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Yes—but only in a carefully engineered laboratory device and over a narrow optical band. A single layer of molybdenum diselenide (MoSe₂) can function as an electrically tunable resonant mirror. A gate voltage changes the monolayer’s charge density, which changes its exciton-related optical response and therefore the amount of light it reflects. The result is a substantial change that can look like switching the mirror “on” and “off,” but it is not broadband reflection and it is not evidence of a ready-to-buy consumer mirror. The original electrical-switching demonstration is reported by Back and colleagues in Physical Review Letters (2018), with a separate high-reflectance demonstration by Scuri and colleagues in Physical Review Letters (2018).
What an “on and off” atom-thin mirror actually does
The phrase describes an electrically induced change in resonant reflectivity, not a sheet that reflects every color when switched on and becomes perfectly transparent when switched off. The active optical feature is tied to an exciton transition in monolayer MoSe₂. At wavelengths close to that transition, the layer can reflect strongly; changing the applied voltage shifts the material between optical states with substantially different reflection.
That distinction matters because reflectivity depends on wavelength, the way the sample is built, temperature, and the exact electrical operating point. A device may show a dramatic contrast at its designed exciton resonance while producing little useful reflection elsewhere in the spectrum. The experiments establish a voltage-controlled narrow-band mirror, not a general-purpose replacement for a metal or dielectric broadband mirror.
The APS Physics summary describes the two 2018 studies as showing more than a factor-of-two change in reflectivity when the voltage is toggled. “On” and “off” are therefore useful shorthand for high- and low-reflection states under the experiment’s specified conditions.
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How one atomic layer can reflect light
Excitons provide the optical resonance
In a semiconductor, absorbing a photon can create a bound electron–hole pair called an exciton. In monolayer MoSe₂, excitons have unusually strong optical interactions because the material is only one atomic layer thick and its electronic structure is strongly confined. Light at the exciton energy drives this resonance, and the excited material re-radiates light.
Backward emission adds to the incoming field
Part of the re-radiated light travels backward. At the resonance, that backward field can interfere constructively with the reflected component of the incident beam. The combined field is much larger than the reflection from an isolated, nonresonant atomic sheet would be. The mirror effect therefore comes from coherent resonant emission, not from the layer’s physical thickness or from a conventional thick reflective coating.
Voltage changes the resonant response
Electrodes around the MoSe₂ provide a gate-controlled carrier density. Adding or removing carriers changes the population and optical response of the excitonic transition. Because the strength and linewidth of the resonance determine how much light is re-radiated into the backward direction, changing charge density changes the measured reflectivity. Back and colleagues summarize the principle as a gate voltage that “modifies the monolayer charge density” and thereby “drastically” modifies mirror reflectivity.
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The two 2018 mirror demonstrations are not the same measurement
Reported percentages are easy to misread unless the optical metric and test conditions stay attached to each number. The following comparison keeps the demonstrations separate.
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| Study or source | Material and structure | Control or operating condition | Reported optical result | What the number means |
|---|---|---|---|---|
| Back, Zeytinoglu, Ijaz, Kroner and İmamoğlu (2018) | Charge-controlled monolayer MoSe₂ heterostructure | Gate-voltage control of monolayer charge density | 87% extinction; maximum reflection coefficient of 41% | Extinction and reflection coefficient are different quantities; 87% extinction is not 87% reflectance |
| Scuri and colleagues (2018) | Monolayer MoSe₂ encapsulated in hexagonal boron nitride (hBN) | Excitonic resonance at cryogenic temperature | Up to 85% reflectance | A high reflectance measured for this encapsulated, low-temperature device at its resonance |
| APS Physics summary (2018) | Summary of the two independent 2018 studies | Voltage toggling | More than a factor-of-two reflectivity change | A cross-study description of switching contrast, not a new reflectance percentage |
The 41% maximum reflection coefficient in the Back experiment should not be ranked directly against the 85% reflectance in the hBN-encapsulated experiment as though they were identical tests. The samples, encapsulation, temperature, optical metrics and measurement details differ. Both results demonstrate electrically controllable resonant reflection, but each percentage applies only to its own architecture and conditions.
