Differential Transmission Signal Research Articles

Terahertz Electromodulation Spectroscopy for Characterizing Electronic Transport in Organic Semiconductor Thin Films

Terahertz (THz) spectroscopy is a well-established tool for measuring the high-frequency conductance of inorganic semiconductors. Its application to organic semiconductors, however, is challenging, because of the low carrier mobilities in organic materials, which rarely exceed 10cm2/Vs. Furthermore, low charge carrier densities in organic field-effect devices lead to sheet conductivities that are often far-below the detection limits of conventional THz techniques. In this contribution, we present the application of THz electromodulation spectroscopy for characterizing charge transport in organic semiconductors. Pulses of THz radiation are transmitted through organic field-effect devices and are time-resolved by electro-optic sampling. A differential transmission signal is obtained by modulating the gate voltage of the devices. This controls charge injection into the semiconductors, where the charge carriers reduce the THz transmission by their Drude response. Advantageous is that a nearly noise-free differential transmission can be obtained. Furthermore, electromodulation allows to sense specifically either injected electrons or holes. Because the method exclusively probes transport of mobile carriers, it provides access to fundamental transport properties, which are difficult to access with conventional characterization methods, such as conductance measurements of organic field-effect transistors. The outstanding property that a relative differential signal is measured allows to obtain charge carrier mobilities with high reliability. Mobilities as small as 1cm2/Vs can be probed, which makes THz electromodulation spectroscopy an attractive tool for studying charge transport in most technologically relevant organic semiconductors.

Electron-phonon scattering in β -Ga2O3 studied by ultrafast transmission spectroscopy

Femtosecond pump-probe experiments in a transmission geometry were performed on Sn-doped n-type β-Ga2O3. With the pump and probe wavelengths below the bandgap, the differential transmission signal was determined by the free electron dynamics. Differential transmission decay times and their spectral dependence were used to evaluate electron-phonon scattering for polar optical (PO) and intervalley phonons. The obtained average electron-PO phonon scattering time is 4.5 ± 0.4 fs, while the electron scattering to and from the side valley is 80 ± 5 fs. The energy between the absolute and second lowest conduction band minima is estimated to be 2.6 ± 0.1 eV.

Effect of strain on exciton dynamics in monolayer WS2**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0202302), the National Natural Science Foundation of China (Grant Nos. 61527817 and 61875236), the Initiative Postdocs Supporting Program of China (Grant No. BX201600013), the General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2017M610756), the Overseas

The exciton dynamics in a WS2 monolayer with strain are studied by transient absorption measurements. We measure the differential transmission signal from monolayer WS2 as a function of the probe wavelength at different levels of strain applied to the sample. The differential transmission spectrum has a positive maximum value at about 614 nm and shows no significant strain dependence. By time-resolving the differential transmission signal, we find that the strain has a minimal effect on the exciton formation process. However, the exciton lifetime is significantly reduced by strain. These results provide useful information for applications of WS2 in flexible electronic and optoelectronic devices where strain is inevitable.

Temporally Resolving Synchronous Degenerate and Nondegenerate Two‐Photon Absorption in 2D Semiconducting Monolayers

AbstractDegenerate and nondegenerate two‐photon absorption (TPA) in WS2 and MoSe2 monolayers are synchronously induced and temporally resolved by femtosecond laser pump‐probe. Differential transmission signals in the first 500 fs consist of negative and positive components, that originate from direct probe depletion via nondegenerate TPA and carrier accumulation via degenerate TPA, respectively. Temporal cross‐correlation of pump and probe pulses allows us to fully decouple the ultrafast nondegenerate and degenerate TPA signals. Subsequently, degenerate and nondegenerate TPA coefficients are calculated as a function of pump irradiance. Under nonresonant pumping, 100 ± 10 and 250 ± 25 cm GW−1 are obtained for degenerate and nondegenerate TPA coefficients of monolayer WS2, respectively, which both present linearly decreasing trends as increasing pump irradiances. However, under resonant pumping of 2p excitonic states in monolayer MoSe2, degenerate TPA coefficients exponentially decrease from 800 to 80 cm GW−1 as increasing pump irradiances, due to the interplay between band‐renormalization and band‐filling effects, while nondegenerate TPA coefficient is about 650 ± 50 cm GW−1. For comparison, a trilayer MoSe2 is also investigated. These results set a foundation for precisely measuring TPA coefficients and actively controlling nonlinear excitonic dynamics via TPA in 2D semiconducting monolayers.

