Buildup Dynamics Research Articles

Transient Leakage Electroluminescence of Quantum-Dot Light-Emitting Diodes.

Parasitic emission or leakage emission caused by electron leakage to a hole transport layer in quantum-dot light-emitting diodes (QLEDs) critically impacts device efficiency and operational stability. The buildup dynamics of such emission channels, however, was insufficiently researched. Herein, we investigate transient electroluminescence dynamics of leakage emission in red/green/blue (R/G/B) QLEDs and reveal notable contrast for R and G. In RQLEDs, leakage emission exhibits delayed turning-on than primary emission, which is attributed to much slower filling up from lower-energy electron states and the initial quenching by nonradiative recombination with trapped holes. For GQLEDs, leakage emission turns on much more concurrently, and emission preferably starts from higher energy states. For R/G QLEDs, under varied offset voltage modulation, the current efficiency of primary emission is invert-correlated to leakage emission, reconfirming leakage emission as a loss indicator.

Pulse buildup dynamics in a self-starting Mamyshev oscillator.

The Mamyshev oscillator (MO) can generate high-performance pulses. However, due to their non-resonant cavities, they usually are not self-starting, and there is almost no effort to reveal the pulse buildup dynamics of the MO. This paper investigates the dynamic of single pulse (SP) and multi-pulse formation in a self-starting MO. It indicated that both SP self-starting and multi-pulse self-starting can be obtained by adjusting the oscillator parameters. More importantly, increasing pump power could only result in bound state pulses (BSPs) if SP self-starting was formed. With the increase of the pump power, the pulse number in BSPs would increase. However, multiple pulses could not be formed only by increasing the pump power, and the BSPs obtained here underwent SP generated from noise, amplified, and then bounded, which is different from conventional passive mode-locked fiber lasers (CPMLFLs). On the other hand, if multiple pulses were self-initiated, BSPs, pulse bunch, and harmonic mode-locked pulses (HMLPs) could be obtained by adjusting the polarization state and pump power in the cavity. Furthermore, once any of the above states are formed, if the oscillator polarization state and filter interval are unchanged, only increasing the pump power from zero, the original state can still be obtained, which is consistent with the characteristics of the CPMLFLs. These findings will provide new insights into the pulse dynamics of self-starting MO, which will be significant for studying ultrafast laser technology and nonlinear optics.

Formation of electron traps in semiconducting polymers via a slow triple-encounter between trap precursor particles

ABSTRACT Already in 2012, Blom et al. reported (Nature Materials 2012, 11, 882) in semiconducting polymers on a general electron-trap density of ≈3 × 1017 cm−3, centered at an energy of ≈3.6 eV below vacuum. It was suggested that traps have an extrinsic origin, with the water-oxygen complex [2(H2O)-O2] as a possible candidate, based on its electron affinity. However, further evidence is lacking and the origin of universal electron traps remained elusive. Here, in polymer diodes, the temperature-dependence of reversible electron traps is investigated that develop under bias stress slowly over minutes to a density of 2 × 1017 cm−3, centered at an energy of 3.6 eV below vacuum. The trap build-up dynamics follows a 3rd-order kinetics, in line with that traps form via an encounter between three diffusing precursor particles. The accordance between universal and slowly evolving traps suggests that general electron traps in semiconducting polymers form via a triple-encounter process between oxygen and water molecules that form the suggested [2(H2O)-O2] complex as the trap origin.

