Feasibility of terahertz lasing in optically pumped epitaxial multiple graphene layer structures

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The modulation depth of 2-D electron gas (2DEG) based THz modulators using AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%. The metal gate not only attenuates the THz signal (> 90%) but also severely degrades the modulation depth. The metal losses can be significantly reduced with an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high mobility of holes that is comparable to electron mobility in conventional semiconductors. The hole conductivity in graphene can be electrostatically tuned in the graphene-2DEG parallel capacitor configuration, thus more efficiently tuning the THz transmission. In this work, we show that it is possible to achieve a modulation depth of > 90% while simultaneously minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac point in graphene.

Section: Manuscript Textmentioning

confidence: 99%

Unique prospects for graphene-based terahertz modulators

Sensale‐Rodriguez¹,

Tian²,

Yan³

et al. 2011

196

128

“…[19,20]). This can provide the superiority of the double-GL lasers under consideration over the GL-based lasers using the intra-GL transitions discussed previously [4][5][6] and QCL lasers [12,31] at elevated temperatures in the low end of the THz range as well as in the range where the operation of QCLs is hampered by the optical phonon absorption.…”

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confidence: 99%

“…In particular, the interband population inversion and the pertinent negativity of the dynamic conductivity in GLs [2,3] due to the optical or injection pumping can be used in GL-based THz lasers [4][5][6][7][8][9]. First experimental results on the THz emission from optically excited GLs [10] (see also review paper [11] and references therein) instill confidence in the realization of such lasers.…”

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confidence: 99%

Injection terahertz laser using the resonant inter-layer radiative transitions in double-graphene-layer structure

Ryzhii

Dubinov

Aleshkin

et al. 2013

Self Cite

We propose and substantiate the concept of terahertz (THz) laser enabled by the resonant electron radiative transitions between graphene layers (GLs) in double-GL structures. We estimate the THz gain for TM-mode exhibiting very low Drude absorption in GLs and show that the gain can exceed the losses in metal-metal waveguides at the low end of the THz range. The spectrum of the emitted photons can be tuned by the applied voltage. A weak temperature dependence of the THz gain promotes an effective operation at room temperature.The gapless energy spectrum of graphene layers (GLs) [1] enables the creation of different terahertz (THz) devices utilizing the interband transition. In particular, the interband population inversion and the pertinent negativity of the dynamic conductivity in GLs [2,3] due to the optical or injection pumping can be used in GL-based THz lasers [4][5][6][7][8][9]. First experimental results on the THz emission from optically excited GLs [10] (see also review paper [11] and references therein) instill confidence in the realization of such lasers. One of the obstacles, limiting the achievement of the negative dynamic conductivity in the range of a few THz, is the reabsorption of the photons with the in-plane polarization emitted at the interband transitions due to the intraband transitions (the Drude absorption). Similar situation takes place in the quantum cascade lasers (QCLs) based on multiple quantum well (MQW) structures [12]. However, in the case of the photon polarization perpendicular to the QW plane the intraband (intrasubband) absorption can be much weaker than that following from the semi-classical Drude formula [13].In this paper, we propose a device structure based on a double-GL structure shown in Fig. 1 (upper panel) with the injection of electrons to one n-doped GL and to another p-doped GL, which can be used for lasing of THz photons with the electric field perpendicular to the GL plane due to the tunneling inter-GL radiative processes. The structure band diagram under the applied bias voltage and tunneling transitions assisted with the emission of photons with the energy ω ∼ ∆, where ∆ is the energy distance (gap) between the Dirac points in GLs, are demonstrated in Fig. 1 (lower panel). These transitions take place from the conduction band of the GL with 2DEG to the empty conduction band of GL with 2DHG. The transition from the filled valence band the former GL to the empty portion of the valence band of the latter GL also contribute to the emission of photons with ω ∼ ∆. The structure comprises two GLs with the side contact at one of the GL edges. This double-GL structure plays the role of the laser active region. The opposite edge of GL is isolated from another contact. A narrow tunneling-transparent barrier separates GLs (its thickness d is about few nanometers). The applied bias voltage V provides the formation of the two-dimensional electron and hole gases (2DEG and 2DHG) in the upper and lower GLs, respectively owing to the injection from the side contacts, so that the ...

“…One of the most challenging and promising problems is the creation of the graphenebased THz lasers [5][6][7]. These lasers are expected to operate at room temperature, particularly, in the 6-10 THz range, where the operation of III-V quantum cascade lasers is hindered by the optical phonons [8].…”

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confidence: 99%

Negative terahertz conductivity in disordered graphene bilayers with population inversion

Svintsov

Otsuji

Mitin

et al. 2015

Self Cite

The gapless energy band spectra make the structures based on graphene and graphene bilayers with the population inversion created by optical or injection pumping to be promising media for the interband terahertz (THz) lasing. However, a strong intraband absorption at THz frequencies still poses a challenge for efficient THz lasing. In this paper, we show that in the pumped graphene bilayer structures, the indirect interband radiative transitions accompanied by scattering of carriers caused by disorder can provide a substantial negative contribution to the THz conductivity (together with the direct interband transitions). In the graphene bilayer structures on high-κ substrates with point charged defects, these transitions almost fully compensate the losses due to the intraband (Drude) absorption. We also demonstrate that the indirect interband contribution to the THz conductivity in a graphene bilayer with the extended defects (such as the charged impurity clusters, surface corrugation, and nanoholes) can surpass by several times the fundamental limit associated with the direct interband transitions and the Drude conductivity. These predictions can affect the strategy of the graphene-based THz laser implementation.The absence of a band gap in the atomically thin carbon structures,such as graphene and graphene bilayers, enables their applications in different terahertz (THz) and infrared devices [1][2][3][4]. One of the most challenging and promising problems is the creation of the graphenebased THz lasers [5][6][7]. These lasers are expected to operate at room temperature, particularly, in the 6-10 THz range, where the operation of III-V quantum cascade lasers is hindered by the optical phonons [8]. Recent pump-probe spectroscopy experiments confirm the possibility of the coherent radiation amplification in the optically pumped graphene [9][10][11][12][13][14][15][16], enabled by a relatively long-living interband population inversion [17]. As opposed to optically pumped graphene lasers, graphenebased injection lasers are expected to operate in the continuous mode, with the interband population inversion maintained by the electron and hole injection from the n-and p-type contacts [18]. A single pumped graphene sheet as the gain medium provides the maximum radiation amplification coefficient corresponding to the quantity 4πσ Q /c = πα = 2.3%, where σ Q = e 2 /4 is the universal optical conductivity of a single graphene layer, e is the electron charge, is the Planck constant and α ≃ 1/137 is the fine-structure constant [17]. The THz