Short Optical Pulses Research Articles

Laser-induced spin dynamics in the iron-yttrium garnet film doped with Si ions

A continuous increase in the volume of stored and processed data leads to stricter requirements for storage media. The most common information storage technology is currently based on magnetic materials, where information in the form of 0 and 1 is associated with the local direction of magnetization, determined by the external magnetic field created by the recording device. It is known that this approach has fundamental limitations on the recording speed which is almost achieved. The requirements for energy efficiency of storage media are also being made stricter. These circumstances lead to the development of alternative approaches to recording information. One of these approaches has been demonstrated in the field of ultrafast opto-magnetism, which has been booming over the past 20 years. It consists in recording information with short optical pulses without the application of an external magnetic field. However, it requires fundamental studies of physical processes, as well as materials, in which magnetization can be controlled by short optical pulses. In this paper, we consider the spin dynamics in a magnetic dielectric: a film of iron - yttrium garnet doped with silicon. The studies were carried out using the pump – probe technique over a time range of up to 800 ns. The spot size was 30 μm, the optical pulse duration was 35 fs, and the pump fluence was about 50 mJ/cm 2 . It is shown that a change in the magnetocrystalline anisotropy constants due to the action of a pump pulse on the structure causes a long-decaying magnetization precession with a period of about 200 ps. The dependences of the amplitude, phase, and decay of the precession on the magnitude of the external magnetic field in the range up to 1.84 kOe were obtained and analyzed. The studied processes can be considered on the basis of the Landau-Lifshitz-Gilbert model, and be of interest for the optical switching of magnetization, as well as the creation of various spintronic devices. It is shown that films of iron-yttrium garnet doped with silicon are a promising material for magnetic information carriers based on ultrafast opto-magnetism.

A Phase Calibration Method for Millimeter-Wave Up-Converter Using Electro-Optic Sampling

A phase measurement method using electro-optic sampling of millimeter-wave 4-tone signals is presented for phase calibration of frequency converters. Phase lock loop (PLL) circuits with high comparison frequencies of 2.6 and 15 GHz are used to synchronize the millimeter-wave 4-tone signal with a short optical pulse for low-jitter equivalent time sampling. A 4-tone separate detection method using a lock-in amplifier is applied for precise phase measurement with high frequency resolution. In addition, a phase-correction scheme using 4-tone signals with different frequency spacings is introduced for wideband phase calibration. The error vector magnitude (EVM) of the 10.2-Gbaud quadrature phase shift keying (QPSK) signal with 292.5-GHz carrier frequency is evaluated to confirm the effectiveness of the wideband phase calibration.

Numerical analysis on capacity improvement in free space optical link employing two-segment quantum cascade laser based repeater

This article investigates the impact of a two-segment Quantum Cascade Laser (QCL) based repeater on the performance of a Free Space Optical (FSO) communication link. The transmitter side of the FSO link consists of a single section QCL which emits short optical pulses at 9 μm into the free space. An optical repeater unit comprising of a Quantum Well Infrared Photodetector (QWIP), high-gain, low-noise transimpedance amplifier and two-segment QCL (gain section and saturable absorber section) regenerates the pulses with increased power levels. The link parameters namely Signal-to-Noise ratio (SNR), Bit Error Rate and Channel Capacity are computed for the FSO link in the presence and absence of repeater. It is found that the two-segment QCL based repeater improves the range of the FSO link by 640.2 m for a BER of 10−9. Further, an SNR enhancement of 28.53 dB is observed in the FSO link due to the introduction of repeater. The link also provides a maximum capacity of 9.48bps/Hz.

Selective bond formation triggered by short optical pulses: quantum dynamics of a four-center ring closure.

We report bond formation induced by an ultrashort UV pulse. The photochemical process is described by quantum dynamics as coherent electronic and nuclear motions during the ultrashort pulse induced ring closure of norbornadiene to quadricyclane. Norbornadiene consists of two ethylene moieties connected by a rigid (CH2)3 bridge. Upon photoexcitation, two new sigma bonds are formed, resulting in the closure of a four-atom ring. As a medium-sized polyatomic molecule, norbornadiene exhibits a high density of strongly coupled electronic states from about 6 eV above the ground state. We report on inducing the formation of the new bonds using a short femtosecond UV pulse to pump a non-equilibrium electronic density in the open form that evolves towards the closed ring form. As the coherent electronic-nuclear coupled dynamics unfold, the excited states change character through non-adiabatic interactions and become valence states for the two new C-C bonds of quadricyclane. Our three-dimensional fully quantum dynamical grid simulations during the first 200 fs show that short UV pulses of different polarization initiate markedly different initial non-equilibrium electronic densities that follow different dynamical paths to the S0/S1 conical intersection. They lead to different initial relative yields of quadricyclane, thereby opening the way to controlling bond-making with attopulses.