Probing the free-carrier absorption in multi-layer black phosphorus

We study the carrier relaxation dynamics in thin black phosphorus (bP) using time-resolved differential transmission measurements. The inter-band and intra-band transitions, relaxation, and carrier recombination lifetimes are revealed by tuning the mid-infrared probe wavelength above and below the bandgap of black phosphorus. When the probe energy exceeds the bandgap, Pauli blocked inter-band transitions are observed. The differential transmission signal changes sign from positive to negative when the probe energy is below the bandgap, due to the absence of inter-band transitions and enhancement in the free-carrier absorption. The minority carrier lifetime and radiative recombination coefficient are estimated to be 1.3 ns and 5.9 × 10−10 cm3/s, respectively. The overall recombination lifetime of bP is limited by radiative recombination for excess carrier densities larger than 5 × 1019 cm−3.

Author Correction: Single-molecule imaging by optical absorption

In the Supplementary Video initially published with this Letter, the right-hand panel displaying the fluorescence emission was not showing on some video players due to a formatting problem; this has now been fixed. The video has also now been amended to include colour scale bars for both the left- (differential transmission signal) and right-hand panels.

Direct Observation of Ultrafast Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy.

Efficient charge separation at the interfaces of the perovskite with the carrier transport layers is crucial for perovskite solar cells to achieve high power conversion efficiency. We present a systematic experimental study on the hole injection dynamics from MAPbI3 perovskite to three typical hole transport materials (HTMs). We extract the carrier dynamics directly related to the hole injection by employing a pump light with short absorption depth and comparing the transient transmission signals excited on the two sides of the sample. The differential transmission signals reveal the hole injections to PTAA and PEDOT:PSS to be complete within 1 and 2 ps, respectively, and that to NiOx to exhibit an additional slow process on a 40 ps time scale. The obtained injection dynamics are discussed in comparison with the device performance of the solar cells containing the same MAPbI3/HTM interfaces.

Profiling wind and greenhouse gases by infrared-laser occultation: results from end-to-end simulations in windy air

Abstract. The new mission concept of microwave and infrared-laser occultation between low-Earth-orbit satellites (LMIO) is designed to provide accurate and long-term stable profiles of atmospheric thermodynamic variables, greenhouse gases (GHGs), and line-of-sight (l.o.s.) wind speed with focus on the upper troposphere and lower stratosphere (UTLS). While the unique quality of GHG retrievals enabled by LMIO over the UTLS has been recently demonstrated based on end-to-end simulations, the promise of l.o.s. wind retrieval, and of joint GHG and wind retrieval, has not yet been analyzed in any realistic simulation setting. Here we use a newly developed l.o.s. wind retrieval algorithm, which we embedded in an end-to-end simulation framework that also includes the retrieval of thermodynamic variables and GHGs, and analyze the performance of both stand-alone wind retrieval and joint wind and GHG retrieval. The wind algorithm utilizes LMIO laser signals placed on the inflection points at the wings of the highly symmetric C18OO absorption line near 4767 cm−1 and exploits transmission differences from a wind-induced Doppler shift. Based on realistic example cases for a diversity of atmospheric conditions, ranging from tropical to high-latitude winter, we find that the retrieved l.o.s. wind profiles are of high quality over the lower stratosphere under all conditions, i.e., unbiased and accurate to within about 2 m s−1 over about 15 to 35 km. The wind accuracy degrades into the upper troposphere due to the decreasing signal-to-noise ratio of the wind-induced differential transmission signals. The GHG retrieval in windy air is not vulnerable to wind speed uncertainties up to about 10 m s−1 but is found to benefit in the case of higher speeds from the integrated wind retrieval that enables correction of wind-induced Doppler shift of GHG signals. Overall both the l.o.s. wind and GHG retrieval results are strongly encouraging towards further development and implementation of a LMIO mission.

