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An UltraThin Mirror Whose Reflectivity Can Be Controlled With Voltage

[Estimated read time: 4 minutes]
  • They reflectivity of ultra-thin material, molybdenum diselenide (MoSe2), can be adjusted with an applied voltage. 
  • MoSe2 shows high reflectivity for specific wavelengths of light. 
  • Reflectivity can be changed by adjusting the material’s electron density.
  • It could be used in reconfigurable equipments in optical and optoelectronic systems. 

Two independent research teams (at Harvard University and the Swiss Federal Institute of Technology, Zurich) have discovered a promising material for building a new type of mirrors whose reflectivity can be controlled electronically. These mirrors would be based on optoelectronic circuits that use electricity to produce and transmit light signals. 

Both teams measured the reflectivity of an atom-sized thin sheet of molybdenum diselenide (MoSe2) using a laser beam. They reflectivity of this ultra-thin material can be adjusted with an applied voltage.

This feat of engineering expands the limits of what we can possibly build in the physical universe. Researchers state that these types of thin mirrors could play a very crucial role in tiny, specialized sensors and computer chips that utilize lasers to carry data.

MoSe2

Molybdenum diselenide shows high reflectivity for specific wavelengths of light. It’s a member of the family of transition metal dichalcogenides (TMDCs). The layered structure of MoSe2 as well as the electrical conductivity of Se offer a decent opportunity for hosting counterions in electrochemical energy storage system like sodium-ion and widely used lithium-ion batteries.

The unsaturated Se atoms at the edge and altered basal plane have high electrocatalytic activity towards hydrogen evolution reaction and similar reactions like in lithium-oxygen batteries. Moreover, the adjustable band gap of molybdenum diselenide makes it a promising candidate for photoelectrochemical solar cells and photocatalysis.

Quasiparticles

As we have mentioned molybdenum diselenide has high reflectivity for specific light frequencies. Light at this ‘resonant’ frequency causes electrons to closely bind to holes, generating quasiparticles known as excitons. These quasiparticles makes light re-radiate in forward as well as backward direction. The light in the backward direction constructively interferes with the incident light, which leads to greater reflectivity.

What Exactly Are Excitons? 

If you smash a photo into an atom, there is a good chance of electron jumping from a lower energy orbit to higher energy orbit. If that happens, an electron hole forms in the electron field. And the material we are talking about, molybdenum diselenide, especially likely to behave in this manner when hit with specific light frequencies.

Electrons contain negative charge, while protons in the atomic nuclei have positive charge. Therefore, electron holes captures some of the proton’s positive charge from the nuclei. This makes hole slightly behave like particles, despite that fact they are nothing but the absence of particles.

The nearby electrons attract those fake particles, and under specific conditions, they pair up with them to create unusual quantum object, named excitons. These excitons are capable of emitting light of their own, constructively interfering with incident light.

Reference: Physical Review Letters | doi:10.1103/PhysRevLett.120.037402 | Harvard University

Measuring Reflectivity

UltraThin Mirror

In order to reflect the light more effectively, Scientists placed the molybdenum diselenide between the two layers of hexagonal boron nitride. They further mounted this stack onto another layer. The research group at the Swiss Federal Institute of Technology selected fused silica and achieved 41% reflectivity. Harvard researchers, on the other hand, chose silicon and reached a reflectivity of up to 85%.

Since the reflectivity of the material is based on the number of excitons, it can be changed by adjusting the material’s electron density. The best way to do this is apply voltage across molybdenum diselenide and the substrate. This makes the electron density increase or decrease, based on the polarity of voltage.

The study showed that switching the applied voltage off and on changed the reflectivity factor by more than 2, which means it could be utilized as an optical switch.

Reference: Physical Review Letters | doi:10.1103/PhysRevLett.120.037401 | Institute for Quantum Electronics

Application

The strong optical response of a MoSe2 opens up new avenues for photonics. The combination of powerful optical response and lightweight mass suggests that a suspended monolayer of MoSe2 could revolutionize the performance of optomechanical mass force sensors.

The possibility of modifying reflection on short time scales and subwavelength length scales utilizing applied electric fields could open up new ways for digital mirror instruments. The valley degree of freedom of quasiparticle polarons can be used to realize chiral devices by introducing a ferromagnetic monolayer next to the MoSe2 layer.

Read: Transparent Materials Can Absorb Light | An Unusual Optical Effect

It could be used in reconfigurable equipments in optical and optoelectronic systems, including active cavities, modulators and metasurfaces. Fast, Intrinsic, strong and engineered nonlinearities make it an excellent candidate for realizing optoelectronic instruments with potential application in traditional and quantum information processing.

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