Picosecond Delay Research Articles

Design and characterization of low loss 50 picoseconds delay line on SOI platform

We design and experimentally demonstrate 50 picoseconds (ps) low loss delay line on 300 nm SOI platform. The delay line unit consists of straight rib waveguide and strip bend section linked by a transition taper waveguide. Low propagation loss of ~0.1 dB/cm is achieved on the straight rib waveguide. With taking into account both low loss and desirable delay, a complete design and characterization process for passive delay line is presented. Our measurement results show that about 0.7 dB excess loss is achievable for 50 ps delay. The loss can be further reduced by adjusting the layout parameters.

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ∼10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.

Intrinsic timing jitter of superconducting nanowire single-photon detectors

We report on an experimental investigation of the timing jitter (TJ) in a fiber-coupled detection system based on a superconducting nanowire single-photon detector (SNSPD). We used a picosecond pulsed laser and fiber delay lines to generate subsequent optical pulsed pairs in each period and measured the TJ between the pulses. Because only the interval time between two pulses in each period was interested, we did not need to know the absolute time of every pulse. Therefore, the TJ from laser and trigger signal can be removed. In order to estimate the intrinsic TJ of SNSPD, we analyzed the TJ caused by time distribution of optical pulse, electronic components, readout noise, kinetic inductance and dispersion in fiber channel. Finally, we obtained the intrinsic TJs for two kinds of detectors considering these impacts.

Intensity correlation measurement system by picosecond single shot soft x-ray laser

We developed a new soft x-ray speckle intensity correlation spectroscopy system by use of a single shot high brilliant plasma soft x-ray laser. The plasma soft x-ray laser is characterized by several picoseconds in pulse width, more than 90% special coherence, and 10(11) soft x-ray photons within a single pulse. We developed a Michelson type delay pulse generator using a soft x-ray beam splitter to measure the intensity correlation of x-ray speckles from materials and succeeded in generating double coherent x-ray pulses with picosecond delay times. Moreover, we employed a high-speed soft x-ray streak camera for the picosecond time-resolved measurement of x-ray speckles caused by double coherent x-ray pulse illumination. We performed the x-ray speckle intensity correlation measurements for probing the relaxation phenomena of polarizations in polarization clusters in the paraelectric phase of the ferroelectric material BaTiO(3) near its Curie temperature and verified its performance.

Localization dynamics of biexcitons and electron–hole plasmas in GaN‐based mixed crystals

AbstractWe report time‐resolved photoluminescence (PL) spectra in highly excited AlxGa1−xN mixed crystals under band‐to‐band excitation. The PL dynamics of excitons and biexcitons reveals that the localization of excitons begins within a few picoseconds delay time and almost all the excitons relax into the localized band‐tail states within 10 ps delay time after laser excitations. The biexciton formation time of ∼2 ps is extracted from the rise time of the PL intensities of excitons and biexcitons. These results indicate that biexcitons are formed from the free excitons and that the exciton localization, due to the potential fluctuations in the mixed crystals, reduces the exciton–exciton collisions and the formation of free biexcitons.

Theory of nonlinear optical response of ensembles of double quantum dots

We study theoretically the time-resolved four-wave mixing (FWM) response of an ensemble of pairs of quantum dots undergoing radiative recombination. At short (picosecond) delay times, the response signal shows beats that may be dominated by the subensemble of resonant pairs, which gives access to the information on the interdot coupling. At longer delay times, the decay of the FWM signal is governed by two rates which result from the collective interaction between the two dots and the radiation modes. The two rates correspond to the subradiant and super-radiant components in the radiative decay. Coupling between the dots enhances the collective effects and makes them observable even when the average energy mismatch between the dots is relatively large.

