Time-resolved spectroscopy on epitaxial graphene in the infrared spectral range: relaxation dynamics and saturation behavior

Metamaterial graphene photodetector with bandwidth exceeding 500 gigahertz

Although graphene has met many of its initially predicted optoelectronic, thermal, and mechanical properties, photodetectors with large spectral bandwidths and extremely high frequency responses remain outstanding. In this work, we demonstrate a >500 gigahertz, flat-frequency response, graphene-based photodetector that operates under ambient conditions across a 200-nanometer-wide spectral band with center wavelengths adaptable from <1400 to >4200 nanometers. Our detector combines graphene with metamaterial perfect absorbers with direct illumination from a single-mode fiber, which breaks with the conventional miniaturization of photodetectors on an integrated photonic platform. This design allows for much higher optical powers while still allowing record-high bandwidths and data rates. Our results demonstrate that graphene photodetectors can outperform conventional technologies in terms of speed, bandwidth, and operation across a large spectral range.

Versatile and active THz wave polarization modulators using metamaterial/graphene resonators

Active modification of the polarization state is a key feature for the next-generation of wireless communications, sensing, and imaging in the THz band. The polarization modulation performance of an integrated metamaterial/graphene device is investigated via a modified THz time domain spectroscopic system. Graphene’s Fermi level is modified through electrostatic gating, thus modifying the device’s overall optical response. Active tuning of ellipticity by >0.3 is reported at the resonant frequency of 0.80 THz. The optical activity of transmitted THz radiations is continuously modified by >21.5o at 0.71 THz. By carefully selecting the transmitted frequency with the relative angle between the incoming linear polarization and the device’s symmetry axis, active circular dichroism and optical activity are almost independently exploited. Finally, this all-electronically tuneable versatile polarization device can be used in all applications requiring an ultrafast modulation such as polarization spectroscopy, imaging, and THz wireless generation.

Ultrafast hot-carrier cooling in quasi freestanding bilayer graphene with hydrogen intercalated atoms

Femtosecond-THz optical pump probe spectroscopy is employed to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene with hydrogen interacalation. We observe longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements show that the supercollision mechanism is not related to the cooling process of the hot carriers, which is ultimately achieved by electron optical phonon scattering.

Impact Ionization Induced by Terahertz Radiation in HgTe Quantum Wells of Critical Thickness

We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency ω and momentum relaxation time τ _l larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to exp ( - E 0 2 / E 2 ) , with the radiation electric field amplitude E and the characteristic field parameter E ₀. As observed in experiment, it exhibits a strong frequency dependence for ω τ ≫ 1 characterized by the characteristic field E ₀ linearly increasing with the radiation frequency ω.

Ultra-long carrier lifetime in neutral graphene-hBN van der Waals heterostructures under mid-infrared illumination

Graphene/hBN heterostructures are promising active materials for devices in the THz domain, such as emitters and photodetectors based on interband transitions. Their performance requires long carrier lifetimes. However, carrier recombination processes in graphene possess sub-picosecond characteristic times for large non-equilibrium carrier densities at high energy. An additional channel has been recently demonstrated in graphene/hBN heterostructures by emission of hBN hyperbolic phonon polaritons (HPhP) with picosecond decay time. Here, we report on carrier lifetimes in graphene/hBN Zener-Klein transistors of ~30 ps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements. We further demonstrate the switching of carrier lifetime from ~30 ps (attributed to interband Auger) down to a few picoseconds upon ignition of HPhP relaxation at finite bias and/or with infrared excitation power. Our study opens interesting perspectives to exploit graphene/hBN heterostructures for THz lasing and highly sensitive THz photodetection as well as for phonon polariton optics.

Long carrier lifetimes are beneficial for graphene-based optoelectronics, but carrier recombination processes in graphene possess sub-picosecond characteristic times. Here, the authors report carrier lifetimes ~30 ps at low energy in graphene/hBN Zener-Klein transistors, attributed to interband Auger processes.

