In most photon systems, losses usually affect the performance of the photon system and increase energy consumption. When we mention "losses", it seems that we cannot think of anything else to use them for besides being used as absorbers. Therefore, in experiments, everyone is thinking about how to eliminate this "trouble". However, in experiments and practical applications, even with advanced processes and materials, not all losses can be completely eliminated. Is loss really just about being a loser? Through research, some scientists have taken a different approach and found ways to turn loss into treasure: by utilizing the exceptional points (EPs) in optical non-Hermitian systems, appropriate control of losses can bring better performance to the system or novel functions that are difficult to achieve with traditional optical systems.

Recently, Professor Cheng-Wei Qiu from the National University of Singapore, invited by the editor-in-chief of Photonics Insights , collaborated with Associate Research Professor Shaohua Dong from Peng Cheng Laboratory and Professor Yang Chen from the University of Science and Technology of China to write a review titled " Exceptional-point optics with loss engineering". (Shaohua Dong, Heng Wei, Zhipeng Li, Guangtao Cao, Kun Xue, Yang Chen, Cheng-Wei Qiu, "Exceptional-point optics with loss engineering," Photon. Insights 4, R02 (2025)).

This review systematically summarizes the research progress on the combination of pure loss systems and EP optics in recent years, focusing on the physical mechanism and application of the combination of EPs and loss control in optical systems. It also categorizes and discusses the design, implementation, and experimental results based on different device types (cavity, waveguide, and metasurface) from the device dimension. In addition, the review also points out some of the challenges and possible solutions currently faced in this field and looks forward to the future development direction of this field.

Through sorting out the research results in this direction, we can clearly see that when combined with EP, Loss can become more personalized and can completely transform from a Loser into a charming Hero. This review not only provides a new perspective for researchers to understand loss but also provides a reference for how to explore the treasure trove of Loss in the future.

1、Mechanism of singularity generation in optical non-Hermitian systems

The problem of losses is prevalent in optical systems in almost all application scenarios. In general, losses can be categorized into non-radiative losses due to material absorption, etc., and radiative losses that radiate to the outside of the structure. In most cases, losses can harm experimental results or device performance, such as overheating the device or reducing the sensitivity of the sensor. Therefore, most experiments are designed to minimize losses, for example by using low-loss materials and more advanced manufacturing processes. However, these methods can only minimize non-radiative losses to a certain extent, which cannot be avoided in practical applications. Realistic optical systems cannot be completely closed due to the need for input and output of results and/or exchange of energy. Therefore, only the possibility of realizing non-Hermitian systems exists. In experiments, it is often necessary to introduce gains to offset the energy loss due to radiative and non-radiative losses.

In 1998, Bender and Boettcher found that in quantum non-Hermitian systems, if the corresponding Hamiltonian quantities satisfy the parity-time symmetry after modulating the distribution and magnitude of gains and losses, the system may have purely real eigenvalues. Under certain circumstances, all the eigenstates may coalesce, and the position of the system in the parameter space corresponding to this state is called an exceptional point. Due to the high similarity between the Schrödinger equation in quantum systems and the wave equation in electromagnetic systems, this phenomenon was quickly generalized to the microwave and optical fields. However, although it is feasible to incorporate gain in electromagnetic systems, the operation still greatly increases the complexity and processing difficulty of devices in experimental and practical applications, especially for metasurfaces or metamaterials.

In addition, numerous theoretical studies have pointed out that the noise brought by the gain can easily lead to the instability of the system near the EP and the generation of energy divergence. To address this issue, researchers have noticed that in the Hamiltonian matrix characterizing a system, if the gain and loss are replaced by two different loss values, the matrix can be equivalently transformed into a superposition of a regular Hamiltonian supporting an EP and a matrix representing a fixed loss. As shown in Fig. 1, this means that EPs may still exist in systems where only losses are present and no gain is added. In other words, one can obtain physical phenomena based on EPs by modulating the loss strength and distribution without relying on the gain.

Fig. 1 Principle and example of passive EP devices based on loss engineering

Therefore, this passive system can overcome some of the drawbacks brought by the aforementioned gains, such as noise problems and instability. At the same time, some anomalous optical phenomena brought by EPs can still be realized by such systems, including but not limited to light-induced transparency (LIT), optical nonreciprocity, asymmetric reflection, etc. Based on the above principles, researchers have realized many novel optical effects by designing different kinds of devices, including cavities, waveguides, and metasurfaces.

