Back Focal Plane Imaging Research Articles

Second harmonic generation in monolithic gallium phosphide metasurfaces

Abstract Gallium phosphide (GaP) offers unique opportunities for nonlinear and quantum nanophotonics due to its wide optical transparency range, high second-order nonlinear susceptibility, and the possibility to tailor the nonlinear response by a suitable choice of crystal orientation. However, the availability of single crystalline thin films of GaP on low index substrates, as typically required for nonlinear dielectric metasurfaces, is limited. Here we designed and experimentally realized monolithic GaP metasurfaces for enhanced and tailored second harmonic generation (SHG). We fabricated the metasurfaces from bulk (110) GaP wafers using electron-beam lithography and an optimized inductively coupled plasma etching process without a hard mask. SHG measurements showed a high NIR-to-visible conversion efficiency reaching up to 10−5, at the same level as typical values for thin-film-based metasurface designs based on III–V semiconductors. Furthermore, using nonlinear back-focal plane imaging, we showed that a significant fraction of the second harmonic was emitted into the zeroth diffraction order along the optical axis. Our results demonstrate that monolithic GaP metasurfaces are a simple and broadly accessible alternative to corresponding thin film designs for many applications in nonlinear nanophotonics.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Reconfiguring the Optical Selection Rule in Ultramicrotome‐Crafted Vertically Aligned InSe Ribbons

AbstractDue to the high carrier mobility and direct bandgap character, indium selenide (InSe) exhibits great advantages for advanced optoelectronic applications. However, the unique optical section rule in InSe weakens the light‐matter interaction when the electric field of illuminated light is perpendicular to the c‐axis of planar InSe flake. To overcome this constraint, ultramicrotome technique is introduced to achieve vertically aligned InSe ribbons, aiming to reconfigure the optical paths for improved optical absorption and device performance. The well‐designed InSe ribbons are acquired with uniform morphology, showing an integrated structure free of cracks or fragments. First, the structural symmetry, crystal orientation, and optical anisotropy of InSe ribbon are carefully revealed by polarization‐dependent Raman spectrum, second harmonic generation (SHG), and photoluminescence measurements. Then, the back focal plane (BFP) imaging of photoluminescence distinguishes the in‐plane and out‐of‐plane dipole emission of InSe ribbons and planar InSe flakes, respectively. Notably, the ribbon exhibits new PL peaks with energy lower than the exciton peak of InSe at low temperature, which is attributed to Se vacancies. The work by introducing vertically aligned InSe nanoribbons provides a new strategy to manipulate the light‐matter interactions and defect engineering for novel optoelectronics, which can be also extended to other 2D semiconductors.

Far-field mapping and efficient beaming of second harmonic by a plasmonic metagrating

Abstract We study numerically and experimentally the second-harmonic generation (SHG) from rectangular metagratings of V-shaped gold nanoantennas. We show that by carefully engineering the array pitch to steer the diffraction orders toward the single antenna emission, the extracted signal is maximized. This enhancement is attributed to the angular overlap between the radiation pattern and array factor and is comparable to the improvement yielded by the coupling of surface lattice resonances (SLRs) with local modes. Moreover, we demonstrate a simple technique to experimentally reconstruct the emission diagram of an antenna from measurements of the collective grating response as a function of the excitation angle. Excellent agreement is obtained with simulations when the sample is immersed either in air or in water, which is crucial in view of future sensing application. Thanks to the high signal-to-noise ratio and low dependence on the statistical particle dispersity, this method constitutes an effective alternative to back-focal plane imaging when very weak signals such as SHG are involved.

