Abstract
1. Introduction
Recent investigations on optical communication[
The chirality of chiral metamaterials is currently the most popular method for CPL detection. Researchers are very interested in special properties of chiral metamaterials, such as circular dichroism (CD), optical rotatory dispersion (ORD), Raman optical activity (ROA)[
In general, self-powered devices have more potential because of their economy and adaptability than those with external power in practical applications. Most self-powered devices based on heterojunctions[
In this paper, we propose and optimize self-powered photodetectors based on chiral interdigital structure, which is a new type of device that can detect CPL through the direction of photocurrent at zero bias. The performance of CPL detection will be good for the devices, even if we do not chase high responsivity. The rotation direction of the CPL electric field can be distinguished by the direction of photocurrent in these devices. The self-powered photodetectors that we propose in this paper are more economical and adaptable than the conventional ones.
2. Structure and physical model
Generally, the higher the responsivity is, the better the performance of conventional devices that detect CPL by the magnitude of photocurrent will be[
Herein, we propose three kinds of devices based on interdigital structures with chiral metamaterial electrodes, as shown schematically in Figs. 1(a)–1(c). These devices are designed by placing an n-type silicon wafer (500-µm thick) in contact with the Ag antenna layer, which is on top of a poly (PMMA) spacer. It should be noted that there is a 1-nm-thick layer of Ti between the silicon and Ag antenna layer, which can lower the Schottky barrier and enhance the adhesion of nanoelectrodes. The bottom layer is an Ag backplane used as a reflector for reflecting the incident light back to the nanoelectrodes and then can enhance the optical absorption and CD of ‘Z’ shaped metamaterial[
Figure 1.(Color online) The thickness of antenna layer, dielectric spacer (PMMA) and Ag backplane are 40, 160, and 100 nm, respectively. The dimensions of the chiral-molecules are
In Fig. 1(a), the ‘Z’-shaped unit cells of the left-handed (LH) chiral antennas allows for an integral electrical connection among all the elements, forming a chiral nanowire[
Although these two devices (device 1, device 2) have unique advantages for CPL detection by the direction of photocurrent, it is only clearly sensitive to LCP or RCP for a single device. The optimized device, which integrates functional advantages of the above two devices while overcoming their respective drawbacks, can be seen in Fig. 1(c). The interdigital structure is composed of LH chiral metamaterials and RH chiral metamaterials. On the one hand, LSPRs can be induced by LCP light on the LH chiral metamaterials, which generates a large amount of hot carriers forming concentration gradient, and the diffusing hot electrons are collected by RH chiral metamaterials. In post-circuit, the current from LH chiral metamaterials to RH chiral metamaterials can be detected. On the other hand, hot carriers created from the RCP light absorption of RH chiral metamaterials are injected into semiconductor and then collected by LH chiral metamaterials, resulting in current from RH chiral metamaterials to LH chiral metamaterials in post-circuit.
To understand clearly how the device distinguishes CPL by the direction of photocurrent, we can depict that the pointer (blue) of the sensitive galvanometer turns right under LCP light illumination as shown in Fig. 1(c). In contrast, the pointer (red) turns left under RCP light illumination. Thus, LCP and RCP light can be distinguished by the opposite direction of photocurrent clearly.
For the optimized device, the generation of photocurrent amid hot electrons across a barrier can be described by five physical steps shown in Fig. 1(d)[
In the second step (S2), hot electrons reach the Ag–Si interface with kinetic energy as shown in Fig. 2(a). The kinetic energy is given by Eq. (1)[
Figure 2.(Color online) (a) Cross-section of the
Here,
where
where
In the third step (S3), some hot electrons transfer across the Schottky barrier and traverse the Ag–Si interface with a certain probability. Hot electrons can escape only when
Some hot electrons reaching the Ag–Si interface may be reflected, while the others may traverse the Ag–Si interface after multiple scattering events in Fig. 2(b). Therefore, we make two assumptions[
where
where L is the mean free path of the electrons[
Once cross the Ag–Si interface, the hot electrons are drifted towards the neutral region in semiconductor by the built-in electric field of the Schottky contact, and then enter the fourth stage (S4).
