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Femtosecond pulsed lasers are widely used in applied technologies, such as laser processing, nonlinear optical microscopy, and time-of-flight measurement, wherein narrowing the pulse duration is particularly important for improving the processing efficiency and signal-to-noise ratio of measurement systems. This paper proposes a method for shortening temporal pulse waveforms using spectral intensity filtering. Symmetrical spectral filtering was specifically applied at the central wavelength of the femtosecond pulsed laser spectrum. To investigate the pulse-narrowing effect, we numerically simulated the pulse duration as a function of the filtering width and cut-off wavelength position. The results of these simulations showed that the pulse duration decreased as the filter wavelength approached the central wavelength of the light. Furthermore, increasing the filter width reduced the pulse duration. Additionally, we implemented a spatial light modulator-based pulse-shaping system to realize a spectral intensity filter. We experimentally demonstrated that the duration of a telecom-band pulse was reduced by 9.8% when using a spectral intensity filter with a width of 1 nm. [ABSTRACT FROM AUTHOR]

Understanding the fundamentals of laser-matter interactions is crucial for developing and optimizing ultrafast laser processing strategies. In optically transparent solids, the key event by which energy is deposited in the material is through the generation of an electron–hole plasma via nonlinear excitation mechanisms. As the energy stored in the plasma relaxes, local distortions of the lattice may occur, such as point defects. These defects give rise to new discrete energy states located in the bandgap. In this study, we investigate how the presence of these energy states influences the transmission of ultrashort near-infrared laser pulses in fused silica. Experimental results of laser pulse transmission and photoluminescence from defects are correlated with optical microscopy of the irradiated spots, allowing us to identify different nonlinear interaction regimes. Numerical simulations indicate that photo-induced defects influence the nonlinear losses of ultrashort laser pulses and explain why a non-destructive damage regime with detectable excitation is only observed for a narrow intensity range in multipulse experiments. [ABSTRACT FROM AUTHOR]

Scattering-type scanning near-field optical microscopy (s-SNOM) has become a powerful tool for subwavelength-resolved imaging and optical spectroscopy. The technique is particularly useful in the terahertz or other long-wavelength regimes, where the diffraction limit prohibits resolving even micron-sized objects. This approach can also drastically improve the spatial resolution of nonlinear measurements such as laser terahertz emission microscopy (LTEM), in which a broadband THz pulse is generated after photoexcitation by a near-infrared (NIR) pulse. However, in all prior near-field LTEM experiments, the pump spot has been localized to the scattering probe, to couple to the generated THz radiation at the center of the illuminated region. Here, we demonstrate the first nonlocal near-field LTEM experiments, in which an ultrafast NIR pulse photoexcites a sample at a location that is laterally shifted from the location of the near-field probe which couples the THz signal to the far field. Increasing lateral shifts of the pump spot produce larger time delays in the arrival of the emitted broadband THz pulse, consistent with drift of the subsurface dipole between the pump location and probe tip. Monte Carlo simulations corroborate the time shift for the dipole formation, which in turn produces the THz emission with a relative delay. The simulation results show excellent agreement with experiments. This nonlocal s-SNOM approach to LTEM offers a new opportunity for studying lateral transport on the nanoscale and may be particularly useful in anisotropic materials. [ABSTRACT FROM AUTHOR]

We present a detailed numerical analysis of the optical parametric amplification (OPA) process in 5 % MgO doped PPLN photonic crystals, for the efficient generation of signal at 3.4 μ m. The current work calls attention to understand the dynamics of femtosecond pump laser for transferring cascaded energy from the pump to signal under the influence of the self-action effect which is a corollary of Kerr nonlinear response, cascaded χ (2) nonlinearities, and higher-order dispersion effects. Here, the dynamics of frequency shift of generated photon in a PPLN crystal is studied through OPA process by solving coupled nonlinear partial differential equation. It is observed that a small digression in the domain width from the coherence length profoundly increases the efficiency of the signal. The optimization condition for this frequency conversion process is discussed in detail using the pump pulse energy, domain size and input pulse width parameters along with the effects of cubic nonlinearity. [ABSTRACT FROM AUTHOR]

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on "three-step scheme"—ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion). [ABSTRACT FROM AUTHOR]

The complete polarization state of second harmonic (SH) light was measured and characterized by collagen type I and skeletal muscle fiber using a Stokes vector-based SHG microscope. The polarization states of the SH signal are analyzed in a pixel-by-pixel manner and displayed through two dimensional (2D) Stokes vector images. Various polarization parameters are reconstructed using Stokes values to quantify the polarization properties of SH light. Also, the measurements are extended for different input polarization states to investigate the molecular structure of second harmonic generation (SHG) active molecules such as collagen type I and myosin. [ABSTRACT FROM AUTHOR]