Improved detection of particle displacement in optical traps using back focal plane interferometry and image processing

Optical tweezers play a crucial role in particle manipulation and force measurement. Therefore, it is essential to calibrate the force measurement parameters, such as the optical trap stiffness and the conversion factor of the sensor deflection signal to the actual displacement of the trapped particle. This calibration is necessary to achieve accurate and efficient parameter calibration. In this study, we employed a suitable image processing method, based on a low-frequency CMOS camera, to calibrate the Displacement Conversion Factor (DCF) of the optical tweezers system. The high accuracy of this image processing method makes it ideal for analyzing the actual displacement of optically trapped microspheres. Interestingly, we discovered that the normalized deflection signal captured by the quadrant photodiode (QPD) through back focal plane interferometry is primarily correlated with the axial height of the trapped microspheres, and is relatively unaffected by laser power variations. Building on this finding, we also examined the linear detection range of the trapped microspheres at different heights. These results address certain limitations associated with the use of optical tweezer systems and provide valuable guidance for utilizing this tool effectively.

High-directivity far-field radiation of quantum dot-based single-photon emitter coupled to polymeric circular waveguide resonant grating

Solid-state single-photon emitters (SPEs) commonly encounter the limitation of quasi-omnidirectional radiation patterns, which poses challenges in utilizing their emission with conventional optical instruments. In this study, we demonstrate the tailoring of the far-field radiation patterns of SPEs based on colloidal quantum dots (QDs), both theoretically and experimentally, by employing a polymer-based dielectric antenna. We introduce a simple and cost-effective technique, namely low one-photon absorption direct laser writing, to achieve precise coupling of a QD into an all-polymer circular waveguide resonance grating. By optimizing the geometry parameters of the structure using 3D finite-difference time-domain simulations, resonance at the emission wavelength of QDs is achieved in the direction perpendicular to the substrate, resulting in photon streams with remarkably high directivity on both sides of the grating. Theoretical calculations predict beam divergence values below 2°, while experimental measurements using back focal plane imaging yield divergence angles of approximately 8°. Our study contributes to the evaluation of concentric circular grating structures employing low refractive index polymer materials, thereby expanding the possibilities for their application.

Detection of refractive index and imperfection in thin film transparent polymer by back focal plane imaging

Emission patterns from molecules at interfaces encode many details about their local environment and their axial position, along the microscope’s optical axis. We introduce an advanced approach that synergizes back focal plane (BFP) imaging with innovative ‘smart’ surfaces make surface imaging more qualitative, more reliable, and more robust. Our method is particularly focused on accurately measuring the refractive index (RI) of transparent thin films and their imperfections close to the interfaces. Our technique utilizes a ‘smart’ surface, which features a uniform fluorescent thin film of about 4 nm thickness together with back-focal plane (BFP) imaging. We manage to detect bubbles or other imperfection in 100 nm thin film of polymer with RI of 1.34.

Strongly polarized color conversion of isotropic colloidal quantum dots coupled to fano resonances.

Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today. However, for LCDs requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge. Here, we show strongly polarized color conversion of isotropic QDs coupled to Fano resonances of v-grooved surfaces compatible with surface-normal LED illumination for next-generation QD-TVs. This architecture overcomes the critically oblique excitation of surface plasmon coupled emission by using v-shapes imprinted on the backlight unit (BLU). With isotropic QDs coated on the proposed v-BLU surface, we experimentally measured a far-field polarization contrast ratio of ∼10. Full electromagnetic solution shows Fano line-shape transmission in transverse magnetic polarization allowing for high transmission as an indication for forward-scattering configuration. Of these QDs coupled to the surface plasmon-polariton modes, we observed strong modifications in their emission kinetics revealed by time-resolved photoluminescence spectroscopy and via dipole orientations identified by back focal plane imaging. This collection of findings indicates conclusively that these isotropic QDs are forced to radiate in a linearly polarized state from the patterned planar surface under surface-normal excitation. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays.

