Liquid phase exfoliation (LPE) is a frequently employed technique for the scalable production of dispersible graphene and graphite nanosheets for electronic, optoelectronic, and photonic applications. Fundamental information on how the liquid exfoliation process and resulting size-distribution of these nanoflakes affects their electrical properties is lacking. To address this gap, terahertz time-domain spectroscopy (THz-TDS) is employed as a non-contact optical approach for determining the AC conductivity behavior and charge transport dynamics of these materials. Cascade centrifugation is employed to separate exfoliated flakes into a range of sizes as confirmed by profilometry measurements. Correlations between the flake area and carrier concentration/scattering time are observed and related to flake morphology through changes to carrier mean free paths and edge defects. A lack of control to the flake chemistry leaves open questions regarding the influence of doping levels on the charge carrier dynamics, which will be the focus of future work.
Nanoparticles of cuprous oxide with and without Pd-coatings are explored for charge carrier dynamics that can influence efficiency in applications of solar photocatalysis. Transient absorption/reflection spectroscopy is used to determine the excited-state dynamics, which depends on size and shape. Nanoparticles of a few tens of nanometers exhibit simple resonances toward the blue end of the visible spectrum, while particles over 100 nm exhibit various Mie resonances that enhance their performance in applications and lead to more complicated dynamics. Global and inversion analysis are used to determine the rate dynamics of the photocarriers. Small spheres exhibit Auger scattering in the high photocarrier injection regime, whereas the low carrier injection regime is characterized by Shockley-Read-Hall dynamics. Adding a near uniform, 2-nm thick Pd coating results in a heterojunction between Cu2O and Pd that modifies the relaxation dynamics. The reduction of slope value from 3, characterizing Auger scattering, to the lower values on Pd coated samples shows the admixture of possibly two or more recombination mechanisms. Also, the intermediate injection regime exhibits radiative recombination/trap-assisted scattering in the Pd-coated Cu2O nanoparticles. Power-dependent analysis, by solving the rate equation at the higher injection regime, validates the suppression of Auger scattering in Pd-coated samples. This result indicates the possibility of charge separation across the heterojunction, either from metal to the semiconductor or vice-versa, lowering the effective photocarrier overlap, reducing Auger scattering, and potentially improving photocatalytic activity.
Light-harvesting technology converts photons into useful charge carriers that can be used to catalyze chemical reactions or be extracted for electricity. We have been examining the hot-carrier dynamics in type-II quantum well heterostructures finding metastable and protracted decay mechanisms in early delay times of transient absorption spectroscopy that may allow for hot carriers to be extracted in order to overcome the detailed-balance limit of single-junction heterostructures. Simultaneously, we have explored dielectric resonances in spherical and cubic nanoparticles that demonstrate coherent signatures in the negative delay time region of the transient absorption data and potentially explain improved photocatalysis for certain reactions excited with broadband light.
Type-II multiple quantum well superlattices based on InAs/AlAsSb are investigated for ground- and excited-state charge carrier transport and excited-state charge carrier dynamics. It is found that ground-state transport matches well to impurity and optical phonon interactions, while the excited-state transport shows increased terahertz photoconductivity for the correct excitation conditions that have previously been linked to a metastability in the early time response after photoexcitation. This regime also shows a reduction in carrier mobility, which is also expected to be due to ambipolar diffusion and increased carrier–carrier scattering. Overall, carrier excited-state dynamics confirm the metastability in early time response and are related to strong Auger scattering. For increased excitation intensities, the Auger-scattering rate increases to obtain a lower carrier density more rapidly. The result is a stronger scattering of carriers energetically deeper into their respective bands, where they exhibit a much slower carrier recombination rate and can maintain their relative temperature as a result of a phonon bottleneck that forces reabsorption of optical phonons. In addition to a previously reported phonon bottleneck, the carrier dynamics offer potential pathways to stabilize hot carriers with further bandgap engineering.
