Ultrafast Optics Research Papers - Academia.edu (original) (raw)

The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence F th $ 0:4 J cm À2 and the volume ablation rate was measured to be 30 < V < 450 mm 3 per pulse in the fluence range 1:4 < F < 21 J cm À2 . The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F > 7 J cm À2 and >85 W cm À2 average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F $ 1:4 J cm À2 with low power density, 3WcmAˋ2producesmuchsuperiorsurfacemicro−structuringwithminimummeltandmeasuredsurfaceroughnessRa3 W cm À2 produces much superior surface micro-structuring with minimum melt and measured surface roughness R a 3WcmAˋ2producesmuchsuperiorsurfacemicro−structuringwithminimummeltandmeasuredsurfaceroughnessRa 1:1 AE 0:1 mm at a depth D $ 50 mm. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm À2 combined with single pulse fluence <4 J cm À2 produces near optimum microstructuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream. #

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- Ultrafast Optics, Femtosecond Laser, Multidisciplinary, Surface Roughness

Three different sites of the Cr 3ϩ ions in the fluoride perovskite KMgF 3 have been identified by absorption, selective optical excitation, and time-resolved emission spectroscopy of the 4 T 2 ↔ 4 A 2 transition. Highpressure measurements showing the crossover from low-crystal field to high-crystal field, allows us to situate the Dq/B values of the different chromium sites clearly below 2.3. The different spin-orbit components associated with the zero-phonon lines of the 4 T 2 ↔ 4 A 2 transition of each type of site are clearly shown on the optical spectra and identified by group-theory analysis. The decay profiles of the 4 T 2 level are exponential and the lifetimes at 15 K are 903, 919, and 473 s for the cubic, quadratic, and trigonal sites, respectively. The trigonal center which presents the lowest energy levels was peculiarly studied. Emission and excitation spectra are compared; their evolution versus temperature is followed and explained by the thermal population of the different spin-orbit sublevels. The phonon sideband of the trigonal site is compared with our previous lattice dynamic studies of the pure compound. The different peaks of the emission broadband are described in terms of phonons of the matrix and normal modes of the ͓CrF 6 ͔ 3Ϫ complex. ͓S0163-1829͑97͒03430-9͔

- by Qiuping A Wang
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- High Pressure, Ultrafast Optics, Group Theory, Optical Spectroscopy

This paper presents recent results in the development of novel ultrafast technologies based on the generation and application of stabilized optical frequency combs. By using novel active resonant cavity injection locking techniques, filtering, modulation and detection can be performed directly on individual components of the frequency comb enabling new approaches to optical waveform synthesis, waveform detection and matched filtering, with effective signal processing bandwidths in excess of 1 THz.

- by Peter Delfyett
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- Quantum Physics, Signal Processing, Ultrafast Optics, Matched Filter

The successful application of finite element analysis to ultrafast optoelectronic devices is demonstrated. Finite element models have been developed for both an alloyed-and surface-contact metal-semiconductor-metal photodetectors. The simulation results agree with previously reported experimental data. The alloyed device, despite having a somewhat larger capacitance, has a non-illuminated region of lower resistance with a more-uniform and deeper-penetrating electric field and carrier transport current. The latter explains, in terms of the equivalent lumped parameters, the experimentally observed faster response of the alloyed device. The model is further used to predict improved responsivity, based on electrode spacing and antireflective coating. We project that increasing the depth of the alloyed contact beyond approximately half of the optical penetration depth will not yield significantly improved responsivity.

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- by Shevelev Artur
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- Ultrafast Optics

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations. FIG. 4. (a) Lattice spacing (a) dependence of the competing AFM Fe-Fe exchange (J Fe ) and the effective Rh-moment mediated FM exchange (J Rh ), calculated for both m Rh 0 (AFM) and m Rh 1 B (FM). (b) Temperature dependence of the exchange parameters: J Fe , J Rh , and the sum J 0 J Fe J Rh . The vertical dashed line separates regions of stable and unstable AFM order.