In the fourth step (S4), the concentration gradient of hot electrons is formed due to asymmetry absorption between the LH and RH chiral metamaterials under LCP light illumination. The diffusion of hot electrons from high concentration to low concentration occurs in semiconductor. Some of the surviving electrons can pass through the gap between the metallic electrodes. In the last step (S5), electrons reach the Si–Ag interface again and are collected by RH chiral metamaterials as detectable photocurrent. The mechanism of hot electrons to traverse the Si–Ag interface is fundamentally the same as that to traverse the Ag–Si interface. Therefore, the total internal quantum efficiency
where
The responsivity of the device is given by Eq. (10) [
where h is Planck’s constant, v is the frequency of the incident photon, q is the elemental charge, and A is the light absorption.
The net current
3. Results and discussion
In device 1, by the simulation of the finite-difference time domain method (Lumerical FDTD Solutions), Fig. 3(a) shows that the LH chiral metamaterials absorb a large amount of photons in the black curve, while collection electrodes absorb few photons in the red curve. Compared with collection electrodes, LH chiral metamaterials with a unique geometric structure can enhance absorption for LCP light, resulting in asymmetric absorption between LH chiral metamaterials and collection electrodes. When the LCP light is incident on the LH chiral metamaterials, the excited LSPRs enhance light-matter interaction that causes strong absorptions and strong electromagnetic fields. The plasmon will decay nonradiatively via intraband or interband transitions, forming “hot” electrons. The photoexcited “hot” electrons with energy over the Schottky barrier and inject into the semiconductor, leading to photocurrent. The net absorption expressed as
Figure 3.(Color online) (a, b) Absorption spectra of LH chiral metamaterials and collection electrodes for LCP light and RCP light, respectively. (c, d) Simulation of the electric field intensity for LCP and RCP light at a wavelength of 1330 nm, respectively.
In Figs. 3(c) and 3(d) electric field intensity diagrams clearly present the obvious CD[
Figure 4.(Color online) (a, b) Optical absorption spectra of RH chiral metamaterials and collection electrodes under LCP light and RCP light illumination, respectively. (c, d) Simulation of the electric field intensity for LCP and RCP light at a wavelength of 1330 nm, respectively.
Simulations of device 3 can be seen in Fig. 5. In Fig. 5(a), LH chiral metamaterials absorb a large amount of photons in the black curve when LCP light is incident, while RH chiral metamaterials absorb a little photon in the red curve. This shows that LH chiral metamaterials are selective to LCP light. We can calculate the net absorption
Figure 5.(Color online) (a, b) Optical absorption spectra of LH and RH chiral metamaterials under LCP light and RCP light illumination, respectively. (c, d) Simulation of the electric field intensity for LCP light and RCP light illumination at a wavelength of 1330 nm, respectively.
Fig. 6(a) shows that the calculated emission probability and internal quantum efficiency will decrease with the increasing wavelength. This implies that the lower the photon energy is, the harder it is to excite hot electrons in chiral metamaterials to generate a photocurrent. Theoretical photoresponsivity spectra is calculated as shown in Fig. 6(b), where photocurrent in the opposite direction can be depicted as the negative values. Although the responsivity of the device is a maximum of 1.9 mA/W at 1322 nm under LCP or RCP light illumination, it is enough to distinguish CPL by the direction of photocurrent. Compared with conventional devices that are aimed to detect CPL by the magnitude of photocurrent, these devices are more effective in distinguishing the rotation direction of the CPL electric field by the direction of photocurrent at the same responsivity. Less affected by low responsivity, the devices we provide are more adaptable in practical applications.
Figure 6.(Color online) (a) Emission probability and internal quantum efficiency of hot electrons as functions of the photon wavelength. (b) Responsivities with different current directions obtained from the interdigital structure of LH and RH metamaterials.
4. Conclusion
In this paper, we have proposed a new kind of CPL detector, which can easily detect the rotation direction of the CPL electric field even if it only has a very weak responsivity. The physical mechanism of hot electrons in chiral metamaterials under CPL illumination was explained and three self-powered devices with different electrode structures were presented. The simulation results showed the self-powered CPL detector we proposed can enhance the performance for distinguishing CPL with a low responsivity. Thus, they have huge application potential in optical communication, optical image processing, and biomedical diagnosis.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 61705065), Hunan Provincial Natural Science Foundation of China (No. 2017JJ3034), Technology Program of Changsha (No. kq1804001), and National Training Program of Innovation and Entrepreneurship for undergraduates (No. S201910532166).
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