Emission Dipole and Pressure‐Driven Tunability of Second Harmonic Generation in vdWs Ferroelectric NbOI2

Abstract2D in‐plane ferroelectric NbOI2 exhibits strong second harmonic generation (SHG) and ultrahigh effective susceptibility. To push forward their applications in nonlinear photonics and optoelectronics, it is highly desirable to understand the emission dipole orientation and tunability of SHG, which is not achieved. Here, by integrating tight focusing from parabolic mirror with back focal plane (BFP) imaging technique, for the first time it is demonstrated that SHG emission of NbOI2 presents purely in‐plane dipole orientation in consistent with numerical simulations, suggesting the in‐plane components of the SHG susceptibility tensor in NbOI2 dominate the emission. Moreover, with the aid of ab‐initio calculations, it is found that the hydrostatic pressure can dramatically change the structure and resultant SHG intensity of NbOI2. Explicitly, SHG intensity endures a slight increase due to the distortion of octahedral at low pressure pressure, and then monotonously decreases due to the improvement of structural symmetry with further increasing pressure, and drastically quenching resulting from the ferroelectric to paraelectric phase transition. This work unambiguously demonstrates the dipole emission behavior of SHG and the relationship between structural evolution and SHG intensity, which provides an avenue for tunable nonlinear optics and optoelectronics.

Back focal plane imaging for light emission from a tunneling junction in a low-temperature ultrahigh-vacuum scanning tunneling microscope.

We report the design and realization of the back focal plane (BFP) imaging for the light emission from a tunnel junction in a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). To achieve the BFP imaging in a UHV environment, a compact "all-in-one" sample holder is designed and fabricated, which allows us to integrate the sample substrate with the photon collection units that include a hemisphere solid immersion lens and an aspherical collecting lens. Such a specially designed holder enables the characterization of light emission both within and beyond the critical angle and also facilitates the optical alignment inside a UHV chamber. To test the performance of the BFP imaging system, we first measure the photoluminescence from dye-doped polystyrene beads on a thin Ag film. A double-ring pattern is observed in the BFP image, arising from two kinds of emission channels: strong surface plasmon coupled emissions around the surface plasmon resonance angle and weak transmitted fluorescence maximized at the critical angle, respectively. Such an observation also helps to determine the emission angle for each image pixel in the BFP image and, more importantly, proves the feasibility of our BFP imaging system. Furthermore, as a proof-of-principle experiment, electrically driven plasmon emissions are used to demonstrate the capability of the constructed BFP imaging system for STM induced electroluminescence measurements. A single-ring pattern is obtained in the BFP image, which reveals the generation and detection of the leakage radiation from the surface plasmon propagating on the Ag surface. Further analyses of the BFP image provide valuable information on the emission angle of the leakage radiation, the orientation of the radiating dipole, and the plasmon wavevector. The UHV-BFP imaging technique demonstrated here opens new routes for future studies on the angular distributed emission and dipole orientation of individual quantum emitters in UHV.

Defocus Effect Correction for Back Focal Plane Ellipsometry for Antivibration Measurement of Thin Films

Back focal plane (BFP) ellipsometry, which acquires the ellipsometric parameters of reflected light at different incident and azimuthal angles through a high-NA objective lens, has recently shown great potential in industrial film measurement. In on-line metrology cases for film manufacturing, the film vibration, which is caused by equipment vibrations or environmental disturbances, results in defocus blur and distortion of the received BFP images. Thus, subsequently extracted ellipsometric spectra and film parameters significantly deviate from the ground truth values. This paper proposes a cost-effective method for correcting vibration-induced BFP ellipsometric spectral errors. The method relies on an initial incident angle calibration of BFP radii at different defocus positions. Then, corresponding ellipsometric spectral errors are corrected by inserting a calibrated Jones compensation matrix into a system model. During measurement, the defocus position of the vibrational film is first determined. Then, BFP ellipsometric spectral errors, including incident angle mapping distortion and ellipsometric parameter variations, are corrected for a bias-free film analysis using the previous calibration results. Experimental results showed that this method significantly improved measurement accuracy without vibrational defocus compensation, from over 30 nm down to less than 1 nm.

Low-Divergence hBN Single-Photon Source with a 3D-Printed Low-Fluorescence Elliptical Polymer Microlens.