- by Bastiaan Bergman
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- Ultrafast Optics, Optical Spectroscopy, Thin Film, Physical sciences

The current status of a fiber-based ultrafast technology is reviewed. Pulse generation techniques capable of producing femtosecond pulses are discussed. Here we describe passive and active-passive mode locking techniques as well as linear and nonlinear fiber amplifiers. Thirtyfemtosecond pulses may be generated directly from fiber oscillators or by implementing pulse compression techniques. The use of cladding-pumping and all-fiber chirped-pulse amplification allows the generation of W-level average powers from conceptually simple fiber laser systems. Nonlinear frequency conversion in highly nonlinear crystals allows a significant extension of the accessible wavelength range. Electronically phase-locked fiber lasers with unprecedented timing accuracy may be constructed by exploitation of the low-noise properties of fiber lasers. In turn, the high timing accuracy possible with fiber lasers enables the demonstration of electronic scanning delay lines for all-electronic pump-probe experiments.

- by Almantas Galvanauskas
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- Mechanical Engineering, Ultrafast Optics, Fiber Laser, Optical physics

Being a Mott type oxide, at a temperature of ~68°C and ambient pressure, stoichiometric VO2 undergoes a first order metal-insulator transition, which is accompanied by a reversible abrupt change in the band gap opening. From an optical point of view, this metal-insulator transition manifests itself by a significant and reversible variation of the refractive index under either a thermal stimuli

- by J. Kana
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- Ultrafast Optics, Pulsed Laser Deposition, Band Gap, Refractive Index

Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. In this article, we review our recent work on

- by Christoph Lienau
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- Nonlinear Optics, Ultrafast Optics, Quantum Logic, Oscillations

The interaction between two quantum bits enables entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots much work has focused on demonstrating single spin qubit control using optical techniques. However, optical control of entanglement of two spin qubits remains a major challenge for scaling from a single qubit to a full-fledged quantum information platform. Here, we combine advances in vertically-stacked quantum dots with ultrafast laser techniques to achieve optical control of the entangled state of two electron spins. Each electron is in a separate InAs quantum dot, and the spins interact through tunneling, where the tunneling rate determines how rapidly entangling operations can be performed. The two-qubit gate speeds achieved here are over an order of magnitude faster than in other systems. These results demonstrate the viability and advantages of optically controlled quantum dot spins for multi-qubit systems.

- by danny kim
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- Quantum Information, Ultrafast Optics, Mathematical Sciences, Physical sciences

Based on two theorems, the importance of the root-mean-square (rms) width for the characterization of ultrashort optical pulses is demonstrated. First, it is shown that one can directly determine the rms width from the autocorrelation without making any assumptions about the specific form of the pulse envelope. Second, it is shown that a bandwidth-limited (unchirped) wave packet has the smallest possible rms time-bandwidth product. This reveals a natural definition for a rms chirp that is easily accessible to experimental measurement and that presents a useful measure for the quality of pulse compression techniques.

- by Evgeni Sorokin
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- Applied Mathematics, Ultrafast Optics, Root-Mean Square Error, Optical physics

We present a compact TW-class OPCPA system operating at 800 nm. Broadband seed pulses are generated and pre-amplified to 25 µJ in a white light continuum seeded femtosecond NOPA. Amplification of the seed pulses to 35 mJ at a repetition... more

- by Rimantas Budriūnas
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- Optics, Ultrafast Optics

Excitonic transitions offer a possible route to ultrafast optical spin manipulation in coupled nanostructures. We perform here a detailed study of the three principal exciton-mediated decoherence channels for optically-controlled electron spin qubits in coupled quantum dots: radiative decay of the excitonic state, exciton-phonon interactions, and Landau-Zener transitions between laser-dressed states. We consider a scheme to produce an entangling controlled-phase gate on a pair of coupled spins which, in its simplest dynamic form, renders the system subject to fast decoherence rates associated with exciton creation during the gating operation. In contrast, we show that an adiabatic approach employing off-resonant laser excitation allows us to suppress all sources of decoherence simultaneously, significantly increasing the fidelity of operations at only a relatively small gating time cost. We find that controlled-phase gates accurate to one part in 10^2 can realistically be achieved with the adiabatic approach, whereas the conventional dynamic approach does not appear to support a fidelity suitable for scalable quantum computation. Our predictions could be demonstrated experimentally in the near future.