2、Cavity-based passive EP devices

Cavity structures belong to one of the fundamental device configurations in electromagnetic systems and have long been widely used in the study of various electromagnetic systems, including microwave, terahertz, and optical ranges, due to their ability to support several resonant modes with rather high-quality factors. Combined with the passive EPs mentioned previously, cavity-based optics can realize novel optical phenomena, including but not limited to coherent perfect absorption (CPA), optical transparency, highly sensitive detection, non-reciprocity, and so on. Among them, CPA is a complete absorption phenomenon realized by a coherent mechanism, when an interference interaction between a light wave and an excited state in a medium occurs. This effect is particularly significant in non-Hermitian systems and is widely used in applications such as photodetectors and photovoltaic cells. CPA can be understood as the time-reversal behavior of the lasing and is, therefore, mostly limited to a relatively narrow absorption bandwidth. However, it is found that the linewidth of the perfect absorption band can be expanded when the system is at the EP.

Figures 2(a)-(e) show two different passive CPA device designs incorporating EP. It can be seen from the absorption spectra that the absorption line shape is closer to a quadratic function when the cavity has a suitable loss and coupling ratio to the waveguide, compared with the conventional CPA device, thus broadening the frequency range in which perfect absorption can be produced. Further, for the device in Fig. 2(c), the eigenstates leave the EP point only when the coupling of the inverse to clockwise modes in the cavity is nonzero. As a result, the device can form an exceptional surface (ES) in the parameter space and thus is robust to undesirable perturbations such as machining errors and material defects, which further enhances the performance in practical applications.

Fig. 2 Realization of coherent perfect absorption and light-induced transparency based on passive EPs

LIT is another important optical phenomenon that can be realized by EP-based devices. Conventional LIT implementations usually resort to magneto-optical or non-linear materials to break the reciprocity. However, the properties and/or processing flow of such materials are not compatible with on-chip integrated devices. Cavity-based passive EP devices are considered to be an ideal platform for the study of electromagnetically-induced transparency (EIT) since they can modulate the resonance spectrum without introducing additional noise and instability. Further, such devices can be combined with other optical phenomena for more flexible response tailoring. As shown in Fig. 2(f)-(h), by modulating the optical loss with the use of a nano tip, the device can exhibit optomechanically-induced transparency (OMIT) with high transmittance in the frequency bands that are otherwise characterized by strong absorption. In the region close to the EP, the transmittance of the device decreases and then increases as the loss increases. Using similar principles and device configurations, loss engineering based on the EP point can also realize the suppression and revival of laser action. As shown in Fig. 3(a)-(c), by placing a Cr-coated nanotip close to a whispering-gallery-mode (WGM) cavity, researchers can introduce additional loss to the system. As the loss increases, the field strength in the cavity shows an anomalous trend of decreasing first and then increasing. Further, with the coupling between the cavities maintained constant, the lasing action of the device is first suppressed and then recovered as the additional introduced loss gradually increases from 0. The experimentally measured Raman spectrum in Fig. 3(c) shows that the recovered lasing intensity may even exceed that of the lossless case. This research work provides strong evidence illustrating the advantages of passive EP devices.

In addition, high-sensitivity detection can also be realized by passive EP devices. The splitting characteristics of the eigenfrequencies in EP systems have been extensively studied. As shown in Fig. 3(d), in second-order systems, unlike the Hermitian environment, the splitting of the eigenfrequency of the EP system in a non-Hermitian system is proportional to the square root of the intensity of the perturbation. Therefore, in detecting minuscule intensity parameter changes, EP-based sensors have a greater advantage because they can provide a stronger response. However, conventional active EP devices are inevitably limited by noise and instability. Therefore, by modulating the losses, passive EP devices possess the potential to provide better feedback. As shown in Fig. 3(e)-(g), by combining the Fabry-Pérot cavity configuration and magneto-optical material, researchers have successfully realized a highly sensitive detection of weak magnetic field changes in the background of a strong magnetic field. By inserting liquid crystal materials into the cavity, the system causes different losses for incident light in different polarization states and can be flexibly regulated by voltage according to different background magnetic field strengths. The experimental result in Figs. 3(g)-(h) shows that the device possesses the EP characteristic of eigenfrequency splitting proportional to the square root of the perturbation intensity. Even considering the effect of noise, the device still has a more sensitive response relative to the Hermitian system. Meanwhile, in the realm of nanometrology, a recent study has achieved a resolution of 2 nm using a similar cavity structure. The device achieves coupling between modes through electrically tuned movable plane mirrors and introduces additional loss through an absorbing material doped with erbium particles. By directly using the shift of the transmission peak to characterize the displacement at the micro-nanometer level, the device avoids errors introduced by spectral fitting as well as the need for a reference signal. Experimental results show that the integrated sensitivity is improved by a factor of 86 and the signal-to-noise ratio is improved by a factor of 5 compared to other work, as the system does not operate exactly at the EP.

