Patent Application
20250224649
Fields of the invention include optoelectronics, electrooptical modulators, optical communication links, and optical transmitters.
Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) have been studied for applications such as excitonic integrated circuits. Jiang et al., “Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures,” Light: Science & Applications (2021) 10:72. Conventional heterostructure devices with spatially direct excitons (DXs), aka intralayer excitons, exhibit short exciton lifetimes, e.g., in the range of picoseconds [Korn et al, “Low-temperature photocarrier dynamics in monolayer MoS2”, Appl. Phys. Lett. 99, 102109 (2011).] Spatially indirect excitons (IXs), aka interlayer excitons, composed of electrons and holes confined in separated layers, can have long lifetimes. Such IXs have been used for excitonic transistor devices. See, Jian et al., Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures,” Light: Science & Applications (2021) 10:72; High et al., “Control of exciton fluxes in an excitonic integrated circuit,” Science 321, 229 (2008); Grosso et al., “Excitonic switches operating at around 100 K,” Nature Photonics volume 3, pages 577-580 (2009).
A Mach-Zehnder modulator (MZM) is an optical interferometric structure made from a material having a strong electro-optic effect. An optical input signal is split into two arms. Applying an electric field to one or both arms modifies the phase of one or both optical signals before they are combined and form an interferometric output. Optical MZMs have applications ranging from sensing to simulation to opto-electronic circuits. Within an MZM, photons are used as carriers. Photons have relatively low sensitivity to electric fields.
A challenge for photonics is the realization of energy-efficient signal control at nanoscale. In optical MZMs, the amplitude of optical wave at the output is controlled by applying voltage that introduces a phase shift in the light wave passing through an MZM arm. The phase shift is determined by the change of the refractive index induced by applied voltage. Even for materials with large electro-optic coefficient, such as lithium niobite, the switching requires devices with significant lengths and large applied voltages. An estimate for switching voltage is given by the voltage required to change the phase by π, Vπ. Even for conventional MZM with a large electro-optic coefficient 30 pm/V, Vπ˜10 V for L˜3 mm (for characteristic parameters). Such large-sized and high-voltage prior photonic MZM devices have limited application because of those characteristics.
A preferred embodiment provides an excitonic Mach Zehnder modulator. The integrated excitonic MZM modulator includes a heterostructure of material to create an exciton or optical input and output and two aims for excitons between the input and the output. An interference control electrode on one of the arms is used to control exciton or optical output. The materials are selected and arranged such that combining excitons at the output is controlled by voltage applied to the interference control electrode such that exciton matter waves are in phase and the interference is constructive when the output signal is on, and when the exciton matter waves are out of phase the interference is destructive and the output signal is off.
The invention provides an excitonic Mach-Zehnder modulator (X-MXM), which can modulate either of excitonic or optical signals. In excitonic devices, input(s)/output(s) can be excitonic, optical, or electronic. For the present X-MZM device, an optical signal can be accepted at X-MZM input and an optical signal output produced at X-MZM output. At an X-MZM input, light transforms to excitons. The excitonic signal is controlled in the X-MZM device, or in an excitonic circuit including a number of the present X-MZM, and excitons can transform to light at an output. Alternatively, an input can be excitonic signal and an output can be an excitonic signal which can be transmitted to another excitonic device. Electronic modulation or switching of optical signals is realized using excitons, which are more sensitive to modulation and responsive to very small electric fields as shown by the estimates below indicating that the switching voltages V, are orders of magnitude smaller for X-MZM in comparison to optical MZM.
The present X-MZM devices can also be used to form hybrid circuits including other optical devices. An output from an X-MZM can be transformed to photons before combining with photonic signals from other optical devices.
Preferred X-MZM devices leverage long-range IX transport. Excitons can travel long distances (tens and hundreds of micrometers) through the present X-MZM devices before recombination. X-MZM devices leverage heterostructures composed e.g. of Group III-V materials or transition metal dichalcogenides (TMDs).
Preferred X-MZM devices enable low-energy signal control and linking electronic computation with optical communication. The present X-MZM devices leverage interference of exciton matter waves and offer both sizes and the energy efficiency orders of magnitude smaller than conventional optical MZM technology. X-MZM devices can modulate excitonic or optical signals at nanoscale (total device size in X-Y plane is nanoscale, e.g. ˜300 nm size between input and output, and height/Z direction is nanoscale, with individual layers typically being a fraction of nanometer to tens of nanometers thick in TMD structures and a few to hundreds of nanometers thick in III-V structures). Modulation can be conducted with voltages orders of magnitude smaller than in optical MZM devices. X-MZM computation can be linked with optical communication.
Preferred embodiments of the invention will now be discussed with respect to experiments and drawings. Broader aspects of the invention will be understood by artisans in view of the general knowledge in the art and the description of the experiments that follows.
A preferred X-MZM 100 in
The materials and length L of the interference control electrode 110 are selected such that when the exciton matter waves are in phase the interference is constructive and the output signal is on, and when the exciton matter waves are out of phase the interference is destructive and the output signal is off. An example L estimated in [0022] is 300 nm and L can be smaller or larger than this value depending on the application. L determines the X-MZM dimensions. The lengths of arms 104 and 106 beyond electrode 110 should be sufficient to complete the geometry of the X-MZM 100. All x-y dimensions of the X-MZM 100 should be larger than the exciton size (Bohr radius). However, the exciton radius is typically small (for TMD˜1 nm, for GaAs˜10 nm) so this limitation does not compromise the small X-MZM dimensions. In the X-MZM 100, the amplitude of an exciton matter wave at the output 108 is controlled by applying voltage to the interference control electrode 110 that introduces a phase shift in the exciton matter wave passing through the MZM arm 106.