We analyze four-wave-mixing experiments with three incident laser pulses performed on a semiconductor quantum well embedded in a microcavity. The coupling of the intracavity field and the exciton transition leads to exciton polaritons. The many-body hierarchy problem that arises due to the Coulomb interaction is treated by the dynamics-controlled truncation scheme, which leads to a set of Bloch equations that contain optical nonlinearities including biexcitonic many-body correlations and contributions beyond the coherent limit, which have not been thoroughly explored for a microcavity yet. A numerical solution of these Bloch equations is performed by projecting onto the 1s-exciton and biexciton states. We present the two-dimensional Fourier transform of the four-wave-mixing signal for different polarization directions of the incident pulses, which allows us to investigate the absorption and emission of the system and the couplings among the different resonances from the lower polariton, the upper polariton, and the biexciton. The numerical results are compared with measurements, in which a GaAs quantum well sample enclosed in distributed Bragg reflectors is investigated for four different polarization configurations, and we find a good agreement.
La0.7Sr0.3MnO3 is a strongly correlated complex oxide. There is limited information about La0.7Sr0.3MnO3 thin films’ transient reflectivity (TR), where differences in ultra-fast dynamics due to surface and interface effects are expected. By decreasing the film thickness, additional energy states emerge, providing extra relaxation channels. Due to the reduced absorption in thin films, observing the effects of these extra states in TR signal is challenging, especially in a hole doped system such as La0.7Sr0.3MnO3. Moreover, in lower thicknesses, sinusoidal behavior superimposed on the TR signal is not analyzable by Fourier transforms. Wavelet transforms are perfect tools to analyze these fast-vanishing oscillations.
Complex oxide thin films, such as La0.7Sr0.3MnO3 (LSMO), are widely studied for a variety of applications. Transient reflectivity (TR) measurements on LSMO indicate enhanced surface recombination of charge carriers in films less than 20 nm in thickness. Oxygen growth pressure variation illustrates that higher oxygen pressures provide more electron dominance in the system, and producing a larger band filling effect which eventually results in higher excitations. Wavelet analysis can distinguish abrupt oscillatory modes with close energy ranges and have been introduced as a method to study sound velocities in ultra-thin films.
Multidimensional coherent spectroscopy measures the third-order polarization response of a system to reveal microscopic electronic and many-body phenomena. Applied to semiconductor nanostructures, it can distinguish homogeneous and inhomogeneous broadening due to disorder or strain gradients, resolve coupling between transitions, and optically access transitions that are either non-radiating or outside the bandwidth of the pulses. Two tools often exploited in this versatile technique are (i) the ability to control the polarization of the excitation and emission and thus the optical selection rules, and (ii) the ability to capture the complex spectrum. Here, the polarization of pulses emerging from a multidimensional optical nonlinear spectrometer (MONSTR) and the resulting four-wave mixing emission are controlled automatically using variable retarders, such that multiple spectra are recorded during a single phase-stabilized scan. This improves the acquisition time by ~3x compared to running separate polarization scans. Importantly, only one phase ambiguity exists in the complex spectra across all sets of polarization states measured. This single ambiguity is resolved by comparing the initial spectrally resolved transient absorption to the complex four-wave mixing spectrum for collinear polarization and then applying it to all spectra. Here, the method is applied to a quantum well embedded in a semiconductor microcavity with an adjustable cavity-exciton detuning. The complex 2DCS spectra we report constitute the first measurements of detuning- and polarization-dependent exciton-polariton lineshape across the strong coupling regime.
Control of electronic, optical and thermal properties in semiconductor nanostructures allows for design of electronic, optoelectronic and thermoelectric devices. For some applications, a prolonged excited-state carrier lifetime is desired without carrier thermalization or recombination. Engineering charge separation and indirect recombination pathways leads to hot electrons dominating the device response. In this work, transient absorption of terhertz (THz) probe pulses measure the recombination dynamics of photoexcited carriers in a type-II InAs/InAsSb multiple quantum well (MQW). THz measures free-photocarrier absorption and lattice expansions within the MQW as a result of phonons or polarons. The carriers are photoexcited close to the fundamental excited-state resonance of the MQW for a range of lattice temperatures between 5 K and 300. Excitation above the MQW resonance at low temperature shows fast (~15 ps), intermediate (~150 ps) and slow (~1500 ps) recombination times. As the lattice temperature is increased, fast recombination subsides and the slower recombination components grow. This switch of recombination components is almost conservative and is agreement with photoluminescence results suggesting that radiative recombination occurs strongly for the entire temperature range. Fast recombination results from direct recombination within the MQW, as conduction electrons combine with localized holes arising from alloy fluctuations that are frozen in low temperature. At higher temperatures, recombination processes are indirect, between the well’s conduction electrons and barrier’s valence holes. The identical temperature dependence of slower recombination contributions indicated this to be a two-step mechanism that is also reliant on the electron-phonon coupling. Type-II MQWs can enhance this recombination times to prolong hot carriers for optoelectronic devices.
