We first built a custom measurement setup in the Fourier plane and investigated the photonic bands in hybrid metal/dielectric nanostructures. We investigate chiral photonic bands in hybrid metal/dielectric (perovskite) nanostructures and exploit them for circularly polarized emission.
Introduction
Strong light confinement in metallic and dielectric nanostructures
In Figure 1.1(b), we can see that the in-plane momentum 𝑘𝑥 grows infinitely as the sum of 𝜀𝑑 and 𝜀𝑚 approaches zero. As shown in Figure 1.3(b), the grating provides additional in-plane momentum and we have the following relationship:
Bound states in the continuum (BICs)
When 𝜀𝑝= −2𝜀ℎ , the momentum 𝒌∥,𝑝 of the nanoparticle diverges, leading to the LSP resonance and strong light confinement.7. The blue spectrum is a continuum of spatially extended states, the green spectrum is discrete levels of bound states, the orange spectrum is leaky resonances, and the red spectrum is bound states in the continuum (i.e. states without radiation).9 Reprinted with permission . from Springer Customer Service Center GmbH: Springer Nature, Nature Reviews Materials, 9, Copyright (2016).
Intrinsic and extrinsic chirality
As shown in the inset of Figure 5.1(a), films coated with J aggregate appear similar to gold due to the metallic response in the visible spectral region. This change in excitonic response results in changes in the optical response of the film.
Chiral emission from hybrid organic-inorganic perovskite (HOIPs)
Optically metallic response in excitonic films
Planar J-aggregated thin films can exhibit an optical metallic response in the visible region due to the strong excitonic response, and can be considered as alternative tunable materials for plasmonics. In Chapter 5 we demonstrate the direct patterning of J-aggregate films using a top-down method (e-beam lithography).
Outline of the thesis: Photonic bands and optical interactions in metal/dielectric hybrid
Furthermore, symmetry breaking of periodic patterns can also induce circularly polarized eigenstates, which could be exploited for circularly polarized light sources. Our work may open new possibilities for circularly polarized light sources based on perovskite materials.
Fourier-plane measurement setup
Introduction
Optical lens as a Fourier transformer
Optical parts for Fourier plane measurements
- Objective lens
- Tube lens and Bertrand lens
- Monochromator
Fourier-plane measurement system
- Overview of the experimental setup
- Fourier-plane pinhole scanning
- Fourier-plane imaging
Circular polarization measurement in Fourier-plane measurement system
- Polarization state of light and Stokes parameters
- Mueller matrix calculation for optical components
- Beamsplitter compensation
Photonic BICs in metal/dielectric hybrid nanostructures
- Introduction
- General theory for BICs from coupled resonances
- Sample fabrication and measurement method
- Reflection spectrum map by Fourier-plane measurement
- Symmetry-protected and non-symmetry-protected bound states in continuums
- Band inversion and BIC position
- Trapping of molecular emission at the hybrid BIC condition
- Fourier plane photoluminescence images
- Conclusions
Both the RCP and LCP emission spectra are strongly altered compared to those of the unsampled reference area (red and blue dotted lines). e) Appropriate PL enhancement factor. The optical metallic behavior gradually deteriorates with increasing electron doses. e), (f) Real and imaginary part of permeability.
Photonic chiral bands and circularly polarized emission in metal/dielectric
Introduction
Device design and fabrication
Fourier plane measurements of differential reflection and photoluminescence
Spectral features of chiral Fano resonances
Reciprocity calculation and chiral field enhancement
Origin of circularly polarized emission
Development of chiral response along the mode dispersion
Analysis of spectral features
Causal connection between reflection and PL spectra
Implications for circularly polarized light sources
Conclusions
Nanopatterning of excitonic films for metallic-to-dielectric response
Introduction
This excitonic absorption line can be tuned over the entire visible range by adjusting the monomer designs.169,. The exciton transport in these natural photosynthetic systems has inspired researchers in the field of "excitonics" to study optical energy transport in excitonic circuits down to the molecular level.181. The Langmuir–Blodgett or spin-coating method can be used to prepare planar J-aggregate films.193 Furthermore, 3D J-aggregate molecular structures can be formed by chemical synthesis using DNA scaffolds.181, 194 However, to the best of our knowledge, a direct method to modify the excitonic and photonic response of J-aggregate films with a nanometer resolution over a large area has not been reported.