Efficiently collecting light from single-photon emitters is crucial for photonic quantum technologies. Here, we develop and use an ultralow fluorescence photopolymer to three-dimensionally print micrometer-sized elliptical lenses on individual precharacterized single-photon emitters in hexagonal boron nitride (hBN) nanocrystals, operating in the visible regime. The elliptical lens design beams the light highly efficiently into the far field, rendering bulky objective lenses obsolete. Using back focal plane imaging, we confirm that the emission is collimated to a narrow low-divergence beam with a half width at half-maximum of 2.2°. Using photon correlation measurements, we demonstrate that the single-photon character remains undisturbed by the polymer lens. The strongly directed emission and increased collection efficiency is highly beneficial for quantum optical experiments. Furthermore, our approach paves the way for a highly parallel fiber-based detection of single photons from hBN nanocrystals.

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that the z-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

Analysis of Deep Learning-Based Phase Retrieval Algorithm Performance for Quantitative Phase Imaging Microscopy.

Quantitative phase imaging has been of interest to the science and engineering community and has been applied in multiple research fields and applications. Recently, the data-driven approach of artificial intelligence has been utilized in several optical applications, including phase retrieval. However, phase images recovered from artificial intelligence are questionable in their correctness and reliability. Here, we propose a theoretical framework to analyze and quantify the performance of a deep learning-based phase retrieval algorithm for quantitative phase imaging microscopy by comparing recovered phase images to their theoretical phase profile in terms of their correctness. This study has employed both lossless and lossy samples, including uniform plasmonic gold sensors and dielectric layer samples; the plasmonic samples are lossy, whereas the dielectric layers are lossless. The uniform samples enable us to quantify the theoretical phase since they are established and well understood. In addition, a context aggregation network has been employed to demonstrate the phase image regression. Several imaging planes have been simulated serving as input and the label for network training, including a back focal plane image, an image at the image plane, and images when the microscope sample is axially defocused. The back focal plane image plays an essential role in phase retrieval for the plasmonic samples, whereas the dielectric layer requires both image plane and back focal plane information to retrieve the phase profile correctly. Here, we demonstrate that phase images recovered using deep learning can be robust and reliable depending on the sample and the input to the deep learning.

Enhanced emission directivity from asymmetrically strained colloidal quantum dots.

Current state-of-the-art quantum dot light-emitting diodes have reached close to unity internal quantum efficiency. Further improvement in external quantum efficiency requires more efficient photon out-coupling. Improving the directivity of the photon emission is considered to be the most feasible approach. Here, we report improved emission directivity from colloidal quantum dot films. By growing an asymmetric compressive shell, we are able to lift their band-edge state degeneracy, which leads to an overwhelming population of exciton with in-plane dipole moment, as desired for high-efficiency photon out-coupling. The in-plane dipole proportion determined by back-focal plane imaging method is 88%, remarkably higher than 70% obtained from conventional hydrostatically strained colloidal quantum dots. Enhanced emission directivity obtained here opens a path to increasing the external quantum efficiencies notably.

Optical dipole orientation of interlayer excitons in MoSe2−WSe2 heterostacks

We report on the far-field photoluminescence intensity distribution of interlayer excitons in ${\mathrm{MoSe}}_{2}\text{\ensuremath{-}}{\mathrm{WSe}}_{2}$ heterostacks as measured by back focal plane imaging in the temperature range between 1.7 and 20 K. By comparing the data with an analytical model describing the dipolar emission pattern in a dielectric environment, we are able to obtain the relative contributions of the in- and out-of-plane transition dipole moments associated to the interlayer exciton photon emission. We determine the transition dipole moments for all observed interlayer exciton transitions to be $(99\ifmmode\pm\else\textpm\fi{}1)%$ in plane for R- and H-type stacking, independent of the excitation power and therefore the density of the exciton ensemble in the experimentally examined range. Finally, we discuss the limitations of the presented measurement technique to observe correlation effects in exciton ensembles.

Comparison of back focal plane imaging of nitrogen vacancy centers in nanodiamond and core-shell CdSe/CdS quantum dots

We report on the characterization of the angular-dependent emission of two different single-photon emitters based on nitrogen-vacancy centers in nanodiamond and on core-shell CdSe/CdS quantum dot nanoparticles. The emitters were characterized in a confocal microscope setup by spectroscopy and Hanbury-Brown and Twiss interferometry. The angular-dependent emission is measured using a back focal plane imaging technique. A theoretical model of the angular emission patterns of the 2D dipoles of the emitters is developed to determine their orientation. Experiment and model agree well with each other.