- by AHSAN NAZIR
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- Ultrafast Optics, New, Physical sciences, Oscillations

We present a method for controlling the spatial properties of high-harmonic beams with high efficiency. The high nonlinearity of harmonic generation allows weak control beams to induce a phase mask for the extreme UV light as it is formed. We fabricate a phase grating and demonstrate efficient diffraction in the far field. Diffractive elements formed in this way are transient. Since they are induced by the subcycle interaction of the medium with the fundamental and control fields, they can be extended to the attosecond time scale.

- by D. Kartashov
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- Quantum Physics, Nonlinear Optics, Ultrafast Optics, High harmonic generation

An overview of the most promising optical clock recovery techniques for all-optical ultrafast communication is presented. A step towards a system view for the various techniques is taken, transferring the clock recovery concepts available from the electronic domain into the optical domain, and leading to block diagram interpretation for both open-loop and closed-loop systems. This is essential for future system analyses that will permit design and comparison of different techniques using established analytical tools.

Time-resolved studies of the dynamics of intersubband transitions are reported in three different strain symmetrized p-Si/SiGe multiple-quantum-well and quantum cascade structures in the far-infrared wavelength range (where the photon energy is less than the optical phonon energy), utilizing the FELIX free-electron laser. The calculated rates for optical and acoustic phonon scattering, alloy disorder scattering, and carrier-carrier scattering have been included in a self-consistent energy balance model of the transient far-infrared intersubband absorption, and show good agreement with our degenerate pump-probe spectroscopy measurements in which, after an initial rise time determined by the resolution of our measurement, we determine decay times ranging from ∼2 to ∼25 ps depending on the design of the structure. In all three samples the lifetimes for the transition from the first light hole subband to the first heavy hole subband are found to be approximately constant in the temperat...

- by C. Pidgeon
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- Quantum Cascade Lasers, Ultrafast Optics, Optical Spectroscopy, Band Structure

Dichroism experiments with 150 fs time resolution on anthracene in benzyl alcohol are presented as a function of viscosity from 14.4 cP ͑274 K͒ to 2.7 cP ͑329 K͒. These measurements test a qualitative prediction of the viscoelastic picture of liquid dynamics, specifically that earlier ''inertial'' dynamics have a viscosity independent rate, whereas later ''diffusive'' dynamics have a rate directly proportional to viscosity. This paper focuses on two components of the dichroism decay that are assigned to rotational motion. A third component is assigned to electronic-state solvation and is analyzed in a companion paper ͓J. Chem. Phys. 115, 4231 ͑2001͔͒. The longest component is due to rotational diffusion and is very well described by a hydrodynamic model with slip boundary conditions. A fast decay component in the subpicosecond region is found and shown to have a viscosity-independent rate. It is assigned to inertial rotation by comparison to the computer simulations of Jas et al. ͓J. Chem. Phys. 107, 8800 ͑1997͔͒. Inertial rotation extends out to at least 1 ps, longer than the range commonly assumed for inertial dynamics. Over much of this range, the inertial rotation is not free-rotor-like, but is strongly modified by interaction with the solvent. The inertial rotation also accounts for the ''missing'' anisotropy found when the rotational diffusion fits are extrapolated to zero time.

