Multilayer Black Phosphorus as a Versatile Mid-Infrared Electro-optic Material

Abstract

We investigate the electro-optic properties of black phosphorus (BP) thin films for optical modulation in the mid-infrared frequencies. Our calculation indicates that an applied out-of-plane electric field may lead to red-, blue-, or bidirectional shift in BP's absorption edge. This is due to the interplay between the field-induced quantum-confined Franz-Keldysh effect and the Pauli-blocked Burstein-Moss shift. The relative contribution of the two electro-absorption mechanisms depends on doping range, operating wavelength, and BP film thickness. For proof-of concept, simple modulator configuration with BP overlaid over a silicon nanowire is studied. Simulation result shows that operating BP in the quantum-confined Franz-Keldysh regime can improve maximal attainable absorption and power efficiency compared to its graphene counterpart.Introduction-The mid-infrared (MIR) regime contains the fingerprints of many common molecular vibrations and covers several atmospheric transmission windows, making it important for spectroscopic molecular analysis, sensing, and free-space optical communications [1, 2]. As such, integrated photonic solutions that can operate between λ = 2-10 µm are of great technological importance. In particular, progress has been made in components such as broadband source and frequency comb with on-chip form-factor [3,4], Si 3 N 4 and SiGebased low-loss optical waveguides, and photodetectors utilizing low-bandgap materials [5]. However, the realization of MIR optical modulators, which require material platforms with versatile opto-electronic properties, remain challenging.

Midinfrared Electro-optic Modulation in Few-Layer Black Phosphorus

“…Our results confirm the viability of using BP to realize an electro-optic modulator in integrated mid-IR photonic systems 35 .…”

“…A recent study, using parameters consistent with our experimental results (see SI), predicts that a modulation level of 0.05 dB/µm is attainable in a waveguide integrated BP modulator. This suggests that a 5 dB modulation depth can be achieved with a 100 µm device 35 . The performance can be further improved with novel device design and integration, as those have been proposed and implemented for graphene-based modulators working in the near-infrared regime [13][14][15]44 .…”

“…As an alternative, BP mid-IR photodetectors have already been demonstrated with promising performance 25,26 . For mid-IR optical modulation, multilayer BP is equally promising 35 . Strong field-effect tuning of its bandgap has been observed in BP, and its infrared optical response has been measured [36][37][38] .…”

Mid-infrared integrated photonics on silicon: a perspective

“…Narrow-gap Van der Waals solids such as black phosphorus (BP) present another choice for on-Si mid-IR modulators. Theoretical analysis shows that optical modulation in BP stems from both quantum confined FKE and Pauli blocking, where their relative magnitude depends on BP layer thickness, doping, field strength, and wavelength [162]. Free-space optical modulation has recently been materialized where light is incident on the BP flake from a surface-normal direction [163,164].…”

Field Effect Optoelectronic Modulation of Quantum-Confined Carriers in Black Phosphorus

“…39 In the bulk limit, the Franz-Keldysh effect refers to electron and hole wavefunctions leaking into the band gap, as described by Airy functions. This behavior introduces oscillatory features to the interband absorption spectrum, and redshifts the band edge.…”