Sign of differential reflection and transmission in pump-probe spectroscopy of graphene on dielectric substrate

Pump-probe differential reflection and transmission spectroscopy is a very effective tool to study the nonequilibrium carrier dynamics of graphene. The reported sign of differential reflection from graphene is not explicitly explained and not consistent. Here, we study the differential reflection and transmission signals of graphene on a dielectric substrate. The results reveal the sign of differential reflection changes with the incident direction of the probe beam with respect to the substrate. The obtained theory can be applied to predict the differential signals of other two-dimensional materials placed on various dielectric substrates.

Resonance enhancement of difference-frequency generation through localized surface plasmon excitation

We report the experimental observation of difference-frequency generation in gold nanoparticles under localized surface plasmon excitation. A zero-delay peak is detected in the differential transmission signal for a gold nanoparticle film with a MgF2 overlayer, showing that the energy transfer from pump light to probe light through the difference-frequency generation is resonantly enhanced by the excitation. This peak of differential transmission decreases in strength with higher probe fluences. Both the enhancement and the power dependence of the difference-frequency generation are explained by modeling the localized surface plasmons as a nonlinear Lorentz resonator.

Modeling picosecond electron dynamics of pump-probe intersubband spectroscopy inn-type Ge/SiGe quantum wells

We present an energy-balance model of the electronic intersubband relaxation in optically excited n-type Ge/SiGe quantum wells with absorption resonance in the THz range. To this aim, the energy relaxation rates of the electron system due to interactions with both nonpolar optical and acoustic phonons are calculated. The time dependence of the relative differential transmission is also evaluated and compared with experimental data from recent pump-probe measurements. The energy relaxation rates due to acoustic and optical phonon are investigated for different electron temperatures, set by the pump beam intensity. We find that the relaxation dynamics strongly depends on the intersubband energy spacing when this is close to the optical phonon energy. Finally, our results evidence that in this material system the time dependence of the depolarization shift may have a strong influence on the relative differential transmission signal.

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.

Phonon‐assisted exciton spin relaxation in (In,Ga)As/GaAs quantum dots

AbstractIn this work, we study the temperature‐dependent spin relaxation of exciton‐bound carriers in (In,Ga)As/GaAs quantum dots. The exciton population mapped by a time‐resolved differential transmission signal reveals a decay on two different time scales, reflecting the fractions of optically active and inactive excitons. The underlying exciton states are split from each other by the exchange interaction. The phonon‐assisted spin‐orbit interaction induces spin flips of an exciton‐bound electron or hole which convert the exciton populations into each other. The temperature‐dependent relaxation rate follows a thermal phonon distribution. Deviations indicate that two‐phonon processes involving higher orbitals may contribute significantly to the total relaxation process. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Homogeneous linewidth of the intraband transition at 1.55 μm in GaN/AlN quantum dots

We present homogeneous line width measurements of the intraband transition at 1.55 μm in GaN/AlN quantum dots by means of nonlinear spectral hole-burning experiments. The square-root dependence of the differential transmission signal with the incident pump power reveals the importance of electron-electron scattering in the population relaxation dynamics. We find on the contrary that this scattering process plays a minor role in the coherence relaxation dynamics since the homogeneous linewidth of 15 meV at 5 K does not depend on the incident pump power. This suggests the predominance of other dephasing mechanisms such as spectral diffusion, and temperature-dependent measurements support this hypothesis.