High-resolution chirped-pulse -OTDR by means of sub-bands processing

Conventional chirped-pulse (CP) phase-sensitive optical time-domain reflectometry (CP φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high accuracies of millikelvin or nanostrain. With respect to standard coherent-detection φ-OTDR, it shows increased robustness to coherent fading and allows a linear and quantitative monitoring of the perturbations acting on the fiber. Its spatial resolution, however, remains a critical parameter and new techniques allowing its improvement without reducing significantly other performances (or increasing the setup complexity) are constantly being researched. In this paper, we propose a method to improve the spatial resolution of CP φ-OTDR without reducing the input pulse width, by means of sub-bands processing. The method is based on adding an optical carrier to the input pulse. Using digital filtering, the spectrum of the fiber backscatter can be split into multiple sub-bands. Each of these sub-bands corresponds to the fiber response generated by a short optical pulse, chirped over a smaller frequency range. This way each sub-band results in φ-OTDR measurements with high spatial resolution, but with a reduced SNR. A dedicated post-processing methodology is proposed to mitigate the SNR reduction obtained from each sub-band, while securing high-resolution measurements of the perturbations acting on the fiber. Experimental results demonstrate the possibility of achieving CP φ-OTDR measurements with a 15-fold spatial resolution improvement over the conventional CP analysis, at the expense of an SNR reduction lower than a factor 2.

Генерация высших гармоник при распространении предельно короткого оптического импульса в среде углеродных нанотрубок с нелинейным поглощением

The problem of the propagation of an extremely short optical pulse in a medium of carbon nanotubes with nonlinear absorption and amplification is considered. Nonlinear absorption is described phenomenologically based on published data. The mode in which the gain is higher than the linear absorption coefficient is considered. This mode is also described phenomenologically. The extremely short optical pulse is considered on the basis of Maxwell's equations, with a microscopically calculated dependence of the current on the pulse field. The possibility of efficient generation of higher harmonics that are not initially present in the pulse is shown.

Spin inertia and polarization recovery in quantum dots: Role of pumping strength and resonant spin amplification

Spin inertia measurements are a novel experimental tool to study long-time spin relaxation processes in semiconductor nanostructures. We develop a theory of the spin inertia effect for resident electrons and holes localized in quantum dots. We consider the spin orientation by short optical pulses with arbitrary pulse area and detuning from the trion resonance. The interaction with an external longitudinal magnetic field and the hyperfine interaction with the nuclear spin bath is considered in both the ground and excited (trion) states of the quantum dots. We analyze how the spin inertia signal depends on the magnetic field (polarization recovery) and on the modulation frequency of the helicity of the pump pulses as well as on their power and detuning. In particular, we elaborate how approaching the saturation limit of the spin polarization influences the measurements. The quantitative description of spin inertia measurements will enable the determination of the parameters of spin dynamics such as the spin relaxation times in the ground and excited states and the parameters of the hyperfine interaction. Finally, we predict the emergence of resonant spin amplification due to the transverse components of the nuclear spin fluctuations, which manifests itself as oscillations of the spin polarization as a function of the longitudinal magnetic field.

High-Spatial-Resolution Brillouin Optical Correlation Domain Analysis Using Short-Pulse Optical Sources

High-spatial-resolution distributed optical fiber sensing based on Brillouin optical correlation domain analysis (BOCDA) using short-pulse optical sources is demonstrated. Short optical pulses phase-modulated with a pseudo random bit stream (PRBS) are used for pump and Stokes probe lights for the correlation-domain measurement of distributions of local Brillouin frequency shift (BFS). Because the excitation of acoustic wave is tightly confined in short length along the fiber owing to the short pulse duration, spatial resolution of the measurement is improved as compared with the case using not-return-to-zero pulses. In this experiment, BFS variation in a 2 mm standard single-mode fiber section is clearly detected by the use of an external cavity mode-locked semiconductor laser emitting 3 ps pulses as the light source. Spatial resolution shorter than 1 mm is obtained. Scanning of measurement location is achieved electrically by changing the clock speed of the PRBS, which is an advantage of the proposed scheme over the high-spatial-resolution BOCDA using amplified spontaneous emission as a light source. Theoretical estimation of Brillouin gain that the short-pulse Stokes probe light acquires is presented.

Unidirectional invisibility and enhanced reflection of short pulses in quasi-PT-symmetric media.

We consider the interaction of a short optical pulse with a layer with periodically modulated permittivity and a periodic gain-and-loss landscape. It is found that if the medium is quasi-parity-time (PT)-symmetric in the vicinity of the exceptional point, the propagation manifests strong unidirectional reflection and invisibility. Due to strong frequency selectivity, quasi-PT-symmetric periodic layers manifest efficient filtering of back-radiation.

