Compact, high-power, and room-temperature terahertz (THz) sources are highly sought after to enable the practical realization of vast applications of THz waves , , , , , such as sensing and non-destructive imaging for safety and security , , , , , medical diagnosis , , , and ultra-broadband wireless communications , , . Several types of THz light sources and/or oscillators (such as THz quantum cascade lasers , , , , , , p-Ge lasers , , and resonant tunneling diode (RTD) oscillators ) have been developed so far. However, laser-type devices, on the one hand, suffer from phonon de-coherency, preventing room-temperature operation . Electron devices like RTDs, on the other hand, suffer from their electron transit times and parasitic capacitance-resistance time constants. Their maximum available output power decreases with increasing frequency by a factor of three orders and is as small as 10 μW order at 1 THz at room temperature . To break through these substantial limitations, graphene has attracted attention owing to its gapless and linear energy spectrum and massless Dirac Fermions, giving rise to superior carrier transport properties , , , , . The latest studies have reported on the potential of relatively simpler multi-layer graphene devices ,  to more complex graphene/h-BN heterostructures ,  for highly sensitive detection as well as emission of THz radiation .
Optical and/or injection pumping of graphene can enable negative-dynamic conductivity in the THz spectral range, which may lead to new types of THz lasers , , , . The THz gain in optically pumped graphene has been experimentally confirmed , . However, optical pumping suffers from carrier heating, preventing from obtaining carrier population inversion and eventual gain , . On the other hand, in the graphene structures with p-i-n junctions, the injected electrons and holes have relatively low energies compared with those in optical pumping, so that the effect of carrier cooling can be rather pronounced, providing a significant advantage of the injection pumping in realization of graphene THz lasers , , , . Recent extensive studies on Auger processes reveal the difficulty of carrier population inversion to be obtained , , , , , which could be dominated in carrier heating regime with breaking the linear dispersive conical band profiles and/or band broadening due to many body effects of the inter-carrier Coulomb scattering , . In this regard, current-injection pumping is the best way to substantially suppress the carrier heating and the Auger processes towards the lasing operation.
Current-injection graphene THz laser, as theoretically proposed by Ryzhii et al. , , , can be constructed with a dual-gate graphene-channel field-effect transistor (DG-GFET) structure, in which electrons and holes are laterally injected between the dual-gated regions by applying complementary gate biases and the populations of electrons and holes are inverted by applying a weak positive drain-source dc bias. Due to the continuum of gapless energy spectra of conduction electrons and valence holes, there are no quantized upper/lower levels that define the photon energy of the spontaneous emission under population inversion. Thus, graphene under carrier population inversion will emit rather broadband photons according to its broad gain spectral profile . Therefore, to make single-mode lasing, one needs to implement a pertinent high-Q laser cavity structure in which the gain medium of the graphene under carrier injection pumping is accommodated.
In this work, a prototype current-injection-type graphene laser device was fabricated in a distributed feedback (DFB) DG-GFET structure and challenged for the first observation of THz radiation oscillations. We experimentally observed amplified spontaneous broadband light emission from 1 to 7.6 THz at 100 K by carrier injection in one sample of the fabricated DFB-DG-GFETs, demonstrating the birth of a new type of THz light-emitting transistors (LETs). In another yet similar device, we also observed a single-mode emission at 5.2 THz corresponding to the fundamental mode of the DFB cavity at 100 K. Both devices exhibited peculiar nonlinear threshold-like behavior with respect to the current-injection level. The result is still at a preliminary level, but the linewidth for the single-mode emission fairly agrees with the calculation based on the DFB-Fabry-Perrot hybrid-mode modeling, suggesting that it could be single-mode lasing. The broadband emission in the former device is interpreted as an amplified spontaneous THz emission being transcended to a single-mode lasing.