- by Yunhan Zhang
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- Engineering, Ultrafast Optics, Optical Spectroscopy, Chemical Physics

The propagation of a laser pulse in a graded-index (GRIN) space lens and in an electro-optic crystal with an appropriate refractive index modulation is studied. It is shown that the crystal functions as a GRIN time lens if the propagation's second-order dispersion is included and if the phase velocity of the modulating wave equals the group velocity of the laser pulse.

- by Francisco Marinho
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- Ultrafast Optics, Ultrafast Spectroscopy

We present a reconstruction algorithm for isolated attosecond pulses, which exploits the phase dependent energy modulation of a photoelectron ionized in the presence of a strong laser field. The energy modulation due to a circularly polarized laser field is manifest strongly in the angle-resolved photoelectron momentum distribution, allowing for complete reconstruction of the temporal and spectral profile of an attosecond burst. We show that this type of reconstruction algorithm is robust against counting noise and suitable for single-shot experiments. This algorithm holds potential for a variety of applications for attosecond pulse sources.

- by Adi Natan and +1
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- Ultrafast Optics, X-ray Physics, Atomic Physics, Attosecond Physics

This letter presents a method aimed at quantifying the dimensions of the heat-affected zone ͑HAZ͒, produced during nanosecond and femtosecond laser-matter interactions. According to this method, 0.1 m thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes ͑i.e., 5 and 2 J/cm 2 , respectively͒. The grain size in the samples was observed near the microholes. The main conclusion is that a 40 m wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 m, which is our observation limit.

- by Ronan Le Harzic
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- Engineering, Applied Physics, Ultrafast Optics, Optical Spectroscopy

Two-photon absorption measurement has been carried out in a single 80 nm×10 μm ZnO nanowire using femtosecond laser pulses in the wavelength range of 700-800 nm. In addition to the deep-level green emission around 530 nm due to surface defects and the near band-edge ultraviolet emission around 360 nm due to the exciton, a second harmonic peak has been observed. The strength of the frequency-doubled component is found to enhance while the two-photon absorption wavelength is tuned towards the exciton wavelength of the nanowire. This behavior can be ascribed to the resonant exciton absorption in ZnO nanowires.

- by Prasanth Ravindran
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- Engineering, Applied Physics, Ultrafast Optics, Optical Spectroscopy

We consider the propagation of ultrashort solitons in noncentrosymmetric quadratically nonlinear optical media described by a general Hamiltonian of multilevel atoms. We use a long-wave approximation to derive a coupled system of Korteweg–de Vries equations describing ultrashort soliton evolution in such materials. This model was derived by using a rigorous application of the reductive perturbation formalism (multiscale analysis). The study of linear eigenpolarizations in the degenerate case and the corresponding formation of half-cycle solitons from few-cycle-pulse inputs are also presented.

- by H. Leblond
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- Ultrafast Optics, Few-Cycle Solitons

Fiber laser technology is leading a revolution in ultrashort-pulse lasers and their applications. Unlike conventional ultrashort-pulse bulk solid-state laser systems, ultrafast fiber amplifiers can be scaled to average powers >100 W without significant beam-quality distortion from thermal lensing. In this paper, we review recent developments in the generation of high-average power and high-pulse energy ultrashort-pulse fiber sources. In particular, we focus upon advances that enhance the practicality of these laser systems. In several application examples, we show how these improvements in laser performance and utility are critical for achieving the high speed and reliability necessary for commercial processing applications.

- by Lawrence Shah
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- Quantum Physics, Ultrafast Optics, Solid State Laser, Fiber Laser

Ultrafast optical probes, photoluminescence spectroscopy, and Raman spectroscopy have been applied to investigate carrier dynamics in nitride-based binary and ternary, and dilute nitride semiconductors. Carrier dynamics in the form of radiative and non-radiative lifetimes in GaN grown on pseudo-in situ TiN and in situ SiN nanonetworks by organometallic vapor phase epitaxy have been investigated and compared with those for freestanding GaN templates which constitute the benchmark values due to the high quality. Room temperature carrier lifetimes as long as 1.86 ns could be achieved with the use of TiN network templates. Time-resolved Raman spectroscopy has been employed to investigate the carrier dynamics, carrier transport and non-equilibrium optical phonons in In-containing nitride-based semiconductors. (1) It has been found that the energy loss rate in In x Ga 1−x As 1−y N y is about 64 meV/ps suggesting that hot electrons lose their energy primarily to the GaAs-like LO phonons in this dilute nitride semiconductor. (2) Both the non-equilibrium electron