Why encapsulation and temperature change the result
hBN protects and modifies the optical environment
Scuri’s high-reflectance device placed the MoSe₂ monolayer between layers of hexagonal boron nitride. Encapsulation can protect an air-sensitive or contamination-sensitive monolayer and provide a more controlled dielectric environment. It also affects the exciton resonance and the way incident and emitted fields interfere. The reported value of up to 85% therefore belongs to that hBN-encapsulated structure, not to every bare MoSe₂ flake.
Cryogenic operation is part of the 85% result
The hBN study operated at cryogenic temperatures. Lower temperature generally reduces several sources of disorder and broadening that weaken a sharp optical resonance. A reader should not interpret the 85% figure as a room-temperature specification. The cited work demonstrates the principle under cryogenic laboratory conditions; it does not establish that the same reflectance is available in an ordinary ambient device.
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Both high contrast and high reflectance occur near the excitonic transition. A different wavelength, detuning from resonance, gate bias, or optical environment can reduce the effect. Any practical design would have to control the illumination wavelength and maintain a stable electrical and thermal state rather than treating the monolayer as a wavelength-independent mirror.
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What the Back experiment adds beyond a reflectance percentage
The Back et al. device demonstrated charge-controlled operation in a monolayer MoSe₂ heterostructure and reported both extinction and a reflection coefficient. The study also found that the measured response was independent of incident intensity up to 400 W/cm² under its tested conditions. That is a statement about the range examined in that experiment, not a universal damage threshold or a guarantee that every MoSe₂ mirror tolerates the same optical power.
Extinction describes how strongly the incident beam is removed from the measured transmitted or reflected optical channel according to the experiment’s definition; reflectance describes the fraction returned into the reflected channel. Absorption, transmission, scattering and the measurement geometry determine how those quantities relate. Treating the 87% extinction value as 87% reflected light would give the wrong picture of the device.
How a voltage-switching experiment is arranged
A typical experiment needs more than a free-standing monolayer. The optical layer is placed in a heterostructure with electrical contacts or gates, and the researchers illuminate it with light selected near the exciton resonance. They then vary the gate voltage while monitoring the reflected signal.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute- Prepare the optical stack. Transfer or grow a monolayer MoSe₂ and, where required, encapsulate it or place it with supporting dielectric and electrode layers.
- Connect the gates. Apply a controllable voltage across the structure so the electric field changes the carrier density in the MoSe₂.
- Select the resonant wavelength. Tune the laser or spectrometer to the excitonic transition; measurements far from resonance will not show the same mirror contrast.
- Record a reference state. Measure reflected intensity at one gate bias, keeping optical alignment, temperature and incident power fixed.
- Toggle the bias. Change the gate voltage to alter charge density, then measure the new reflected intensity and the spectral line shape.
- Report the correct metric. State whether the result is reflectance, a reflection coefficient, extinction, contrast ratio or another quantity, together with the sample and temperature.
This is a research measurement sequence, not a consumer installation procedure. Fabricating atomically clean contacts, preserving a high-quality monolayer and operating at the required temperature demand specialized materials and cryogenic or nanofabrication equipment.
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“On” and “off” are conditional optical states
In a conventional household mirror, a user expects a broad range of visible wavelengths to reflect continuously. The MoSe₂ device instead has a designed optical bandwidth centered on an exciton. Its “on” state means that the resonant reflected field is strong under the chosen illumination; its “off” state means the electrically modified resonance produces much less reflected light at that same measurement condition.
Several effects can reduce the apparent switching contrast:
- Wavelength mismatch: illumination away from the exciton resonance weakens the resonant contribution.
- Line broadening: disorder, temperature and environmental fluctuations spread the resonance and lower its peak response.
- Different optical stacks: substrates, hBN encapsulation and nearby conductors alter interference and field coupling.
- Electrical limitations: leakage, contact resistance or an insufficient gate range can prevent the intended carrier-density change.
