Investigation of Dynamically Tunable Devices Controlling Free-Space Mid-Infrared Beams

The vibrational frequencies of various molecules for mid-infrared applications such as environmental monitoring, defense and security, and medical diagnosis [2]. a) The mid-infrared perfect absorber device based on the metal (Cu)-insulator (Al2O3)-metal (Cu) structure [19] and (b) The mid-infrared perfect absorber device based on the Al-Al2O3-Al structure for selective thermal emitters [33]. a). The optically tunable switching devices with GST layer tunable by fs-order pulses. a) The mid-infrared perfect absorber structure for thermal imaging devices with GST [15]. The schematic of the case when visible light is incident from above. a) The square of the electric field magnitude of the structure that the visible light incident from above cannot reach the shaded region of the GST layer.

Schematic of the MDM structure for the analysis of optical properties in the visible and mid-infrared regions. a) Schematic of the overall structure for the analysis of device properties in the mid-infrared region. Reflection spectrum for three different thicknesses of Ag. b) Compromise ratio between the transmission spectrum in the visible and the reflection spectrum in the mid-infrared range. a) Reflectance of the proposed device as a function of Al disk diameter and wavelength. Spectrum of radiation emitted by a device when the device temperature is 400 K (a) and a blackbody when the temperature is 373 K (b). c) Integrated emissivity, which is the area under the emission spectrum in a specific mid-infrared region as a function of temperature for the three cases, blackbody, amorphous and crystalline GST phase device.

Introduction

• Mid-infrared photonics
• Perfect absorber based on the metal-insulator-metal structure
• Phase change material Ge 2 Sb 2 Te 5
• Tunable perfect absorber with phase change materials
• Motivation for a new optically-tunable perfect absorber structure

Finally, the tunable properties of the device by varying the GST phase are demonstrated. The transmission of the MDM structure as a function of the thickness of the silver layer is depicted in Fig. Transmission spectrum as a function of wavelength for different thicknesses of Ag (a) and SiO2 (b).

The figure shows that the thickness of the silver layer affects the reflectance of the MDM structure. The thickness of the GST layer is designed for minimal device reflection at the resonance wavelength. 2.13(b) is the reflectance at the resonance wavelength as a function of GST layer thickness for three different cases.

Then the optimal value of the GST layer thickness is 40 nm when the target wavelength is 3.15 μm. a). Then the optimum value of Al disk thickness is fixed at 30 nm. It shows that the thickness of the Al disk is independent of the optical properties of the proposed device.

3.1(a) shows that the emissivity of the device as a function of wavelength for two phases of the GST layer. The radiant temperature of the device for both phases of the GST layer is described in Figure. The optical properties of the fabricated MDM structure in the visible range (a) and in the mid-infrared range (b).

And the reflectance of the device will be measured as changing phases of GST layer in the mid-infrared range.

Fig. 1. 1. The vibration frequencies of various molecules for mid-infrared applications such as environmental monitoring, defense and security, and medical diagnositcs [2]

Proposal of the tunable perfect absorbers in the mid-infrared range

Schematic of the proposed device

In this MIM structure, the insulator is replaced by a GST layer, the bottom metal layer is replaced by the metallodielectric mirror (MDM) structure, and the Al disks are arranged in the form of a hexagonal array on the GST layer. The MDM structure consists of twice-repeated stacks of an Ag layer and a ZnO layer with a SiO2 layer in between. The MDM structure has two unique optical properties: first, visible light incident from the bottom transmits through the MDM structure and is absorbed by the GST layer so that the phase of the GST can be changed effectively; second, the MDM structure has high reflectance in the mid-infrared, which falls from the top of the structure, so it can be used for the perfect infrared absorber instead of the thick metal layer below.

The Al disk array, the GST layer and the MDM structure form the mid-infrared perfect absorber similar to the conventional MIM structure. However, the period of the disk array and the GST layer do not affect the resonance wavelength, but the reflectance of the structure is affected by them. To investigate the optical properties of the device, the simulation and numerical analysis were performed.