- by David Ferry
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- Condensed Matter Physics, Raman Spectroscopy, Ultrafast Optics, Time Resolved

A basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit 1-6 . For qubits based on electron spin, a universal single-qubit gate is realized by a rotation of the spin by any angle about an arbitrary axis. Driven, coherent Rabi oscillations between two spin states can be used to demonstrate control of the rotation angle. Ramsey interference, produced by two coherent spin rotations separated by a variable time delay, demonstrates control over the axis of rotation. Full quantum control of an electron spin in a quantum dot has previously been demonstrated using resonant radio-frequency pulses that require many spin precession periods 7-10 . However, optical manipulation of the spin allows quantum control on a picosecond or femtosecond timescale 11-18 , permitting an arbitrary rotation to be completed within one spin precession period 6 . Recent work in optical singlespin control has demonstrated the initialization of a spin state in a quantum dot , as well as the ultrafast manipulation of coherence in a largely unpolarized single-spin state 17 . Here we demonstrate complete coherent control over an initialized electron spin state in a quantum dot using picosecond optical pulses. First we vary the intensity of a single optical pulse to observe over six Rabi oscillations between the two spin states; then we apply two sequential pulses to observe high-contrast Ramsey interference. Such a two-pulse sequence realizes an arbitrary single-qubit gate completed on a picosecond timescale. Along with the spin initialization and final projective measurement of the spin state, these results demonstrate a complete set of all-optical single-qubit operations.

- by Yoshihisa Yamamoto
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- Pharmacology, Biochemistry, Bioinformatics, Evolutionary Biology
- by vu xuan quang
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- Engineering, Applied Physics, Ultrafast Optics, Optical Spectroscopy

We present results on development and experimental implementation of a 1−kHz, coherent extreme ultraviolet (XUV) radia− tion source based on high−order harmonic generation of the femtosecond, near−infrared laser pulses produced by the tita− nium−doped sapphire laser system (35 fs, 1.2 mJ, 810 nm) at the Institute of Physics AS CR / PALS Centre. The source com− prises a low−density static gas cell filled with a conversion medium, typically argon. The comprehensive optimization of the XUV harmonic source has been performed with respect to major parameters such as gas pressure in the cell, cell length, po− sition of the focus of the driving laser field with respect to the gas cell position, size of the driving near−infrared laser beam, chirp of the femtosecond pulse, and the focal length of the lens deployed in the experimental setup. Harmonic spectra were recorded using an XUV transmission grating spectrometer developed specifically for this purpose. Detailed characterization of the XUV source has been performed including measurement of the XUV beam profile, M 2 parameter of the beam, absolute energy, and spatial coherence.

- by Ladislav Pina and +1
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- Ultrafast Optics, Titanium, Near Infrared, Optical physics

We demonstrate laser action from a waveguide fabricated in an Er:Yb-doped silicate glass using femtosecond laser inscription. Waveguides were fabricated using a diode-pumped Yb:glass oscillator at a 600-kHz repetition rate. The multiscan technique was used to control the waveguide cross section. Textbook threshold characteristics are observed for both 980 nm and dual-wavelength excitation.

- by H. Bookey and +1
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- Photonics, Ultrafast Optics, Femtosecond Laser, Optical Waveguide

Currently, direct-write waveguide fabrication is probably the most widely studied application of femtosecond laser micromachining in transparent dielectrics. Devices such as buried waveguides, power splitters, couplers, gratings and optical amplifiers have all been demonstrated. Waveguide properties depend critically on the sample material properties and writing laser characteristics. In this paper we discuss the challenges facing researchers using the femtosecond laser direct-write technique with specific emphasis being placed on the suitability of fused silica and phosphate glass as device hosts for different applications.

