Ultrafast Photocurrent Research Articles

(Invited) Directing Interlayer Photocurrent Dynamics By Twisting and Stacking Van Der Waals Materials

Recently, careful stacking of 2D materials and heterostructures has shown that new interlayer electronics state can have lifetimes that are orders of magnitude longer than present in the monolayer.[1,2] Using time-space resolved ultrafast microscopy on individual 2D crystal grains, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state as a consequence of the symmetrized rehybridization of constrained interlayer 2p orbitals.[3,4] Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly-bound (450-550 meV), stable interlayer exciton states. Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer directed E-field, inducing electron-hole dissociation. Our lab [5] and others [6] measure that stacked WSe2 TMD devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. Using a first-principle kinetic master equation, our methods analytically extracts both the E-field-dependent interlayer escape velocity and rate-limiting exciton dissociation time. Remarkably our photocurrent response function produces the same E-field-dependent electronic escape and dissociation rates for both the optical and PC addressed ultrafast measurements. As confirmation, the resulting ratio of the electronic rates accurately matches our overall WSe2 device IQE in the intensity limit of zero Auger recombination. Thus through time-space resolved microscopy, we now obtain a timeline selective to the interlayer electronic dynamics of TMDCs and tBLG van der Waals materials. We show how this novel scanning microscopy approach, combines ultrafast photocurrent and transient absorption to identify new long-lived and metastable interlayer electronic states in emerging twisted and stacked 2D materials and devices.

Effects of photogenerated carrier scattering on the decay process of coherent longitudinal optical phonons in an undoped GaAs/n-type GaAs epitaxial structure investigated by terahertz time-domain spectroscopy

The authors investigated the scattering effect of photogenerated carriers on the decay of the coherent longitudinal optical (LO) phonons in an undoped GaAs/n-type GaAs epitaxial structure using a terahertz time-domain spectroscopic technique. The terahertz wave from the coherent LO phonon was observed together with those emitted both by the ultrafast photocurrent and by the coherent LO-phonon-plasmon coupled (LOPC) mode. The simultaneous observation of the coherent LO phonon and coherent LOPC mode originates from the fact that the photogenerated carrier density laterally distributes around the surface of the undoped GaAs layer owing the Gaussian profile of the pump beam. The authors found that the terahertz wave from the coherent LO phonon lives up to 5.0 ps, whereas those from the coherent LOPC mode and ultrafast photocurrent disappear within 1.0 ps. The decay time of the coherent LO phonon monotonically decreases with an increase in the pump power. This finding indicates that the dephasing of the terahertz wave from the coherent LO phonon is dominated by the photogenerated carrier scattering in the time range shorter than 1.0 ps in which photogenerated carriers remain.

Topological Insulators as Ultrafast Auston Switches in On-Chip THz-Circuits

We demonstrate that the three-dimensional topological insulator Bi2Se3 can be exploited as an ultrafast photoswitch in high-frequency optoelectronic circuits without the need to apply a voltage bias to the topological insulator. Bi2Se3-films are optically excited with a circularly polarized laser pulse. The circular photogalvanic effect gives rise to an ultrafast surface current. We use coplanar THz-striplines and an on-chip pump-probe scheme to sample the ultrafast transient photocurrent. For such subpicosecond electromagnetic transients, the stripline circuit introduces significant broadening due to dispersion and attenuation. Therefore, we compute the effective refractive index of the striplines and introduce a back-transformation of the distorted signal to explore the original ultrafast currents in the topological insulator. Our results show how to use topological insulators as unipolar THz-sources in on-chip stripline circuits.

Ballistic and resonant negative photocurrents in semiconducting carbon nanotubes

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

Ultrafast photocurrent measurements of a black phosphorus photodetector

With its high mobility, narrow bandgap, and unique anisotropy, black phosphorus (BP) is a promising material for optoelectronic applications. Waveguide-integrated photodetectors with RC-limited speeds up to 3 GHz have been recently demonstrated at telecom wavelengths. To truly be competitive, however, BP photodetectors must reach speeds of tens of GHz. Here, we use BP's nonlinear photoresponse to measure the intrinsic speed of a BP photodetector using ultrafast pump-probe measurements. With this technique, we are able to observe how the detection speed depends on both the incident power and applied source-drain bias. A minimum response time of 60 ps was observed which corresponds to an intrinsic bandwidth of 9 GHz.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3.

