Terahertz light emission and lasing in graphene-based heterostructure 2D material systems -theory and experiments

IntroductionGraphene has attracted attention due to its massless and gapless energy spectrum. Carrier-injection pumping of graphene enables negative-dynamic conductivity in the terahertz (THz) range, which may lead to new... [ view full abstract ]

Introduction

Graphene has attracted attention due to its massless and gapless energy spectrum. Carrier-injection pumping of graphene enables negative-dynamic conductivity in the terahertz (THz) range, which may lead to new types of THz lasers [1].

Method

The dual-gate graphene channel transistor (DG-GFET) structure serves carrier population inversion in the lateral p-i-n junctions under complementary dual-gate biased and forward drain biased conditions, promoting spontaneous incoherent THz light emission. A laser cavity structure implemented in the active gain area can transcend the incoherent light emission to the single-mode lasing. 

Results

We designed/fabricated the distributed feedback (DFB) DG-GFET (Fig. 1) [2]. The GFET channel consists of a few layer (non-Bernal) epitaxial graphene [3], providing an intrinsic field-effect mobility exceeding 100,000 cm²/Vs [4]. The teeth-brash-shaped DG forms the DFB cavity having the fundamental mode at 4.96 THz. The modal gain and the Q factor at 4.96 THz were simulated to be ~5 cm^-1 and ~240, respectively (Fig. 2) [2].THz emission from the sample was measured using a Fourier-transform spectrometer with a 4.2K-cooled Si bolometer. Broadband rather intense (~10~100 μW) amplified spontaneous emission from 1 to 7.6 THz (Fig. 3) and weak (~0.1~1μW) single-mode lasing at 5.2 THz (Fig. 4) [2] were observed at 100K in different samples. 

Discussion

When the substrate-thickness dependent THz photon field distribution could not meet the maximal available gain-overlapping condition, the DFB cavity cannot work properly, resulting in broadband LED-like incoherent emission. To increase the operating temperature and lasing radiation intensity, further enhancement of the THz gain and the cavity Q factor are mandatory. Plasmonic metasurface structures promoting the superradiance and/or instabilities [5] are promising for giant THz gain enhancement.

Acknowledgements: JSPS KAKENHI (16H06361), Japan.

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