- by Vladimir Mezentsev
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- Quantum Physics, Ultrafast Optics, Femtosecond Laser, Optical Waveguides

Soliton trains can be parametrically generated in fiber loops wherever an incoming soliton train interacts with a cw pump in the nonlinear loop. The phase-conjugated signal may either copy or multiply the incoming repetition rate, depending on whether the corresponding cavity harmonic is exactly matched or differs by an amount equal to an integer fraction of the longitudinal mode spacing. It is also shown that the phase relationship between adjacent solitons in the multiplicative conditions continuously evolves on propagation. This effect limits the range of potential application of multiplicative loops.

- by Flavio Fontana
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- Quantum Physics, Nonlinear Optics, Ultrafast Optics, Optical Communication

We present an all-optical in-band optical signal-to-noise ratio (OSNR) monitor using a nonlinear optical loop mirror. Monitoring is enabled from the nonlinear power transfer function of the loop mirror. Experimental results are provided at 40 Gb/s for three modulation formats: nonreturn-to-zero, carrier-suppressed return-to-zero, and return-to-zero. The monitor discriminates the various OSNR levels over a dynamic range of more than 25

- by Martin Rochette
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- Nonlinear Optics, Ultrafast Optics, Performance Monitoring, Transfer Function

We present femtosecond gain measurements in microcavity-embedded quantum wells. When the excitonic transition is saturated by an intense pump field, the spectrum measured with a weak probe pulse is strongly modulated by pump-probe wave mixing processes. A giant probe amplification occurs within the bandwidth of the empty-cavity mode. The optical feedback provided by the microcavity is responsible for a net gain of the broadband probe pulse. ͓S0163-1829͑99͒50424-4͔

- by Benoit Deveaud
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- Ultrafast Optics, Optical Spectroscopy, Physical sciences, Time Resolved

We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.

- by Christoph Lienau
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- Ultrafast Optics, Optical Spectroscopy, Physical sciences, Band Gap
- by Antonio Pifferi and +1
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- Biomedical Engineering, Ultrafast Optics, Optical Imaging, Biomedical Optics

The first-order semiconductor–metal Mott
transition in single nano-crystal of VO2 has been
observed using scanning tunneling spectroscopy. The
variation of the band gap Eg with an external thermal
stimulus on a single VO2 nano-crystal in the temperature
range of 293.5–361.0 K is reported for the first
time. The corresponding tuneable I–V characteristics
versus temperature could be applied in thermally or
optically tunable electronic nano-gating in the femtosecond
regime in view of the ultrafast dynamic in VO2.

- by Malik Maaza
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- Ultrafast Optics, Nanotechnology, Optoelectronics
- by Benoit Deveaud
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- Physics, Thermodynamics, Ultrafast Optics, Optical Spectroscopy
- by bahram jalali
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- Ultrafast Optics, Silicon Photonics, Mathematical Sciences, Physical sciences

Low-energy coherent charge-density wave excitations are investigated in blue bronze (K 0.3 MoO 3 ) and red bronze (K 0.33 MoO 3 ) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K 0.33 MoO 3 . The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.

