Ultra-precise optical-frequency stabilization with heterogeneous III-V/Si lasers.

The demand for low-noise, continuous-wave, frequency-tunable lasers based on semiconductor integrated photonics has advanced in support of numerous applications. In particular, an important goal is to achieve a narrow spectral linewidth, commensurate with bulk-optic or fiber-optic laser platforms. Here we report on laser-frequency-stabilization experiments with a heterogeneously integrated III/V-Si widely tunable laser and a high-finesse, thermal-noise-limited photonic resonator. This hybrid architecture offers a chip-scale optical-frequency reference with an integrated linewidth of 60 Hz and a fractional frequency stability of 2.5×10-13 at 1 s integration time. We explore the potential for stabilization with respect to a resonator with lower thermal noise by characterizing laser-noise contributions such as residual amplitude modulation and photodetection noise. Widely tunable, compact and integrated, cost-effective, stable, and narrow-linewidth lasers are envisioned for use in various fields, including communication, spectroscopy, and metrology.

Magnetically Controlled Atomic-Plasmonic Fano Resonances.

Following the efforts of size reduction and the integration of light and vapor systems, great promise is held in the integration of vapor and confined electromagnetic waves. By confining light to nanoscale dimensions, fundamental properties of light-vapor interactions may vary significantly. For example, the state of polarization may be modified as compared with weakly focused beams. Specifically, in transverse magnetic modes, the existence of a longitudinal field component, which is in quadrature to the transverse field, generates a "circular-like" polarized light. Here, by taking advantage of this very property, we study the interaction of confined light and vapor in a coupled system of plasmons and atomic vapors in the presence of magnetic fields. Our results show that the spectroscopic nature and Fano resonances of the hybrid plasmonic-atomic system are greatly altered. In parallel, we also exploit the existence of the atoms in proximity to the plasmonic mode to probe the polarization state of the electromagnetic field and reveal the longitudinal-to-transverse ratio between the plasmonic modes components in the near field. Interestingly, our system maps the amplitude and phase information of the electromagnetic modes to the spectral domain. As such, combining magnetic fields with the coupled plasmonic-atomic system has the potential for future applications in high spatial resolution magnetometry, near-field vectorial imaging, and magnetically induced switching and tuning.

Dynamic Control over the Optical Transmission of Nanoscale Dielectric Metasurface by Alkali Vapors.

In recent years, dielectric and metallic nanoscale metasurfaces are attracting growing attention and are being used for variety of applications. Resulting from the ability to introduce abrupt changes in optical properties at nanoscale dimensions, metasurfaces enable unprecedented control over light's different degrees of freedom, in an essentially two-dimensional configuration. Yet, the dynamic control over metasurface properties still remains one of the ultimate goals of this field. Here, we demonstrate the optical resonant interaction between a form birefringent dielectric metasurface made of silicon and alkali atomic vapor to control and effectively tune the optical transmission pattern initially generated by the nanoscale dielectric metasurface. By doing so, we present a controllable metasurface system, the output of which may be altered by applying magnetic fields, changing input polarization, or shifting the optical frequency. Furthermore, we also demonstrate the nonlinear behavior of our system taking advantage of the saturation effect of atomic transition. The demonstrated approach paves the way for using metasurfaces in applications where dynamic tunability of the metasurface is in need, for example, for scanning systems, tunable focusing, real time displays, and more.

Controlling the interactions of space-variant polarization beams with rubidium vapor using external magnetic fields.

Space variant beams are of great importance for a variety of applications that have emerged in recent years. As such, manipulation of their degrees of freedom is highly desired. Here, we study the general interaction of space variant beams with a magnetically influenced Rb medium exploiting the atoms versatile properties in terms of frequency and intensity dependent circular dichroism and circular birefringence. We present the particular cases of radially polarized and hybrid polarized beams where the control of the polarization states is demonstrated experimentally. Moreover, we show that such an atomic system can be used as a tunable analyzer for space variant beams. Finally, exploiting the non-linear properties of Rb vapor, we show that we can control the circular birefringence all optically, and thus modulate the polarization of the radial polarized beam.

Direct observation of optical near field in nanophotonics devices at the nanoscale using Scanning Thermal Microscopy.

