Introduction
Graphene
The π band is responsible for the low-energy electronic band structure that causes many interesting physical phenomena. The low-energy excitations in graphene are massless, chiral Dirac fermions, which exhibit many unusual properties of quantum electrodynamics (QED), but at about a 300-fold slower speed.
Graphene plasmons
The sensitivity of the Drude weight to electron temperature enables ultrafast optical control of plasmons in graphene on femtosecond time scales. The observed phenomenon was explained with an increase in the Drude weight and was consistent with the resolved dispersion relation of graphene plasmons at an elevated electron temperature [13].
Scope of this thesis
The dispersion of the optical modes of the graphene/h-BN/SiO2 nanoresonator can be observed in Figure Ef(E, µ, Te)D(E)d E (3.8) The quantities, N0 and Np, are the initial gated and photoexcited carrier densities, respectively.
Active Control of Graphene Plasmons
Tunable large absorption in graphene
An atomic force microscopy (AFM) image of the resulting graphene nanoresonators is shown in the inset of figure. Figures 2.2(b) and (c) show that the graphene plasmonic absorption intensity always occurs at 1400 cm−1.
Perfect absorption in graphene
The type A structure depicted in Figure 2.3(a) consists of periodically arrayed 100-nm-wide gap/100-nm-wide GPRs on the 150-nm-thick SiO2/1-µm-thick SiNx/Au substrate. In the type B structure, a 100 nm wide GPR is located in the center of a 200 nm wide metallic gap. In the type C structure, a 50 nm wide GPR is located on one side of the 100 nm wide metallic gap.
The widths of the metal strips in the type B and C structures are 910 and 615 nm, respectively. The narrower metal slits in type C result in greater field enhancement by trapping the radiation more efficiently.
Tunable graphene plasmon dispersion relation and emergence of
The narrow peak seen in the bare h-BN spectrum near 1370 cm-1 has been assigned in previous studies as an in-plane optical phonon of h-BN [20]. The two modes above 1200 cm−1 show anticrossing behavior near the 1370 cm−1 optical phonon energy of h-BN due to the coupling of graphene plasmon mode and h-BN phonon mode. For the 80 nm band, the four different observable optical modes are labeled with the symbols used to indicate experimental data points in Fig. Right axis, bottom spectrum) Infrared transmission of the bare monolayer h-BN on SiO2 normalized to transmission through the SiO2 (285 nm)/Si wafer.
The relative intensities associated with these two modes change with the widths of the graphene nanoresonators. When the graphene plasmon mode is brought into resonance with the h-BN phonon, the polarizations of the two modes cancel each other, creating a transparency window where no absorption occurs in the plasmonic modes.
Tunable Planckian Thermal Emission
The power density of the incident ultrafast pulse, I(t), is thus scaled by a parameter, β, which has the same units as specific heat (eVm−2K−1) [11]. When the imaginary part of the plasmon frequency, γp, is positive (negative), plasmons by definition experience gain (loss) via net stimulated emission (absorption). The calculated graphene Fermi level-dependent absorptivity (emissivity) of the device is in good agreement with the measured absorptivity at ambient temperature as shown in fig.
Most of the gate dependence comes from the SiNx layer due to the Fabry-Perot mode formed in the SiNx layer sandwiched between the graphene and ITO layers. Snapshots of the plasmon decay rate at a given time for a given gate-controlled graphene Fermi level of 0.34 eV in the spectral range of interest (i.e., where the pulsed laser-induced emission increases with level of Fermi graphene) are shown in Fig.
Non-equilibrium Graphene Plasmons and Gain
Carrier dynamics in graphene upon ultrafast optical excitation . 47
The polarizability for the two-component plasma system with finite temperatures can be represented as the sum of the zero-temperature quasi-equilibrium polarizability and the correction terms that account for smearing of the Fermi edge due to finite temperatures as Eq. The zero-temperature quasi-equilibrium polarizability is defined as Eq. S12) accounts for the Fermi edge smearing as quantified by δf(E)|Tµ, and the evaluation of the integrand of Eq. S12) requires the derivative of ΠµT=0, which can be written as Eq. The temporal evolution of the carrier temperature as the carriers relax from a quasi-equilibrium state to a full equilibrium state is described with a phenomenological two-temperature (2T) model.
The specific heat of the SCOPs, Cp, and the electron-SCOP exchange rate, Γe-p, are expressed in Ref. The time evolution of the chemical potentials is described phenomenologically based on the rate equation studies, solving the relaxation dynamics.
Graphene Fermi level and time dependence
Since the emission rate varies with γp, the calculations show that the plasmon emission increases with increasing graphene hole doping. The increase in emission with increased hole doping of graphene can be intuitively understood as the result of an increase in the phase space for the release of excited carriers by plasmon emission. Since plasmon emission is an interband process, controlling the Fermi level of graphene through an electrostatic transition can greatly improve the observation of plasmon emission, since hole doping graphene not only increases the phase space for plasmon emission but also raises the (Pauli blocking) barrier for plasmon absorption.