Antiferromagnets are an important class of ordered spin systems, common in spintronic applications and providing a testbed for studying magnetism. Recently, the injection of magnons – coherent spin waves – has been explored by broadband terahertz pulses in antoferromagnets, such as MnO. Here, terahertz time-domain spectroscopy is used to detect magnon resonances in MnF2, which is a model antiferromagnet with uniaxial anisotropy and a Néel temperature of 67 K. Temperature dependence of a one-magnon resonances is examined from 5 K to 70 K. The center frequency of the one-magnon is recorded below the Néel temperature and fit to a Brillouin function. It is found that the degree of correlation between neighboring spins is j = 1.1. Namely, a weak correlation and appropriately modeled by mean-field theory befitting this simple system. From low temperature to room temperature, a two-magnon resonance is observed to broaden and strengthen as the temperature increases. Two-magnon modes arise due to zone-edge magnons being stimulated with -k and +k momenta and do not require magnetic ordering. Over this same temperature range, THz transients are used to monitor the time-of flight through the crystal, the refractive index, the internal energy and the heat capacity. Overall these quantities decrease with decreasing temperature, with behavior that falls into three regimes: a thermal dominated region above the Néel temperature, a magnetic regime below the Néel temperature; and a hyperfine interaction region at temperatures below 6 K. The latter is the first direct observation of the hyperfine interaction using terahertz time-domain spectroscopy.
Semiconductors systems exhibiting excitonic properties are discussed in terms of their coherent response, which is
extracted using two-dimensional coherent spectroscopy. This control method allows for separation of quantum pathways
that comprise the optical response, such as interactions between excitons, their dephasing rates, the effects of many-body
interactions and the role of structure on the microscopic electronic environment. Additional controls, such as polarization
can be used to further distinguish biexcitons and suppress many-body interactions. These result are compared and
contrasted with those from a semiconductor microcavity where the excitons form polaritonic modes due to normal-mode
splitting. Rephrasing spectra map the detuning dependence of the exciton-polariton branches. Increasing the detuning
shifts all features to higher energy and the expected anti-crossing is observed. An isolated biexciton is seen only at
negative detuning, separated by a binding energy. For positive detuning, the spectral weight of the off-diagonal features
swap, as the lower polariton branch and biexciton come into resonance. This indicates that the off-diagonal features are
sensitive to the interactions of the exciton-polaritons and other resonances in the system.
Multidimensional coherent spectroscopy maps the detuning dependence of the upper (UP) and lower (LP) excitonpolariton branches1 in a wedged microcavity with a single InGaAs quantum well at 5 K. Features on the diagonal correspond to intra-action coherences of the UP and LP branches. Off-diagonal peaks are interaction coherences between the UP and LP branches. With increasing detuning (Δ), all peaks move to higher energy, the exciton-like (EEX) and cavity-like (Eγ) modes swap position and have maximum intensity near the anti-crossing at Δ=0. An isolated biexciton (B) is only seen at Δ<0, separated by a binding energy of approximately 2 meV. For Δ>0, the spectral weight of the off-diagonal features swap, as the LP and B come into resonance. This indicates that the off-diagonal features are sensitive to the interactions including two-quantum contributions and that a situation similar to a Feshbach resonance exists.2 Polarization of two-quantum contributions show spin sensitive two-polariton and new biexciton correlations. The latter likely influence the Feshbach resonance between biexcitons and two-polariton states. The two-quantum signatures also demonstate that biexcitons perturb the light-matter coupling in the microcavity to reduce the mixed two-polariton contributions. Detuning dependence of zero-quantum contributions show Raman-like coherences that are enhanced near zero detuning. Asymmetry of the Raman coherences are indicative of many-body interactions, which also grow stronger as the light-matter interactions are enhanced near zero deuning.