- by Yuhang Ren
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- Engineering, Ultrafast Optics, Optical Spectroscopy, Chemical Physics

We present innovative concepts related to the realization of ultrafast optical switches to be used for obtaining all-optical switching. We review the construction and the achievements of Civcom Incorporated during its research and... more

- by Nadav Cohen
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- Ultrafast Optics, Optical Switch, Optical Networking, Optical

In this paper, single-mode 200-mW laser diodes have been demonstrated to be very effective pump devices for low-power Nd:glass lasers, yielding the remarkable continuous wave (cw) slope efficiency of 46.5% for silicate and 58.2% for phosphate glasses, respectively. Femtosecond operation has been investigated with both semiconductor saturable absorber mirrors (SESAMs) and a single-walled carbon nanotube SAM (SWCNT-SAM). Furthermore, a detailed comparison of the mode-locking performance with Nd:phosphate and Nd:silicate, employing either one of the SA devices is presented. Although not fully optimized for this particular application yet, SWCNT-SAs yielded sub-100-fs pulses for the first time in Nd:glass. With SESAM mode locking and a single-prism resonator for dispersion compensation, pulse duration as short as 92 fs has been measured, whereas shorter pulses down to 80 fs have been obtained with a two-prism resonator. Tuning range as broad as 30 nm and output power up to 55 mW have also been achieved, confirming the effectiveness of the proposed laser architecture.

- by Giancarlo Reali
- •
- Quantum Physics, Ultrafast Optics, Carbon Nanotubes, Carbon Nanotube

A photonic ADC based on balanced detection, phase encoded optical sampling, wavelength multiplexing, and electronic quantization is demonstrated. It achieves 7.0 ENOB resolution at a 2GSa/s sub-sampling rate for a 40 GHz input analog... more

- by Michael Peng
- •
- Ultrafast Optics, Radio Frequency, High Speed, Optical Filters

We review waveform analysis and optical performance monitoring of ultrabroadband signals using a photonicchip-based radio-frequency spectrum analyzer. This approach offers the potential for fast integrated monitoring and characterization... more

- by Barry Luther-davies
- •
- Quantum Physics, Nonlinear Optics, Ultrafast Optics, Optical Waveguide

Photoinduced absorption data for polyaniline films in two forms, emeraldine base (EB) and emeraldine salt (ES), are presented and discussed. Electrochemically synthesized films were excited with laser pulses of large intensity (the energy =2 eV, the pulse duration = 8 ps, the pulse energy up to 10 mJ/cm'). Optical spectra of the EB and ES films in the temperature range from 298 to 363 K are also studied. The polymer film is presumed to be a three-dimensional (3D) system of long finite conjugated fragments of polymer chains. Considering each fragment integrally and taking into account the electronic polarization energy ( =1.5 eV) for a charge in the film, the 3D model of a charge-transfer exciton is proposed. The low-energy absorption of EB and ES and the electronc processes in photoexcited films are interpreted within the framework of the proposed model, avoiding the commonly adopted approach of an isolated polymer chain.

