSFM based on a micro-mirror array (MMA) to enable light sheet imaging with a single objective lens is reported. The planar fluorescent emission excited by the light sheet illumination is collected by the same objective, relayed onto an MMA and detected by a side-view camera. The proposed scheme makes LSFM compatible to single objective imaging system and shows promising candidacy for high spatiotemporal imaging.

Proposal and demonstration of a spectrometer using a diffractive optical element with dual dispersion and focusing functionality.

We propose an optical spectrometer using a hybrid grating-Fresnel (G-Fresnel) diffractive optical element. Theoretical simulation shows that a spectral resolution of approximately 1 nm can be potentially achieved with a millimeter-sized G-Fresnel. A proof-of-concept G-Fresnel-based spectrometer with subnanometer spectral resolution is experimentally demonstrated. The proposed method provides a promising new way for realizing compact optical spectrometers.

Demonstration of a PDMS based hybrid grating and Fresnel lens (G-Fresnel) device.

A hybrid device that we term G-Fresnel (i.e., grating and Fresnel) is demonstrated. It fuses the functions of a grating and a Fresnel lens into a single device. We have fabricated the G-Fresnel device by using polydimethylsiloxane (PDMS) based soft lithography. Three-dimensional surface profilometry has been performed to examine the device quality. We have also conducted optical characterizations to confirm its dual focusing and dispersing properties. The G-Fresnel can be useful for the development of miniature optical spectrometers as well as emerging optofluidic applications.

Chromatic second harmonic imaging.

We report a non-axial-scanning second harmonic imaging technique, in which the chromatic aberration of a Fresnel lens is exploited to focus different wavelengths of a fundamental beam into different axial positions to effectively realize axial scanning. Since the second harmonic signals at different axial positions are generated by different fundamental wavelengths and hence accordingly have different center wavelengths, they can be resolved and detected in parallel by using a spectrometer without axial mechanical scanning. We have demonstrated a system capable of achieving about 8 µm effective axial scanning range. Proof-of-concept imaging results are also presented.

Supercontinuum trap stiffness measurement using a confocal approach.

We report a novel method for characterizing the stiffness of white light supercontinuum tweezers, in which the nonlinear photonic crystal fiber used for supercontinuum generation is also utilized as an effective confocal pinhole to track the motion of a trapped bead and as a scan head to realize rapid scanning of the optical trap. By measuring the phase of the bead's motion in following the trap, a lateral stiffness value of about 7.9 µN/m was obtained with supercontinumm power of about 75 mW. Our technique can potentially allow for trap stiffness calibration along an arbitrary direction in three dimensions.

Inline holographic coherent anti-Stokes Raman microscopy.

We demonstrate a simple approach for inline holographic coherent anti-Stokes Raman scattering (CARS) microscopy, in which a layer of uniform nonlinear medium is placed in front of a specimen to be imaged. The reference wave created by four-wave mixing in the nonlinear medium can interfere with the CARS signal generated in the specimen to result in an inline hologram. We experimentally and theoretically investigate the inline CARS holography and show that it has chemical selectivity and can allow for three-dimensional imaging.