The real part and (b) the imaginary part of the plasmon complex frequency spread for different gated initial graphene-Fermi levels at the time when the quasi-equilibrium has just been established. The real part and (b) the imaginary part of the plasmon complex frequency for a given initial graphene Fermi level of 0.34 eV as time progresses since the quasi-equilibrium is established.
Collision loss dependence
Effects of underlying substrate
For this reason, no plasmon emission is expected in the spectral range of wavelengths >8 µm when graphene is deposited on SiNx.
The ratio of stimulated to spontaneous plasmon emission rates 64
When the plasmonic distribution function is greater than unity (i.e. above the dotted line in Figure 3.10), the stimulated emission dominates the spontaneous emission. On a time scale of 100 fs, the stimulated emission dominates the spontaneous emission when the collision time, τcoll, is sufficiently long. Once the inversion is exhausted, absorption begins to deplete the plasmon population exponentially, and spontaneous emission begins to dominate stimulated emission.
When the plasmon distribution function (ie the ratio between the stimulated and spontaneous emission rate) is time-averaged up to t−t0=250 fs, stimulated emission is dominant over the frequency range between 4.5 µm and 6 µm as shown in fig. Amplification and lasing of terahertz radiation by plasmons in graphene with a planar distributed Bragg resonator”.
Experimental Setup
Understanding the optical properties of the device structure is important when analyzing the emission spectra. The optical properties of graphene are determined by the conductivity model within the random phase approach [3]. The deviation of the optical properties of ITO at room temperature above a sufficiently high electronic temperature (which is the case under the pulsed laser excitation) is calculated based on the previous research on the temperature-dependent plasma frequency of ITO, ωp, due to the non-parabolity of the conduction band [5].
We use a finite element method to calculate the temperature-dependent optical absorption (emission) of the device, consisting of a planar graphene layer on a 1 µm-thick SiNx and a 50 nm-thick ITO, under the incident wave directly using the temperature-dependent optical properties described above. In the wavelength range longer than 8 µm, most of the emission comes from the SiNx layer.
Mid-infrared emission phenomena under pulsed laser excitation . 76
Gate dependence arising from graphene Fermi level modulation. We note that the gate dependence in the observed emission between 4.5 µm and 8 µm under pulsed laser excitation arises only from graphene. When illuminated with pulsed laser excitation with a fluence of 1.12 J m−2 , negligible gate dependence was observed as shown in Fig. This trend is consistent with the gate dependence observed in emission under pulsed laser excitation (Fig. 4.4).
The time-integrated spontaneous plasmon emission spectra for different gate-controlled graphene Fermi levels are shown in Fig. The calculated spontaneous emission spectra predict the same gate-dependent trend in the spectral region of interest as seen in the experimentally measured emission under pulsed laser excitation.
Plasmon-coupled far-field radiation
For a direct comparison with the experimentally measured emission spectra, the calculated spontaneous emission spectra are then multiplied by the decoupling efficiency of plasmons. As the laser fluence increases, more plasmon-coupled radiation was observed experimentally in the spectral region where net plasmon enhancement is predicted (γp>0). As we demonstrate in the next section, such a limitation can be easily overcome with suitable nanophotonic structures.
Atmospheric impurities likely to be present in the purge gas have previously been shown to cause hysteresis effects in the conductance curves of graphene FET devices. Such charge traps can cause aberrant behavior in the conductance curves of the graphene FET devices, similar to what has been observed in the presence of metal impurities [13].
Roles of gold nanodisks
However, in addition to these heating effects, the resonant and non-resonant NDs cause a large gate-dependent deviation of the thermal emission profiles between 4.5 µm and 8 µm. Compared to the observed emission spectra under CW laser excitation (Fig. 4.20), the large gate-dependent deviation of the measured thermal emission profiles is between 4.5 µm and 8 µm, as shown in Fig. Since the emission spectra measured under CW laser excitation resemble the measured thermal emission profiles shown in Fig.
Measured thermal emission spectra from the device for given temperatures of 70 ◦C, 95 ◦C and 100◦C (black dashed lines). Measured thermal emission spectra from the device for given temperatures of 70 ◦C, 115 ◦C and 150 ◦C (black dashed lines).
Concluding Remark
Thermal emission is a form of spontaneous emission, and thus the rate of thermal emission via graphene nanoresonators can be increased by a factor of 106 to 107. Even faster excitations of plasmons can be achieved on a femtosecond time scale via optical excitation [4, 6 ]. The spectral flux of spontaneously emitted plasmons per pulse can be several orders of magnitude higher than that of photons emitted from a blackbody at various representative temperatures of 500 K, 1,000 K and 2,000 K, as shown in Fig.
The collected spectral flux can be significantly improved by increasing the repetition of pulses or performing time-resolved measurements. To facilitate the coupling of graphene plasmons with the probe pulse, various mid-infrared resonant nanophotonic structures can be explored.
Conclusion
Super-Planckian radiation
In Chapter 2, rate modulation of the thermal emission on the order of kHz from the original Salisbury display device was demonstrated by electronically switching the plasmonic modes on and off. The modulation speed can be further improved by engineering the device structure to reduce the RC constant of the device.
Bright spontaneous emission sources
Coherent graphene plasmon amplification