Contrasting Phosphorus Build-up and Drawdown Dynamics in Soils Receiving Dairy Processing Sludge and Mineral Fertilisers

Sustainable utilisation of waste from the food industry is required to transition to a circular economy. The dairy industry relies on high phosphorus (P) inputs and produces large quantities of P-rich dairy processing sludge (DPS). Recycling DPS into P fertilisers provides an opportunity to decrease the reliance on chemical P fertilisers. However, current soil nutrient management planning (NMP) is based on chemical P and does not account for recycled alternatives. A pot trial using a novel isotope pool dilution technique was used to describe build-up and drawdown cycles of P in soils fertilised with DPS. Changes in available, exchangeable, and Mehlich3 P (M3-P) pools were recorded over 36 weeks of grass growth. Results demonstrated that in the period of high P demand (12 weeks), these P pools were depleted. As crop growth and demand decreased, available P recovered through mobilisation of P from exchangeable P and M3-P reserves. DPS allowed available P to recover and build up to agronomic target levels after 24 weeks. Using DPS, build-up of available and exchangeable P was slower but P use efficiency was higher at stages of slow growth. Dairy waste created a more stable P pool which could be utilised by crops over a growing season indicating that NMP needs to account for this in the decision support for growers. Isotope studies revealed that extractive agronomic tests do not capture drawdown in P reserves.

Bidirectional mode-locked erbium-doped fiber laser based on an all-fiber gold nanofilm saturable absorber.

We demonstrate a bidirectional mode-locked erbium-doped fiber laser by incorporating gold nanofilm as a saturable absorber (SA). The gold nanofilm SA has the advantages of high stability and high optical damage threshold. Besides, the SA exhibits a large modulation depth of 26% and a low saturation intensity of 1.22 MW/cm2 at 1.56 μm wavelength band, facilitating the mode-locking of bidirectional propagating solitons within a single laser cavity. Bidirectional mode-locked solitons are achieved, with the clockwise pulse centered at 1568.35 nm and the counter-clockwise one at 1568.6 nm, resulting in a slight repetition rate difference of 19 Hz. Moreover, numerical simulations are performed to reveal the counter-propagating dynamics of the two solitons, showing good agreement with the experimental results. The asymmetric cavity configuration gives rise to distinct buildup and evolution dynamics of the two counter-propagating pulses. These findings highlight the advantage of the gold nanofilm SA in constructing bidirectional mode-locked fiber lasers and provide insights for understanding the bidirectional pulse propagation dynamics.

Fiber optical parametric oscillator pumped by a 16 GHz tunable frequency comb

We demonstrate a 16 GHz repetition rate fiber-based optical parametric oscillator, pumped by an electro-optical comb tunable around 1.03 µm, delivering picosecond and wavelength tunable signal and idler pulses. Build-up and relaxation dynamics of these pulses are reported.

Build-up dynamics of a self-mode-locked Thulium-doped fiber laser explored by dispersive Fourier transformation

Build-up dynamics of a self-mode-locked Thulium-doped fiber laser explored by dispersive Fourier transformation

Single-shot dynamics of dual-comb generation in a polarization-multiplexing fiber laser

Dual optical frequency combs have been a recurrent case of study over the last decade due to their wide use in a variety of metrology applications. Utilizing a single cavity laser to generate a dual comb reduces system complexity and facilitates suppression of common noise. However, a dual-comb regime in single cavity lasers tends to be more unstable and difficult to achieve. Therefore, having a better understanding about the way they are generated could improve and automate their generation and control. In this paper, we investigate the build-up dynamics and collision of dual comb in a polarization-multiplexing ring-cavity fiber laser using DFT (Dispersive Fourier Transform) method. We observe a bunch of meta-stable short-lived mode-locking states before the laser entered the dual-comb mode-locking state. The energy level of this short-lived initial pulses determines its evolution. If it decreases too much, the pulse will eventually collapse while if it stays above certain level, it will be successfully generated. The results presented in this paper increase the understanding of dual-comb generation inside a single cavity laser and may contribute in future attempts to increase the stabilization of this regime.

Revealing the pulse dynamics in a Mamyshev oscillator: from seed signal to oscillator pulse.