2 Device design and fabrication
2.1 Design and fabrication details
The cross-sectional schematic of the device is shown in Figure 1A. First, epitaxial graphene was synthesized by the thermal decomposition of a SiC substrate; this method provides both easiness of fabricating the device and high crystallinity of the film . C-face is specifically chosen to obtain non-Bernal stacking of multiple undoped graphene layers excluding the heavily doped buffer layer that is substantially made for Si-face decomposition. Lattice structure and crystal quality were first characterized by Raman spectroscopy at 300 K. The Raman shift spectrum obtained is shown in Figure 1B and C. No peak at the D band and sharp mono-peaks at the G and G′ bands with G′/G peak intensity ratio being 2.11 confirmed a high-quality, few-layer non-Bernal stacked graphene. Then the surface morphology of the synthesized epitaxial graphene was investigated by low-energy electron microscope (LEEM) at 300 K. Very large grains (>50 μm) consisting mainly of two-layer graphene were observed. Figure 1D shows that graphene is homogeneous in a rather large area; the lighter central area consists of two layers, whereas the darker area outside its edges contains three to six graphene layers . This bilayer nature of the central grain area was also confirmed by the presence of the two dips in electron reflectance spectra between 0 and 8 eV, as shown in the inset in Figure 1D. Figure 1E shows an angle resolved photoelectron spectroscopy (ARPES) image, manifesting that the main band holds a linear dispersion and is divided into two groups and Fermi level lies at 0 eV, confirming the high-quality, non-Bernal stacked, double-layer graphene with no unintentional doping [52, 54]. By using this epitaxial graphene, GFET was fabricated by a standard photolithography process involving a gate stack of SiN dielectric layer, providing an excellent intrinsic field-effect mobility exceeding 100,000 cm2/Vs at 300 K at the maximal transconductance . Throughout this gate stack process, the epitaxial graphene layer was encapsulated between the SiC substrate and the SiN gate dielectric layer, preventing the exposure of the graphene surface to the air and thus from the chemical reactions of unintentional doping even under varying the temperatures.
A pair of toothbrush-shaped gate electrodes was patterned to form a DFB cavity in which the active gain area and corresponding gain coefficient are spatially modulated  (see Figure 2A and B). The number of DFB periods NDFB was designed to be NDFB=16, resulting in active graphene channel width of ~200 μm, as shown in Figure 2B. Designed grating period Λ, the effective refractive index neff, the Bragg wavelength λdfb, and the principal mode fP are 12 μm, 2.52, 60.5 μm, and 4.96 THz, respectively, as shown in Figure 2C. In this work, we carried experiments on two devices that are similar in design but slightly differ in the substrate thickness and DFB design parameters. The DFB modulation indices for device 1 (showing broadband emission) and device 2 (showing single mode emission) are Lint/Ldfb=20/15 and 20/18, respectively. Also, the carrier momentum relaxation time of these two devices, which were extracted from the slope of the ambipolar current-voltage curves, has different values: device 1 having longer values (ranging from 1.5 to 4 ps at temperatures 300~100 K) than those for device 2 (ranging from 0.8 to 2 ps at temperatures 300~100 K).
2.2 Principles of operation
The operation principle called current injection THz lasing is as follows: when the dual-gate electrodes are applied with complementary biases (Vg=−Vg1=+Vg2), electrons or holes are injected underneath each gate electrode. The level of Vg determines the carrier injection level and corresponding Fermi level εF. With no applied source to drain bias Vd, the electrons and holes diffuse into the ungated i region and are recombined together to annihilate. Application of Vd, on the other hand, shifts the quasi-Fermi level of electrons and holes injected into the ungated i region to form a population inversion, and the recombination of those electrons and holes having an energy difference of several meV gives rise to spontaneous THz photon emission .
The real part of the dynamic conductivity Re σ(ω) in graphene under current-injection pumping is given by the sum of the intraband Drude-like component and the interband transition-related component as follows , :
where e is the elementary charge, f(ε) is the Fermi-Dirac distribution function for electrons, ħ is the reduced Planck’s constant, kB is the Boltzmann constant, T is the carrier temperature, τ is the momentum relaxation time, vF is the Fermi velocity, κ is the permittivity of the gate dielectric layer, and d is the gate-dielectric layer thickness. Re σintra(ω) always takes positive values and has the dependence of
Figure 3 plots the simulated real part of the dynamic conductivities of the DG-GFET at a constant carrier temperature of 300 K for different complementary gate biases (Vg=−Vg1=+Vg2) and drain-source biases Vd conditions according to Eqs. (1)–(4). The values of κ, d, τ, and T are assumed to be 4.7, 50 nm, 1 ps, and 300 K, respectively. When Vg=1.0 V, a weakly biased condition giving rise to doping with a Fermi level εF of 24.5 meV, as shown in Figure 3A, rather weak Vd beyond the threshold level of ~20 mV causes carrier population inversion and real net gain (negative dynamic conductivity) around 2 THz. Increasing the Vd value up to a certain level (Vd~30 mV in Figure 3A), which shifts the quasi-Fermi level equal to the magnitude of the original Fermi level at zero-Vd, can increase both the gain and its bandwidth. The lower and upper cutoff frequencies are situated between 0.7 and 7 THz. Further increase in Vd increases the channel electric field intensity resulting in carrier heating and weakening the gain. With increasing Vg, the carrier injection increases so that larger and wider gain bandwidths are obtained, as shown in Figure 3B and C. As abovementioned, a longer τ, meaning a higher carrier mobility, gives a higher THz gain with its peak at lower frequency, as shown in Figure 3D in comparison with Figure 3B. Hence, to realize a current injection THz lasing, realization of GFET with high carrier mobility is indispensable. In real operating conditions, as is described in detail in Ref. , carrier temperature T may change depending on the current-injection levels, modifying the gain spectral profiles. This effect will be discussed to interpret the experimental results in the next section.