In this work, we demonstrate that the excitonic response of J-aggregate films can be locally modified by exposure to electron beams. We show that the optical constants of the films can be gradually changed from optically metallic to dielectric. The results indicate that nanopatterned excitonic films can be used as functional elements in a variety of photonic devices with tailored optical responses.
Therefore, the present work may have a major impact on both excitonics and photonics and may contribute to the innovative integration of excitonic and photonic elements on a single platform.164.
Sample fabrication and measurement setup
In this way, we were able to significantly change the spectral response of the films. Plasmonics based on metal nanostructures can enable light concentration and manipulation down to the size of tens of nanometers, which is well beyond the diffraction limit in pure dielectric photonics.54 The introduction of molecular excitonic elements to photonic platforms can further push this limit; it can enable the control of light-matter interactions and the transfer of optical energy down to the molecular or true nanometer scale. A UV/visible microspectrometer was used for reflectance and transmission measurements (20/20/PV, CRAIC) with a white light spot size of 6 µm.
A diode laser at 516 nm was used as pump light, and the laser power was maintained at ≈1 µW. A microscope objective lens (NA:0.42) was used to focus the incident laser light onto a small spot (M Plan Apo NIR, Mitutoyo) to ensure that PL was collected only from the patterned area (20 µm × 20 µm). A long-pass filter at 550 nm was used before the monochromator to filter the pump laser and collect PL only with a silicon CCD camera (DU420A-BVF, Andor).
For all measurements, the incident laser was polarized perpendicular to the grating lines (TM-polarized) and the polarization-dependent PL spectra were obtained using a visible-region linear polarizer.
Metallic-to-dielectric transition with electron beam exposure
A blue shift of the Im[ε] peak is also visible. g) J-aggregate photoluminescence (PL) gradually weakens and also shows a gradual blue shift (inset: the green dot in the figure is the laser spot). h) Comparison of blue shifts in dielectric constants and PL spectra. a) Molecular structure of TDBC monomer. Before exposure to the electron beam, the optical reflection and transmission from the J-aggregate film showed clear metallic behavior due to the strong excitonic response of the film (see the red curve in Fig. 5.1(c), (d); the reflection is strongly enhanced near the metallic region (Re[ε] < 0), while the transmission is suppressed. To further understand this behavior, we extracted the dielectric constants of the J-aggregate film from the reflection and transmission spectra (see Appendix C).
More PL data from dose test samples are given in Figure 5.3 in the Supporting Information. This small structural deviation of the J-aggregates can lead to a sharp decrease in the absorption peak. This laser-induced modification of the absorption spectrum is also similar to the "spectral hole burning" observed in various dye molecules.
In the present work, we enabled direct nanoscale patterning on J-aggregate films by locally controlling the excitonic response of the aggregates.
Electron-beam-induced nanoscale patterning and polarization-dependent optical
The period in the x-direction is fixed as 450 nm, while the y-direction period gradually changes from 400 nm to 480 nm. The period in the x-direction is fixed as 450 nm, while the y-direction period gradually changes from 350 nm to 550 nm. In AFM height images, there is a dielectric constant change as well as a slight height change in the J-aggregate film (10 ~ 20nm) due to the electron beam exposure. b) and (c) are numerical simulations for checking the effects of dielectric layer height fluctuations (for dielectric layer heights 140 nm and 120 nm).
Clear spectral features in the metallic region (shaded) are present in agreement with experimental results. Clear spectral features in the metallic region (shaded) are also present, consistent with experimental results. We also check the case (d) when there is no dielectric layer at all and there is only a height change in the metallic J aggregate film (i.e. we consider what happens when there is only a film height variation without changes in dielectric constant ). e), (f) Transmission in the metallic spectral region is strongly suppressed and no notable spectral features exist.
Therefore, these simulations demonstrate that changes in the dielectric constant of J-aggregates induced by the electron beam are mainly responsible for the strong polarization-dependent optical spectra observed in the experiment.