We demonstrated real-time, in-situ operation of a single-shot, frequency-time-encoding burst imaging method of sequentially timed all-optical mapping photography utilizing spectral filtering (SF-STAMP) by employing a high-speed camera with the frame rate of a 1000 fps as a detector. We obtained single-shot burst images of a pulse-by-pulse femtosecond laser ablation of a glass by a chirped probe laser pulse with a 1–2-ps frame interval or by a spectrally sweeping burst laser pulse train with a 300-ps interval. We observed burst images of plasma generation during the early stage of laser ablation in a glass with a short frame interval by a chirped probe laser pulse. We also captured burst images of plume and shock-wave generation in air with a 1.5-ns observation time window by a spectrally sweeping probe laser pulse train. [ABSTRACT FROM AUTHOR]

We propose a feasible temporal filtering (or spectrum weighting) system to implement optical differentiation with arbitrarily-tunable order. Differentiation order is tuned by programming the temporal weighting mask (TWM) as power function of different exponents, not limited by the structure of the system. Assembly of the system is feasible, since the active parts are implemented by electro-optic elements and some key elements are reused between sub-systems involved in the scheme. Tuning of the system is simple, since electronically pre-stored TWMs are easy to switch among each other. The order-tunable functionality is verified by numerically testing the differentiation cases of normalized input pulse duration ranging from 0.1 to 0.4, and of differentiation order tuned from 1 to 10. Good accuracy is achieved, and measured by root mean square error lower than 0.1. Feasibilities of the system are also verified in terms of accuracy maintaining and distortion avoiding. Compared with existing methods, apart from the arbitrary order-tunable capacity and good accuracy, our proposed optical differentiator also possesses properties of good compactness and constant complexity. The proposed differentiator has unique applications on multi-peak waveform generation for optical communications and photonics-aided Radars with enhanced agility and feasibility. [ABSTRACT FROM AUTHOR]

Standing on the potential for high-speed modulation and switching in the terahertz (THz) regime, all-optical approaches whose response speeds mainly depend on the lifetime of nonequilibrium free carriers have attracted a tremendous attention. Here, we establish a novel bi-direction THz modulation experiment controlled by femtosecond laser for new functional devices. Specifically, time-resolved transmission measurements are conducted on a series of thin layers Bi2Se3 films fabricated straightforwardly on Al2O3 substrates, with the pump fluence range from 25 μJ/cm2 to 200 μJ/cm2 per pulse. After photoexcitation, an ultrafast switching of THz wave with a full recovery time of ~10 ps is observed. For a longer timescale, a photoinduced increase in the transmitted THz amplitude is found in the 8 and 10 quintuple layers (QL) Bi2Se3, which shows a thickness-dependent topological phase transition. Additionally, the broadband modulation effect of the 8 QL Bi2Se3 film is presented at the time delays of 2.2 ps and 12.5 ps which have a maximum modulation depth of 6.4% and 1.3% under the pump fluence of 200 μJ/cm2, respectively. Furthermore, the absorption of α optical phonon at 1.9 THz shows a time-dependent evolution which is consistent with the cooling of lattice temperature. [ABSTRACT FROM AUTHOR]

The layered MoS2 has recently attracted significant attention for its excellent nonlinear optical properties. Here, the ultrafast nonlinear optical (NLO) absorption and excited carrier dynamics of layered MoS2 (monolayer, 3-4 layers, and 6-8 layers) are investigated via Z-scan and transient absorption spectra. Our experimental results reveal that NLO absorption coefficients of these MoS2 increase from-27 × 103 cm/GW to-11 × 103 cm/GW with more layers at 400-nm laser excitation, while the values decrease from 2.0 × 103 cm/GW to 0.8 × 103 cm/GW at 800 nm. In addition, at high pump fluence, when the NLO response occurs, the results show that not only the reformation of the excitonic bands, but also the recovery time of NLO response decreases from 150 ps to 100 ps with an increasing number of layers, while the reductive energy of A excitonic band decreases from 191.7 meV to 51.1 meV. The intriguing NLO response of MoS2 provides excellent potentials for the next-generation optoelectronic and photonic devices. [ABSTRACT FROM AUTHOR]

A novel scheme of ultrashort pulse characterization is proposed that is capable of reconstructing the complex-valued envelope of a pulse in the time domain by use of a second harmonic generation system with two Gaussian gate pulses. In this scheme, we apply an analytic (noniterative) phase-retrieval method for coherent diffractive imaging in the space domain to the pulse characterization, and so the spectral phase of an unknown pulse can be retrieved from only two spectra of the unknown pulse modulated with a Gaussian gate and its time-shifted Gaussian one. Computer-simulated examples demonstrate that the pulse retrieval is robust. In particular, it is shown that the reconstruction of an unknown pulse using approximate Gaussian gates derived from replicas of the pulse itself is also effective, provided the spectrum of the unknown pulse has a gentle slope within the spectral bandwidth of the gate pulse. [ABSTRACT FROM AUTHOR]