Another important criterion of the device operation is choosing the lasing frequency. The conductivity profile shows negative gain for a wide range of THz frequencies. To make single-mode lasing, one needs to implement a pertinent high-Q laser cavity structure in which the gain medium of the graphene under carrier injection pumping is accommodated. Such a laser cavity can be realized by using the gate electrodes as a DFB type waveguide structure. As shown in Figure 2B, the DFB structure periodically modulates the distance between the two gate electrodes along the length of the device. This results in periodic modulation of the gain coefficient given by the conductivity and the gain overlapping factor (the ratio of the areal integration of the THz photon electric field overlapped with the gain area to that of the entire area) . At the Bragg wavelength (λB=2neffλdfb), determined by the modulation period λdfb and the effective refractive index neff of the gain coefficient, the spontaneously emitted THz photons intensify each other, in each period, along the dual-gate direction. At that resonance frequency, single-mode oscillation can be obtained if the current-injection gain at only the maximal mode exceeds the laser oscillation threshold. Figure 4A shows the quality factors for in-plane waveguided modes of the DFB-DG-GFET structure for different quality graphene samples having τ of 0.1, 1.0, 3.0, 5.0, and 7.0 ps at a weak drain bias of 30 mV. For longer τ of 5~7 ps, the spectra exhibit high Q factors up to 150 over a broader frequency range, suggesting LED-like broadband spontaneous emission. The modal gain for this case is ~−30 cm−1 (losses are greater than gain, limiting the possible operation to spontaneous emission). Varying the DFB design parameters, quality of graphene layer, and thickness of substrate can largely affect the confinement of THz photon electric fields on to the active gain area, which can hugely alter the gain profile. The other important parameter is the drain bias Vd that determines the level of carrier population inversion. Figure 4B shows the numerically calculated quality factors for the case of device 2, with different Vd levels of 30, 50, 70, 100, and 150 mV for a relatively shorter fixed τ value of 3.5 ps. When Vd is relatively high at around 70~100 mV, the spectra exhibit single peak at the designed fundamental DFB mode at ~5 THz, suggesting the single-mode lasing operation. The modal gain in this case is ~5 cm−1. Further increase in Vd to 150 mV totally vanishes the peak profile, reflecting the carrier heating and making the net THz gain disappear. These results suggest that graphene quality and carrier temperature reflecting τ values, drain bias reflecting the level of population inversion, and DFB modulation index reflecting the Q factor determine the overall spectral profile of the THz photon emission.
3 Results and discussions
THz emission from the sample was measured at temperatures ranging from 300 K down to 100 K using a Fourier-transform far-infrared (FTIR) spectrometer with a 4.2 K-cooled Si bolometer (see Supplement 1 for details). The background blackbody radiation was first observed under the zero-bias condition, which was subtracted from the one observed under biased conditions. The drain-current-to-gate-voltages (Vg1, Vg2) characteristics were first measured for the fabricated samples. They exhibited ambipolar characteristics as expected, as shown in Figure 5A. The relationship between drain voltage (Vd) and drain current (Id) when a positive voltage was applied to one gate and the negative voltage was applied to the other gate was found to be linear, as shown in Figure 5B. Unlike ordinary semiconductor p-i-n diodes, immediate current flow is observed, irrespective of a positive or negative voltage change due to the gapless and symmetrical conical band dispersion for the conduction and valence bands. With complimentary biased gate 1 (Vg1<0) and gate 2 (Vg2>0), with a positive drain-source bias Vd (as designated by the square dots in Figure 5A), the carrier population can be inverted at the intermediate channel region . Figure 5C shows the temperature dependence of τ in these devices. With decreasing temperature, thermionic current reduces, i.e. the level of drain current decreases but the transconductance (gm), the slope of the ambipolar curves in Figure 5A, increases, reflecting the increase of τ. Hence, at low temperatures, τ increases so that one can expect negative conductivities to be obtained more easily (see Figure 3D), enabling enhanced emission at low temperatures.