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

(Invited) Generation of Hot Plasmonic Electrons and Heat in Nanocrystals with Hot Spots

The efficiency of generation of energetic plasmonic carriers in metal nanostructures strongly depends on the optical design and material composition. This study demonstrates the ability to generate large numbers of hot plasmonic carriers in specially-designed hybrid nanostructures [1,2]. The hot-electron generation becomes especially efficient in plasmonic nanostructures with electromagnetic hot spots [1]. The energy distribution of optically-excited plasmonic carriers is very different in metal nanocrystals with large and small sizes [2]. For metal nanocrystals with smaller sizes (less than 10nm) or in nanostructures with hot spots, the excited state gets a large number of carriers with high energies. Nanostructures with a strong plasmonic enhancement generate unusually large numbers of energetic electrons, which can be observed using ultra-fast spectroscopy, photo-currents and photo-chemistry [1,3,4]. Along with the hot-electron generation, which is a quantum effect, the electromagnetic radiation typically causes efficient heating in metal nanostructures, which may also be concentrated in thermal hot spots [5]. The results obtained in this study can be used to design plasmonic hot-electron nanodevices for photo-catalysis, photo-detectors and solar-energy applications. [1] H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, L.V. Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015). [2] A.O. Govorov, H. Zhang, V. Demir, and Y. K. Gun’ko, NanoToday 9, 85 (2014). [3] ] W. Li, Z. J. Coppens, L. Vázquez, W. Wang, A. O. Govorov, and J. Valentine, Nature Communications 6, 8379 (2015). [4] L. Weng, H. Zhang, A. O. Govorov, and M. Ouyang, Nature Communications 5, 4792 (2014). [5] A. Govorov and H.H. Richardson, NanoToday 2, 20 (2007).

Ultrafast magneto-photocurrents as probe of anisotropy relaxation in GaAs

We induce ultrafast photocurrents in a GaAs crystal exposed to a magnetic field by optical femtosecond excitation. The magneto-photocurrents are studied by time-resolved detection of the simultaneously emitted THz radiation. We find that their dynamics differ considerably from the dynamics of other photocurrents which are expected to follow the temporal shape of the optical intensity. We attribute this difference to the influence of carrier-anisotropy relaxation on the magneto-photocurrents. Our measurements show that the anisotropy relaxation for carrier densities ranging between 1016cm−3 and 5×1017cm−3 occurs on two different time scales. While the slow time constant is approximately 100 fs long and most likely governed by electron-phonon scattering, the fast time constant is on the order of 10 fs and presumably linked to the valence band. Our studies not only help to better understand the microscopic origins of optically induced currents but—being even more important—show that magneto-photocurrents can be employed as a probe of anisotropy relaxation in GaAs. This technique is applicable to all non-centrosymmetric bulk semiconductors.

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

Charge separation and recombination are key processes determining the performance of organic optoelectronic devices. Here we combine photoluminescence and photovoltaic characterization of organic solar cell devices with ultrafast multipulse photocurrent spectroscopy to investigate charge generation mechanisms in the organic photovoltaic devices based on a blend of an alternating polyquinoxaline copolymer with fullerene. The combined use of these techniques enables the determination of the contributions of geminate and bimolecular processes to the solar cell performance. We observe that charge separation is not a temperature-activated process in the studied materials. At the same time, the generation of free charges shows a clear external field and morphology dependence. This indicates that the critical step of charge separation involves the nonequilibrium state that is formed at early times after photoexcitation, when the polaronic localization is not yet complete. This work reveals new aspects of molecul...

Observation of localized states in bulk GaAs using ultrafast photocurrent beat spectroscopy

We report photocurrent (PC)-generation processes in bulk GaAs at low temperatures revealed by PC beat spectroscopy and photoluminescence (PL) measurements. Because the time-domain PC spectroscopy is highly sensitive to PC-generation processes, several PC generation sites, including localized states, were identified. The difference between the PC beat and PL spectra shows that a non-luminescent bound-exciton state predominantly generates PC at intermediate temperatures, which is difficult to observe using steady-state PC and PL measurements. Thus, PC beat spectroscopy provides detailed understanding of the PC-generation processes in semiconductors.

Femtosecond upconverted photocurrent spectroscopy of InAs quantum nanostructures

The carrier upconversion dynamics in InAs quantum nanostructures are studied for intermediate-band solar-cell applications via ultrafast photoluminescence and photocurrent (PC) spectroscopy based on femtosecond excitation correlation (FEC) techniques. Strong upconverted PC-FEC signals are observed under resonant excitation of quantum well islands (QWIs), which are a few monolayer-thick InAs quantum nanostructures. The PC-FEC signal typically decays within a few hundred picoseconds at room temperature, which corresponds to the carrier lifetime in QWIs. The photoexcited electron and hole lifetimes in InAs QWIs are evaluated as functions of temperature and laser fluence. Our results provide solid evidence for electron–hole–hole Auger process, dominating the carrier upconversion in InAs QWIs at room temperature.

Ultrafast Lateral Photo-Dember Effect in Graphene Induced by Nonequilibrium Hot Carrier Dynamics.