The Mamyshev oscillator (MO) is a promising platform to generate high-peak-power pulse with environmentally stable operation. However, rare efforts have been dedicated to unveil the dynamics from seed signal to oscillator pulse, particularly for the multi-pulse operation. Herein, we investigate the buildup dynamics of the oscillator pulse from the seed signal in a fiber MO. It is revealed that the gain competition among the successively injected seed pulses leads to higher pump power that is required to ignite the MO, hence resulting in the higher optical gain that supports buildup of multiple oscillator pulses. The multiple oscillator pulses are identified to be evolved from the multiple seed pulses. Moreover, the dispersive Fourier transform (DFT) technique is used to reveals the real-time spectral dynamics during the starting process. As a proof-of-concept demonstration, a highly intensity-modulated pulse bunch was employed as the seed signal to reduce the gain competition effect and avoid the multi-pulse starting operation. The experimental results are verified by numerical simulations. These findings would give new insights into the pulse dynamics in MO, which will be meaningful to the communities interested in ultrafast laser technologies and nonlinear optics.

Transient dynamics in mode-locked all-PM Er-doped fiber laser with NALM

In this study, we utilized the Time-Stretch Dispersive Fourier Transform (TS-DFT) method to explore rapid transient states in both time and frequency domains of an all-polarization-maintaining dispersion-managed figure-eight mode-locked laser with Nonlinear Amplifying Loop Mirror (NALM). We detail the different stages of build-up dynamics at varying pump powers, encompassing the Q-switching (QS) stage, transient single pulse formation, pulsation beating patterns, bound states, and the final transition to a single pulse state. Our findings reveal that the mode-locked bound state regime emerges from a single pulse stage following QS, different from previously reported directly resulting from QS pulses or relaxation oscillations. In a novel contribution to the field, we have experimentally demonstrated the transient single pulse stage of build-up dynamics and compared it with the fundamental mode-locked single pulse.

Build-up dynamics of different pulse types in a dispersion-managed Yb-doped fibre laser

We present a dispersion-managed mode-locked ytterbium fibre laser based on the nonlinear polarization evolution technique. Four single-pulse states could be generated by changing group delay dispersion values from anomalous (−0.0038 ps2) to normal (0.051 ps2) with a grating pair. In the normal dispersion regime, the pulse width was gradually decreased with the decrease of the net dispersion. In the anomalous dispersion region, the typical soliton was obtained, which time-bandwidth product could be calculated as 0.526. In addition, the dynamics of four states were also investigated by using the dispersive Fourier transformation technique. Different pulse evolution dynamics were observed from chaotic state to mode-locking.

Real-Time Exploration on Buildup Dynamics of Diode-Pumped Passively Mode-Locked Nd:YVO4 Laser with SESAM

The buildup dynamics of diode-pumped passively mode-locked solid-state laser is thoroughly explored using the real-time measurement with temporal sampling rate of up to 40 GHz. A concise cavity is developed to ensure the transient dynamics purely arising from the gain medium and saturable absorber. Experimental results reveal that the laser output in the buildup process exhibits numerous passively Q-switched pulses followed with a damped relaxation oscillation prior to the stable mode locking. Furthermore, it is confirmed that the laser output has already displayed single clean mode-locked pulses inside the first several Q-switched envelopes before stepping into the stage of relaxation oscillation. The present real-time exploration is expected to provide important information for practical applications with temporal modulation of the pump intensity.

Transient multi-soliton dynamics between adjacent-order harmonic mode-locking states in a hybrid mode-locked fiber laser

The buildup dynamics from noise-seeded relaxation oscillation to harmonic mode-locking (HML) solitons generation have been well investigated in ultrafast fiber lasers. However, the transient multi-soliton dynamics in higher-order HML states are still attractive yet largely unexplored. We reveal here that, the experimental observation of the multi-soliton dynamics including soliton beating dynamics, soliton annihilation, transient bound state, two-soliton resonance state, soliton splitting, and multi-soliton repulsion dynamics are analyzed by the emerging time-stretched dispersive Fourier transform (TS-DFT) technique. Particularly, the excessive intracavity energy result in the double-pulse pulsating with transient bound states and three types of soliton molecules states can be observed in the earlier and middle part of the transition process. Numerical results further reveal that several similar multi-soliton dynamics operating at stable 2nd and 3rd states could be caused by a fine tuning of the pump power level in a hybrid mode-locked laser. Our results unveil a diverse set of ultrafast transient process in fiber lasers, and open up new ways into obtaining and controlling the generation of high-repetition-rate ultrashort pulses in fiber lasers.

