Pump-probe microscopy is an emerging nonlinear imaging technique based on high repetition rate lasers and fast intensity modulation. Here we present new methods for pump-probe microscopy that keep the beam intensity constant and instead modulate the inter-pulse time delay, the relative polarization, or the pulse length. These techniques can improve image quality for samples that have poor heat dissipation or long-lived radiative states, and can selectively address nonlinear interactions in the sample. We experimentally demonstrate this approach and point out the advantages over conventional intensity modulation.
We present a new imaging method for pump-probe microscopy that explores non-collinear excitation. This method (crossed-beam pump-probe microscopy, or CBPM) can significantly improve the axial resolution when imaging through low-NA lenses, providing an alternative way for depth resolved, large field-of-view imaging. We performed a proof-of-concept demonstration, characterized CBPM's resolution using different imaging lenses, and measured an enhanced axial resolution for certain types of low-NA lenses.
A pump-probe dark-field X-ray microscopy (DFXM) experiment was carried out at the Advanced Photon Source (APS) with nanosecond drive laser pulses and hybrid mode X-ray probe pulses. We observe a thermal decay due to laser-induced heat diffusion in a Germanium single crystal which matches the theoretical prediction. The single pulse DFXM imaging in combination with the laser-pump X-ray probe method could reveal thermal strain formation and propagation of the acoustic wave "on-the-fly" generated by a laser-induced lattice deformation. This will open a new avenue of materials research on laser-in…
We show that the emission of broad area semiconductor amplifiers and lasers can be efficiently stabilized by introducing, two-dimensional periodic modulations simultaneously on both the refractive index and the pump (gain-loss) profiles, in the transverse and longitudinal directions. The interplay between such index and gain-loss modulations efficiently suppresses the pattern forming instabilities, leading to highly stable and bright narrow output beams from such sources. We also determine the stabilization performance of the device as a function of pump current and linewidth enhancement facto…
Pump-probe microscopy of melanin in tumors has been proposed to improve diagnosis of malignant melanoma, based on the hypothesis that aggressive cancers disaggregate melanin structure. However, measured signals of melanin are complex superpositions of multiple nonlinear processes, which makes interpretation challenging. Polarization control during measurement and data fitting is used to decompose signals of melanin into their underlying molecular mechanisms. We then identify the molecular mechanisms that are most susceptible to melanin disaggregation and derive false-coloring schemes to highli…
The long-term lasing dynamics of Brillouin Fiber Lasers was recently shown to be governed by an intrinsic thermal feedback which stabilizes the lasing frequency at the lower half of the gain line. We show that this feedback can be utilized by means of pump modulation to greatly control this dynamics. Specifically, we demonstrate experimentally a bi-stable fiber laser where switching between lasing modes is possible by either controlling the cavity length or pump power. An inherent hysteretic response ensures the long-term stability of the laser in each mode. Excellent agreement is found betwee…
We have performed polarization spectroscopy and sub-Doppler DAVLL on the (5s5s) 1^S_0 -> (5s5p) 1^P_1 transition of atomic strontium. Both techniques generated a dispersion-type lineshape suitable for laser stabilization, without the need for frequency modulation. In both cases the signal is generated primarily by saturation effects, rather than optical pumping. The dependence of the amplitude and gradient on intensity and magnetic field were also investigated.
We investigate the large phase shifts of the bi-color spatial soliton pair in a cylindrical lead glass rod. The theoretical study suggests a synchronous propagation of a strong pump beam and a weak signal beam under the required initial condition. We experimentally obtain a $π$ phase shift of the signal beam by changing the power of the pump beam by about 14 mW around the soliton critical power, which agrees qualitatively with our theoretical result. The ratio of the phase shift rate of the signal soliton to that of the pump soliton shows a close match to the theoretical estimation.
