Vector Exceptional Points with Strong Superchiral Fields

Exceptional points (EPs), branch points of complex energy surfaces at which eigenvalues and eigenvectors coalesce, are ubiquitous in non-Hermitian systems. Many novel properties and applications have been proposed around the EPs. One of the important applications is to enhance the detection sensitivity. However, due to the lack of single-handed superchiral fields, all of the proposed EP-based sensing mechanisms are only useful for the nonchiral discrimination. Here, we propose theoretically and demonstrate experimentally a new type of EP, which is called a radiation vector EP, to fulfill the homogeneous superchiral fields for chiral sensing. This type of EP is realized by suitably tuning the coupling strength and radiation losses for a pair of orthogonal polarization modes in the photonic crystal slab. Based on the unique modal-coupling property at the vector EP, we demonstrate that the uniform superchiral fields can be generated with two beams of lights illuminating the photonic crystal slab from opposite directions. Thus, the designed photonic crystal slab, which supports the vector EP, can be used to perform surface-enhanced chiral detection. Our findings provide a new strategy for ultrasensitive characterization and quantification of molecular chirality, a key aspect for various bioscience and biomedicine applications.

Figure 1

(a) Diagram of the asymmetric photonics crystal slab. The (b) real and (c) imaginary parts of the eigenvalues for the system in the two parameters space, where the background refractive index and the thickness of the photonic crystal slab are swept. Other parameters are chosen as $a=259\mathrm{nm}$, $d=136\mathrm{nm}$, $n_p=2.02$, and $n_s=1.47$, respectively. (d) The real and imaginary parts of eigenvalues for the system as a function of the refractive index of the background medium when the thickness of the photonic crystal slab is fixed at $h=154.2\mathrm{nm}$. (e) The optical chirality of the eigenmode at the vector EP. The red and green arrows correspond to the real and imaginary parts of the electric and magnetic fields.

Figure 2

(a) Side and top views of the scanning electron microscope images for the fabricated sample. The geometric parameters of the fabricated samples are, $a=259$, $d=136$, and $h=154\mathrm{nm}$, respectively. (b),(d) The calculated and measured transmission spectra of the structure possessing the vector EP. The black, red, and blue lines and dots correspond to the conditions with the structure immersing into air, water, and sucrose solution, respectively. (c) Experimental setups for the measurement of transmission spectra.

Figure 3

The variation of absolute values for the normalized excitation strength of TE-like (blue dot) and TM-like (red dot) modes as functions of the wavelength for the incident light propagating along the $+z$ (a) and (b) $-z$ directions. The structural parameters are chosen as $a=259\mathrm{nm}$, $d=136\mathrm{nm}$, $n_p=2.02$, $n_s=1.47$, $n_b=1.385$, and $h=154.2\mathrm{nm}$, where the vector EP exists in the system.

Figure 4

(a) The scheme to generate the superchiral field at the vector EP by using two beams of CPL exciting the system from opposite directions. (b) The left chart shows the averaged enhancement of $C/C_0$ in the cylindrical hole of the photonic crystal slab with different thickness. The blue line corresponds to the structure sustaining EP with $h=154.2\mathrm{nm}$. The black, red, green, and pink lines correspond to cases deviating from the vector EP. (c) Near-field distribution of optical chirality near the photonic crystal slab at the vector EP under two beams of excitation from opposite directions.