Performance of a picosecond x-ray delay line unit at 839 keV

A prototype device capable of splitting an x-ray pulse into two adjustable fractions, delaying one of them with the aim to perform x-ray photon correlation spectroscopy and pump-probe type studies, was designed, manufactured, and tested. The device utilizes eight perfect silicon crystals in vertical 90 degrees scattering geometry. Its performance has been verified with 8.39 keV synchrotron radiation. The measured throughput of the device with a Si(333) premonochromator at 8.39 keV under ambient conditions is 0.6%. Time delays up to 2.62 ns have been achieved, detected with a time resolution of 16.7 ps.

Four‐wave mixing optical response of an ensemble of quantum dot molecules

AbstractFour‐wave mixing (FWM) optical response from a sample containing a non‐homogeneously broadened ensemble of vertically stacked pairs of quantum dots is studied theoretically. The effects of collective interaction between the dots and their electromagnetic environment are analyzed. It is shown that the shape of the FWM echo is affected by the coupling between the two dots and, to some extent, by the statistics of the ensemble.In the time‐integrated signal, oscillations on picosecond delay time scale appear which lead to a reduction of the coherent response for coupled dots. For longer delays, the evolution is dominated by radiative decay, with the decay rate increasing from Γ to 2Γ(superradiance effect) as the coupling becomes stronger. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast self-modulation of the optical absorption spectrum under conditions of both the ultrashort optical pumping and superluminescence in GaAs

Self-modulation of the optical absorption spectrum is observed during the picosecond photogeneration of charge carriers and intense superluminescence in GaAs. As the picosecond delay τ of the probing pulse with respect to the pump pulse is varied in the region of τ < 0, the local points of the absorption intensification (juts) shift along the spectrum (the modulation resembles a running wave). As the value of τ is varied in the vicinity of τ = 0, the juts in the spectrum arise and disappear at approximately fixed photon energies (the modulation resembles a standing wave). At certain photon energies, the dependence of the rate of variation in the absorption coefficient dα/dτ on τ is found to be modulated by pulsations, similarly to the previously observed modulation of the picosecond stimulated emission from GaAs. Presumably, the spectrum self-modulation represents (and, thus, reveals) the modulation of the electron distribution in the conduction band. This modulation is caused by the fact that the evolution of the electron-population depletion at the bottom of the conduction band during superluminescence reflects (due to the electron-phonon interaction) on the population of the upper energy levels in the band.

Novel Assessment of Bone Using Time-Resolved Transcutaneous Raman Spectroscopy

With fragility fractures increasing as the population ages, there is a need for improved means to estimate risk of fracture. We recorded Raman spectra of both the mineral and organic phases of bone transcutaneously, a technology with potential to enhance bone quality and fracture risk assessment. The current "gold standard" assessment of bone quality is BMD determined by DXA. However, this accounts for only 60-70% of bone strength. X-rays are absorbed by the mineral phase of bone, whereas the organic phase remains essentially invisible; however, bone strength is critically dependent on both phases. We report, for the first time, a Raman spectroscopic technique that analyses both phases of bone beneath unbroken skin by eliminating spectral components of overlying tissues. We used an 800-nm laser (1-kHz, 1-ps pulses) with a synchronized 4-ps Kerr gate with variable picosecond delay that effectively shuttered out photons from overlying tissues. We measured bone Raman spectra at a point 2 mm above the carpus from two mouse genotypes with extreme differences in bone matrix quality: wildtype and oim/oim (matched for age, sex, and weight). Typical depth was 1.1 mm. We repeated the measurements with overlying tissues removed down to bone. Oim/oim mice produce only homotrimeric collagen, which results in poorly mineralized bone tissue. The main spectral features were present from both bone phases. The spectral bands were in similar ratios when measured through the skin or directly from bone (in both genotypes). The band of the mineral phase (phosphate nu1) was smaller in oim/oim mice when measured directly from bone and through skin. The band associated with a particular vibrational mode of organic phase collagen (CH2 wag) showed a frequency shift between the genotypes. This novel technique allowed us, for the first time, to make objective transcutaneous spectral measurements of both the mineral and the organic phases of bones and distinguish between normal and unhealthy bone tissue. After further optimization, this technology may help improve fracture risk assessments and open opportunities for screening in anticipation of the predicted increase in fragility fractures.