Here we report nonlinear focal modulation microscopy (NFOMM) to achieve super-resolution imaging. Abandoning the previous persistence on minimizing the size of Gaussian emission pattern by directly narrowing (e.g. Minimizing the detection pinhole in Airyscan, Zeiss) or by indirectly peeling its outer profiles (e.g., Depleting the outer emission region in STED, stimulated emission microscopy) in pointwise scanning scenarios, we stick to a more general basis------ maximizing the system frequency shifting ability. In NFOMM, we implement a nonlinear focal modulation by applying phase modulations w…
We studied generation and dynamics of solitonic pulses, platicons, at normal group velocity dispersion due to the pump amplitude modulation. We proposed, that if the required frequency of amplitude modulation is too large for available modulators, it is possible to use subharmonic phase modulation for platicon generation. It was also demonstrated that it is possible to control the repetition rate of platicons by tuning modulation frequency. Tuning range for platicons was found to be much wider than for bright solitons. We also studied the influence of the high-order dispersion on platicon prop…
We demonstrate that flat-topped dissipative solitonic pulses, platicons, and corresponding frequency combs can be excited in optical microresonators with normal group velocity dispersion using either amplitude modulation of the pump or bichromatic pump. Soft excitation may occur in particular frequency range if modulation depth is large enough and modulation frequency is close to the free spectral range of the microresonator.
Spatial modulation microscopy is a technique originally developed for quantitative spectroscopy of individual nano-objects. Here, a parallel implementation of the spatial modulation microscopy technique is demonstrated based on a line detector capable of demodulation at kHz frequencies. The capabilities of the imaging system are shown using an array of plasmonic nanoantennas and dendritic cells incubated with gold nanoparticles.
We present a 100 MHz repetition rate Yb-fiber-laser-pumped synchronously pumped optical parametric oscillator (SPOPO) delivering tunable frequency combs covering almost the entire visible spectral range. By intracavity doubling both the signal and idler combs and using collinear residual pump light, nearly continuous tuning over the range of 420-700 nm is achieved with only small gaps near the OPO degeneracy condition. Output powers range from 10 mW to 200 mW, depending on wavelength, with pulse durations below 150 fs without external compression. Frequency locking of all three collinearly out…
We vary the time-frequency mode structure of ultrafast pulse-pumped modulational instability (MI) twin beams in an argon-filled hollow-core kagomé-style PCF by adjusting the pressure, pump pulse chirp, fiber length and parametric gain. Compared to solid-core systems, the pressure dependent dispersion landscape brings increased flexibility to the tailoring of frequency correlations and we demonstrate that the pump pulse chirp can be used to tune the joint spectrum of femtosecond-pumped sources. We also characterize the resulting mode content, not only by measuring the multimode second-order cor…
In this paper we describe the technical implementation of pump-probe cathodoluminescence microscopy (PP-CL), a novel technique for studying ultrafast dynamics in materials using combined excitation of electron and laser pulses. The PP-CL setup is based on a commercial scanning electron microscope combined with a femtosecond laser. Here we discuss the design, technical details, and characterization of the PP-CL setup. The luminescence emitted by the sample (either photoluminescence, cathodoluminescence or a combination of both) can be analyzed in terms of spectrum, temporal statistics, and freq…
We investigate multi-wave mixing associated with the strongly pump depleted regime of induced modulation instability (MI) in optical fibers. For a complete transfer of pump power into the sideband modes, we theoretically and experimentally demonstrate that it is necessary to use a much lower seeding modulation frequency than the peak MI gain value. Our analysis shows that a record 95 % of the input pump power is frequency converted into the comb of sidebands, in good quantitative agreement with analytical predictions based on the simplest exact breather solution of the nonlinear Schrödinger eq…
Transient Dynamics and Duty-Cycle Optimization in a Pulse-Wwidth Modulation Bell--Bloom Pumping
School of Physics, Hefei University of Technology（合肥工业大学物理学院）；Key Laboratory of Opto-Electronic Information Acquisition and Manipulation (Anhui University), Ministry of Education（教育部光电信息获取与控制重点实验室（安徽大学））
We experimentally demonstrate a negative Kerr nonlinearity for quasi-undoped graphene. Hereto, we introduce the method of chirped-pulse-pumped self-phase modulation and apply it to graphene-covered silicon waveguides at telecom wavelengths. The extracted Kerr-nonlinear index for graphene equals n2,gr = -10^(-13) m^2/W. Whereas the sign of n2,gr turns out to be negative in contrast to what has been assumed so far, its magnitude is in correspondence with that observed in earlier experiments. Graphene's negative Kerr nonlinearity strongly impacts how graphene should be exploited for enhancing the…
Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but li…
We experimentally show that the self-phase modulation of picosecond pump pulses, induced by both the optical Kerr effect and free-carrier refraction, has a detrimental effect on the maximum on-off Raman gain achievable in silicon on insulator nanowaveguides, causing it to saturate. A simple calculation of the Raman gain coefficient from the measured broadened output pump spectra perfectly matches the saturated behavior of the amplified Raman signal observed experimentally at different input pump powers.