3.1 Broadband THz light emission
The emission results from device 1 are summarized in Figure 6. A rather strong emission at 100 K, stronger than that at higher temperatures from 300 to 150 K (Figure 6A), was observed in 1–7.6-THz range when Vd is forward-biased to a certain level under symmetric electron-hole injection conditions leading to carrier population inversion (see inset of Figure 5B: Vg1=−2.28 V and Vg2=4.56 V at 100 K). The emission power from the device integrated over 1–7.6 THz is of the order of ~80 μW at most. The shape of emission spectra reflects superimposed effects of the graphene-original conductivity profile, which gives larger gain at lower frequencies  and the DFB cavity effects that have the fundamental mode peak at ~4.96 THz. Regarding the former, as the device is kept at low temperatures, scattering processes in graphene are suppressed, which means longer momentum relaxation time of the carriers present in the channel (see Figure 5D), thereby enabling the achievement of higher negative conductivity in the wide THz range and resultant broadband spontaneous THz emission (see Figure 3D). To obtain single-mode lasing, improvement on the Q factor of the DFB cavity with larger numbers of the DFB periods and/or deeper DFB modulation indices would be a possible solution (see Figure S2 in Supplemental Material). Further precise identification of the threshold temperature to the spontaneous THz emission is a next subject.
With increasing Vd, the emission power increases non-monotonically while preserving its spectral shape, as shown in Figure 6B. Apart from temperature-dependent spontaneous THz emission, the device also exhibited double-threshold-like behavior with respect to Vd and drain injection current, as shown in Figure 6C. The first threshold current to the spontaneous THz emission stays below 0.1 mA, whereas the second threshold current does around 0.9 mA. Such a double-threshold-like behavior may be due to the carrier overcooling effect , . As theoretically predicted in Refs.  and , when carriers are excited, their energy is relaxed mainly via emission of optical phonons (including interband emission). The optical phonon decay rate, which depends on the overall thermal conductivity of the device, modifies the energy relaxation dynamics and carrier cooling decay. The existence of the scattering factors with a finite value of τ gives rise to the imaginary part of the conductivity, Im σ, which is known as the origin of the kinetic inductance of carriers. Such an inductive inertia of carriers may cause an overshoot on their energy transfer to the lattice phonons, resulting in carrier overcooling even below the lattice temperature in a limited time scale in a rather weak carrier-injection pumping regime . Continuous injection pumping at a low Vd level may cause carrier overcooling steadily, which in turn enhances the carrier population inversion and contributes to promote the THz gain. When the carrier injection level rises further with increasing Vd and dc power consumption (see Figure 6C), carrier heating is dominated, which prevails over the cooling effect, resulting in net carrier heating and reduction of the THz gain. Further increase in Vd increases the level of population inversion, recovering the net THz gain. As a consequence, the carrier temperature changes non-monotonically along with the increase in Vd . This is thought to be a possible interpretation for the cause of such non-monotonic double-threshold-like behavior.
Also, the conductivity profile highly depends on the applied biases, as shown in Figure 3. Increasing the gate voltage increases the number of injected charge carriers, which thereby increase the gain bandwidth and the lower cutoff for gain . The drain bias is also very important as it determines the shifting of quasi-Fermi level; higher voltage causes more injection current and hence more gains. However, it also increases the band slope and causes carrier heating, which sacrifices the injection beyond the Fermi level , . Hence, there is both a lower and upper limit on gain with respect to Vd, as also observed in our experimental results.
Another factor causing the broadband nature of emission from device 1 is the poor DFB cavity effects (substrate-thickness-dependent THz photon field distribution  could not meet the maximal available gain-overlapping condition), which do not work properly as single-mode lasing but give a tendency of being transcended from spontaneous broadband THz emission to stimulated emission with a central peak at the DFB fundamental mode of ~5 THz. The simulated quality factor for the DFB cavity for excellent quality GFET at low value of Vd shows gain over a wide frequency range (as seen in Figure 4A) supporting the possibility of observing broadband emission spectra from these devices. If the photon electric fields could be concentrated effectively on the graphene gain area, by optimizing the substrate thickness as well as additional thick high-K dielectric layer on top of the gate stack, one can expect single-mode lasing from these structures.