We present the first results on experimentally measured ultrafast X-ray scattering of strongly driven molecular Iodine and analysis of high-order anisotropic components of the scattering signal. We discuss the technical details of retrieving high fidelity high-order anisotropy components from measured scattering data and outline a method to analyze such signals using Legendre decomposition. We describe how anisotropic motions can be extracted from the various Legendre orders using simulated anisotropic scattering signals and Fourier analysis. We implement the method on the measured signal and observe a multitude of dissociation and vibration motions simultaneously arising from various multiphoton transitions occurring in the sample. We use the anisotropic scattering information to disentangle the different processes and assign their dissociation velocities on the Angstrom and femtosecond scales de-novo. Capturing motions in space and time at the atomic scale is fundamental to the understanding of chemical reactions and structural dynamics of molecules of different complexities. Some of the emerging tools that allow such studies are ultrafast scattering modalities, primarily by X-rays and relativistic electrons, that were made feasible in recent years. In these studies, the motions from excited molecules are captured in a pump-probe scheme, where the excitation pump pulse is often an ultra-short linearly polarized optical laser pulse with a duration shorter than the typical timescales of motion of interest, as illustrated in Fig 1. The scattering signal is then usually integrated over angle for improved fidelity and subtracted from the scattering signal of the unexcited system, to allow tracing changes of signal positions and infer structural dynamics 1. This approach was used successfully to demonstrate coherent motions and dynamics in molecules in the gas phase 2,3 , as well as structural changes in molecules in solution after the electronic excitation 4-6. In these experiments, the optical pulse parameters were carefully chosen to ensure that only single-photon absorption is taking place and that the molecular system is photoexcited to a specific electronic state. This is often done by varying the pump intensity in the pump-probe setup and finding conditions where linearity of the excitation, as manifested by the scattering signal is achieved. The motion is inferred by measuring the scattering difference as a function of delay and applying modeling and simulations. Using angle integrated scattering signal is well justified as it captures all types of motions that take place in the photoexcited system. However , angle-dependent signals can be dramatically attenuated if only the isotropic component is being analyzed. In many cases, there is an inherent anisotropy in the scattering signal when a sample is excited by linearly polarized light due to an optically induced dipole moment transition. This interaction creates geometric alignment in the ensemble, and can be used to filter and enhance the specific processes under study, such as in the case of a single-photon absorption process 7-9 , as well as perturbative two-photon excitation 10,11. Higher orders of anisotropy play a significant role in understanding and probing cases where the molecular system is in the presence of multi-photon absorption and strong laser fields, such as dissociation due to bond softening 12 , above-threshold dissoci-ation 13 , quantum coherent control 14 , and light-induced conical intersections 15. In addition, the interaction of ultrashort pulses with molecules with anisotropic polarizability will generate non-adiabatic (or impulsive) alignment 16. The broad bandwidth of an ultrashort pulse creates rotational wavepackets that evolve and rephase at periodic time delays, forming molecular alignment, manifested by high order anisotropy in the sample under field-free conditions. Molecular alignment is often used to probe diverse phenomena in the molecular frame, such as, polyatomic J o u r n a l N a me , [ y e a r ] , [ v o l. ] ,1-9 | 1

The optical components industry stands at the threshold of a major expansion that will restructure its business processes and sustain its profitability for the next three decades. This growth will establish a cost effective platform for the partitioning of electronic and photonic functionality to extend the processing power of integrated circuits. BAE Systems, Lucent Technologies, Massachusetts Institute of Technology, and Applied Wave Research are participating in a high payoff research and development program for the Microsystems Technology Office (MTO) of DARPA. The goal of the program is the development of technologies and design tools necessary to fabricate an application-specific, electronicphotonic integrated circuit (AS-EPIC). As part of the development of this demonstration platform we are exploring selected functions normally associated with the front end of mixed signal receivers such as modulation, detection, and filtering. The chip will be fabricated in the BAE Systems CMOS foundry and at MIT's Microphotonics Center. We will present the latest results on the performance of multi-layer deposited High Index Contrast Waveguides, CMOS compatible modulators and detectors, and optical filter slices. These advances will be discussed in the context of the Communications Technology Roadmap that was recently released by the MIT Microphotonics Center Industry Consortium.

- by Milos Popovic
- •
- Ultrafast Optics, Femtosecond Laser, Technology development, Silicon Photonics

- by Andreas Wieck
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- Ultrafast Optics, Optical Spectroscopy, Physical sciences, Band Gap

Recent advances in the generation and characterization of extreme-ultraviolet pulses, generated either by intense femtosecond lasers or by free electron lasers, are pushing the frontier of time-resolved investigations down to the attosecond domain, the relevant timescale for electron motion. The quantum nature of the intertwined electronic and nuclear motion requires theoretical models going beyond the Born-Oppenheimer approximation and taking into account electron correlation, representing a challenge for the computational power available nowadays. Understanding how the electron dynamics inside molecules can influence chemical reactions presents important implications in several fields and allows for the development of new technologies. In this paper, we report on experimental and theoretical results of an investigation in H2/D2, where for the first time control of molecular dynamics with attosecond resolution was achieved. The data represent the first evidence of the control of the electron motion in a molecule undergoing a chemical reaction on the subfemtosecond scale.

- by Felipe Morales
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- Quantum Physics, Ultrafast Optics, Femtosecond Laser, Dissociation