Nonlinear optical metasurfaces offer a possibility to perform frequency mixing without the phase-matching constraints of bulk nonlinear crystals and with control of the local nonlinear response at a sub-wavelength scale. Nonlinear inter-subband polaritonic metasurfaces created by combining the semiconductor heterostructures with quantum-engineered inter-subband nonlinear response and electromagnetically engineered metal-clad nanoresonators offer by far the largest second-order nonlinear response of all condensed matter systems reported to date. However, the nonlinear optical response of these metasurfaces is limited by optical intensity saturation in the nanoresonator hot spots that prevented the achievement of power conversion efficiencies over 0.2% in three-wave mixing experiments. In this study, we propose and experimentally demonstrate dielectric inter-subband polaritonic metasurfaces for second-harmonic generation that achieve 0.37% power conversion efficiency. Our structure is created by a new design approach that combines dielectric resonators inducing Mie resonant modes with a lattice resonance to achieve a uniform and high field enhancement throughout the meta-atom volume.

Nonlinear frequency mixings have shown an alternative way to create new electromagnetic sources in frequency ranges that are difficult to access with conventional techniques. To simultaneously use the fundamental frequency pump beam and multiple harmonic signals generated in the same channel, a device capable of separating each frequency component is required. Here, we propose and experimentally demonstrate metasurface-based spatial filters for the pump frequency and multiple harmonic frequencies. The metasurface was designed using eight different split ring resonator-based phase elements with 45° phase spacing, which allows wavefront shaping. The metasurface designed to have a one-dimensional gradient phase array produces cross-polarized reflection waves with different beam steering angles at the third- and fifth-harmonic frequencies (15 and 25 GHz) and operates as a metallic mirror at the fundamental frequency of 5 GHz. Our work suggests a new method to enable simultaneous use of broadband multi-frequency sources based on nonlinear frequency mixing.

We report giant nonlinear response and relaxed intensity saturation for second-harmonic-generation (SHG) from nonlinear dielectric polaritonic metasurfaces. Experimentally, 21 mW of SHG peak power and 0.37% of SHG conversion efficiency at 10.55 μm were achieved.

We demonstrate orbital-angular-momentum (OAM) multiplexing and demultiplexing at E-band frequency using a metasurface. With off-axis integration, two Gaussian beams are converted to two orthogonal OAM beams, which are in turn demultiplexed into two off-axis directions.

Orbital angular momentum (OAM) has received considerable attention regarding high‐capacity communication owing to its spatial orthogonality. However, it is still challenging to build a compact communication system that can generate multiple coaxial OAM beams and receive information from each. In this work, OAM multiplexing and demultiplexing at the E‐band frequency using a single metasurface structure is proposed and experimentally demonstrated. For OAM multiplexing, the metasurface used as a transceiver generates two orthogonal coaxial OAM beams for Gaussian incident beams with different incidence angles. For OAM demultiplexing, the same metasurface flipped 180° as a receiver forms a Gaussian beam in different off‐axis directions depending on the topological charge of the coaxially incident OAM beam. The Gaussian beam measured at the receiver end exhibits a high signal‐to‐noise ratio of more than 33 dB compared with the background OAM beam. OAM multiplexing and demultiplexing based on a single metasurface may provide a route for high‐capacity and compact free‐space communication systems.

Raman spectroscopy of gaseous molecules has been challenging, requiring complicated experimental procedures and peripheral devices for concentrating the analytes. Here, Raman spectroscopy of gaseous molecules at parts-per-billion (ppb) levels is demonstrated using aqueous microlenses of LiCl solution that spontaneously absorb water-soluble gas molecules from the environment. The lenses are easily formed by filling the microwells of an elastomeric stamp with an aqueous solution of LiCl and stamping onto a substrate. Because LiCl is hygroscopic, the aqueous lenses maintain their liquid states under various ambient conditions. Gaseous molecules in the air dissolve in the aqueous microlens and can be identified based on their Raman fingerprints. Lowering the humidity causes the aqueous lens to transition into a salt crystal, while preserving the dissolved molecules within the crystal and facilitating the long-term storage and analysis of gaseous analytes. By forming the aqueous microlenses on a surface-enhanced Raman scattering (SERS) substrate, 800 ppb dimethyl methylphosphonate, a nerve agent simulant, is detected in a collection time of only 5 s. Aqueous microlenses that are responsive to the chemical environment are useful for analyzing various water-soluble gaseous molecules and therefore have broad implications for healthcare, food safety, and environmental-monitoring applications.