Fig. 3 Cavity-based passive EP devices for more efficient laser effects and detection

In addition to the above work, cavity-based EP devices can also be used to achieve additional functionality by combining different mechanisms. For example, Mie scatterers can be used to break the rotational symmetry of a micro-ring cavity and tune the system to EP, thus allowing the system to exhibit a high-quality chiral response. In addition, in the quantum realm, one-photon or two-photon blocking effects may arise when EP-induced asymmetric coupling is combined with resonant cavities with Kerr nonlinearities, and EP can also be used to facilitate magnon-photon or magnon-phonon coupling through cavity-based magnetically coupled systems, which can lead to a more flexible and efficient tuning of optical effects.

3、Waveguide-based passive EP devices

Compared to cavity-based devices, waveguides can introduce additional spatial degrees of freedom into the system, thus offering the possibility to exhibit transition processes between different modes. Various system parameters, e.g., shape, loss rate, refractive index, etc., can be modulated along the propagation path of the electromagnetic wave to represent different positions in the parameter space, thus allowing the positions of the system eigenstates in the parameter space to vary continuously along certain paths. For example, the trajectory of the system eigenstate in Fig. 4(a) forms a closed path around the EP in the parameter space. Devices corresponding to such paths usually exhibit chiral responses, i.e., the output state depends only on the encircling direction and is independent of the input state.

As shown in Fig. 4(b)-(d), the researchers designed an L-shaped waveguide supporting two propagation modes and realized the encircling of EP by adjusting the geometrical parameters. Another waveguide located nearby can couple part of the energy out, thus equivalently realizing an adjustable loss. Using coordinate transformations, the waveguide encircles the EP in the parameter space through the infinity boundary. Experimental results show that the device can realize nearly lossless inter-mode transitions and maintains crosstalk below -20 dB when the wavelength of the input light is 1550 nm. Based on a similar path through the infinity boundary, researchers also designed another dual-coupled silicon waveguide to relax the adiabatic requirement in system evolution and achieve fast inter-mode transitions.

As shown in Fig. 4(e)-(g), the device adds a layer of chromium above one of the waveguides to introduce position-dependent losses. Experimental results show that the device successfully achieves mode-to-mode transitions in a footprint of 57 µm with less than -15 dB crosstalk. The above work shows that the introduction of losses in waveguide-based passive EP devices allows one to flexibly regulate the position of the system eigenstates in the parameter space and achieve efficient output mode locking through encircling EPs. The size of the device can also be further shrunk, which is more conducive to the integration with on-chip systems.

Fig. 4 Waveguide-based passive EP-encircling devices

In addition to this, loss-modulated passive EP devices can also realize unidirectional reflectionless propagation of light. If the waveguide is considered an optical device with two ports, the relationship between the incident and the outgoing electric or magnetic fields can be described by a scattering matrix. Similar to the Hamiltonian matrix, the scattering matrix also has corresponding eigenstates and eigenfrequencies. A simple calculation shows that when the eigenvalues are merged, at least one of the reflectances at the forward or backward port will become 0, implying that reflectionless propagation occurs. This phenomenon arises from the destructive interference of the two system eigenstates occurring at the EP, which suppresses the reflection in one of the two directions. In 2013, researchers used germanium and chromium to periodically modulate the loss along the waveguide propagation direction to simulate a passive EP system with PT-symmetry as in Fig. 1(a), shown in Fig. 5(b)-(e). Experimentally measured reflectance spectra with an inverse ratio of up to 0.7 were obtained using incident light at 1550 nm.

Fig. 5 Unidirectional reflectionless propagation of light using waveguide-based passive EP devices

4、Metasurface-based passive EP devices

Metasurfaces are planar optical elements consisting of subwavelength artificial microstructures capable of manipulating electromagnetic waves with subwavelength resolution. Due to the high degree of design freedom of the unit cells, metasurfaces are widely used in various electromagnetic wave modulation applications, such as polarization control, asymmetric reflection/transmission, and sensing. For the polarization of an electromagnetic wave, two different polarization states can be regarded as a set of bases for the eigenvectors of the system.

Therefore, the different responses of the system to these two polarizations can be represented as a matrix similar to Hamiltonians, thus obtaining an EP solution in this parameter space. For example, researchers used rectangular metal strips as antennas to modulate the polarization states in each of the two directions and generated EPs in the parameter space by breaking the mirror symmetry. As shown in Figs. 6(a)-(c), in the parameter space consisting of displacement perturbations in the x and y directions, the asymmetric transmittance for the circularly polarized light suggests the presence of EP, and the eigenmodes of the EP points are almost perfectly circularly polarized. To further enhance the flexibility of the device, a work has successfully constructed reconfigurable passive EP metasurfaces by combining niobium nitride (NbN), a superconducting material with variable conductivity, with metal structures. The temperature-dependent property of the conductivity of NbN allows it to be regarded as a tunable loss, which can dynamically adjust the polarization state of the EP.

