the most widely applied holographic method. [2] Though SLMs possess a huge number of pixels with highly tunable flexibility and fast tuning speed, the pixel size is commonly larger than the working wavelength. Such feature results in unwanted holographic images generated at higher diffraction orders, thus bringing down the efficiency and the field of view. Recently, metasurfaces with unprecedented wavefront engineering ability have gained enormous interest. [3,4] Being composed of specially designed subwavelength structures, metasurfaces control the interfacial abrupt changes of phase, amplitude, and polarization distributions in a flexible manner, thus breaking many limitations in conventional optics. Many intriguing and unique demonstrations beyond their conventional counterparts have been presented using metasurfaces, such as beam deflection, lensing, special beam generation, surface plasmon coupling, etc. [5][6][7][8][9] Besides those with continuous wavefront distributions, metasurfaces are also promising in complex wavefront control, for instance, holography. In particular, the subwavelength resolution of metasurfaces perfectly avoids the problems in SLMs mentioned above. Meanwhile, the capability of synchronously engineering diverse electromagnetic parameters also endows Metasurfaces have drawn enormous attention in both the physics and engineering communities owing to their capabilities of arbitrarily manipulating the electromagnetic waves at subwavelength scale. One of the promising applications of metasurfaces is the realization of high-quality holograms. However, due to the passive response of metasurface resonators, previous demonstrations on meta-holography were mainly limited to generating static images. Seeking new approaches to realize active control over meta-holograms is highly demanded for display applications, such as holographic movies. Here, a broadband active meta-holography which enables temperature-dependent dynamic holographic imaging is experimentally demonstrated. The metahologram consists of two sets of resonators: set 1 is passive, being composed of simple metallic C-shape split-ring resonators (CSRRs) and set 2 is active, being composed of vanadium dioxide (VO 2 ) integrated CSRRs (V-CSRRs). Each set generates certain holographic images with predesigned amplitude and phase distributions. Under external heating, the insulator-to-metal transition property of VO 2 will gradually short out the V-CSRRs and their responses, thus dynamically changing the overall generated holographic image. The proposed approach presents a new way of tunable metasurface design and holds promise for active meta-devices with versatile functionalities.

Accessing the nonradiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resolution imaging far beyond the diffraction limit. At far-infrared and terahertz (THz) wavelengths (e.g., 300 μm = 1 terahertz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multilayer graphene flakes by using a state-of-the-art scattering-type near-field optical microscope (s-SNOM). We experimentally demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metal films at the same frequency range. Further modeling and analysis considering the nonlocal graphene conductivity indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temperature. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.

We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a strong broadening of the THz emission spectrum.

Due to its fast and high resolution characteristics, dual-comb spectroscopy has attracted an increasing amount of interest since its first demonstration. In the terahertz frequency range where abundant absorption lines (finger prints) of molecules are located, multiheterodyne spectroscopy that employs the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate compact terahertz dual-comb spectroscopy based on quantum cascade lasers (QCLs). In our experiment, two free-running QCLs generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that ∼490 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a short air path at room temperature. Spectra are shown of semiconductor samples and of moist air, the latter allowing rapid monitoring of the relative humidity. Our devices should be readily extendable to perform imaging, microscopy and near-field microscopy in the terahertz regime. arXiv:1904.03330v5 [physics.optics]

Passive mode locking was achieved at 1.3 μm in oxide-confined, two-section, bistable quantum dot (QD) lasers with an integrated intracavity QD saturable absorber. Fully mode-locked pulses at a repetition rate of 7.4 GHz with a duration of 17 ps were observed under appropriate bias conditions. No self-pulsation accompanied the mode locking. These results suggest that a carefully designed QD laser is a candidate for ultrashort pulse generation.

Two novel red thermally activated delayed fluorescence (TADF) emitters [triazatruxene (TAT)−dibenzo-[a,c]phenazine (DBPZ) and TAT−fluorine-substituted dibenzo-[a,c]phenazine (FDBPZ)] were developed by incorporating TAT as the electron donor (D) and DBPZ or FDBPZ as the electron acceptor (A). Both compounds showed aggregation-induced emission behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structure of TAT and DBPZ, TAT−DBPZ and TAT−FDBPZ realized high photoluminescence quantum yields in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet−triplet energy splitting (ΔE ST ), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing ( RISC) rate (>10 6 s −1 ) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT−DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT−FDBPZ-based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low-efficiency roll-off ratios of 1.0% at 100 cd m −2 and 19% at 1000 cd m −2 . Furthermore, their solution-processable nondoped devices displayed EQEs of 5.6 and 2.9% with the red-shifted emission peaks at 626 and 641 nm, respectively. These results indicate the huge potential of utilization of TAT as the donor unit to achieve highly efficient and low-efficiency roll-off solution-processable red TADF OLEDs.

Uncooled infrared photodetectors have evoked widespread interest in basic research and military manufacturing because of their low‐cost, compact detection systems. However, existing uncooled infrared photodetectors utilize the photothermoelectric effect of infrared radiation operating at 8–12 µm, with a slow response time in the millisecond range. Hence, the exploration of new uncooled mid‐wavelength infrared (MWIR) heterostructures is conducive to the development of ultrafast and high‐performance nano‐optoelectronics. This study explores a van der Waals heterojunction on epitaxial HgCdTe (vdWs‐on‐MCT) as an uncooled MWIR photodetector, which achieves fast response as well as high detectivity for spectral blackbody detection. Specifically, the vdWs‐on‐MCT photodetector has a fast response time of 13 ns (77 MHz), which is approximately an order of magnitude faster than commercial uncooled MCT photovoltaic photodetectors. Importantly, the device exhibits a photoresponsivity of 2.5 A W‐1, quantum efficiency as high as 85%, peak detectivity of 2 × 1010 cm Hz1/2 W‐1 under blackbody radiation at room temperature, and peak detectivity of up to 1011 cm Hz1/2 W‐1 at 77 K. Thereby, this work facilitates the effective design of high‐speed and high‐performance heterojunction uncooled MWIR photodetectors.

The characteristics of polarization self-modulation in a vertical-cavity surface-emitting laser (VCSEL) were studied for frequencies up to ≈9 GHz both experimentally and theoretically. Polarization self-modulation was obtained by rotating the linearly polarized output of the VCSEL by 90° and reinjecting it into the laser. Experimentally we simultaneously recorded time traces, optical and radio-frequency spectra. We found for increasing modulation frequencies that the output characteristics changed from square-wave to sinusoidal and the VCSEL system assumed new polarization eigenstates that are different from the free-running VCSEL eigenstates. We modeled polarization self-modulation as an interband process and found a good qualitative agreement between our experimental and numerical results.