Negative Differential Transmission Research Articles

Trapping and exciton-exciton annihilation assisted ultrafast carrier dynamics in nanosheets of 2H-MoSe2 and Cr doped 1T/2H-MoSe2.

Nanosheets of transition metal dichalcogenides with prospects of photocatalysis and optoelectronics applications have significant potential in device fabrication due to their low-cost production and easily controllable morphology. Here, non-degenerate pump-probe differential transmission studies with varying pump-fluence have been carried out on single-phase 2H-MoSe2 and mixed-phase 1T/2H-MoSe2 nanosheets to characterize their excited carrier dynamics. For both the samples, the differential probe transmission data show photo-induced bleaching at earlier pump-probe delay followed by photo-induced absorption unveiling signatures of exciton-state filling, exciton trapping, defect-mediated photo-induced probe absorption and recombination of defect bound excitons. The exciton trapping and photo-induced absorption by the trapped-carriers are estimated to occur with time constant of ∼430 to 500fs based on multi-exponential modelling of the differential transmission till pump-probe delay of ∼3.5ps. Biexponential modeling of the subsequent slow-recovery of the negative differential transmission at pump-probe delay ≳3.5ps reveals that the exciton recombination happens via two distinct decay channels with ∼25 to 55ps (τ1) and ≳1ns (τ2) time constants. Pump-fluence dependent reduction in τ1 and further modelling of exciton population using higher order kinetic rate equation reveals that the two-body exciton-exciton annihilation governs the exciton recombination initially with a decay rate of ∼10-8 cm3s-1. The detailed analysis suggests that the fraction of total excitons that decay via long decay channel decreases with increasing exciton density for 2H-MoSe2, in contrast to 1T/2H-MoSe2 where the fraction of excitons decaying via long decay channel remains constant.

All-optical negative differential transporter of Bose-Einstein condensates in cavity.

We present a new and interesting physical phenomenon of optical negative differential transmission (ONDT, whose output intensity decreases with the increasing of input field intensity for an arbitrary optical system) in present BEC-cavity coupling system which pumped by a strong light and probed by a weak light. Theoretical results show that the transmission of the probe can be suppressed or promoted greatly by the pump due to optical nonlinearity and the Stokes/anti-Stokes scattering. To our most interest, two kinds of ONDT respectively induced by the nonlinear incoherent light-controlling and the nonlinear coherent interference have been uncovered, which have promising prospect in producing hyper-stable light source since it provides an unusual negative feedback.

Microscopic analysis of saturable absorbers: Semiconductor saturable absorber mirrors versus graphene

Fully microscopic many-body calculations are used to study the influence of strong sub-picosecond pulses on the carrier distributions and corresponding optical response in saturable absorbers used for mode-locking—semiconductor (quantum well) saturable absorber mirrors (SESAMs) and single layer graphene based saturable absorber mirrors (GSAMs). Unlike in GSAMs, the saturation fluence and recovery time in SESAMs show a strong spectral dependence. While the saturation fluence in the SESAM is minimal at the excitonic bandgap, the optimal recovery time and least pulse distortion due to group delay dispersion are found for excitation higher in the first subband. For excitation near the SESAM bandgap, the saturation fluence is about one tenth of that in the GSAM. At energies above the bandgap, the fluences in both systems become similar. A strong dependence of the saturation fluence on the pulse width in both systems is caused by carrier relaxation during the pulse. The recovery time in graphene is found to be about two to four times faster than that in the SESAMs. The occurrence of negative differential transmission in graphene is shown to be caused by dopant related carriers. In SESAMs, a negative differential transmission is found when exciting below the excitonic resonance where excitation induced dephasing leads to an enhancement of the absorption. Comparisons of the simulation data to the experiment show a very good quantitative agreement.

Microscopic description of intraband absorption in graphene: the occurrence of transient negative differential transmission.

We present a microscopic explanation of the controversially discussed transient negative differential transmission observed in degenerate optical pump-probe measurements in graphene. Our approach is based on the density matrix formalism allowing a time- and momentum-resolved study of carrier-light, carrier-carrier, and carrier-phonon interaction on microscopic footing. We show that phonon-assisted optical intraband transitions give rise to transient absorption in the optically excited hot carrier system counteracting pure absorption bleaching of interband transitions. While interband transition bleaching is relevant in the first hundreds of fs after the excitation, intraband absorption sets in at later times. In particular, in the low excitation regime, these intraband absorption processes prevail over the absorption bleaching resulting in a zero crossing of the differential transmission. Our findings are in good qualitative agreement with recent experimental pump-probe studies.

Negative differential transmission in graphene

By using the Kubo linear response theory with the Keldysh Green function approach, we investigate the mechanism leading to the negative differential transmission in system with the equilibrium electron density much smaller than the photon-excited one. It is shown that the negative differential transmission can appear at low probe-photon energy (in the order of the scattering rate) or at high energy (much larger than the scattering rate). For the low probe-photon energy case, the negative differential transmission is found to come from the increase of the intra-band conductivity due to the large variation of electron distribution after the pumping. As for the high probe-photon energy case, the negative differential transmission is shown to tend to appear with the hot-electron temperature being closer to the equilibrium one and the chemical potential higher than the equilibrium one but considerably smaller than half of the probe-photon energy. We also show that this negative differential transmission can come from both the inter- and the intra-band components of the conductivity. Especially, for the inter-band component, its contribution to the negative differential transmission is shown to come from both the Hartree-Fock self-energy and the scattering. Furthermore, the influence of the Coulomb-hole self-energy is also addressed.