A time-to-frequency converter for measuring the shape of short optical pulses.

A time-to-frequency converter was constructed using an electro-optic phase modulator as a time lens, allowing the pulse shape in time to be transferred to the frequency domain. We used such a device to record the temporal shape of infrared pulses at a wavelength of 1053 nm (width about 7 ps) and compared these measurements to those made by using both a streak camera and an autocorrelator. This side-by-side comparison illustrates the benefits and limitations of each of the measurement methods. Numerical simulations were used to establish that our time-lens-based system can accurately measure the shape of infrared pulses between 3 ps and 12 ps. We also use our numerical model to determine how such a system can be modified to measure pulses whose width lies in the range of 1-30 ps, a range of interest for the OMEGA-EP laser at the Laboratory for Laser Energetics.

Broadband mid-infrared supercontinuum generation in three-level multiple quantum wells using short optical pulses

The paper presents a theoretical study of the electromagnetically induced transparency (EIT) facilitated mid-infrared supercontinuum generation using femtosecond optical pulses at low input power in multiple quantum well (MQW) nanostructures. The MQWs exhibit large nonlinearity, low dispersion and suppressed attenuation due to EIT. A multiple time scale method has been adopted to obtain modified nonlinear Schrödinger equation that governs the dynamics of the supercontinuum generation. Spectral broadening of about6.0μm is achievable at the end of a 1.38μm long MQW by injecting 50fs pulses of 200W peak power at 6.79μm wavelength. The spectrum is asymmetric, and attributed to the complex interplay of nonlinearity, group velocity dispersion and third-order dispersion. The result of the present investigation maybe useful in the fabrication of supercontinuum sources on integrated platforms with other semiconductor devices.

Fundamentals and Applications of Chalcogenide Phase‐Change Material Photonics

AbstractRecently, chalcogenide phase‐change materials (PCMs) photonics have become an emerging research field because the optical properties of the PCMs undergo a dramatic change during the amorphous–crystalline phase transition. The chalcogenide PCMs can be efficiently switched by electrical or short optical pulses, offering versatility in photonic applications and extraordinary capability to engineer light. The authors introduce the fundamentals of chalcogenide PCM‐tuned photonics. The progress in the field is reviewed. Particularly, recent developments of metal–chalcogenide–metal (MCM) trilayered plasmonic nanostructures are presented. The authors then discuss the methodology of designing reconfigurable MCM‐based photonic devices, their advantages, and their existing challenges. Finally, requirements and prospects to successfully implement chalcogenide PCMs in growing areas of photonics are discussed.

Photonic integrated circuit extended cavity passively mode-locked dual absorber symmetric ring laser

A photonic integrated circuit extended cavity passively mode-locked semiconductor ring laser with two saturable absorbers in a symmetric ring geometry fabricated using an InP generic integration technology platform is presented for the first time, to the best of our knowledge. The laser emits 1.4 ps short optical pulses with a time-bandwidth-product of 0.96 (Gaussian-shaped pulses) at a fundamental repetition rate of 23.3 GHz. The laser exhibits a beat-note line width of 80 kHz, corresponding to a pulse-to-pulse timing jitter of 31.7 fs. The emission is centered at 1570 nm and maximum spectral bandwidths amount to 10.2 nm (−3 dB) and 19.7 nm (−20 dB).

Spatiotemporal Metrology of Broadband Optical Pulses

Accurately characterizing the field of broadband optical pulses is required for developing new optical sources, performing successful experiments, and interpreting experimental results. Temporal diagnostics for short optical pulses are now well established. The most recent developments in ultrafast metrology focus on determining the field as a function of time (conversely frequency) and space, potentially revealing the field's nonhomogeneity and coupling between these variables. After a review of common situations where a nonhomogeneous spatiotemporal field is used as a tool or is the outcome of nonideal conditions, historical and state-of-the-art techniques for spatiotemporal ultrafast metrology are described. The different classes of techniques demonstrate the richness of the fundamental and technical developments in this domain.

1064-nm-resonant gold nanorods for photoacoustic theranostics within permissible exposure limits.