Geometric Control Over the Edge Diffraction of Electrically Excited Surface Plasmon Polaritons by Tunnel Junctions

Plasmonic metal–insulator–metal tunnel junctions (MIM-TJs) readily excite surface plasmon polaritons (SPPs) by inelastic electron tunneling, well below the diffraction limit, eliminating the need for bulky optical elements. The highly confined MIM cavity mode (MIM-SPP) excited by the tunneling electrons dominantly outcouples to photons and single-interface SPPs which further outcouple as photons and give characteristic features in the Fourier plane of observation. Here, we employ SPP waveguides, directly integrated with MIM-TJs, to explore the diffraction of single-interface SPPs at the edges of the metal stripe waveguides. In addition to the leakage of the SPP modes through the metal electrodes, SPP edge diffraction presents as an additional channel for SPP outcoupling, not limited by the thickness of the waveguide. Edge diffraction manifests as a straight-line feature in the Fourier plane of the far-field and by systematically varying the width of the waveguides, we show that the in-plane momentum of the SPPs can be controlled, which in turn leads to control over the edge diffraction. We fabricated MIM-TJs with integrated plasmonic stripe waveguides of different widths, and the propagation and confinement of the SPP modes along the edges of the SPP waveguides are directly identified by the experimental back focal plane imaging, which are further corroborated by numerical simulations. Our findings can be exploited for applications ranging from sensing to nonlinear plasmonics, where focusing and localization of SPP fields are necessary.

Probing guided monolayer semiconductor polaritons below the light line

In this work, we demonstrate an approach to study exciton-polaritons supported by transition metal dichalcogenide monolayers coupled to an unstructured planar waveguide below the light line. In order to excite and probe such waves propagating along the interface with the evanescent fields exponentially decaying away from the guiding layer, we employ a hemispherical ZnSe solid immersion lens (SIL) precisely positioned in the vicinity of the sample. We visualize the dispersion of guided polaritons using back focal (Fourier) plane imaging spectroscopy with the high-NA objective lens focus brought to the center of SIL. This results in the effective numerical aperture of the system exceeding an exceptional value of 2.2 in the visible range. In the experiment, we study guided polaritons supported by a WS2 monolayer transferred on top of a Ta2O5 plane-parallel optical waveguide. We confirm room-temperature strong light-matter coupling regime enhanced by ultra-low intrinsic ohmic and radiative losses of the waveguide. Note that in the experiment, total radiative losses can be broadly tuned by controlling SIL-to-sample distance. This gives a valuable degree of freedom for the study of polariton properties. Our approach lays the ground for future studies of light-matter interaction employing guided modes and surface waves.

Bloch Surface Waves Mediated Micro-Spectroscopy.

Micro-spectroscopy is a critical instrument for spectrum analysis in various applications such as chemical and biological analysis, environment detection, and hyperspectral imaging. However, current micro-spectral technique requires bulky and costly spectrometer. In this report, a new type of Bloch surface wave (BSW) based micro-spectrometer is proposed. A single silicon nanoparticle sitting on a dielectric multilayer substrate is used to excite the BSW which acts as a nanoscale unknown source. Taking advantage of the dispersion relations of BSWs, an abundant spectrally related database is formed that is useful for spectrum retrieval applications. Back-focal plane images are used to monitor the change of angular spectrum corresponding to the dispersion relationship of Bloch surface waves. Combined with an iterative algorithm, experimental retrieval of visible-range monochromatic and broadband light spectrums can be obtained. The resolution of the spectrometers can reach 2 nm across a wavelength range of 130 nm. The method in this work is CMOS compatible, enabling spectra retrieval for nanoscale radiators and can also be used to measure and retrieve the microscopic spectrum signal rapidly and timely without conventional scanning monochromator spectrometer.