In recent years, following the miniaturization and integration of passive and active nanophotonic devices, thermal characterization of such devices at the nanoscale is becoming a task of crucial importance. The Scanning Thermal Microscopy (SThM) is a natural candidate for performing this task. However, it turns out that the SThM capability to precisely map the temperature of a photonic sample in the presence of light interacting with the sample is limited. This is because of the significant absorption of light by the SThM probe. As a result, the temperature of the SThM probe increases and a significant electrical signal which is directly proportional to the light intensity is obtained. As such, instead of measuring the temperature of the sample, one may directly measure the light intensity profile. While this is certainly a limitation in the context of thermal characterization of nanophotonic devices, this very property provides a new opportunity for optical near field characterization. In this paper we demonstrate numerically and experimentally the optical near field measurements of nanophotonic devices using a SThM probe. The system is characterized using several sets of samples with different properties and various wavelengths of operation. Our measurements indicate that the light absorption by the probe can be even larger than the light induced heat generation in the sample. The frequency response of the SThM system is characterized and the 3 dB frequency response was found to be ~1.5 kHz. The simplicity of the SThM system which eliminates the need for complex optical measurement setups together with its broadband wavelength of operation makes this approach an attractive alternative to the more conventional aperture and apertureless NSOM approaches. Finally, referring to its original role in characterizing thermal effects at the nanoscale, we propose an approach for characterizing the temperature profile of nanophotonic devices which are heated by light absorption within the device. This is achieved by spatially separating between the optical near field distribution and the SThM probe, taking advantage of the broader temperature profile as compared to the more localized light profile.

Ultrahigh-Q silicon resonators in a planarized local oxidation of silicon platform.

We describe a platform for the fabrication of smooth waveguides and ultrahigh-quality-factor (Q factor) silicon resonators using a modified local oxidation of silicon (LOCOS) technique. Unlike the conventional LOCOS process, our approach allows the fabrication of nearly planarized structures, supporting a multilayer silicon photonics configuration. Using this approach we demonstrate the fabrication and the characterization of a microdisk resonator with an intrinsic Q factor that is one of the highest Q factors achieved with a compact silicon-on-insulator platform.

Doppler-based flow rate sensing in microfluidic channels.

We design, fabricate and experimentally demonstrate a novel generic method to detect flow rates and precise changes of flow velocity in microfluidic devices. Using our method we can measure flow rates of ~2 mm/s with a resolution of 0.08 mm/s. The operation principle is based on the Doppler shifting of light diffracted from a self-generated periodic array of bubbles within the channel and using self-heterodyne detection to analyze the diffracted light. As such, the device is appealing for variety of "lab on chip" bio-applications where a simple and accurate speed measurement is needed, e.g., for flow-cytometry and cell sorting.

Nanoscale plasmonic memristor with optical readout functionality.

We experimentally demonstrate for the first time a nanoscale resistive random access memory (RRAM) electronic device integrated with a plasmonic waveguide providing the functionality of optical readout. The device fabrication is based on silicon on insulator CMOS compatible approach of local oxidation of silicon, which enables the realization of RRAM and low optical loss channel photonic waveguide at the same fabrication step. This plasmonic device operates at telecom wavelength of 1.55 µm and can be used to optically read the logic state of a memory by measuring two distinct levels of optical transmission. The experimental characterization of the device shows optical bistable behavior between these levels of transmission in addition to well-defined hysteresis. We attribute the changes in the optical transmission to the creation of a nanoscale absorbing and scattering metallic filament in the amorphous silicon layer, where the plasmonic mode resides.

Generation of a periodic array of radially polarized Plasmonic focal spots.

This paper demonstrates experimentally the tight focusing of a 3X3 array of radially polarized diffraction orders, and the coupling of this array of spots to surface plasmon polaritons (SPPs), propagating on a uniform metal film, and effectively generating a periodic structure of plasmonic sources by the use of structured illumination pattern, rather than by structuring the plasmonic sample. Using near field measurements, we observed coherent interactions between these multiple plasmonic sources as they propagate towards each other. The demonstrated setup exploits the previously demonstrated advantages of radially polarized light in coupling to SPPs and in generating sharper plasmonic hot spots and expends its use towards mitigating parallel processing challenges. The experimental results are in good agreement with the theory, showing interference fringes having periodicity compatible with the plasmonic SPP wavelength. The demonstrated approach of generating array of hot spots on flat metallic films is expected to play a role in variety of applications, e.g. microscopy, lithography, sensing and optical memories.

Transmission and time delay properties of an integrated system consisting of atomic vapor cladding on top of a micro ring resonator.

