Sensitive and selective detection of prostate-specific antigen using a photonic crystal nanolaser

Photonic and Iontronic Sensing in GaInAsP Semiconductor Photonic Crystal Nanolasers

The GaInAsP semiconductor photonic crystal nanolaser operates at room temperature by photopumping and emits near-infrared light at a wavelength longer than 1.3 μm. Immersion of the nanolaser in a solution causes its laser characteristics to change. Observation of this phenomenon makes it possible to perform biosensing without a fluorescent label or a chromogenic substrate. The most common phenomenon between many photonic sensors is that the resonance wavelength reflects the refractive index of attached media; an index change of 2.5 × 10−4 in the surrounding liquid can be measured through an emission wavelength shift without stabilization. This effect is applicable to detecting environmental toxins and cell behaviors. The laser emission intensity also reflects the electric charge of surface ions. The intensity varies when an electrolyte or a negatively charged deoxyribonucleic acid (DNA), which is positively or negatively charged in water, is accumulated on the surface. This effect allows us to detect the antigen-antibody reaction of a biomarker protein from only the emission intensity without any kind of spectroscopy. In detecting a small amount of DNA or protein, a wavelength shift also appears from its concentration that is 2–3 orders of magnitude lower than those of the conventional chemical methods, such as the enzyme-linked immuno-solvent assay. It is unlikely that this wavelength behavior at such low concentrations is due to the refractive index of the biomolecules. It is observed that the electric charge of surface ions is induced by various means, including plasma exposure and an electrochemical circuit shifting the wavelength. This suggests that the superhigh sensitivity is also due to the effect of charged ions. Thus, we call this device an iontronic photonic sensor. This paper focuses on such a novel sensing scheme of nanolaser sensor, as an example of resonator-based photonic sensors, in addition to the conventional refractive index sensing.

Label-Free Optical Single-Molecule Micro- and Nanosensors

Label-free optical sensor systems have emerged that exhibit extraordinary sensitivity for detecting physical, chemical, and biological entities at the micro/nanoscale. Particularly exciting is the detection and analysis of molecules, on miniature optical devices that have many possible applications in health, environment, and security. These micro- and nanosensors have now reached a sensitivity level that allows for the detection and analysis of even single molecules. Their small size enables an exceedingly high sensitivity, and the application of quantum optical measurement techniques can allow the classical limits of detection to be approached or surpassed. The new class of label-free micro- and nanosensors allows dynamic processes at the single-molecule level to be observed directly with light. By virtue of their small interaction length, these micro- and nanosensors probe light-matter interactions over a dynamic range often inaccessible by other optical techniques. For researchers entering this rapidly advancing field of single-molecule micro- and nanosensors, there is an urgent need for a timely review that covers the most recent developments and that identifies the most exciting opportunities. The focus here is to provide a summary of the recent techniques that have either demonstrated label-free single-molecule detection or claim single-molecule sensitivity.

Design and Fabrication of a Visible-Light-Compatible, Polymer-Based Photonic Crystal Resonator and Waveguide for Sensing Applications

In this paper, we have proposed a polymer-based photonic crystal (PhC) resonator, with multiple sizes of cavities, and a waveguide to be used as highly sensitive optical sensor components. Properties of the proposed PhC were simulated by the finite-difference time-domain method, and the polymer-based PhC resonator and waveguide were fabricated on a photoresist (polymer) by electron beam lithography, which was prepared on an Au-layer-deposited Si substrate. We detected the resonant light that penetrated through the waveguide and was trapped in the PhC resonator. Optical characteristics of the fabricated PhC were evaluated by detecting the polymer layer deposition process by using the layer-by-layer (LbL) method to deposit polymer layers. As a result, by using an optimized design of a polymer-based PhC resonator with a long cavity (equivalent to a defect of three holes), the PhC structure changes caused by LbL deposition lead to changes in resonant light wavelength (peak shift: 5.26 nm/layer). Therefore, we suggest that a PhC resonator and a waveguide is applicable as an optical sensor.

Ion-sensitive photonic-crystal nanolaser sensors

In general, biochemical sensors based on photonic cavities are used to detect changes in the refractive index of the environment. In this study, however, a GaInAsP semiconductor photonic-crystal nanolaser sensor that we recently developed was found to detect not only the environmental refractive index but also the surface charge. In contrast to the pH sensitivity we reported previously, this is an ultra-sensitive detection mechanism capable of identifying proteins and deoxyribonucleic acids (DNA) at a femtomolar-order or lower concentrations. When the device is exposed to plasma or DNA solutions, the laser wavelength simultaneously changes with the zeta potential and the flat-band potential of the semiconductor surface. This indicates that the charged functional groups on the surface, which are formed by these treatments, modify the Schottky barrier near the semiconductor surface, trap the excited carriers in the barrier, and change the refractive index of the semiconductor via the carrier effects. These findings also suggest that some other photonic sensors may also exhibit similar electrochemical and optoelectronic effects.

Thresholdless quantum dot nanolaser

Thresholdless lasing is an outstanding challenge in laser science and is achievable only in devices having near unity quantum efficiency even when not lasing. Such lasers are expected to exhibit featureless linear light output curves. However, such thresholdless behavior hinders identification of the laser transition, triggering a long-lasting argument on how to identify the lasing. Here, we demonstrate thresholdless lasing in a semiconductor quantum dot nanolaser with a photonic crystal nanocavity. We employ cavity resonant excitation for enabling the thresholdless operation via focused carrier injection into high cavity field regions. Under conventional (above bandgap) excitation, the same nanolaser exhibits a typical thresholded lasing transition, thereby facilitating a systematic comparison between the thresholdless and thresholded laser transitions in the single device. Our approach enables a clear verification of the thresholdless lasing and reveals core elements for its realization using quantum dots, paving the way to the development of ultimately energy-efficient nanolasers.