InAs-Nanowire-Based Broadband Ultrafast Optical Switch

Due to their tunable optical properties with various shapes, sizes, and compositions, nanowires (NWs) have been regarded as a class of semiconductor nanostructures with great potential for photodetectors, light-emitting diodes, gas sensors, microcavity lasers, optical modulators, and converters. Indium arsenide (InAs), an attractive III-V semiconductor NW with the advantages of narrow bandgap and large electron mobility, has attracted considerable interest in infrared optoelectronic and photonic devices. Here, we studied the ultrafast carrier dynamics and nonlinear optical responses of InAs NWs ranging from 1.0 to 2.8 μm and demonstrated the InAs-NW-based ultrafast broadband optical switch for passively Q-switching in all-solid-state laser systems. Furthermore, we achieved ultrafast optical modulation for laser mode-locking at 1.0 μm, paving the way for their applications in the field of ultrafast optics. These exotic optical properties indicate that InAs NWs have significant potential for various optoelectronic and photonic devices, especially in the mid-infrared wavelength range.

Experimental Demonstration of Ultrafast THz Modulation in a Graphene-Based Thin Film Absorber through Negative Photoinduced Conductivity

We present an experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on an SU-8 dielectric substrate that is grounded by a metallic ground plate. The structure shows enhanced absorption originating from constructive interference of the impinging and reflected waves at the absorbing graphene sheet. The modulation of this absorption, which is demonstrated via a THz time-domain spectroscopy setup, is achieved by applying an optical pump signal, which modifies the conductivity of the graphene sheet. We report an ultrafast (on the order of few ps) absorption modulation on the order of 40% upon photoexcitation. Our results provide evidence that the optical pump excitation results in the degradation of the graphene THz conductivity, which is connected with the generation of hot carriers, the increase of the electronic temperature, and the dominant increase of the scattering rate over the carrier concentration as found in highly doped samples.

Nonlocal transport in bounded two-dimensional systems: An iterative method

The concept of transport mediated through the dynamics of "jumping" particles is used to develop an iterative method for obtaining steady-state solutions to the nonlocal transport equation in two dimensions. The technique is self-adjoint and capable of correctly treating spatially nonuniform, asymmetric systems. An appropriate reduced version of the iteration method is used to compare with results obtained with a self-adjoint one-dimensional transport matrix approach [Maggs and Morales, Phys. Rev. E 94, 053302 (2016)10.1103/PhysRevE.94.053302]. The transport "jump" probability distribution functions are based on Lévy α-stable distributions. The technique can handle the entire Lévy α-parameter range from one (Lorentz distributions) to two (Gaussian distributions). Cases with α=2 (standard diffusion) are used to establish the validity of the iterative method. The capabilities of the iterative method are demonstrated by presenting examples from systems with various source configurations, boundary shapes, boundary conditions, and spatial variations in parameters.

Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas

Time- and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation, a sequence of time frames is resolved that are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent nonthermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes.

Unconventional double-bended saturation of carrier occupation in optically excited graphene due to many-particle interactions

Saturation of carrier occupation in optically excited materials is a well-established phenomenon. However, so far, the observed saturation effects have always occurred in the strong-excitation regime and have been explained by Pauli blocking of the optically filled quantum states. On the basis of microscopic theory combined with ultrafast pump-probe experiments, we reveal a new low-intensity saturation regime in graphene that is purely based on many-particle scattering and not Pauli blocking. This results in an unconventional double-bended saturation behaviour: both bendings separately follow the standard saturation model exhibiting two saturation fluences; however, the corresponding fluences differ by three orders of magnitude and have different physical origin. Our results demonstrate that this new and unexpected behaviour can be ascribed to an interplay between time-dependent many-particle scattering and phase-space filling effects.

All-polarization-maintaining, stretched-pulse Tm-doped fiber laser, mode-locked by a graphene saturable absorber

In this Letter, we demonstrate an all-polarization-maintaining, stretched-pulse Tm-doped fiber laser generating ∼200  fs pulses centered at 1945 nm. As a saturable absorber, a graphene/poly(methyl methacrylate) composite was used. To the best of our knowledge, this is the first demonstration of stretched-pulse operation of a graphene-based fiber laser at 2 μm.