Therapeutic and diagnostic methods based on photomechanical effects are attracting much current attention in contexts as oncology, cardiology and vascular surgery, for such applications as photoacoustic imaging or microsurgery. Their underlying mechanism is the generation of ultrasound or cavitation from the interaction of short optical pulses with endogenous dyes or targeted contrast agents. Among the latter, gold nanorods are outstanding candidates, but their use has mainly been reported for photoacoustic imaging and photothermal treatments. Conversely, much less is still known about their value as a precision tool for photomechanical manipulations, such as to impart local damage with high spatial resolution through the expansion and collapse of microbubbles. Here, we address the feasibility of gold nanorods exhibiting a distribution of surface plasmon resonances between about 900 to above 1100 nm as a contrast agent for photoacoustic theranostics. After testing their cytotoxicity and cellular uptake, we discuss their photostability and use to mediate cavitation and the photomechanical destruction of targeted cells. We find that the choice of a plasmonic band peaking around 1064 nm is key to enhance the translational potential of this approach. With respect to the standard alternative of 800 nm, at 1064 nm, relevant regulations on optical exposure are less restrictive and the photonic technology is more mature.

Invite paper: Self-imaging in multimode graded-index fibers and its impact on the nonlinear phenomena

Abstract The phenomenon of periodic self-imaging of optical beams, occurring inside any graded-index (GRIN) medium, was studied during the decade of the 1970s and was exploited to commercialize the GRIN lens. It has been found in recent years that the periodic self-imaging also affects the nonlinear propagation of optical pulses inside multimode GRIN fibers. In this paper, we first present the theory of self-imaging in linear GRIN fibers using a modal expansion approach. It is shown that the optical field at any point inside the fiber can be written without any reference to the fiber modes as a two-dimensional integration over the input field using a propagation kernel that is similar to that found in diffraction theory. However, this kernel has a specific property that reproduces the input field precisely in a periodic fashion along the length of a GRIN fiber (self-imaging). We apply this kernel to study the propagation of a Gaussian beam and discuss how self-imaging is modified by self-focusing produced by the Kerr nonlinearity. We then consider propagation of the continuous and pulsed Gaussian beams inside a GRIN fiber and discuss how self-imaging affects the modulation instability, leads to the formation of GRIN solitons, and produces novel temporal and spectral features when short optical pulses are launched that are intense enough to form high-order solitons.

Three-Dimensional Extremely Short Optical Pulses of Airy in a Photonic Crystal with Carbon Nanotubes

The problem of the propagation dynamics of three-dimensional optical Airy pulses (light bullets) in a photonic crystal with carbon nanotubes is considered. It is numerically shown that the expected type of pulse exhibits a steady and stable propagation.

Optically injected intensity-stable pulse source for secure quantum key distribution.

The security of decoy-state quantum key distribution (QKD) highly depends on the accurate control of multiple intensity states. Although several theoretical studies on the QKD with loosely controlled source intensities have been proposed, there is still a large gap between the experimental realization and the theoretical analysis. In this paper, we adopt the gain-switching method to generate short optical pulses, and the corresponding intensity stabilities are quantitatively measured. The method via optical injection is proposed to make effective reductions of the intensity fluctuations from 6.47%∼1.59% to 1.95%∼1.15% at different optical powers. QKD performance adopting the experimental results is also analyzed and discussed. For a typical 40 dB high-attenuation QKD system, the relative increase on the secure key rates reaches 51.89% for the corresponding intensity fluctuations of 1.15% with optical injection and 1.59% without optical injection. The presented intensity-stable optical pulse source can find wide applications in different QKD protocols, such as BB84, DPS, COW, etc.

Superresolution Microscopy of Cold Atoms in an Optical Lattice

Super-resolution microscopy has revolutionized the fields of chemistry and biology by resolving features at the molecular level. Such techniques can be either "stochastic," gaining resolution through precise localization of point source emitters, or "deterministic," leveraging the nonlinear optical response of a sample to improve resolution. In atomic physics, deterministic methods can be applied to reveal the atomic wavefunction and to perform quantum control. Here we demonstrate super-resolution imaging based on nonlinear response of atoms to an optical pumping pulse. With this technique the atomic density distribution can be resolved with a point spread function FWHM of 32(4) nm and a localization precision below 1 nm. The short optical pumping pulse of 1.4 $\mu$s enables us to resolve fast atomic dynamics within a single lattice site. A byproduct of our scheme is the emergence of moir\'{e} patterns on the atomic cloud, which we show to be immensely magnified images of the atomic density in the lattice. Our work represents a general approach to accessing the physics of cold atoms at the nanometer scale, and can be extended to higher dimensional lattices and bulk systems for a variety of atomic and molecular species.

Light Bullets in a Periodically Inhomogeneous Medium of Oriented Carbon Nanotubes in an Optical Cavity

The problem of the propagation dynamics of three-dimensional ultimately short optical pulses in an optical cavity in a periodically inhomogeneous medium of oriented carbon nanotubes is considered. It is shown numerically that such pulses demonstrate stable and sustainable propagation. In addition, it is shown that one can control for the pulse propagation rate and modify the pulse shape by varying parameters of the inhomogeneous medium.