Probing near Dirac point electron-phonon interaction in graphene

Carrier dynamics in graphene films on CaF2 have been measured in the mid infrared region by femtosecond pump-probe spectroscopy. The relaxation kinetics shows two decay times. The fast time component is ~0.2 ps, which is attributed to the mixture of initial few ultrafast intraband and interband decay channels. The slow component is ~1.5 ps, which is primarily assigned to optical phonon-acoustic phonon scattering. The contribution of fast component exhibits an increase trend in the probe photon frequencies from 2600 to 3100 cm−1. At the probe frequency of 2700 cm−1, the accelerated carrier relaxation was detected, which resulted from the interband triple-resonance electron-phonon scattering in graphene. At the probe frequency of 3175 cm−1, a clear instant negative differential transmission signal was observed, which is due to stimulated two-phonon emission involved with G phonons in graphene. This result indicates that graphene can be used as a source of coherent ultrashort sound-wave emission.

FDTD Analysis of Two-Photon Absorption and Free-Carrier Absorption in Si High-Index-Contrast Waveguides

The two-photon absorption (TPA), the free-carrier absorption (FCA), and the carrier plasma effect play important roles in Si-based photonic devices. These effects have been incorporated directly into a finite-difference time-domain (FDTD) simulator for the first time. To check the validity of the FDTD simulator, it has been applied to the simulation of nonlinear transmission of an ultrashort optical pulse in a short Si high-index-contrast waveguide. The results are consistent with those shown in the literature. The cause of the slight negative slope transmission has been explained as follows. Since the TPA-induced FCA makes the pulse-shape asymmetric, a pulse with a higher optical input intensity tends to have a longer tail. Therefore, a stronger optical pulse is subject to FCA for a longer period, resulting in the negative differential transmission.

Two-photon absorption in InP substrates in the 1.55μm range

The nonlinear optical absorption has been studied in three different InP substrates (semi-insulating Fe-doped, n-type S-doped, and p-type Zn-doped) by subpicosecond pump-probe differential transmission experiments at 1.6μm. A strong negative differential transmission peak is observed at zero delay, induced by an autocorrelation effect: it is related to two-photon absorption, but not to the occurrence of transitions involving any midgap level (noticeably the Fe-related one). The experimental two-photon absorption coefficient β for InP stands in the range between 24 and 33cm∕GW.

Nonlinear transmission of 1.5 �m pulses through single-mode silicon-on-insulator waveguide structures

An 80 MHz pulse train of ~ 100 fs optical pulses centred at ~ 1.5 microm is propagated through a variety of high-index-contrast silicon-on-insulator waveguide structures less than 1 mm long. All-optical power limiting and negative differential transmission, based only on the intrinsic nonlinear response of the untextured waveguides near 1.5 microm, are demonstrated for average in-guide power levels of ~ 1 mW. Superlinear transmission is observed in a textured silicon waveguide for power levels less than 20 microW.

Potential of femtosecond chirp control of ultrabroadband semiconductor continuum nonlinearities

We present an experimental study of the influence of frequency chirp in 20-fs optical pulses on broadband semiconductor continuum nonlinearities. Differential-transmission (DT) measurements were performed in which either the exciting pump pulse or the readout probe pulse, or both pump and probe, were chirped. We demonstrate that in certain chirp configurations the spectrally integrated DT is enhanced on an ultrafast time scale compared with measurements with unchirped pulses. Therefore pulse chirping has the potential to improve and optimize all-optical ultrafast switching. Spectrally resolved DT measurements explain these findings. Positive and negative DT contributions are observed in different spectral ranges. The spectral position and the magnitude of these contributions change in time. Proper chirping of the pulses optimizes the readout of the positive contributions and maximizes the spectrally integrated DT. A simple quantitative model confirms these considerations.

Chirp-controlled ultrafast optical nonlinearities in semiconductors

We experimentally demonstrate that the differential transmission (DT) response of bulk semiconductors excited well above the band edge can be manipulated by chirping of the broadband excitation and readout pulses. In particular, the maximum transmission change in spectrally integrated DT experiments can be modified on the 20 fs time scale. Spectrally resolved DT studies explain this chirp dependence. Depending on the sign of the chirp, positive or negative DT contributions at low or high photon energies are probed with varying efficiency around zero time delay. These results demonstrate that chirp can become an additional degree of freedom for the optimization of device performance in ultrafast all-optical switching.

Femtosecond degenerate and nondegenerate pump-probe experiments in bulk GaAs below the band gap

We perform degenerate and nondegenerate pump-probe experiments on bulk GaAs at 100 K below the band gap. We mostly observe a negative differential transmission signal both in the degenerate and nondegenerate experiments. We interpret our signal as due to two-photon absorption. This negative signal has a different origin from the normally considered band gap renormalization for resonant excitations.