Nonlinear polaritonic metasurfaces created by the coupling of intersubband nonlinearities in semiconductor heterostructures with optical modes in nanoresonators have recently demonstrated efficient frequency mixings at very low pumping intensities of the order of a few tens of kilowatts per square centimetre. In these subwavelength structures, the efficiency, spectral bandwidth and local nonlinear phase of wave mixing do not depend on phase matching but only on the nonlinear response of the constituent meta-atoms. We exploit this property to demonstrate an electrically tunable nonlinear metasurface that combines a plasmonic nanocavity and a quantum-engineered semiconductor heterostructure, in which the magnitude and phase of the local nonlinear responses are controlled by a bias voltage through the quantum-confined Stark effect. We demonstrate spectral tuning, dynamic intensity modulation and dynamic beam manipulation for second-harmonic generation. Our work suggests a route for electrically reconfigurable flat nonlinear optical elements with versatile functionalities. By coupling plasmonic resonators with a semiconductor heterostructure, researchers control the nonlinear response by a bias voltage, thereby enabling spectral tuning, dynamic intensity modulation and dynamic beam manipulation for second-harmonic generation.

We report giant nonlinear circular dichroism for second- and third-harmonic generation from nonlinear chiral polaritonic metasurfaces. Experimentally, over 86% of circular dichroisms for the two harmonic generations on one-chip system around 10 pm were achieved.

Nonlinear metasurfaces are advancing into a new paradigm of "flat nonlinear optics" owing to the ability to engineer local nonlinear responses in subwavelength-thin films. Recently, attempts have been made to expand the design space of nonlinear metasurfaces through nonlinear chiral responses. However, the development of metasurfaces that display both giant nonlinear circular dichroism and significantly large nonlinear optical response is still an unresolved challenge. Herein, we propose a method that induces giant nonlinear responses with near-unity circular dichroism using polaritonic metasurfaces with optical modes in chiral plasmonic nanocavities coupled with intersubband transitions in semiconductor heterostructures designed to have giant second and third order nonlinear responses. A stark contrast between effective nonlinear susceptibility elements for the two spin states of circularly polarized pump beams was seen in the hybrid structure. Experimentally, near-unity nonlinear circular dichroism and conversion efficiencies beyond 10-4% for second- and third-harmonic generation were achieved simultaneously in a single chip.

Optically-thin nonlinear polaritonic metasurfaces created by coupling of intersubband nonlinearities in semiconductor heterostructures with optical modes in nanoresonators have recently demonstrated efficient three-wave-mixing at very low pumping intensities of the order of few tens of kW cm-2. In these subwavelength structures, the efficiency and the spectral bandwidth of the wave mixing depends solely on nonlinearity of the constituent meta-atoms. Here we exploit this property to demonstrate an electrically-tunable nonlinear metasurface that combines a plasmonic nanocavity and a quantum-engineered semiconductor heterostructure, in which the magnitude and the spectral characteristics of the nonlinear response are controlled by bias voltage through the quantum-confined Stark effect. We demonstrate tuning the peak second-harmonic-generation (SHG) efficiency in the range of 8.7 – 10.75 μm and modulation of SHG intensity at a fixed pump wavelength by applying bias voltage. An SHG power conversion efficiency of 0.082 % was achieved using a peak pump intensity of only 80 kW cm-2.

Vortex beams carrying orbital angular momentum (OAM) have attracted considerable attention for the development of high-capacity wireless communication systems due to their infinite sets of orthogonal modes. However, the practical applications of Laguerre-Gaussian type vortex beams are limited due to the fact that the divergence angle increases as the order of the OAM mode increases. In this work, we present metasurfaces that generate vortex beams carrying OAM modes with reduced divergence angles in the E-band frequency range. The metasurfaces were designed using eight different meta-atom phase elements, including a spiral phase distribution for OAM modes l = 1 and 2, a phase gradient array to avoid interference with the source beam, and a lens pattern array to reduce the divergence angle. Through simulation and experimental measurement, it was confirmed that the divergence angle of the vortex beam generated by the metasurface with the lens pattern was reduced from 13° to 9° and 14° to 11° for OAM modes l = 1 and 2, respectively, in comparison with the metasurface without the lens pattern. Our results provide new design methods for various applications based on OAM multiplexing especially in high frequency E-band range.

We propose an electrically tunable nonlinear intersubband polaritonic metasurface for third-harmonic generation (THG). Numerically, 7300% of THG modulation depth, over 300° of local phase tuning, and dynamic THG diffraction signal modulation were achieved in mid-infrared.

Metasurfaces that generate nonlinear optical responses provide new degrees of freedom for applications such as nonlinear holography, wave mixing, and new frequencies generation. To extend the utility of flat nonlinear optics, metasurfaces providing both giant nonlinear response for efficient frequency mixing in subwavelength films and a single-beam output with a local wavefront control need to be developed. Metasurfaces with giant nonlinear response have recently been realized using the concept of intersubband polaritons. Separately, nonlinear metasurfaces providing a single-beam nonlinear output with a local wavefront control have been studied. However, a metasurface platform that possesses both of these properties has yet to be demonstrated. Herein, the first such platform is presented and its operation for three- and four-wave mixing processes is demonstrated. This approach is based on using Pancharatnam–Berry phase control of local nonlinear response and on employing meta-atoms with specific symmetries to enable generation of only a single nonlinear beam. Experimentally, 400 nm-thick metasurfaces with a wavefront-controlled single-beam output are demonstrated for second- and third-harmonic generation at pump wavelength of approximately 10 µm. Power conversion efficiencies of 7.6 × 10 −4 % and 3.6 × 10 −4 % are obtained for the two nonlinear processes, respectively, at peak pumping intensity of only 80 kW cm −2 .