In this paper we analyze the transmission and time delay properties of light propagating through a microring resonator (MRR) consisting of a solid core waveguide surrounded by an atomic vapor cladding. Using the atomic effective susceptibility of Rubidium we derive the complex transmission spectrum of the integrated system. We show, that when the system is under-coupled, the transmission can exceed the standalone MRR's background transmission and is accompanied by enhanced positive time delay. It is shown that in this case the contrast of the atomic lines is greatly enhanced. This allows achieving high optical densities at short propagation length. Furthermore, owing to its features such as small footprint, high tunability, and high delay-transmission product, this system may become an attractive choice for chip scale manipulations of light.

Frequency locked micro disk resonator for real time and precise monitoring of refractive index.

We experimentally demonstrate a frequency modulation locked servo loop, locked to a resonance line of an on-chip microdisk resonator in a silicon nitride platform. By using this approach, we demonstrate real-time monitoring of refractive index variations with a precision approaching 10(-7) RIU, using a moderate Q factor of 10(4). The approach can be applied for intensity independent, dynamic and precise index of refraction monitoring for biosensing applications.

Near field phase mapping exploiting intrinsic oscillations of aperture NSOM probe.

An innovative, simple, compact and low cost approach for phase mapping based on the intrinsic modulation of an aperture Near Field Scanning Optical Microscope probe is analyzed and experimentally demonstrated. Several nanoscale silicon waveguides are phase-mapped using this approach, and the different modes of propagation are obtained via Fourier analysis. The obtained measured results are in good agreement with the effective indexes of the modes calculated by electromagnetic simulations. Owing to its simplicity and effectiveness, the demonstrated system is a potential candidate for integration with current near field systems for the characterization of nanophotonic components and devices.

Generation and tight focusing of hybridly polarized vector beams.

We propose and experimentally demonstrate the generation of hybridly polarized beams by transmitting radially polarized light through a wave plate. We show that such beams span a closed circle on the surface of the Poincaré sphere whose center coincides with the center of the sphere. In addition we numerically investigate the field and energy density distribution across the focal plane of a high NA lens illuminated by such a hybrid beam. The results show an interesting polarization distribution with 3D orientation and space variant ellipticity. This kind of polarization distributions may be used for a variety of applications, e.g. particle orientation analysis, microscopy and in atomic systems.

A Chip-Scale Optical Frequency Reference for the Telecommunication Band Based on Acetylene.

Lasers precisely stabilized to known transitions between energy levels in simple, well-isolated quantum systems such as atoms and molecules are essential for a plethora of applications in metrology and optical communications. The implementation of such spectroscopic systems in a chip-scale format would allow to reduce cost dramatically and would open up new opportunities in both photonically integrated platforms and free-space applications such as lidar. Here the design, fabrication, and experimental characterization of a molecular cladded waveguide platform based on the integration of serpentine nanoscale photonic waveguides with a miniaturized acetylene chamber is presented. The goal of this platform is to enable cost-effective, miniaturized, and low power optical frequency references in the telecommunications C band. Finally, this platform is used to stabilize a 1.5 µm laser with a precision better than 400 kHz at 34 s. The molecular cladded waveguide platform introduced here could be integrated with components such as on-chip modulators, detectors, and other devices to form a complete on-chip laser stabilization system.

Direct Kerr frequency comb atomic spectroscopy and stabilization.

Microresonator-based soliton frequency combs, microcombs, have recently emerged to offer low-noise, photonic-chip sources for applications, spanning from timekeeping to optical-frequency synthesis and ranging. Broad optical bandwidth, brightness, coherence, and frequency stability have made frequency combs important to directly probe atoms and molecules, especially in trace gas detection, multiphoton light-atom interactions, and spectroscopy in the extreme ultraviolet. Here, we explore direct microcomb atomic spectroscopy, using a cascaded, two-photon 1529-nm atomic transition in a rubidium micromachined cell. Fine and simultaneous repetition rate and carrier-envelope offset frequency control of the soliton enables direct sub-Doppler and hyperfine spectroscopy. Moreover, the entire set of microcomb modes are stabilized to this atomic transition, yielding absolute optical-frequency fluctuations at the kilohertz level over a few seconds and <1-MHz day-to-day accuracy. Our work demonstrates direct atomic spectroscopy with Kerr microcombs and provides an atomic-stabilized microcomb laser source, operating across the telecom band for sensing, dimensional metrology, and communication.

A novel antitumor prodrug platform designed to be cleaved by the endoprotease legumain.