We show that the modulation of the phases of the laser beams of ultra-short pulses leads to modulation of the two photon fluorescence intensity. The phase modulation technique when used in multi-photon microscopy can improve the signal to noise ratio. The technique can also be used in multiplexing the signals in the frequency domain in multi-focal raster scanning microscopy. As the technique avoids the use of array detectors as well as elaborate spatiotemporal multiplexing schemes it provides a convenient means to multi-focal scanning in axial direction. We show examples of such uses. Similar …
In this paper we propose a dark-state-based trapping strategy to break the optical diffraction limit for microscopy. We utilize a spatially dependent coupling field and a probe laser field with temporal and spatial modulation to interact with three-level atoms. The temporal modulation allows us to reduce the full width at half maximum (FWHM) of point spread function, and the spatial modulation help us obtain better spatial resolution than Gaussian beam. In addition, we also propose a proof-of-principle experiment protocol and discuss its feasibility.
We investigate the interaction of two pulses (pump and probe) scattered by a nonlinear epsilon-near-zero (ENZ) slab whose thickness is comparable with the ENZ wavelength. We show that when the probe has a narrow spectrum localized around the ENZ wavelength its transmission is dramatically affected by the intensity of the pump. Conversely, if the probe is not in the ENZ regime, its propagation is not noticeably affected by the pump. Such all-optical modulation is due to the oversensitive character of the ENZ regime and it is so efficient to even occur in a wavelength thick slab.
Spatiotemporal dynamics of excitons in isolated semiconducting single-walled carbon nanotubes are studied using transient absorption microscopy. Differential reflection and transmission of an 810-nm probe pulse after excitation by a 750-nm pump pulse are measured. We observe a bi-exponentially decaying signal with a fast time constant of 0.66 ps and a slower time constant of 2.8 ps. Both constants are independent of the pump fluence. By spatially and temporally resolving the differential reflection, we are able to observe a diffusion of excitons, and measure a diffusion coefficient of 200 cm2/…
The ultrafast response of metals to light is governed by intriguing non-equilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them gives rise to nonlinear diffusion behavior on ultrashort timescales. Here, we use scanning ultrafast thermo-modulation microscopy to image the spatio-temporal hot-electron diffusion in a thin gold film. By tracking local transient reflectivity with 20 nm and 0.25 ps resolution, we reveal two distinct diffusion regimes, consisting of an initial rapid diffusion during the first few picoseconds after optical excitation, fo…
We report the suppression of intermodal modulation instability peaks as a consequence of Kerr induced self beam-cleaning in a 90m long graded-index multimode optical fiber under various specific launching conditions. The output spectrum and the modal beam profiles for three modes (LP01, LP11, and LP21) have been recorded for several values of pump pulse energy. Experimental findings establish that the nonlinear coupling among the guided modes leads to reshaping the output speckle pattern into a bell shape or higher-order spatially clean beam profile as the pump peak power increases, whereas in…
We derive analytical expressions for the coherence in the onset of modulation instability, in excellent agreement with thorough numerical simulations. As usual, we start by a linear perturbation analysis, where broadband noise is added to a continuous wave (CW) pump; then, we investigate the effect of adding a deterministic seed to the CW pump, a case of singular interest as it is commonly encountered in parametric amplification schemes. Results for the dependence of coherence on parameters such as fiber type, pump power, propagated distance, seed signal-to-noise ratio are presented. Finally, …
Analytical formulation of sub-cycle modulation (SCM) of dielectrics including electron excitation is presented. The SCM is sensitive to not only the time-resolved dynamical Franz-Keldysh effect (Tr-DFKE) [T. Otobe, et al., Phys. Rev. B 93, 045124 (2016)], which is the nonlinear response without the electron excitation, but also the excited electrons. The excited electrons enhance the modulation with even-harmonics of pump laser frequency, and generate the odd-harmonics components. The new aspect of SCM is a consequence of i) the interference between the electrons excited by the pump laser and …
Continuously pumped passive nonlinear cavities can be harnessed for the creation of novel optical frequency combs. While most research has focused on third-order "Kerr" nonlinear interactions, recent studies have shown that frequency comb formation can also occur via second-order nonlinear effects. Here, we report on the formation of quadratic combs in optical parametric oscillator (OPO) configurations. Specifically, we demonstrate that optical frequency combs can be generated in the parametric region around half of the pump frequency in a continuously-driven OPO. We also model the OPO dynamic…