Serial Polarization Transfer by Combination of Cross-Relaxation and Rotational Resonance for Sensitivity-Enhanced Solid-State NMR.

Methods which induce site-specificity and sensitivity enhancement in solid-state magic-angle spinning NMR spectroscopy become more important for structural biology due to the increasing size of molecules under investigation. Recently, several strategies have been developed to increase site specificity and thus reduce signal overlap. Under dynamic nuclear polarization (DNP) for NMR signal enhancement, it is possible to use cross-relaxation transfer induced by select dynamic groups within the molecules which is exploited by SCREAM-DNP (Specific Cross Relaxation Enhancement by Active Motions under DNP). Here, we present an approach where we additionally reintroduce the homonuclear dipolar coupling with rotational resonance (R2) during SCREAM-DNP to further boost the selectivity of the experiment. Detailed analysis of the polarization buildup dynamics of 13C-methyl polarization source and 13C-carbonyl target in 2-13C-ethyl 1-13C-acetate provides information about the sought-after and spurious transfer pathways. We show that dipolar-recoupled transfer rates greatly exceed the DNP buildup dynamics in our model system, indicating that significantly larger distances can be selectively and efficiently hyperpolarized.

Enhanced Extreme Ultraviolet Free Induction Decay Emission Assisted by Attosecond Pulses

We demonstrate the extreme ultraviolet free induction decay emission that can be significantly enhanced by employing isolated attosecond pulses. The near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecond pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.

Real-time buildup dynamics of additive dissipative soliton in mode-locked fiber lasers

Multi-soliton operation is a universal phenomenon in mode-locked lasers, which is generally considered to be generated from pulse splitting or pulse shaping of dispersive waves. Here, we experimentally unveiled the formation dynamics of additive dissipative soliton (ADS) in mode-locked fiber lasers by means of the time stretched technique. Two distinct evolution ways can be observed during the ADS formation process. One way is pulse splitting which corresponds to the soliton molecules with relatively close separation, while the other one is generated from background noise which corresponds to multi-solitons with loose separation. Due to the environmental perturbation, the former process undergoes transient quasi-periodic dissipative soliton explosion before the soliton molecules are stable and the latter way may suffer transient standstill before the formation of additive soliton. These findings could provide some assistance for further enhancing the comprehension of the buildup of multiple soliton and soliton molecules in dissipative systems.

Spotlighting the Simultaneous Formation of Coherent and Incoherent Dissipative Solitons in an Er-Doped Bidirectional Ultrafast Fiber Laser

The emergence of localized structures originating from instabilities is of particular interest to scientists interested in exploring the rich nonlinear dynamics associated with complex dynamical systems. In the context of ultrafast lasers, the localized structures can be broadly divided into two categories: coherent structures of the dissipative soliton (DS) and incoherent structures such as the noiselike pulse (NLP). These two different types of structures have unique properties and applications and their physical origins are markedly different. Having both in a single fiber laser is an intriguing but challenging problem. Here we demonstrate the formation of the DS and NLP in a bidirectional ultrafast fiber laser and study their buildup dynamics from $Q$-switched instabilities. The clockwise light is a DS while the counterclockwise light is a NLP. The synchronized buildup dynamics of a DS or NLP from $Q$-switched instabilities provides vital evidence of the $Q$-switched instabilities as a universal indicator for the generation of localized structures in ultrafast lasers. The evenly spaced $Q$-switched spikes before the onset of a DS (NLP) exhibit intensity fluctuations, which is strikingly different than in the mode-locking regime in unidirectional oscillators, hence providing decisive evidence of the interaction between the counterpropagating light fields in the bidirectional lasers. The spectra cross-correlation and the histogram of the spectral intensity confirm the concurrent formation of a DS and NLP. Numerical simulations confirmed experimental observations. These findings help deepen understanding of the behavior of bidirectional ultrafast lasers and also demonstrate their potential as multifunctional ultrafast laser sources.