We report on the measurement and characterization of polarization properties of second harmonic (SH) light from starch granule using a Stokes vector based microscopy. The four-element Stokes parameters are the most complete description of the polarization state of SH light, including any depolarization effects. Various polarization parameters, such as the degree of polarization, the degree of linear polarization, the degree of circular polarization, as well as anisotropy are extracted by implementing a pixel by pixel image analysis from the 2D reconstructed SH Stokes images of starch granule. Furthermore, we implemented the technique by varying the polarization states of excitation light and recording the resulting Stokes parameters at micro-meter depths of resolution in order to investigate the molecular structure and orientation of the samples. [ABSTRACT FROM AUTHOR]

Terahertz (THz) filters based on extraordinary optical transmission from periodical hole array structures fabricated on aluminum slab have been experimentally investigated by using THz time-domain spectroscopy. The incident THz pulses with frequency from 0.1 to 2.7 THz could be partly filtered, and the central peak was at ∼0.26. The high frequency signal could be observed to decrease, especially for the frequency above ∼1 THz. Moreover, the transmission peak from small-size sample with less hole arrays shifts to high frequency at ∼0.53 THz due to both the effects of boundary condition and insufficient periodical extension. Furthermore, finite element method with surface plasmon polariton theory is employed to analyze this extraordinary optical transmission and filter phenomena. [ABSTRACT FROM AUTHOR]

Terahertz (THz) wave imaging for biomaterial samples such as cells requires real-time acquisition and high spatial resolution beyond the diffraction limit. The existing THz near-field microscopes are based on raster-scanning techniques, and are therefore not able to image and trace morphological changes in a large area. With the recent advances in high-power THz sources, we demonstrated how to achieve high spatial resolution over a large size using a conventional charge-coupled-device (CCD) camera with the electro-optic (EO) sampling technique. In this paper, we determine a limiting factor that restricts spatial resolution in our near-field microscope. By calculating the imaging performance of the probe beam together with THz wave diffraction, we show that the most relevant factor is the diffraction inside the EO crystal. Near-field imaging of metal patterns using EO crystals with different thicknesses supports this calculation. A thin EO crystal is essential for achieving THz images with high spatial resolution. [ABSTRACT FROM AUTHOR]

We have performed a two-color time-resolved magneto-optical Kerr effect (MOKE) study of a ferromagnetic InMnAs/GaSb heterostructure. We observed ultrafast photoinduced changes in the MOKE signal induced by a large density of spin-polarized transient carriers created only within the InMnAs layer using intense 140 fs mid-infrared pulses. Our data clearly demonstrates that magnetic properties, e.g., remanence and coercivity, can be strongly modified. The dependence of these changes on the time delay, pump polarization, pump intensity, and sample temperature is discussed. [ABSTRACT FROM AUTHOR]

To realize an ultra-fast spatio-temporal photonic interface system, we propose direct time–space conversion of ultra-fast pulses based on the interferometric time-of-flight cross correlation technique using a fast optical addressable spatial light modulator (OA-SLM). We show preliminary experiments using transverse-field photorefractive MQW devices as an OA-SLM compatible with ultra-fast pulses. [ABSTRACT FROM AUTHOR]

For the application of high repetition rate wavelength division multiplexed (WDM) pulse train generation from a femtosecond modelocked fiber laser, the direct space-to-time pulse shaper and a properly designed arrayed waveguide grating (AWG) are equivalent. The analogy between the bulk optics and integrated configuration is explored for this application. The critical design parameters of the AWG are the free spectral range and the pathlength difference between adjacent guides in the array. [ABSTRACT FROM AUTHOR]

We discuss Fourier methods for shaping and processing femtosecond optical signals. Examples of applications in optical communications and in generation of terahertz radiation are presented. [ABSTRACT FROM AUTHOR]

Compact semiconductor-based ultrafast optoelectronic devices are crucial for networks with a throughput in the 1–10 Tb/s range. A variety of ultrafast phenomena in semiconductors are attractive for developing such new optoelectronic devices. This paper discusses the requirements of ultrafast optical-communication systems and necessary optoelectronic devices. Recent progress of ultrafast semiconductor-based optoelectronic devices are described with a focus on all-optical switching devices, including novel devices using electron spin polarization relaxation and intersubband transition both in multiple quantum well structures. [ABSTRACT FROM AUTHOR]

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E3+ r -doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E3+ r -doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power. [ABSTRACT FROM AUTHOR]

Light-induced insulator-to-metal phase transition of vanadium dioxide films was studied by ultrafast optical pump-probe spectroscopy. The transient optical reflection measurement shows that both heating and laser illumination contribute to the phase transition of VO2. Within 10-11-10-9 sec, these two mechanisms are competitive. Excited-state dynamics were found to be strongly dependent on the concentration of structural defects and the pump laser power as well. Comparison of the transient reflection of VO2 films deposited on different substrates suggests that the excitonic-controlled light induced insulator-to-metal phase transition in VO2 proceeded through an intermediate state. The transient reflection measurement of VO2 in metallic phase shows a three-stage relaxation process. A polaron excitation model is introduced to describe the dynamical process for metallic VO2. [ABSTRACT FROM AUTHOR]