- Measurement geometry: the collected angle and polarization affect the reported reflected signal.
These constraints explain why a single headline percentage cannot serve as a universal performance specification.
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A separate route: changing the mirror distance
Not every atomically thin optical switch uses charge-density control. A 2020 study, “Controlling Excitons in an Atomically Thin Membrane with a Mirror,” suspended a MoSe₂ heterostructure above a metallic mirror and reversibly changed their separation. Moving the membrane changes the coupling between excitons and the electromagnetic field reflected by the metal.
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This is electromechanical modulation of exciton–photon coupling. It is physically distinct from the 2018 electrical mirror, where a gate changes carrier density directly in the MoSe₂. Both approaches can tune optical behavior, but their actuators, speed limits, fabrication challenges and failure modes are different. A spacing-controlled membrane should not be presented as proof that voltage-induced charge switching occurred in that experiment.
Related atomically thin devices can steer light
Later work extended the idea from changing reflection amplitude to changing the spatial phase of reflected light. The 2022 Nature Communications study “Beam steering at the nanosecond time scale with an atomically thin reflector” used a MoSe₂ monolayer with graphene split gates to create a tunable phase profile. It reported a beam-deflection range of 10° and switching times down to 1.6 ns for that particular architecture.
Those figures describe a related atomically thin reflector, not the switching speed or steering range of every electrically tunable mirror. Phase control requires spatially patterned gating and optical design in addition to changing the local reflection amplitude. The result nevertheless shows why electrically tunable monolayers are interesting for beam steering, modulators and compact optical systems: one layer can be integrated with electrodes that reshape the reflected wavefront.
What has and has not been demonstrated
Established by the cited experiments
- Monolayer MoSe₂ can act as an electrically tunable resonant mirror.
- Gate voltage can modify charge density and the exciton-related optical response.
- Independent 2018 studies reported substantial voltage-controlled changes, with a 41% maximum reflection coefficient in one device and up to 85% reflectance in a cryogenic hBN-encapsulated device.
- The effect is narrow-band and tied to the excitonic resonance.
- Related gated architectures can control reflected phase and steer a beam, with 10° deflection and 1.6 ns switching reported in the 2022 study.
Not established by these results
- A broadband, room-temperature mirror that switches from perfect reflection to perfect transparency.
- A universal reflectance value independent of wavelength, encapsulation, temperature or device geometry.
- A consumer product, standard module, market price or commercial supply chain for complete atomically thin mirror systems.
- That the electromechanical spacing experiment and the electrical charge-density experiments use the same switching mechanism.
Why the result matters for future optical hardware
Conventional optical components often rely on fixed coatings, moving parts or relatively bulky spatial-light hardware. A gate-controlled monolayer offers a different design strategy: put the optical resonance and its electrical control in an extremely thin stack. Arrays of independently gated regions could, in principle, vary amplitude or phase across a surface, enabling compact modulators, adaptive reflectors and beam-steering elements.
The practical path is demanding. Engineers must solve contact and dielectric losses, uniformity across large areas, thermal management, wavelength selectivity and integration with a usable light source. Cryogenic demonstrations also need a route to operation at higher temperature if they are to leave the laboratory. The existing results are best understood as validated device physics and architectural building blocks, not as finished mirrors for windows, vehicles or household displays.
How to interpret a claim about a switchable atomic mirror
- Check the material. Confirm that the active layer is monolayer MoSe₂ or identify a different material explicitly.
- Identify the actuator. Determine whether the experiment changes charge density with a gate or changes membrane spacing mechanically.
- Locate the resonance. Ask which wavelength and exciton transition were used.
- Read the metric. Distinguish reflectance, reflection coefficient, extinction and switching contrast.
- Check conditions. Look for cryogenic temperature, hBN encapsulation, substrate design, incident intensity and polarization.
- Separate functions. A demonstration of amplitude switching does not automatically establish phase control, beam steering or a particular switching speed.
Using those checks prevents the most common error: turning a narrow-band, condition-dependent laboratory measurement into a claim about an all-purpose mirror.
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