Modulation of the Characteristics of Intense Picosecond Stimulated Emission from GaAs

Intense picosecond stimulated emission from the face of a thin GaAs film is studied. The emission is observed when GaAs is pumped with high-power picosecond optical pulses. It is found that the dependences of the emission energy on the photon energy, the picosecond delay between two pump pulses, and the distance between the active region and the face are modulated. Modulation is taken to mean the appearance of protrusions or peaks in any of the mentioned dependences. The modulation parameters for the dependences under consideration are found to be related by expressions that make it possible to suggest the following. The modulation of characteristics is caused by a common (not yet conclusively identified) mechanism of self-modulation of the emission spectrum. This mechanism is related to an ultrafast nonlinear interaction between a highly photoexcited semiconductor and the pump radiation and stimulated emission. There is indirect evidence that this mechanism also gives rise to an amplitude modulation of the emission in a picosecond time interval.

Thermal response of nanocomposite materials under pulsed laser excitation

The optical properties of nanocomposite materials made of matrix-embedded noble metal nanoparticles strongly depend on thermal effects from different origins. We propose a classical model describing the energy exchanges within the nanoparticles and between the latter and the surrounding dielectric host subsequent to a light pulse absorption. This model, which accounts for the thermal interactions between neighboring particles, allows us to calculate numerically the temperature dynamics of the electrons, metal lattice and matrix as functions of particle size, and metal concentration of the medium, whatever be the pulsed excitation temporal regime. It is illustrated in the case of Au:SiO2 materials under femtosecond and nanosecond pulse excitation. It is shown that, in the femtosecond regime, the heat transfer to the matrix cannot be neglected beyond a few picosecond delay from which particle size and metal concentration play a significant role in the electron relaxation. In the nanosecond regime, these morphologic parameters influence crucially the material thermal behavior with the possibility of generating a thermal lens effect. The implications in the analysis of experimental results regarding both the electron relaxation dynamics and the nonlinear optical properties are also discussed. Finally, a method to adapt the model to the case of thin nanocomposite film is proposed.

A UV–Visible–NIR fluorescence lifetime imaging microscope for laser-based biological sensing with picosecond resolution

This article describes the design and characterization of a wide-field, time-domain fluorescence lifetime imaging microscopy (FLIM) system developed for picosecond time-resolved biological imaging. The system consists of a nitrogen-pumped dye laser for UV–visible–NIR excitation (337.1–960 nm), an epi-illuminated microscope with UV compatible optics, and a time-gated intensified CCD camera with an adjustable gate width (200 ps-10-3 s) for temporally resolved, single-photon detection of fluorescence decays with 9.6-bit intensity resolution and 1.4-μm spatial resolution. Intensity measurements used for fluorescence decay calculations are reproducible to within 2%, achieved by synchronizing the ICCD gate delay to the excitation laser pulse via a constant fraction optical discriminator and picosecond delay card. A self-consistent FLIM system response model is presented, allowing for fluorescence lifetimes (0.6 ns) significantly smaller than the FLIM system response (1.14 ns) to be determined to 3% of independently determined values. The FLIM system was able to discriminate fluorescence lifetime differences of at least 50 ps. The spectral tunability and large temporal dynamic range of the system are demonstrated by imaging in living human cells: UV-excited endogenous fluorescence from metabolic cofactors (lifetime ∼1.4 ns); and 460-nm excited fluorescence from an exogenous oxygen-quenched ruthenium dye (lifetime ∼400 ns).

