DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.

Single Nanoparticle Detection Using Optical Microcavities.

Optical whispering gallery mode (WGM) microresonators, confining resonant photons in a microscale resonator for long periods of time, could strongly enhance light-matter interaction, making it an ideal platform for all kinds of sensors. In this paper, an overview of optical sensors based on WGM microresonators is comprehensively summarized. First, the fundamental sensing mechanisms as well as several recently developed enhanced sensing techniques are introduced. Then, different types of WGM structures for sensing as well as microfluidics techniques are summarized. Furthermore, several important sensing parameters are discussed. Most importantly, a variety of WGM sensing applications are reviewed, including both traditional matter sensing and field sensing. Last, we give a brief summary and perspective of the WGM sensors and their role in future applications.

Whispering gallery microsensors: a review

Whispering gallery mode biosensors have been widely exploited over the past decade to study molecular interactions by virtue of their high sensitivity and applicability in real-time kinetic analysis without the requirement to label. There have been immense research efforts made for advancing the instrumentation as well as the design of detection assays, with the common goal of progressing towards real-world sensing applications. We therefore review a set of recent developments made in this field and discuss the requirements that whispering gallery mode label-free sensors need to fulfill for making a real world impact outside of the laboratory. These requirements are directly related to the challenges that these sensors face, and the methods proposed to overcome them are discussed. Moving forward, we provide the future prospects and the potential impact of this technology.

Towards next-generation label-free biosensors: recent advances in whispering gallery mode sensors

Abstract Nanophotonic device building blocks, such as optical nano/microcavities and plasmonic nanostructures, lie at the forefront of sensing and spectrometry of trace biological and chemical substances. A new class of nanophotonic architecture has emerged by combining optically resonant dielectric nano/microcavities with plasmonically resonant metal nanostructures to enable detection at the nanoscale with extraordinary sensitivity. Initial demonstrations include single-molecule detection and even single-ion sensing. The coupled photonic-plasmonic resonator system promises a leap forward in the nanoscale analysis of physical, chemical, and biological entities. These optoplasmonic sensor structures could be the centrepiece of miniaturised analytical laboratories, on a chip, with detection capabilities that are beyond the current state of the art. In this paper, we review this burgeoning field of optoplasmonic biosensors. We first focus on the state of the art in nanoplasmonic sensor structures, high quality factor optical microcavities, and photonic crystals separately before proceeding to an outline of the most recent advances in hybrid sensor systems. We discuss the physics of this modality in brief and each of its underlying parts, then the prospects as well as challenges when integrating dielectric nano/microcavities with metal nanostructures. In Section 5, we hint to possible future applications of optoplasmonic sensing platforms which offer many degrees of freedom towards biomedical diagnostics at the level of single molecules.

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Advances in optoplasmonic sensors - combining optical nano/microcavities and photonic crystals with plasmonic nanostructures and nanoparticles

Ultrasensitive detection of nanoscale particles has applications in important fields ranging from environmental monitoring to analysis of viral structures. The authors show that the dissipative interaction in an optical microcavity of high quality factor allows the detection of single nanoparticles, even when the real part of an analyte's polarizability approaches zero. This innovative approach presents a significant step towards practical optical sensors for use in physics, analytical chemistry, environmental science, and molecular biology.

Detection of Single Nanoparticles Using the Dissipative Interaction in a High-Q Microcavity

The invention discloses a free-space optical micro-cavity Raman laser sensing device and a sensing method thereof The sensing device comprises a laser light source, a first focusing object lens, an optical micro-cavity, a second focusing object lens, a first collimating lens, a second collimating lens, a first photoelectric detector, an oscilloscope, a filter sheet, a third focusing object lens, a second photoelectric detector and a base spectrum analyzer Raman laser of an optical micro-cavity is triggered through free-space frequency-tunable laser, and by observing frequency of beat frequency of the Raman laser, information of nanoscale-dimension particles attached to the surface of the optical micro-cavity is obtained; in the free-space Raman laser sensing technology, a whispering gallery mode and the Raman laser can be directly triggered in the free space through the frequency-tunable laser; and through avoiding the technical problems due to the introduction of an optical fiber taper, the flexibility of the device is improved, and signal to noise ratio is reduced, so that sensitivity of the sensing device is improved, and detection limit is reduced

Free-space optical micro-cavity Raman laser sensing device and sensing method thereof

Ultrasensitive optical detection of nanometer-scaled particles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring, and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode microcavities, which can enhance significantly the light-matter interaction, have emerged as promising platforms for label-free detection of nanoscale objects. Different from the conventional whispering-gallery-mode sensing relying on the reactive (i.e., dispersive) interaction, here we propose and demonstrate to detect single lossy nanoparticles using the dissipative interaction in a high-$Q$ toroidal microcavity. In the experiment, detection of single gold nanorods in an aqueous environment is realized by monitoring simultaneously the linewidth change and shift of the cavity mode. The experimental result falls within the theoretical prediction. Remarkably, the reactive and dissipative sensing methods are evaluated by setting the probe wavelength on and off the surface plasmon resonance to tune the absorption of nanorods, which demonstrates clearly the great potential of the dissipative sensing method to detect lossy nanoparticles. Future applications could also combine the dissipative and reactive sensing methods, which may provide better characterizations of nanoparticles.

Different from the conventional whispering-gallery-mode sensing using reactive interaction, the dissipative interaction is proposed and demonstrated to detect single nanoparticles which are lossy and with an ultra-small real part of the polarizability.

Yanyan Zhi

Papers

Detection of Single Nanoparticles Using the Dissipative Interaction in a High-Q Microcavity

Free-space optical micro-cavity Raman laser sensing device and sensing method thereof

Detection of Single Nanoparticles Using the Dissipative Interaction in a High-Q Microcavity

Singe Nanoparticle detection using the dissipative interaction with a high-Q microcavity