Simulation method

Design procedure

This means that the optimal range of silver thickness is from 8 to 12 nm, and the thickness of the SiO2 layer is from 60 to 100 nm. 2.8(a) shows that the absorption by the GST layer is dependent on the thickness of the SiO2 layer when the thickness of the silver layer is fixed at 10 nm. This means that the optimal value of the thickness of the SiO2 layer is fixed at 100 nm, and the optimal range of the thickness of the GST layer is above 30 nm.

It shows that the flat and almost 95 % of the reflectance in the target wavelength of 3 to 5 µm. 2.10(b) shows that the transmission in the visible range at left y-axis and the reflectance in the mid-infrared range at right y-axis as a function of the thickness of the silver layer. This shows that the transmittance decreases as the thickness of the silver increases, but the reflectance increases.

Then the optimal value of the thickness of the silver layer is fixed at 10 nm, the cross point of the two lines. 2.11(b) plots the wavelengths of the minimum reflection points as a function of Al disk diameters for three different thicknesses of the GST layer. It shows that the resonance wavelengths of the device are almost linearly proportional to the diameters of the Al disk.

The period of the Al disk array is designed to have minimal reflection from the device at the resonance wavelength. 12(b), which is the reflectance at the resonance wavelength as a function of the period of the Al disk array for GST layers with three different thicknesses. 2.13(a) is the color map of the reflectance as functions of the wavelength from 1 to 6 μm and the thickness of GST.

This figure shows that the resonance wavelength is almost independent of the thickness of the Al wafer when it is above 20 nm.

Fig. 2.6. The transmittance spectrum as a function of the wavelength for different thickness of Ag (a) and SiO 2 (b)

Reflectance spectra of the proposed device for two different phases of GST layer

Investigation of the proposed devices for infrared scene projectors

Radiance of the proposed device in the mid-infrared range

The emissivity of the device can be deduced from the reflection spectrum obtained from Chapter 2. The radiance of the device for the respective phases of GST is shown in Figure 3.1(b) as a function of wavelength when the temperature of the device is 400 K. The radiation for the crystalline phase of the GST layer is larger than that for the amorphous phase case. A).

In this section, the calculation method for the mid-infrared camera radiation temperature is explained. The second step is for the camera to compare between the integrated radiance of the device at a temperature of X K and the black body at a temperature of Y K. Then the camera determines that the device temperature is 373 K when the device temperature is 400 K. .

It shows the integrated emissivity as a function of temperature for a blackbody (black), an amorphous phase (red), and a crystalline (blue) device. Then, the device radiation temperature for both phases of the GST layer as a function of device temperature is presented in Figure 3.2 (d). The radiation temperature of the device with the crystalline GST phase is higher than the amorphous phase by about 20 Kelvin.

The radiation temperature of the device changes from 342 to 373 K at the phase transition when the temperature of the device is 400 K. Finally, the thermal properties of the device are demonstrated by calculating a temperature of emitted light.

Fig. 3. 2. The radiance spectrum emitted from the device when device temperature is 400 K (a) and the black body when temperature is 373 K (b)

Calculation method of the radiation temperature in mid-infrared camera

False color map for radiation temperature of the proposed device

The y-axis in the figure represents the phase of the GST layer, the x-axis is around the device temperature, and the radiation temperature is shown as a color axis, where the temperature range is 280 K (blue) to 400 K (red). This means that the proposed perfect absorbing device can project the tunable thermal image by controlling a phase of the GST layer.

Preliminary experiment and future works

Measured optical properties of the metallodielectric mirror

The MDM structure is fabricated with the geometric parameters that are defined in the second part of the thesis and its optical properties are measured in the visible with the UV-Vis microspectrometer (UV-Vis) and the mid-infrared range with the Fourier transform infrared spectroscopy. (FTIR). The blue line shows the measured transmission spectrum and its value is 80% at the target wavelength of 500 nm. The blue line shows the measured reflectance spectrum by FTIR and its value is 85% at the target wavelength of 3.15 µm.

The difference of reflectance between measured and simulated one comes from the mismatch of the optical properties of used materials because the refractive indices of the materials that are actually deposited are exactly equal to the values ​​of the references.

Problem: exfoliation of the GST layer on the metallodielectric mirror

Future works

Conclusion

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