3.2 Towards single-mode lasing
We repeated the experiment on another sample device 2 (having a bit smaller DFB modulation index and shorter momentum relaxation time than those in device 1, as mentioned in 2.1) and observed no emission at 300 K, but lowering the temperature down to 100 K showed current-injection-dependent (Vg2=7.3 V, Vg1=−4.8 V) emission, as shown in Figure 7A. This single-mode-like emission also exhibited a non-monotonic threshold behavior, i.e. the emission at 5.2 THz grows with increasing Vd from 0 to 0.1 V but weakens with further increasing Vd until 0.2 V. The emission grows again after increasing Vd from 0.2 to 0.5 V, giving a single-mode emission with the highest power of ~0.1 μW. Spectral narrowing with increasing the carrier injection around the threshold was also observed, as shown in Figure 7B. The emission spectra at Vd=0.5 V can fit to the Lorentzian curve with a Q factor of 170 (a linewidth of 30.6 GHz), which fairly agrees with the simulated values ~210, as shown in Figure 4B. Compared to the broadband emission obtained from device 1, such a sharp single-mode emission is thought to be caused by the shorter momentum relaxation time of graphene carriers in device 2, which could allow the net THz gain only at around the DFB fundamental mode frequency ~5 THz, as is simulated in Figure 4B. From the good correspondence between the experimental result and the analytical results for the gain spectra, it could be inferred that the obtained radiation is due to laser oscillation.
Although device 2 shows single-mode emission, the peak power of emission is 3.5 times lower than that for device 1, which implies that high-quality GFET when paired with carefully designed DFB cavity could result in efficient THz lasing at higher THz frequencies. For example, numerical simulations suggest that an increase of NDFB from 16 to 24 and an increase of the DFB cavity modulation index Lint/Ldfb from 20/15 to 20/8 give rise to an increase of the quality factor of the DFB cavity by two orders of magnitude, respectively (see Supplement 2 in detail). Also, pertinent waveguide structures with a thick dielectric layer on top of the GFET and/or ridge-waveguiding with thick dual-gate metal pillars as well as surface plasmon-polaritonic waveguiding may improve the THz photon field spatial confinement and the gain-overlapping factor.
Further investigation is needed to confirm if the observed emission is truly single-mode lasing. Also, as the drain bias dependence and device structure dependence of radiation spectrum are yet to be fully understood, we need to elucidate them by future experiments along with improving the device processing and integration to achieve high-power THz emission at room temperature.
A forward-biased graphene structure with a lateral p-i-n junction was implemented as a DFB-DG-GFET in which the current injection mechanism was realized using a dual-gate structure and the laser action mechanism was realized using a DFB cavity structure. Using our original process technology with epitaxial graphene and SiN gate dielectric, we achieved easy device integration and very high carrier mobilities exceeding 100,000 cm2/Vs at room temperature. A rather intense carrier-injection level-dependent amplified spontaneous emission in the 1–7.6-THz range peaking at ~5 THz was observed at 100 K with integrated emission output power of ~80 μW. The results demonstrate the operation as a broadband THz light-emitting transistor. We also observed indications of single-mode emission at 5.2 THz at 100 K in another device. Although the results obtained are still preliminary level and a concrete dependence of emission power on the Vd is yet to be established, our results highlight that single-mode continuous wave THz lasing with an output power of the order of ~10 μW could be feasible by the carrier injection pumping in the DG-GFET with a carefully designed laser cavity structure.
The authors acknowledge Tetsuya Suemitsu, Hirokazu Fukidome, and Maki Suemitsu for their contributions on device processes and graphene synthesis and characterization. The epitaxial graphene was synthesized by Prof. H. Fukidome, K. Tashima, and Prof. M. Suemitsu, for which detailed characterizations of the structural and electronic properties of the epitaxial graphene were performed as the contracted cooperation studies between Prof. H. Fukidome, Prof. M. Suemitsu, Photon Factory (2015S2-005, and 2015G536), and SPring-8 (2015A1278 and 2015B1232) with devoted contributions from and insightful discussions with staffs at Photon Factory and SPring-8 (Prof. H. Kumigashira, Prof. K. Horiba, and Prof. M. Kotsugi). They also thank Vladimir Ya Aleshkin, Stephane Boubanga-Tombet, Vladimir Mitin, and Michael S. Shur for valuable discussions. The device process was carried out at the Laboratory for Nanoelectronics and Spintronics at RIEC in Tohoku University. The part of the works primarily contributed by D.Y., A.S., T.W., M.R., V.R., and T.O. was financially supported by JSPS KAKENHI (#23000008, #16H06361, and #16K14243), Japan. The part of the works primarily contributed by V.R. was supported by the Russian Scientific Foundation (#14-29-00277), and the part of the works by A.A.D. was supported by the Russian Foundation of the Basic Research (#18-52-50024).
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2017-0106).