Polariton-hybridization of light-matter oscillations can emerge from various quasiparticles, such as phonon, plasmon, exciton and magnon. Particularly, exciton polaritons are bosonic quasiparticles with half-light, halfmatter nature, which are originated from strong coupling between excitons and microcavity photons. The half-light nature results in extremely small effective mass, making it feasible to achieve high temperature even room-temperature Bose-Einstein condensation (BEC). Meanwhile, the half-matter nature leads to strong nonlinear interaction, which is missing between photons and can promote the polaritons relaxation to ground state and give rise to low threshold polariton lasing, compared to photonic lasing. The exciton polaritons are of great importance in applications of quantum simulation, topological quantum optics, ultrafast optical switch and low threshold lasers.

The concept of cloaking has never failed to fascinate scientific researchers. In recent years, the idea has ignited a whole new trend of research effort when extended to temporal domain. A typical temporal cloak conceals events from probing light by creating a temporal intensity gap (cloaking time) that will later be closed back by manipulating the speed of light leveraging the chromatic dispersion in dielectric media. Therefore, the observer will only see a continuous probing light but will never realize the temporal events that happens during the temporal gap. Previous experimental demonstrations primarily relied on parametric- or phase-modulator (PM)-based time lens to realize temporal cloaking, which had finite temporal aperture and only concealed events at a fixed time period. Consequently, the cloaking time is so far limited to around 200 ps and practical applications are still unforeseeable.Recently, Zhou et al. [1] has demonstrated a programmable temporal cloak with significantly enhanced cloaking time using a brand-new type of time lens. The time lens first generates a coherent broad-band optical frequency comb and a following electrically-tuned microring resonator (ET-MRR) provides linearly-scanned filtering. Consequently, the output is linearly chirped, which is equivalent to a conventional time lens. Most importantly, the ET-MRR not only achieves large modulation depth as the parametric time lenses, it is also programmable by generating arbitrary electrical driving signals. With the combined strength, ET-MRR opens temporal cloaks at arbitrary time and the cloaking time is also tunable from 0.449 to 3.365 ns. The cloaking capacity is 17 times larger than the previous record, finally bringing temporal cloaking technique to the nanosecond regime.Admittedly the current performance might not directly lead to practical applications, but it will definitely inspires more subsequent research effort. The scalability of the cloaking time is related to the bandwidth of frequency comb generator and the free spectral range of the ET-MRR, which has not been fully explored yet. Moreover, the application of the new ET-MRR-based time lens is certainly not restricted to temporal cloaking. It would be technically fascinating to explore its application in temporal magnification or temporal Fourier transform, which might also bring new breakthroughs. Last but not least, for more useful application, the programmability of temporal cloaking should not only be realized in the time domain, but also in the spatial domain: i.e., concealing events at arbitrary positions along the transmission link, which we expect to see in the near future.

The idea of photonic crystals and photonic band gap was first introduced by both Yablonovitch [1] and John in 1987 [2]. Photonic crystals are man-made periodic optical media in which the dispersion of light is strongly modified due to the scattering of periodically arranged dielectric or metal inclusions in the unit cell. Photonic band gaps, a frequency range in which light cannot propagate, can form as a consequence of Bragg scattering or the resonance of the inclusions in the unit cell. The existence of band gaps means that photonic crystals can serve as low-loss distributed feedback mirrors and as such, they can confine light and can be used to realize high fidelity resonant cavities that can facilitate the observation of quantum electronics phenomena. The application of such ideas to realize strong coupling between photon and exciton is achieved using planar dielectric Si periodic structures [3]. When combined with a gain material, photonic crystals are obviously good platforms to realize lasing and indeed photonic crystal based lasers have attracted great interest in past three decades. The technical challenges and progress in distributed feedback organic lasers based on photonic crystals are discussed and reviewed by Fu and Zhai [4]. For practical applications, nonlinear photonic crystals with different superlattices has been successfully used in quasi-phase matching and nonlinear diffraction harmonic generation. This is reviewed by Li and Ma [5].

Femtosecond laser writing is a powerful three dimensional (3D) engineering technique for materials processing. It has been used to modify the refractive index of dielectric materials (e.g., glass, crystals) to construct optical waveguides for diverse applications. Lithium niobate (LiNbO3) is a multifunctional crystal with excellent nonlinear optical properties. Nonlinear photonic crystal (NPC) could control nonlinear optical interactions through quasi-phase matching (QPM) due to the periodical nonlinear coefficients. Although the 1D and 2D NPCs have been fabricated by electric-field poling, the implementation of 3D NPCs remains as a major challenge in nonlinear optics because of the limitation of traditional poling methods.

Optical spectroscopy is a versatile characterization technique for a wide range of applications. Developing miniaturized spectrometers is the trend for applications in which small footprint takes precedence over high resolution. However, development of micro-spectrometers based on miniaturized or integrated optics is approaching a bottleneck toward submillimeter scales because of the inherent scale limitation of their optical components or path lengths. Although these constraints can be circumvented with computational spectral reconstruction by addressing a full range of spectral components simultaneously at multiple detectors, complex millimeter-scale arrays of individually prepared filters arranged over charge-coupled device or complementary metal-oxide semiconductor detectors are difficult to be miniaturized.