Device with a quantum well layer

1. A method of optimising a layer thickness and composition of a quantum-well layer for a device for guiding and absorbing electromagnetic radiation, the device comprising a substrate; at least one resonator located on the substrate for absorbing the electromagnetic radiation, the or each of the at least one resonators being resonant at a predetermined wavelength of radiation; a waveguide on the substrate, the waveguide being coupled to the at least one resonator for guiding the electromagnetic radiation to the at least one resonator, wherein the waveguide and the at least one resonator are formed from a structure comprising a first cladding layer, a second cladding layer over the first cladding layer, and the quantum-well layer between the first and second cladding layers, the quantum-well layer being formed of a material having a different composition to the first and second cladding layers, the method comprising:

determining an appropriate quantum well ground state transition energy for the quantum-well layer for absorption of the electromagnetic radiation in the at least one resonator; and

determining the thickness and the composition of the quantum well that are configured to provide the desired ground state transition energy and provide an acceptable level of absorption in the waveguide, while maximizing a quality Q factor of resonance of the at least one resonator and keeping the strain within the quantum-well layer lower than a predetermined limit,

wherein determining the thickness and the composition of the quantum well comprises:

selecting an initial thickness and composition of the quantum-well layer from a predetermined range of thicknesses and compositions;

determining a bend loss in the at least one resonators based on the initial thickness and composition;

obtaining a value of the Q factor for the resonator, based on the bend loss;

determining whether the obtained value of the Q factor is a maximum available value of the Q factor within the predetermined range of thicknesses and compositions;

obtaining a value of strain in the quantum-well layer based on the selected thickness and composition;

determining whether the obtained value of the strain is below the predetermined acceptable limit; and

using the selected thickness and composition as the final thickness and composition of the quantum-well layer, if it is determined that the value of the Q factor is a maximum available value, and if the obtained strain is below the predetermined acceptable limit.

2. The method of claim 1 , wherein the predetermined acceptable limit is 1.5%.

3. The method of claim 1 , further comprising, if it is determined that the value of the Q factor for the initial thickness and composition is not a maximum value or if the obtained strain is not below the predetermined acceptable limit, adjusting the initial thickness and composition to obtain a new thickness and composition and repeating the steps of obtaining a bend loss, determining a Q factor value, determining whether the obtained value is a maximum, obtaining a strain value and determining whether the obtained strain value is below a predetermined acceptable limit for the new thickness and composition.

4. The method of claim 3 , wherein the initial thickness and composition are selected based on a target value of a band gap for the quantum-well layer.

5. The method of claim 4 , wherein the at least one resonator has a minimum free-spectral range FSR value, the method further comprising:

obtaining a wavelength difference value that is less than a minimum FSR value of the plurality of resonators; and

obtaining the target value of the band gap by obtaining a value corresponding to the energy of radiation at a wavelength equal to the sum of the wavelength difference value and the predetermined wavelength.

6. The method of claim 1 , wherein the acceptable level of absorption in the waveguide is a minimum level of absorption obtainable within a predetermined range of thicknesses and compositions of the quantum-well layer, such that the thickness and composition of the quantum-well is optimised to minimise absorption in the waveguide.

7. The method of claim 1 , wherein the thickness of the quantum-well layer is determined to be substantially less than a thickness of the waveguide.

8. The method of claim 1 , wherein the device is a spectrometer.

9. The method of claim 8 , wherein the thickness and the composition of the quantum well are determined so as provide a band-gap that is less than or equal to a predetermined energy, the predetermined energy corresponding to a maximum wavelength Amax of electromagnetic radiation that the spectrometer is configured to detect.