Modulating the electric potential on a conducting electrode is presented to generate an optical contrast for scattering microscopy that is sensitive to both surface charge and local topography. We dub this method Electric-Double-Layer-Modulation microscopy. We numerically compute the change in the local ion concentration that is the origin of this optical contrast for three experimentally relevant geometries: nanosphere, nanowire, and nanohole. In absence of plasmonic effects and physical absorption, the observable optical contrast is proportional to the derivative of the ion concentration wit…
A novel polarization-modulation transient method has been developed for studying fast anisotropic relaxation in electronic excited states of polyatomic and biologically relevant molecules under excitation with femtosecond laser pulses. The method is based on the modulation of pump beam polarization with a photo-elastic modulator and detection of an anisotropic contribution to the transient signal by a highly-sensitive demodulation balanced scheme. The method was tested on aqueous solution of coenzyme NADH (nicotin-amideadenine-dinucleotide) pumped at 360 nm and probed at 720 nm. Anisotropic vi…
The electric double layer (EDL) formed around charged nanostructures at the liquid-solid interface determines their electrochemical activity and influences their electrical and optical polarizability. We experimentally demonstrate that restructuring of the EDL at the nanoscale can be detected by dark-field scattering microscopy. Temporal and spatial characterization of the scattering signal demonstrates that the potentiodynamic optical contrast is proportional to the accumulated charge of polarisable ions at the interface and its time derivative represents the nanoscale ionic current. The mate…
We report an angle-tilted, wavelength-multiplexed ptychographic modulation approach for multispectral lensless on-chip microscopy. In this approach, we illuminate the specimen with lights at 5 wavelengths simultaneously. A prism is added at the illumination path for spectral dispersion. Lightwaves at different wavelengths, thus, hit the specimen at slightly different incident angles, breaking the ambiguities in mixed state ptychographic reconstruction. At the detection path, we place a thin diffuser in-between the specimen and the monochromatic image sensor for encoding the spectral informatio…
Nanolasers are considered ideal candidates for communications and data processing at chip-level thanks to their extremely reduced footprint, low thermal load and potentially outstanding modulation bandwidth, which in some case has been numerically estimated to exceed hundreds of GHz. The few experimental implementations reported to date, however, have so-far fallen very short of such predictions, whether because of technical difficulties or of overoptimistic numerical results. We propose a methodology to study the physical characteristics which determine the system's robustness and apply it to…
A simple and flexible technique for achieving quasi-phase-matching in integrated photonic waveguides without periodic poling is proposed and experimentally demonstrated, referred to as mode-shape-modulation (MSM). It employs a periodic variation of waveguide width to modulate the intensity of the pump wave, effectively suppressing out-of-phase light generation. This technique is applied to the case of second-harmonic generation in thin-film lithium niobate ridge waveguides. MSM waveguides are fabricated and characterized with pulsed-pumping in the near-infrared, showing harmonic generation at …
Chaos in semiconductor lasers or other optical systems has been intensively studied in the past two decades. However, modulation around threshold has received much less attention, in particular in gain-modulated semiconductor lasers. In this article, we investigate the bifurcation sequence which appears with pump modulation in the threshold region with a large amplitude and different values of modulation frequency. Modulation around threshold necessarily includes ``below-threshold'' dynamics, which can be effectively displayed only through through a nonlinear visualization of the oscillations.…
We present a simple method and its experimental implementation to completely determine the characteristics of the pump-and-probe pulse and the Stokes pulse in a coherent anti-Stokes Raman scattering microscope at sample level without additional autocorrelators. Our approach exploits the delay line, ubiquitous in such microscopes, to perform a convolution of the pump-and-probe and Stokes pulses as a function of their relative delay and it is based on the detection of the photons emitted from an appropriate non-linear sample. The analysis of the non-resonant four-wave-mixing and sum-frequency-ge…