Chemotherapeutic treatment of neoplastic diseases is often restricted by adverse systemic toxicity, which limits the dose of drug that can be administered, or by the appearance of drug resistance. Therefore, novel targeted therapeutic approaches are being developed to improve current conventional therapy in order to increase specificity and biocompatibility, and decrease toxicity. Legumain represents a recently identified lysosomal protease that has been reported to be overexpressed in the majority of human solid tumors, to promote cell migration and is associated with enhanced tissue invasion and metastases. Therefore, it serves as a promising candidate for prodrug therapy. We synthesized a novel legumain-cleavable prodrug, carbobenzyloxy-alanine-alanine-asparagine-ethylenediamine-etoposide, which releases the chemotherapeutic agent, etoposide, as the active drug. The prodrug was characterized and analyzed by (1)H NMR and HPLC. 293 Human embryonic kidney (293 HEK) cells were stably transfected with human legumain, to achieve overexpression in vitro (293 HEK-Leg). 293 HEK-Leg cells expressed both active and inactive legumain and secreted it to the medium. Legumain expression was found to be elevated because of serum starvation in both 293 HEK cells and PC3 human prostate carcinoma cells. The commercial substrate of legumain, carbobenzyloxy-alanine-alanine-asparagine-amino-4-methyl coumarin (CBZ-Ala-Ala-Asn-AMC) and the synthesized prodrug were both cleaved by recombinant human legumain (rhlegumain) and legumain expressed in the 293 HEK-Leg cell lysate. Upon cleavage by rhlegumain, the prodrug showed an inhibitory effect on the proliferation of 293 HEK and 293 HEK-Leg cells. This study suggests a novel platform for prodrug therapy activated by legumain as a promising approach for cancer therapy.

Chip-scale atomic diffractive optical elements.

The efficient light-matter interaction and discrete level structure of atomic vapors made possible numerous seminal scientific achievements including time-keeping, extreme non-linear interactions, and strong coupling to electric and magnetic fields in quantum sensors. As such, atomic systems can be regarded as a highly resourceful quantum material platform. Recently, the field of thin optical elements with miniscule features has been extensively studied demonstrating an unprecedented ability to control photonic degrees of freedom. Hybridization of atoms with such thin optical devices may offer a material system enhancing the functionality of traditional vapor cells. Here, we demonstrate chip-scale, quantum diffractive optical elements which map atomic states to the spatial distribution of diffracted light. Two foundational diffractive elements, lamellar gratings and Fresnel lenses, are hybridized with atomic vapors demonstrating exceptionally strong frequency-dependent, non-linear and magneto-optic behaviors. Providing the design tools for chip-scale atomic diffractive optical elements develops a path for compact thin quantum-optical elements.

Publisher Correction: Chip-scale atomic diffractive optical elements.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Strong coupling and high-contrast all-optical modulation in atomic cladding waveguides.

In recent years, there has been marked increase in research aimed to introduce alkali vapours into guided-wave configurations. Owing to the significant reduction in device dimensions, the increase in density of states, the interaction with surfaces and primarily the high intensities carried along the structure, a plethora of light-vapour interactions can be studied. Moreover, such platform may exhibit new functionalities such as low-power nonlinear light-matter interactions. One immense challenge is to study the effects of quantum coherence and shifts in nanoscale waveguides, characterized by ultra-small mode areas and fast dynamics. Here, we construct a highly compact 17 mm long serpentine silicon-nitride atomic vapour cladding waveguide. Fascinating and important phenomena such as van-der-Waals shifts, dynamical stark shifts and coherent effects such as strong coupling (in the form of Autler-Townes splitting) are observed. Some of these effects may play an important role in applications such as all-optical switching, frequency referencing and magnetometry.

Fano resonances and all-optical switching in a resonantly coupled plasmonic-atomic system.

The possibility of combining atomic and plasmonic resonances opens new avenues for tailoring the spectral properties of materials. Following the rapid progress in the field of plasmonics, it is now possible to confine light to unprecedented nanometre dimensions, enhancing light-matter interactions at the nanoscale. However, the resonant coupling between the relatively broad plasmonic resonance and the ultra-narrow fundamental atomic line remains challenging. Here we demonstrate a resonantly coupled plasmonic-atomic platform consisting of a surface plasmon resonance and rubidium ((85)Rb) atomic vapour. Taking advantage of the Fano interplay between the atomic and plasmonic resonances, we are able to control the lineshape and the dispersion of this hybrid system. Furthermore, by exploiting the plasmonic enhancement of light-matter interactions, we demonstrate all-optical control of the Fano resonance by introducing an additional pump beam.