On-chip picosecond delay measurement of RSFQ digital logic gates

Because RSFQ circuits are intended to operate at multi-GHz frequencies, a logical requirement for developing the technology is a method of accurately measuring the picosecond delays associated with individual logic circuits. A technique has been developed for on-chip measurements of such RSFQ gate delays. The central element in this scheme is a race between a path of calibrated variable delay and a path of unknown delay. Modification of the canonical RSFQ RS flip-flop circuit yields a multiple state destructive readout cell (MDRO), in which one can configure the number of flux quanta to be stored. This circuit has been experimentally verified for groups of two flux quanta. Used in concert with the confluence buffer, this scheme can provide the RSFQ designer with sub-picosecond pulse arrival delay information crucial for higher order circuit simulation. A detailed experimental process is presented from which this timing information can be extracted using basic low-speed measurement techniques. >

Photodissociation of aminophenyldisulfide and aminophenylthiol in the liquid phase. Geminate recombination and the dimer formation of p-aminophenylthiyl radical

Measuring picosecond time-resolved absorption spectra in the visible region, the photodissociation of p-aminophenyldisulfide and p-aminophenylthiol in various solutions has been investigated. In tens of picoseconds delay times a decay of p-aminophenylthiyl radical ascribed to the geminate recombination was observed for p-aminophenyldisulfide in nonpolar solvents, while the decay of the radical was not detected for p-aminophenylthiol in the same time region. In addition to a slight change in the spectral shape of p-aminophenylthiyl radical, the rise of the broad absorption band, which should be assigned to the radical dimer, follows the geminate recombination in nonpolar solvents.

Femtosecond carrier dynamics in the presence of a cold plasma in GaAs and AlGaAs

The femtosecond dynamics of hot carriers interacting with a cold high density electron-hole plasma are investigated in GaAs and Al0.2Ga0.8As. Studies are performed using a three pulse pump-probe technique where a cold plasma is first generated by a femtosecond pulse, then pump-probe transmission measurements are performed after a few hundred picoseconds delay. The results indicate only a small increase of the hot carrier thermalization rate even for plasma densities as high as 1018 cm−3.

Optoelectronic measurement of picosecond turn-on delay in InGaAsP laser diodes

A tandem photoconductive switch, producing nearly rectangular electrical pulses with variable duration from 25 ps to 1 ns, was used for accurate turn-on delay measurements of a laser diode. Maximum electrical pulse amplitude was 75 V and both rise and fall times were 15 ps. The shortest delay recorded was 60 ps. The carrier lifetime at the lasing threshold was found to be 3.89 ns.

Spectroscopic demonstration of picosecond, phase-shifted laser multiple-pulse sequences

We show that the concept of phase-shifted laser pulse trains is meaningful only for purely amplitude-modulated pulses, and probably cannot be generalized to use the output of a normal pulsed laser. We demonstrate a laser system which gives picosecond pulse lengths, picosecond or nanosecond interpulse delays, and complete phase control. This makes most molecular transitions amenable to the coherent techniques previously restricted to nuclear magnetic resonance. Fluorescence suppression by π phase shifts is demonstrated for the first time with picosecond pulses and long delays.

Direct measurement of picosecond propagation delays in individual logic gates by a differential optoelectronic technique

We describe the first direct measurement of single-gate propagation delays in gigabit GaAs digital IC's. Our technique uses picosecond light pulses to generate short on-chip logic-level-switched pulses and infers single logic gate delays by differential measurement of output waveforms. In the ∼ 2-GHz clock-rate D-flip-flop selected for these measurements, single-gate propagation delays of ∼ 100 ps were measured in specific NOR gates internal to the flip-flop (FF) with this new measurement technique; the technique is easily extendible to measurement of gate delays of the order of a few picoseconds.

A new technique for the measurement of speeds of gigahertz digital IC's

We describe a new technique for the measurement of picosecond switching speeds and propagation delays in gigahertz digital IC's. This technique, based on logic-level sampling with pairs of ultrashort optical pulses at variable interpulse delays, provides accurate and reliable values of picosecond propagation delays without requiring the coupling of high-frequency electrical signals to and from the IC chip. Since this optical technique dispenses with the usual need for dicing and mounting the IC wafer in high-frequency (multigigahertz bandwidth) test fixtures, a promising application of its use is in the precise temporal characterization of multigigabit logic circuits in low-frequency (< 100-MHz) probe stations.