It has been established for some time that high-power pump can trap a probe beam of lower intensity that is simultaneously propagating in a Kerr-type optical medium, inducing a focusing of the probe with the emergence of modes displaying solitonic properties. To understand the mechanism by which such self-sustained modes are generated, and mainly the changes on probe spectrum induced by the cross-phase-modulation effect for an harmonic probe trapped by a multiplex of temporal pulses, a linear equation (for the probe) and a nonlinear Schrödinger equation (for the pump) both coupled by a cross-p…
A scheme is proposed to achieve substantial controllable phase modulation for a probe field propagating through a thermal atomic vapor in double-$Λ$ configuration. The phase modulation is based on the linear susceptibility of the probe field, paraxial diffraction is eliminated by exploiting the thermal motion of atoms, and residual absorption is compensated via an incoherent pump field. As a result, a strong controllable uniform phase modulation without paraxial diffraction is achieved essentially independent of the spatial profile or the intensity of the probe field. This phase shift can be c…
We report a new coherent imaging technique, termed ptychographic structured modulation (PSM), for quantitative super-resolution microscopy. In this technique, we place a thin diffuser (i.e., a scattering lens) in between the sample and the objective lens to modulate the complex light waves from the object. The otherwise inaccessible high-resolution object information can thus be encoded into the captured images. We then employ a ptychographic phase retrieval process to jointly recover the exit wavefront of the complex object and the unknown diffuser profile. Unlike the illumination-based super…
In wide-field nonlinear microscopy, wavefront modulation by means of phase-only spatial light modulators (SLMs) allows achieving simultaneous two-photon excitation and fluorescence emission from specific region-of-interests (ROIs) of biological specimens. This is basically accomplished at the illumination path of the microscope by the reconstruction of computer generated holograms (CGHs) onto the sample plane. However, as two-photon absorption (TPA) is inherently an intensity-square dependent process and iterative Fourier transform algorithms (IFTAs) can only approximate the illumination of se…
We observe competition between the modulation instability (MI) and stimulated Brillouin scattering (SBS) in a 9.2-GHz broadband SBS slow light device, in which a standard 20-km-long single-mode LEAF fibre is used as the SBS medium. We find that MI is dominant and depletes most of the pump power when we use an intense pump beam at ~1.55 μm, where the LEAF fibre is anomalously dispersive. The dominance of the MI in the LEAF-fibre-based system suppresses the SBS gain, degrading the SBS slow light delay and limiting the SBS gain-bandwidth to 126 dB \cdot GHz. In a dispersion-shifted highly nonline…
We present an experimental study of the effects of temporal modulation of the pump intensity on a random laser. The nanosecond pump pulses exhibit rapid intensity fluctuations which differ from pulse to pulse. Specific temporal profiles of the pump pulses produce extraordinarily strong emission from the random laser. This process is deterministic and insensitive to the spatial configuration of the scatterers and spontaneous emission noise.
We report a novel lensless on-chip microscopy platform based on near-field blind ptychographic modulation. In this platform, we place a thin diffuser in between the object and the image sensor for light wave modulation. By blindly scanning the unknown diffuser to different x-y positions, we acquire a sequence of modulated intensity images for quantitative object recovery. Different from previous ptychographic implementations, we employ a unit magnification configuration with a Fresnel number of ~50,000, which is orders of magnitude higher than previous ptychographic setups. The unit magnificat…
An experiment demonstrating single-pixel single-arm complementary compressive microscopic ghost imaging based on a digital micromirror device (DMD) has been performed. To solve the difficulty of projecting speckles or modulated light patterns onto tiny biological objects, we instead focus the microscopic image onto the DMD. With this system, we have successfully obtained a magnified image of micron-sized objects illuminated by the microscope's own incandescent lamp. The image quality of our scheme is more than an order of magnitude better than that obtained by conventional compressed sensing w…
We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 μm were used as high quality factor (Q>10^6) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The…
The recent work by Chmyrov et al. (Nature Methods 2013) presents a capstone for the current major super-resolution microscopy techniques. In optical super-resolution microscopy, two pathways are commonly taken: targeted illumination modulation, or stochastic single-molecule localization. Incoherent cross-standing microscopy has utilized the concept of structured illumination in generating patterns, to generate effective 100,000 "doughnuts" as with STED; and used the photo-switchable dye to decrease the requirement of modulation intensity. It has combined the key elements of all these major sup…