A frequency stabilization technique for diode lasers based on frequency-shifted beams from an acousto-optic modulator

We present a simple method for diode laser frequency stabilization that makes use of a Doppler-broadened vapor cell absorption signals of two frequency-shifted laser beams. Using second-order-diffracted, double-passed beams from an acousto-optic modulator, we achieve a frequency separation roughly equal to the Doppler half width. The differential transmission signals of the two beams provide an error signal with a very large linear feature, allowing frequency stabilization over a range of greater than 1 GHz by means of standard proportional-integral-derivative servo feedback to the piezoelectric control of the grating in our external cavity diode laser. We have applied this technique to two different diode laser systems, one used to lock to the 410 nm E1 transition in indium and another for locking to the M1/E2 transition in thallium at 1283 nm. In both cases the technique reduces frequency fluctuation to roughly 1 MHz over time scales from 10(-3) to 10(2) s.

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers: Effects of high fluence excitation

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x=0.07, 0.15, and 0.33, over a fluence range of 1–12mJ∕cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation.

Femtosecond Differential Transmission Spectroscopy of α-Sexithienyl Single Crystals at Low Temperature

Spectral relaxation of photogenerated excitations and stimulated emission is investigated in an α-sexithienyl single crystal. We have performed femtosecond differential transmission spectroscopy, probing the spectral region from 1.85 to 2.6 eV, and time-resolved photoluminescence measurements at low temperature. Stimulated emission with amplification of the probe light which results in a positive differential transmission signal is observed at two spectral positions for polarization parallel to the molecular c-axis. The observed stimulated emission decays with a time constant of 85 ps. Different kinetics of the rise of the stimulated emission and the superimposed excited-state absorption has been observed for excitation with 3.1 eV. This difference vanishes for excitation with 2.3 eV. Stimulated emission proceeds also via trap luminescence.

Femtosecond dynamics and absorbance of self-organized InAs quantum dots emitting near 1.3 μm at room temperature

High-sensitivity, femtosecond differential transmission measurements on self-organized InAs quantum dots at room temperature allow us to determine the dynamics of resonantly excited electron-hole pairs, as well as the absorbance (α0d) of quantum dots. The room temperature differential transmission signal decreases with a time constant of 65 ps, determined by the rate of excitation from the initial photoexcited states to higher excited states. From the magnitude of the differential transmission, we deduce that the absorbance, and hence the maximum gain/pass in the direction normal to the layers, is ≈2.1×10−3 at the spectral peak for our sample with six layers of quantum dots.

A pump and probe study of photoinduced internal field screening dynamics in an AlGaN/GaN single-quantum-well structure

Photogenerated carrier dynamics in an AlGaN/GaN single quantum well has been studied using a conventional degenerate pump and probe technique at room temperature. Photoinduced absorption at the exciton resonance has been observed. It is explained by the absorption coefficient change, through the quantum-confined Stark effect and the quantum-confined Franz–Keldish effect, caused by the photoinduced internal electric-field screening. In comparison with biased GaAs multiple quantum wells, a slower time evolution of differential transmission signals has been also found. Its origin is attributed to the longer carrier sweep-out time due to the potential profile of the sample in conjunction with the longer carrier recombination time.

Ultrafast Relaxation of Photoexcited Carriers in Semiconductor Superlattices

The differential transmission signal (DTS) of a pump-probe transmission experiment on the GaAs/AlGaAs superlattice is calculated using the effective semiconductor Bloch equation and by assuming that the main mechanism of the relaxation for a low carrier density is the carrier LO-phonon scattering. From the theoretical analysis of the DTS signal, the intrasubband and intersubband relaxation times of the carriers which are excited optically to the second miniband are estimated. The results are compared to those of recent femtosecond pump-probe experiments on the GaAs/AlGaAs superlattice.