Self-adjoint integral operator for bounded nonlocal transport

An integral operator is developed to describe nonlocal transport in a one-dimensional system bounded on both ends by material walls. The "jump" distributions associated with nonlocal transport are taken to be Lévy α-stable distributions, which become naturally truncated by the bounding walls. The truncation process results in the operator containing a self-consistent, convective inward transport term (pinch). The properties of the integral operator as functions of the Lévy distribution parameter set [α,γ] and the wall conductivity are presented. The integral operator continuously recovers the features of local transport when α=2. The self-adjoint formulation allows for an accurate description of spatial variation in the Lévy parameters in the nonlocal system. Spatial variation in the Lévy parameters is shown to result in internally generated flows. Examples of cold-pulse propagation in nonlocal systems illustrate the capabilities of the methodology.

All-fiber Ho-doped mode-locked oscillator based on a graphene saturable absorber

In this Letter, we demonstrate a graphene mode-locked, all-fiber Ho-doped fiber laser generating 1.3 nJ energy pulses directly from the oscillator. The graphene used as a saturable absorber was obtained via chemical vapor deposition on copper substrate and immersed in a poly(methyl methacrylate) support. The laser generated ultrashort soliton pulses at 2080 nm with bandwidth up to 6.1 nm. The influence of the output coupling ratio and the SA modulation depth on the mode-locking performance was also investigated.

Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator

Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.

THz saturable absorption in turbostratic multilayer graphene on silicon carbide

We investigated the room-temperature Terahertz (THz) response as saturable absorber of turbostratic multilayer graphene grown on the carbon-face of silicon carbide. By employing an open-aperture z-scan method and a 2.9 THz quantum cascade laser as source, a 10% enhancement of transparency is observed. The saturation intensity is several W/cm2, mostly attributed to the Pauli blocking effect in the intrinsic graphene layers. A visible increase of the modulation depth as a function of the number of graphene sheets was recorded as consequence of the low nonsaturable losses. The latter in turn revealed that crystalline disorder is the main limitation to larger modulations, demonstrating that the THz nonlinear absorption properties of turbostratic graphene can be engineered via a proper control of the crystalline disorder and the layers number.

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.

Transient charge and energy balance in graphene induced by ultrafast photoexcitation

Ultrafast optical pump-probe spectroscopy measurements on monolayer graphene reveal significant optical nonlinearities. We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac-fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates for absorption loss in the graphene layer. To quantitatively investigate this transient, extremely dense photoexcited Dirac-fermion state, we construct a two-chemical-potential model, in addition to a time-dependent transient carrier temperature above the lattice temperature, to describe the population inverted electronic state metastable on the time scale of tens of femtoseconds generated by a strong exciting pulse. The calculated transient optical conductivity reveals a complete bleaching of absorption, which sets the saturation density during the pulse propagation. In particular, the model calculation reproduces the negative optical conductivity at lower frequencies in the states close to saturation, corroborating the observed femtosecond stimulated emission and optical gain in the wide near-infrared window.

Photothermal response in dual-gated bilayer graphene

The photovoltaic and bolometric photoresponse in gapped bilayer graphene was investigated by optical and transport measurements. A pulse coincidence technique at 1.5 μm was used to measure the response times as a function of temperature. The bolometric and photovoltaic response times were found to be identical implying that the photovoltaic response is also governed by hot electron thermal relaxation. Response times of τ ∼ 100-20 ps were found for temperatures from 3-100 K. Above 10 K, the relaxation time was observed to be τ = 25 ± 5 ps, independent of temperature within noise.

The impact of pump fluence on carrier relaxation dynamics in optically excited graphene

The carrier dynamics in optically excited graphene is determined by an interplay between the Coulomb- and phonon-induced scattering processes. The pump fluence of the excitation pulse plays a crucial role: for high fluences the ultrafast relaxation dynamics is determined by carrier–carrier scattering, whereas for low fluences the carrier–phonon relaxation channels are predominant. On the basis of a microscopic approach, we shed light on the influence of the pump fluence on the temporal evolution of carrier and phonon occupations as well as on the many-particle dephasing of the microscopic polarization. The new insights obtained contribute to a better understanding of the ultrafast carrier dynamics in graphene and can guide future pump–probe experiments.