Observation of a Transient Decrease in Terahertz Conductivity of Single-Layer Graphene Induced by Ultrafast Optical Excitation

Jnawali, Giriraj; Rao, Yi; Yan, Hugen; Heinz, Tony F.

doi:10.1021/nl303988q

Cited by 261 publications

(316 citation statements)

References 73 publications

(123 reference statements)

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“…So a lower cooling rate (longer lifetime of hot electrons) leads to a larger photocurrent. The electron heating and cooling dynamics in bulk graphene have been studied using pump-probe measurements, such as optical pumpprobe [22,31,33,34], femtosecond time-resolved angleresolved photo-electron spectroscopy (ARPES) [30,32], and time-resolved optical pump-terahertz (THz) probe spectroscopy [26,[35][36][37][38][39][40][41]. The photoexcited carrier dynamics have also been studied in graphene-based devices through time-resolved photocurrent scanning microscopy [7,8,17,29].…”

Section: Time-resolved Photocurrentmentioning

confidence: 99%

Hot-carrier photocurrent effects at graphene–metal interfaces

Tielrooij¹,

Massicotte²,

Piątkowski³

et al. 2015

J. Phys.: Condens. Matter

102

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Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.

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Section: Time-resolved Photocurrentmentioning

confidence: 99%

Hot-carrier photocurrent effects at graphene–metal interfaces

Tielrooij¹,

Massicotte²,

Piątkowski³

et al. 2015

J. Phys.: Condens. Matter

102

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“…This technique has been applied to study transient photoconductivity (PC) in graphene, but conflicting results have been reported [9][10][11][12][13][14][15]. Positive PC was observed in epitaxial graphene on SiC (Ref.[15]), while negative PC was seen in graphene grown by chemical vapor deposition (CVD) [11][12][13][14]. It has been argued that the opposite behavior in these samples arises from their different charge densities.…”

mentioning

confidence: 99%

“…The resulting ratio −ΔE τ =E 0 (referred to as "differential field") approximately represents the PC, Δσ τ;1 (Refs. [9][10][11][12]17]). …”

mentioning

confidence: 99%

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

et al. 2014

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We investigate the transient photoconductivity of graphene at various gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations can be accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Notably, we observe nonmonotonic fluence dependence of the photoconductivity at low carrier density. This behavior reveals the nonmonotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. DOI: 10.1103/PhysRevLett.113.056602 PACS numbers: 72.80.Vp, 72.40.+w, 73.40.Qv, 78.20.−e Charge carriers in graphene mimic two-dimensional (2D) massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties [1]. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range [2]. Interband transitions in graphene give rise to pronounced optical absorption in the midinfrared to visible spectral range, where the optical conductivity is close to a universal value σ 0 ¼ πe 2 =2h [3]. Free-carrier intraband transitions, on the other hand, cause lowfrequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density [4]. In addition to this density dependence, the massless Dirac particles in graphene are predicted to exhibit a distinctive nonmonotonic temperature dependence of the intraband absorption strength, or Drude weight, due to their linear dispersion [5,6]. This behavior contrasts with the temperature-independent Drude weight expected in conventional systems of massive particles with parabolic dispersion [7,8]. Although the unique behavior of the Drude weight in graphene has been considered theoretically, experimental signatures are still lacking.The intrinsic properties of Drude absorption in graphene can be revealed by studying its dynamical response to photoexcitation. In particular, optical-pump terahertz-probe spectroscopy provides access to a wide transient temperature range via pulsed optical excitation, and allows measurement of the ac Drude conductivity by a timedelayed terahertz probe pulse [9]. This technique has been applied to study transient photoconductivity (PC) in graphene, but conflicting results have been reported [9][10][11][12][13][14][15]. Positive PC was observed in epitaxial graphene on SiC (Ref.[15]), while negative PC was seen in graphene grown by chemical vapor deposition (CVD) [11][12][13][14]. It has been argued that the opposite behavior in these samples arises from their different charge densities. Here we study...

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“…They are particularly useful for investigations of Dirac materials, as demonstrated in studies of the photoinduced carrier dynamics in graphene. [20][21][22][23][24][25] Recently, THz-TDS provided evidence of the surface carrier conductivity at THz frequencies with a relatively minor contribution from the bulk carriers in thin films of Bi 2 Se 3 . [7,8] Here, we take advantage of the unique characteristics of OPTP spectroscopy to unravel the differences in the dynamics of hot surface and bulk carriers following photoexcitation in TI Bi 2 Se 3 .…”

mentioning

confidence: 99%

“…This clearly demonstrates that an increase in Γ D dominates the photoinduced conductivity changes, as previously reported in graphene. [24,25] We now turn to the signature of the bulk phonon in the 10 QL film in Fig. 3d, where the phonon frequency ω o versus τ pp at all measured fluences is shown.…”

mentioning

confidence: 99%

Time-resolved terahertz dynamics in thin films of the topological insulator Bi2Se3

Aguilar

Brahlek

et al. 2015

Applied Physics Letters

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We use optical pump-THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi2Se3 films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.Topological insulators (TI) represent a new state of matter [1][2][3] and promise numerous applications in optoelectronics and spintronics. [4][5][6] In an ideal TI, insulating bulk does not contribute to the charge transport, and conductivity is determined solely by surface carriers. The surface state is topologically protected from backscattering and exhibits very low transport scattering rates (the inverse transport lifetime). For example, measurements of AC conductivity of thin films [7,8] and single crystals [9,10] of exemplary TI material, Bi 2 Se 3 , found surface scattering rates of at most 2 THz which indicated, at least partially, the effect of the topological protection of the surface state.However, the existing generation of TI materials, specifically Bi 2 Se 3 , exhibit significant contributions from the bulk carriers to the total conductivity. One reason is the chemical potential shift into the conduction band due to electron doping, typically by Se vacancies and anti-site defects.[11] Another effect interfering with ideal TI response occurs when the bulk bands cross the Fermi level near the material surface (band bending) and generate two dimensional electron gas (2DEG). This 2DEG is confined within ∼20 nm from the surface [12,13] and coexists with the topological surface states. Although such 2DEG has been observed in single crystals, [12,13] there is no definite evidence for its existence in thin films of Bi 2 Se 3 . As we will show below, we observed significant changes in the transport dynamics even as we varied the film thickness well below the range expected for the effects of the 2DEG to occur, which suggests that it does not play a significant role in our measurements.Optoelectronic functionality of TI relies on the dynamic response of non-equilibrium charge carriers in topologically protected surface states, making it important to unambiguously separate surface response from the interfering bulk. Although electronic measurements on FET structures have unveiled a plethora of TI transport parameters (scattering rate, plasma frequency, etc.), they cannot reliably distinguish signatures of the surface carriers from that of the bulk carriers present in no...

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Observation of a Transient Decrease in Terahertz Conductivity of Single-Layer Graphene Induced by Ultrafast Optical Excitation

Cited by 261 publications

References 73 publications

Hot-carrier photocurrent effects at graphene–metal interfaces

Hot-carrier photocurrent effects at graphene–metal interfaces

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

Time-resolved terahertz dynamics in thin films of the topological insulator Bi2Se3

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