Ultrafast Buildup Dynamics of Terahertz Pulse Generation in Mode-Locked Quantum Cascade Lasers

Ultrashort terahertz pulse generation is essential for a range of proven terahertz applications, from time-resolved spectroscopy of fundamental excitations to nondestructive testing and imaging. Recently, it has been shown that semiconductor-based terahertz quantum cascade lasers (QCLs) can be used to generate pulses as short as a few picoseconds through active mode locking. However, further progress for subpicosecond and high peak power pulse generation is hampered by poor knowledge on how the electric field actually forms in these lasers. Here, we theoretically and experimentally show the amplitude- and phase-resolved buildup of pulse generation through active mode locking, from initiation of pulse generation to the nanosecond steady state. The experimental results, using an ultrafast coherent seeding technique to probe the laser from femtosecond to nanosecond time scales, are in full agreement with the theoretical calculations based on a theoretical model using multimode reduced rate equations. In particular, we show that the electric field buildup to achieve short pulse operation is extremely fast, requiring only a few photon round trips, owing to the ultrafast gain dynamics of the lasers. Further, this shows a gain recovery time of the order of a few picoseconds, an order of magnitude smaller than the photon round-trip time, highlighting that terahertz QCLs are categorically class-A lasers. This demonstration marks an important formulism for future progress towards exploring the ultrafast pulse generation buildup dynamics of these complex semiconductor lasers.

Investigations on pulse dynamics and offset spectral filtering in Er-doped Mamyshev fiber oscillator

We have experimentally established an all fiber Er-doped Mamyshev oscillator that is self-started with an external seed laser. The Mamyshev fiber oscillator emitted pulses with a repetition rate of 6.54 MHz and pulse duration of 1.61 ps. Both the buildup dynamics and intracavity evolution dynamics are numerically modeledwith nonlinear Schrödinger equation. The impact of offset spectral filtering including frequency offset and spectrum bandwidth on the Mamyshev fiber oscillator is simulated. Numerical results show that these parameters can influence the physical properties of output pulses in Mamyshev oscillator, which is analogous to the saturable absorber. These findings provide a deep understanding of the significance of offset spectrum filtering in Er-doped Mamyshev fiber oscillators, as well as a foundation for the design of fiber lasers in practical applications.

Generation of different mode-locked states in a Yb-doped fiber laser based on nonlinear multimode interference.

We demonstrated an ultrafast Yb-doped fiber laser with a single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure based saturable absorber. The GIMF was placed in the groove of an in-line fiber polarization controller to adjust its birefringence, enabling the SMF-GIMF-SMF structure to realize efficient saturable absorption based on nonlinear multimode interference without strict length restriction. By adjusting two intra-cavity polarization controllers, stable dissipation solitons and noise-like pulses were achieved in the 1030 nm waveband with pulse durations of 10.67 ps and 276 fs, respectively. We also realized Q-switched mode-locked pulses in the same fiber laser cavity. By the dispersive Fourier transform method, the real-time spectral evolution in the buildup process of the Q-switched mode-locked state was captured, which showed that the continuous-wave in this laser could gradually evolved into the stable Q-switched mode-locked pulses through unstable self-pulsation, relaxation oscillation and rogue Q-switching stage. To the best of our knowledge, our work reveals the buildup dynamics of the Q-switched mode-locked operation in a fiber laser for the first time. And we also studied the real-time spectral evolution of the stable Q-switched mode-locked pulses, which exhibited periodic breathing property.