The heterostructures 120 and 122 can include TMD or Group III-V semiconductor and barrier materials. Preferred TMD semiconductor materials include MoS2, MoSe2, and WSe2, and hBN is the preferred barrier/cap material. Generally, TMD material that is stable at ambient conditions with a cap layer can be used. Preferred Group III-V semiconductor materials include AlAs and GaAs, a preferred barrier material include AlGaAs, and a preferred cap material include AlGaAs followed by GaAs. The TMD materials are selected such that excitons have sufficiently high binding energies to be stable at ambient conditions. Preferred heterostructures for the X-MZM include MoS2/hBN heterostructure with electron confinement and hole confinement layers in MoS2 separated by an hBN barrier. MoS2 can be replaced by other TMD material such as MoSe2 or WSe2. Another preferred structure uses GaAs for electron confinement and hole confinement layers and AlGaAs for a barrier. Another preferred structure is MoSe2/WSe2 heterostructure with the electron confinement layer in MoSe2 and hole confinement layer in WSe2. Another preferred structure is an AlAs/GaAs heterostructure with the electron confinement layer in AlAs and hole confinement layer in GaAs.
Excitons in heterostructures 120 or 122 have built-in electric dipoles ed (e is electron charge, dis the separation between the electron and hole layers) and their energy is effectively controlled by voltage applied to the device electrodes, such as the interference control electrode 110 and the shaped electrode 112. The recombination rate is also effectively controlled by applied voltage(s). Switching the recombination off allows excitons to travel over long distances reaching and exceeding hundreds of microns. Excitons can therefore travel through the entire X-MZM 100 device. Excitons can transform to photons to provide an optical output at the output 108, or can be transmitted to another X-MZM devices as excitons. An optical readout of the X-MZM 100 can be controlled by voltage applied to output region 108 to increase exciton conversion to photons.
An exciton (X) wave is confined in the heterostructure layers of the structures 120 or 122 and guided in x-y plane by shaped-electrode excitonic guide 112 or physical heterostructure shaped guide 114. The layers 126 and 128 confine excitons composed from holes (+) and electrons (−) in the z direction. Optical guides can also be used to guide photons to the source and out of the drain of X-MZM 100.
The switching of the X-MZM 100 is achieved with both lengths and voltages that are much smaller than for conventional optical MZM. The X-MZM 100 can provide nanoscale devices operating with low switching voltages, with the energy efficiency orders of magnitude smaller than conventional optical MZM technology.
In a conventional optical MZM the amplitude of optical wave at the output is controlled by applying voltage that introduces a phase shift in the light wave passing through the optical MZM arm. The phase shift Δψ˜ΔnkL is determined by the change of the refractive index induced by applied voltage Δn∞rV. Even for materials with large electro-optic coefficient r, such as lithium niobite, the switching requires devices with large lengths L and large applied voltages V. An estimate for switching voltage is given by the voltage required to change the phase by π: Vπ˜D/(n3rL). Even for MZM with large r=30 pm/V, Vπ˜10V for L˜3 mm (for typical parameters: wavelength λ=1 μm, separation between the electrodes D=10 μm). The photonic MZM devices thus have large sizes and require high applied voltage.
In contrast, the present IXs in X-MZM 100 include built-in electric dipoles ed, and IX energy is controlled by voltage ΔE∞Vd. The amplitude of an exciton matter wave at the output 108 is controlled by applying voltage at electrode 110 that introduces a phase shift in the exciton matter wave passing through the arm 106. The phase shift Δψ˜ΔknL is determined by the change of wave vector Δk˜dk/dE·ΔE induced by applied voltage, which controls ΔE. When the X-MZM 100 is fabricated in van der Waals transition metal dichalcogenide (TMD) heterostructures composed of MoSe2/WSe2 bilayer encapsulated by hBN, ΔE˜VdeεhBN/(DεTMD) and
DεTMD/(dεhBN). The estimate gives Vπ˜10 meV for L˜300 nm (for typical parameters: d˜1 nm, D˜20 nm, m˜m0, εhBN=3.8, εTMD=7.4) The exciton energy corresponding to the thermal energy at room temperature is E˜25 meV. Switching of the example TMD X-MZM 100 is achieved with small L and Vπ. For the example TMD X-MZM, Vπ·L is ˜107 smaller than the conventional photonic MZM. With respect to the electrode length and length of the X-MZM 100, Vπ∞1/L, therefore a smaller size L of electrode 110 and, in turn, of the entire X-MZM can be used when a higher voltage Vπ is used for X-MZM switching, or a smaller switching voltage Vπ can be used when L is larger. L and Vπ can be optimized for the application
While preferred embodiments have been described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
Various features of the invention are set forth in the appended claims.
The application claims priority under 35 U.S.C. § 119 and all applicable statutes and treaties from prior U.S. provisional application Ser. No. 63/329,148, which was filed Apr. 8, 2022.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/065273 | 4/3/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63329148 | Apr 2022 | US |