As shown in Fig. 6(d)-(e), the eigenstates of the metasurface have different polarizations at different temperatures, thus enabling flexible and efficient manipulation for different incident light. In addition, by combining plasmonic nanoantennas, researchers realized the modulation of the reflected light with a topological phase range. Different antenna structure designs correspond to different states surrounding the EP along a certain path in the parameter space, thus realizing the accumulation of the topological phase. As shown in Fig. 6(f), by arranging different antenna structures, the metasurface can realize the independent modulation of left- and right-handed circularly polarized incident light and present different reflected holograms.

Fig. 6 Hypersurface-based passive EP devices

At the same time, passive EP devices can also produce asymmetric transmission/reflection properties. For example, by combining the concepts of non-Hermitian systems and phase-gradient metasurfaces, researchers have realized efficient unidirectional retroreflection. As shown in Fig. 7(a)-(d), by arranging lossless and lossy unit cells in a regular pattern to form a tri-meta-atom containing three structural units, the metasurface successfully exhibits passive PT-symmetry as shown in Fig. 1(a). Experimental measurements show that the metasurface exhibits strong unidirectional retro-reflection for incident light from the left and right sides. In addition, other anomalous optical phenomena can also be realized by the passive EP metasurface through the combination with other physical phenomena, such as Fano resonance. In the devices in Figs. 7(e)-(g), the researchers added a layer of graphene between two layers of silver gratings to obtain tunable loss magnitude and further unidirectional invisibility located at the EP.

Fig. 7 More anomalous optical phenomena that can be realized by passive EP devices based on metasurfaces

Passive EP devices based on metasurfaces can also be used in applications where high-sensitivity detection is required. A plasmonic EP-based biochemical detector is illustrated in Fig. 8(a)-(d). The device utilizes a double layer of periodically arranged nano-sized gold rods to obtain plasmonic resonance modes with different losses, and the EP is generated by a precise arrangement that allows the two different modes to coalesce. The experimental results show that, as predicted by the theory, the device's response is proportional to the square root of the perturbation strength. In addition, EP can also be combined with other materials to amplify the corresponding response. For example, vanadium dioxide (VO₂) has a temperature-dependent conductivity. In a metasurface, this material can be treated as a temperature-dependent lossy material. As shown in Fig. 8(e)-(g), utilizing this property enables the temperature probing ability of this metasurface. At the same time, since the eigenmodes of the metasurface are affected by the surrounding refractive index, the variation of the latter causes a splitting of the eigenfrequencies at the EP. As a result, the device realizes sensitive detection of both temperature and refractive index at the same time.

Fig. 8 Metasurface-based passive EP devices for high-sensitivity detection

Finally, in addition to the examples mentioned above, passive EP devices based on metasurfaces are also used in many other application scenarios, such as imaging and holograms, thermal emission, diffraction control, etc.

5、Conclusion and outlook

This review systematically and comprehensively summarizes passive devices supporting EPs, including different configurations such as cavities, waveguides, and metasurfaces. The article introduces the theoretical foundations of passive EP devices and analyzes their advantages over conventional active counterparts. For different device configurations, the article highlights several representative examples that fully illustrate that the inclusion of losses provides a higher degree of freedom for the device in specific cases and even leads to better overall performance, especially in terms of noise control and stability.

At the same time, passive EP devices still have many aspects that need further research. First, the EP effect itself on the system is still controversial in the academic community. Theoretical studies have shown that due to the non-orthogonality of the eigenmodes near the EP, the linewidths of the system spectra will increase significantly, which ultimately makes it impossible to identify the eigenfrequency splitting from the experiment. However, the authors believe that there are suitable ways to solve the existing problems and take full advantage of passive EP devices. As an example, they can be combined with physical phenomena, including optical non-reciprocity, and break the limitations of conventional non-Hermitian systems that observe reciprocity. In addition, higher-order EPs have not yet been widely realized in passive devices. Compared to common second-order EPs, higher-order EPs require more stringent system design and manufacturing accuracy due to their higher dimensionality and complex parameters. Researchers may need to utilize more delicate principles such as exceptional surfaces to design experimentally accessible devices.

Finally, loss-engineered EP devices possess the potential to combine with more physical phenomena, such as the quantum Mpemba effect, quantum coherence improvement, and so on. These directions emerging in the field of traditional active EPs may also drive the development of passive EPs, leading to even more novel phenomena and advancing their widespread use in cutting-edge optics.