The photo-Dember effect arises from the asymmetric diffusivity of photoexcited electrons and holes, which creates a transient spatial charge distribution and hence the buildup of a voltage. Conventionally, a strong photo-Dember effect is only observed in semiconductors with a large asymmetry between the electron and hole mobilities, such as in GaAs or InAs, and is considered negligible in graphene due to its electron-hole symmetry. Here, we report the observation of a strong lateral photo-Dember effect induced by nonequilibrium hot carrier dynamics when exciting a graphene-metal interface with a femtosecond laser. Scanning photocurrent measurements reveal the extraction of photoexcited hot carriers is driven by the transient photo-Dember field, and the polarity of the photocurrent is determined by the device's mobility asymmetry. Furthermore, ultrafast pump-probe measurements indicate the magnitude of photocurrent is related to the hot carrier cooling rate. Our simulations also suggest that the lateral photo-Dember effect originates from graphene's 2D nature combined with its unique electrical and optical properties. Taken together, these results not only reveal a new ultrafast photocurrent generation mechanism in graphene but also suggest new types of terahertz sources based on 2D nanomaterials.

Plasmonics enhanced photomixing for generating quasi-continuous-wave frequency-tunable terahertz radiation.

We experimentally demonstrate an order of magnitude enhancement in the quasi-continuous-wave radiated power from a photomixer with plasmonic contact electrodes in comparison with an analogous conventional photomixer without plasmonic contact electrodes in the 0.25-2.5THz frequency range when pumped at an optical wavelength of 1550nm. The significant efficiency enhancement results from the unique capability of the plasmonic contact electrodes to reduce the average transport path of photocarriers to the device contact electrodes, increasing the ultrafast photocurrent that drives the terahertz antenna.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

Ultrafast carrier dynamics in single-crystal In2Se3 thin layers

Carrier dynamics in single-crystal In2Se3 thin layers with various thicknesses was studied by femtosecond optical pump-probe reflectivity and ultrafast photocurrent measurements. The results suggest that, in thinner (thicker) layers, the carrier recombination dynamics is dominated by three-carrier (bimolecular) Auger process. The Auger time constant was found to decrease with deceasing layer thickness. Surface states were suggested to be the origin of the transition between different Auger processes as the layer thickness varies.

Terahertz beam steering by optical coherent control

We demonstrate optical coherent control of the emission direction of THz radiation. Femtosecond laser pulses are used to excite different types of ultrafast photocurrents along different directions in a bulk GaAs sample. The overall emission pattern can be modified by changing the phase of the optical excitation. With this method, THz beam steering of about 8° is realized. A simple dipole-based model allows us to relate the size of the steering effect to the amplitude ratio between the different photocurrent contributions and to diffraction effects resulting from the excitation spot size.

Polarization-induced phase shift of ultrafast photocurrents

Shift and injection currents are known to occur for linearly and circularly polarized optical excitations of semiconductors, respectively. Here, we show with room-temperature experiments that for excitation of discrete transitions the frequency dynamics of the coherent polarization changes this phase rule significantly. For above-bandgap optical excitation of semiconductors two main components of all-optically induced photocurrents have been recognized and are known as shift and injection currents (1). These currents can be linked to a second-order optical nonlinear process associated with difference- frequency mixing. Microscopically, shift currents and injection currents result from a polar distribution of carriers in real and momentum space, respectively. They are also known as linear and circular photogalvanic currents, since they occur for optical excitation with linearly and circularly polarized light, respectively (1-3). Here, we show that the frequency dynamics of the coherent polarization resulting from excitation of discrete transitions is capable of significantly altering this phase rule such that shift and injection currents occur for both, linear and circular polarizations. The measurements are performed on exciton transitions in a (110)-oriented, 5-nm wide GaAs quantum well (QW) sample, with the x, y, and z directions being parallel to the (001), (1 0), and (110) crystallographic directions, respectively.

Ultrafast photocurrents and THz generation in single InAs‐nanowires

To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAs‐nanowire‐based circuits, an on‐chip photocurrent pump‐probe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photo‐thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAs‐nanowires, which is interpreted in terms of a dominating photo‐Dember effect. The results are relevant for nanowire‐based optoelectronic and photovoltaic applications as well as for the design of nanowire‐based THz sources.

All-Optically Induced Ultrafast Photocurrents: Beyond the Instantaneous Coherent Response

It is demonstrated that the noninstantaneous response of the optically induced coherent polarization tremendously influences the real-space shift of electronic charges in semiconductors. The possibility of coherently controlling this real-space shift with temporally nonoverlapping excitation pulses allows for the observation of a new type of shift current, which only exists for certain polarization-shaped excitation pulses and vanishes in the continuous-wave limit. In contrast to previously studied shift currents, the new current requires a phase mismatch between two orthogonal transition dipole moments and leads, within a nonlinear second-order description, to a tensor which is antisymmetric with respect to the order of the two exciting electric field amplitudes. These observations, which can even be made at room temperature and are expected to occur in a variety of semiconductor crystal classes, contribute to a better understanding of light-matter interaction involving degenerate bands. Thus, they are expected to prove important for future studies of coherent and nonlinear optical effects in semiconductors.

Ultrafast Photocurrent Measurement of the Escape Time of Electrons and Holes from Carbon Nanotubep−i−nPhotodiodes

Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V(SD), and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.