Quantum cascade external cavity laser with metasurfaces
A metasurface reflector for quantum cascade lasing is disclosed. The metasurface reflector uses an array of subcavities disposed on a substrate and spaced with a sub-wavelength period. Each of the subcavities includes a layer of quantum-cascade-laser-active material sandwiched between two metallic layers. The array of subcavities reflect an incident light of a resonant frequency with amplification. When used with an output coupler, a quantum cascade laser beam can be generated.
1. A metasurface reflector for quantum cascade lasing, the metasurface reflector comprising:
a substrate;
an array of subcavities disposed on the substrate and spaced with a period, each of the subcavities comprising:
a first metallic layer disposed on the substrate;
a layer of quantum-cascade-laser-active material disposed on the first metallic layer; and
a second metallic layer disposed on the layer of quantum-cascade-laser-active material;
wherein the array of subcavities reflect an incident light of a resonant frequency with amplification,
wherein the period of the array of subcavities is less than a wavelength of the incident light of the resonant frequency, and
wherein the array of subcavities include a first group of subcavities and a second group of subcavities interlaced with each other, wherein the first group of subcavities selectively reflects first incident light of a first polarization state with amplification, wherein the second group of subcavities selectively reflects second incident light of a second polarization state with amplification, and wherein the first polarization state and the second polarization state are different.
2. The metasurface reflector of claim 1 , wherein the substrate comprises a GaAs substrate.
3. The metasurface reflector of claim 1 , wherein the layer of quantum-cascade-laser-active material comprises a GaAs/AlGaAs material system or a InGaAs/InAlAs material system.
4. The metasurface reflector of claim 1 , wherein a gain peak of the layer of quantum-cascade-laser-active material is in a range of about 1 Terahertz (THz) to about 10 THz.
5. The metasurface reflector of claim 4 , wherein the gain peak is in a range of about 2 THz to about 3 THz.
6. The metasurface reflector of claim 1 , wherein a width of each subcavity substantially determines a cutoff of the resonant frequency.
7. The metasurface reflector of claim 1 , wherein a width of each subcavity is in a range of about 10 micrometer (μm) to about 15 μm, and wherein a height of each subcavity is in a range of about 5 μm to about 20 μm.
8. The metasurface reflector of claim 1 , wherein the period of the array is in a range of about 50 μm to about 100 μm.
9. The metasurface reflector of claim 1 , wherein each subcavity tapers at edges of the metasurface reflector and terminates at an unbiased region of the metasurface reflector.
10. The metasurface reflector of claim 1 , wherein a reflectance of the array of subcavities is more than unity (1) at a gain peak of the layer of quantum-cascade-laser-active material.
11. The metasurface reflector of claim 1 , wherein the first polarization state is a linear polarization, a right-hand circular polarization, or a left-hand circular polarization.
12. The metasurface reflector of claim 1 , wherein the second polarization state is a linear polarization, a right-hand circular polarization, or a left-hand circular polarization.
13. The metasurface reflector of claim 1 , wherein the first polarization state and the polarization state are orthogonal to each other.
14. The metasurface reflector of claim 1 , wherein each of the first group of subcavities includes a plurality of first connectors and a plurality of parallel first patches electrically connected by the plurality of first connectors, wherein each of the second group of subcavities includes a plurality of second connectors and a plurality of parallel second patches electrically connected by the plurality of second connectors, and wherein the first patches and the second patches have different orientations.
15. A quantum cascade laser comprising:
a metasurface reflector comprising:
a substrate;
an array of subcavities disposed on the substrate and spaced with a period, each of the subcavities comprising:
a first cladding layer disposed on the substrate;
a layer of quantum-cascade-laser-active material disposed on the first cladding layer; and
a second cladding layer disposed on the layer of quantum-cascade-laser-active material;
wherein the array of subcavities reflect an incident light of a resonant frequency with amplification,
wherein the period of the array of subcavities is less than a wavelength of the incident light of the resonant frequency, and
wherein the array of subcavities include a first group of subcavities and a second group of subcavities interlaced with each other, wherein the first group of subcavities selectively reflects first incident light of a first polarization state with amplification, wherein the second group of subcavities selectively reflects second incident light of a second polarization state with amplification, and wherein the first polarization state and the second polarization state are different; and
an output coupler coupled to the metasurface reflector which forms an external cavity with the metasurface reflector for generating a quantum cascade laser beam.
16. The quantum cascade laser of claim 15 , wherein a reflectance of the metasurface reflector is more than unity (1) at a gain peak of the layer of quantum-cascade-laser-active material, and wherein the gain peak of the layer of quantum-cascade-laser-active material is in a range of about 1 Terahertz (THz) to about 10 THz.
17. The quantum cascade laser of claim 16 , further comprising:
a heat sink that holds the metasurface reflector; and
a cryostat that houses the heat sink and the metasurface reflector, wherein the cryostat includes a window for transmission of the quantum cascade laser beam.
18. The quantum cascade laser of claim 15 , wherein the output coupler is an external reflector, wherein the quantum cascade laser beam is reflected between the external reflector and the metasurface reflector before emitting.
19. The quantum cascade laser of claim 15 , wherein the output coupler is a THz wire-grid polarizer, and wherein an output power of the quantum cascade laser is adjusted by varying a wire orientation angle on the THz wire-grid polarizer.
20. The quantum cascade laser of claim 15 , wherein the external cavity is a plano-plano Fabry-Perot cavity, and wherein a configuration of the output coupler determines a spectrum of the quantum cascade laser beam.
21. The quantum cascade laser of claim 15 , wherein the quantum cascade laser beam is a Gaussian mode laser beam with a divergence of about 4.3°×5.1° or lower.
22. The quantum cascade laser of claim 15 , wherein the first polarization state and the polarization state are orthogonal to each other.