Efficient solar energy conversion in photovoltaics and solar to chemical conversion is hindered by large band gaps and poor absorption in thin films. The easily tunable absorption and scattering cross section of localized surface plasmon resonance (LSPR) make it an ideal solution to capturing lost light. For above band edge light, scattering and light trapping can be used to increase absorption in thin semiconductor films, improving photoconversion without sacrificing recombination times. Below the band edge, plasmonic hot electrons can transfer to the semiconductor directly or resonant energy transfer can non-radiatively induce charge separation, allowing photoconversion where the semiconductor cannot absorb. In this brief review, we explore the mechanisms and efficiency of light recovery in plasmonics. Surface plasmon polaritons are used to increase light trapping in semiconductor nanowires using a metal nanohole array. Metal-semiconductor nanostructures with varying energy alignment, insulating barrier thickness, and spectral overlap are systematically varied to differentiate hot electron and resonant energy transfer. Transient absorption spectroscopy and action spectrum analysis are applied to track plasmonic charge creation and transfer, linking short and long time scale behavior. Guidelines are given for achieving optimal plasmonic light capturing and enhancement across the solar spectrum.
Optical multidimensional coherent spectroscopy is a powerful tool for studying structure and dynamics in complex
systems, such as semiconductors. In optical two-dimensional coherent spectroscopy (2DCS), where the spectrum
is presented in a two-dimensional (2D) plane with two frequency axes, an important advantage is the ability
to isolate quantum pathways by unfolding a one-dimensional spectrum onto a 2D plane. For many systems,
however, the quantum pathways are only partially separated in a 2D spectrum. In order to completely isolate
the quantum pathways, we extend 2DCS into a third dimension to generate three-dimensional (3D) spectra
in which the spectrum is further unfolded. A 3D spectrum provides complete and well-isolated information of
the third-order optical response of the system. The information can be used to fully characterize the quantum
pathways and to determine the system’s Hamiltonian. Quantitative knowledge of the Hamiltonian enables
prediction and control of quantum processes. For instance, such information is essential for deterministic control
and improved performance of coherent control schemes.
Exciton dephasing and relaxation dynamics are studied in a GaAs quantum dot ensemble using optical twodimensional
Fourier transform spectroscopy. We measure the temperature and excitation-density dependence of
the exciton ground-state homogeneous lineshape of quantum dots within the ensemble and show that acoustic
phonon sidebands are absent. The linewidth increases nonlinearly with temperature from 6 to 50 K and the
behavior is well-described by an Arrhenius equation with an offset. The absence of a phonon-activation peak in
the spectra reveals that elastic exciton-phonon scattering is the primary dephasing mechanism and the results can
be explained qualitatively using an extension of the independent Boson model that includes quadratic coupling in
the phonon displacement coordinates. At temperatures ≥ 35 K, spectral features associated with phonon-assisted
population transfer of excitons out of the quantum dots and into quantum wells states begin to appear.
Electronic structure and dynamics are captured by optical 2D-Fourier-transform (2DFT) spectroscopy, which
tracks the phase of the nonlinear signal during two time delays of a multi-pulse excitation sequence. These
Fourier-transformed spectra separate and isolate overlapping and competing contributions to the coherent
response. We have developed an ultra-stable platform consisting of nested interferometers with active phase
control, allowing for exploration of single- and two-quantum coherences. Phase-resolved spectra are retrieved
by all-optical determination of experimental phase ambiguities. GaAs quantum wells show suppression of
many-body interactions in cross-linear polarized 2DFT spectra and many-body two-quantum coherences.
Potassium vapor also shows unexpected two-quantum coherences.
Two-dimensional Fourier transform spectroscopy, an extension of four-wave mixing (FWM), is able to resolve numerous aspects of many-body effects and higher-order Coulomb interactions that contribute to the ultrafast dynamics of quantum wells and bulk GaAs resonances. Coherent oscillations between heavy-hole and light-hole excitons - or quantum beats - can be unfolded from the exciton populations by Fourier transforming FWM data with respect to two time-axes. Excitation conditions such as pulse ordering, polarization, tuning and pulse energy can isolate Feynman pathways and highlight the coherent many-body correlations, including those from biexcitons. In addition, the bulk exciton and continuum states are studied more carefully for their dynamics.
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