Optical modes in the metallic and dielectric spectral regions
For a clearer understanding of these optical spectra, we normalized the transmission spectra (TTE/TTM) in the metallic region [Figure 5.9(b)]. A clear spectral peak is present in the metallic region, which gradually redshifts and sharpens as the lattice period increases. This drastic change shows that the two polarizations (TM and TE) behave very differently in the metallic region.
A clear spectral peak appears in the metallic region, and this peak gradually redshifts and becomes sharper as the annealing period increases. We now consider the optical response in the dielectric region (Re[ε] > 0), especially beyond λ = 600 nm. The dielectric constant profile in the patterned J-aggregate film is again shown in Figure 5.11(a) (λ. = 750 nm).
As shown in Figure 5.1(g) in the main text, the PL from the metallic J aggregate is much stronger than that from the dielectric J aggregate region.
Polarization-dependent photoluminescent spectra
TM-polarized PL from the linear grating film shows a strong peak at 592 nm with a broad, weak shoulder, similar to that of the unpatterned film [Figure 5.13(a)]. However, the TE-polarized PL spectra show additional peaks in the shoulder region [Figure 5.13(b)], consistent with the transmission measurements. Note that the spectral features in the transmission spectra [Figure 5.11(b)] are quite broad, and it is therefore difficult to identify the exact resonance positions.
Although transmission and PL are independent optical measurements, they show similar diffraction characteristics confirming the excitation of the optical mode in the grating pattern. -The aggregates themselves have strong PL with a very small Stokes shift from the excitonic absorption line. In the nanopatterned film, TE polarized PL spectra show additional and sharp peaks in the shoulder region consistent with our transmission measurements.
For all measurements, the incident laser was TM polarized and the polarization-dependent PL spectra were acquired using a linear visible-range polarizer.
Further confirmation of electron-beam-induced response changes
A PVA polymer matrix may affect the film thickness change, but we do not believe that PVA plays a critical role in the optical response change. The gradual blue-shift observed in our PL measurement [Figure 5.1(g), (h)] also indicates that there is a gradual change in the excitonic transition energy with electron beams. This suggests that the electron beam exposure can be used more widely for different molecular aggregates.
The transmission and PL spectra are shown in Fig. 5.15 and we can clearly see that there are no significant spectral features like those observed in metallic films. The PVA solutions were mixed in a volume ratio of 3:1 and spin-coated onto the substrate (film thickness ~60 nm). In contrast to Figure 5.4(a), no clear color change is observed in the microscope image in the main text.
There are no special spectral features like those observed in metallic J-aggregate films because the dielectric constants of the non-metallic film are always positive and their variation in the patterned sample is also relatively small.
Conclusions
Our study suggests that nanopatterned excitonic films can be used as functional elements in various photonic systems with tailored optical responses. This work can contribute to the integration of excitonic and photonic elements on a single platform for light harvesting and various photonic applications.
Conclusions and Outlook
Ši, L.; Zi, J., Generation of optical vortex beams by momentum-space polarization vortices centered in bound states in the continuum. Zhang, S.; Gu, H.; Liu, J.; Jiang, H.; Chen, X.; Zhang, C.; Liu, S., Characterization of beam splitters in six-channel Stokes polarimeter calibration. Taghizadeh, A.; Chung, I.-S., Quasi-bound states in the continuum with few unit cells of a photonic crystal slab.
Xiang, J.; Xu, Y.; Chen, J.-D.; Lan, S., Tailoring the spatial localization of bound state in the continuum in plasmonic-dielectric hybrid system. Carletti, L.; Koshelev, K.; De Angelis, C.; Kivshar, Y., Giant nonlinear response at the nanoscale driven by bound states in the continuum. Ndao, A.; Hsu, L.; Cai, W.; Ha, J.; Park, J.; Contractor, R.; Lo, Y.; Kanté, B., Differentiation and quantification of exosome secretion from a single cell using quasi-bound states in the continuum.
Wang, Y.; Fan, Y.; Zhang, X.; Tang, H.; Poem, Q.; Han, J.; Xiao, S., Highly controllable etch-free perovskite microlasers based on bound states in the continuum.