Nonlinear responses in metasurfaces have shown great promise for various applications, such as nonlinear holography, spectroscopy, and novel nonlinear light sources. Metasurfaces that produce nonlinear responses have the advantage of greatly relaxed phase‐matching conditions, which can simplify nonlinear optical systems. However, the practical use of nonlinear metasurfaces with an accessible optical pump source requires giant nonlinear responses. A multi‐quantum‐well‐loaded nonlinear metasurface designed for third‐harmonic generation at λ ≈ 8.9 µm with a record‐high third‐order nonlinear optical response is presented for condensed matter systems in the infrared spectral range of up to 4.4 × 10 −14 m 2 V −2 . About 1.8 × 10 −3 % third‐harmonic generation power conversion efficiency is experimentally achieved in 400 nm thick metasurfaces with a pump intensity of only 210 kW cm −2 . The multi‐quantum‐well structure can be used to produce second‐harmonic generation when combined with properly designed nanoresonators to allow multiple and simultaneous harmonic generations on the same chip.

We report the first experimental observation of low-threshold lasing in strained germanium nanowires. The lasing threshold is ~3.0 kW cm -2 which is more than one order of magnitude lower than the state-of-the-art germanium-tin Fabry-Perot laser.

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavourable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be a successful demonstration of lasing from this seemingly promising material system. Here we demonstrate a low-threshold, compact group IV laser that employs a germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently overcome optical losses at 83 K, thus allowing the observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm −2 . Our demonstration opens new possibilities for group IV lasers for photonic-integrated circuits.

Obtaining a high-quality germanium layer on silicon has recently garnered much attention for photonic-integrated circuits. Epitaxial growth of germanium on silicon has made significant progress toward suppressing the propagation of threading dislocations that are inevitably created at the germanium/silicon interface due to a large lattice mismatch. Recently, there have been numerous reports on successful demonstration of high-performance MOSFETs fabricated on this improved germanium layer. However, unlike MOSFETs in which charge carriers are transported primarily at the defect-free top interface, the performance of optical devices may be significantly hampered by the defects at the germanium/silicon interface. In this talk, I will present our research works on the effect of the germanium/silicon interface on germanium-based optical devices. We demonstrate methods to improve structural and optical properties of germanium by effectively reducing the defective interface. By using various characterization techniques including TEM, time-resolved photoluminescence and Raman spectroscopy, we show that the minority carrier lifetime and internal quantum efficiency can be significantly improved. We also show improved performances from devices integrated on our germanium platform.

We investigated the optical, electrical, and structural properties of epitaxially grown Ge-on-Si substrates after phosphorous implantation. Ion implantation increases n-type doping in Ge for an on-chip light source. However, its effects on Ge should be carefully studied as implantation may increase the recombination sites, and possibly reduce light-emitting efficiency. We studied the light-emitting efficiency of implanted Ge using various material characterizations. We found that phosphorous implantation increased the doping concentration of in situ doped Ge-on-Si, which boosted the photoluminescence by 12–30%. It is therefore critical to optimize the post-annealing and implantation doses to increase light-emitting efficiency of Ge.

Germanium has been an important material in the field of group IV photonics and photonic-integrated circuits because of its superior optical properties over silicon such as high responsivity up to 1550 nm (C-band). Although the material quality of germanium plays an important role in improving the performance of germanium-based photonic devices, it has been a technical challenge to obtain a high-quality germanium layer on silicon due to large lattice mismatch between silicon and germanium. In this paper, we discuss the effect of germanium/silicon interface on germanium-based optical devices. We present a couple of approaches to obtain a high quality germanium layer on a silicon platform. By using various characterization techniques including transmission electron microscopy, time-resolved photoluminescence and steady-state photoluminescence, we show that the minority carrier lifetime and light emitting efficiency can be significantly improved in our engineered germanium-on-silicon platforms.

We present germanium microdisk optical resonators under a large biaxial tensile strain using a CMOS-compatible fabrication process. Biaxial tensile strain of ~0.7% is achieved by means of a stress concentration technique that allows the strain level to be customized by carefully selecting certain lithographic dimensions. The partial strain relaxation at the edges of a patterned germanium microdisk is compensated by depositing compressively stressed silicon nitride layer. Two-dimensional Raman spectroscopy measurements along with finite-element method simulations confirm a relatively homogeneous strain distribution within the final microdisk structure. Photoluminescence results show clear optical resonances due to whispering gallery modes which are in good agreement with finite-difference time-domain optical simulations. Our bandgap-customizable microdisks present a new route towards an efficient germanium light source for on-chip optical interconnects.