Cheng Liu

Smartphone based hand-held quantitative phase microscope using the transport of intensity equation method

In order to realize high contrast imaging with portable devices for potential mobile healthcare, we demonstrate a hand-held smartphone based quantitative phase microscope using the transport of intensity equation method. With a cost-effective illumination source and compact microscope system, multi-focal images of samples can be captured by the smartphones camera via manual focusing. Phase retrieval is performed using a self-developed Android application, which calculates sample phases from multi-plane intensities via solving the Poisson equation. We test the portable microscope using a random phase plate with known phases, and to further demonstrate its performance, a red blood cell smear, a Pap smear and monocot root and broad bean epidermis sections are also successfully imaged. Considering its advantages as an accurate, high-contrast, cost-effective and field-portable device, the smartphone based hand-held quantitative phase microscope is a promising tool which can be adopted in the future in remote healthcare and medical diagnosis.

Tunable plasmonically induced transparency with unsymmetrical graphene-ring resonators

The multi-wavelength tunable plasmonically induced transparency (PIT) phenomena in two-ring and three-ring systems at infrared range were theoretically and numerically investigated. In the two-ring system, changing the bias voltage of graphene ring or the separation between graphene rings would induce an off-to-on PIT optical response. An asymmetry factor has been introduced to explain the corresponding transmission spectra. In the three-ring system, by bringing in a new asymmetry factor, multiple new PIT windows would arise at the left or right side of the original PIT windows. Numerical simulation by finite element method was conducted to verify our designs. Those proposed structures hence have potential in ultra-compact graphene optoelectronic devices at the infrared range.

Multi-mode Plasmonically Induced Transparency in Dual Coupled Graphene-Integrated Ring Resonators

We propose a highly wavelength-tunable multi-mode plasmonically induced transparency (PIT) device based on monolayer graphene and graphene rings for the mid-IR region. The proposed PIT systems explore the near-field coupling and phase coupling between two graphene resonators. The multi-mode transparency windows in the spectral response have been observed in the graphene-integrated configurations. By varying the Fermi energy of the graphene, the multi-mode PIT resonance can be actively controlled without reoptimizing the geometric parameters of the structures. Based on the coupled mode theory and Fabry-Perot model, we numerically investigated the two kinds of coupling in the graphene-based PIT systems. This work may pave the ways for the further development of a compact high-performance PIT device.

Plasmonic-induced transparency of unsymmetrical grooves shaped metal–insulator–metal waveguide

The plasmonic waveguides with unsymmetrical grooves shaped metal-insulator–metal (MIM) structures are proposed in theory. For symmetrical and unsymmetrical groove structures, the transmission varies with the increasing of the groove depths and groove lengths. The filtering characteristics due to the destructive interference of the plasmonic modes are found in those subwavelength structures. The transmission line theory is utilized to interpret the transmittance and filtering phenomena. The transmission formulas are also achieved by the transmission line theory. It is found that the slow light effects are emerged in the unsymmetrical groove structures. A small group velocity (c/80) can be achieved. Finite Element Method (FEM) is conducted to verify our design.

A dual-way directional surface-plasmon-polaritons launcher based on asymmetric slanted nanoslits

We theoretically design a device composed of two asymmetric slanted nanoslits to achieve the directionality of surface plasmon polaritons (SPPs). With proper inclination of the two slits, the desirable relative phase delay can be obtained. When the structure is illuminated by normal incident light, the SPPs can be controlled to deflect the specific direction due to light interference. The SPPs can be altered to the opposite direction when the illuminating light is changed inversely. We develop another way to tailor the relative phase delay by choosing the specific effective index for each slanted slit. In order to acquire higher directional excitation efficiency, our designs have been extended to periodic structures with the pairs of slanting slits. The finite element method is carried on to verify our designs. The simulations show that the best proportion of the SPP field intensity along two opposite directions reaches to around 30.

Polarization-Controlled Tunable Multi-focal Plasmonic Lens

A polarization-controlled tunable plasmonic lens which can generate different multi-focal combinations with exciting sources of left and right circular polarizations is proposed in this paper. Both position and intensity of each focal point can be adjusted by modulating the structure of the plasmonic lens. It is believed that the polarization-controlled tunable plasmonic multi-focal lens can be potentially used for optical switches and multi-channel couplers in future logic photonic and plasmonic systems.

Ultra-high speed digital micro-mirror device based ptychographic iterative engine method

To reduce the long data acquisition time of the common mechanical scanning based Ptychographic Iterative Engine (PIE) technique, the digital micro-mirror device (DMD) is used to form the fast scanning illumination on the sample. Since the transverse mechanical scanning in the common PIE is replaced by the on/off switching of the micro-mirrors, the data acquisition time can be reduced from more than 15 minutes to less than 20 seconds for recording 12 × 10 diffraction patterns to cover the same field of 147.08 mm2. Furthermore, since the precision of DMD fabricated with the optical lithography is always higher than 10 nm (1 μm for the mechanical translation stage), the time consuming position-error-correction procedure is not required in the iterative reconstruction. These two improvements fundamentally speed up both the data acquisition and the reconstruction procedures in PIE, and relax its requirements on the stability of the imaging system, therefore remarkably improve its applicability for many practices. It is demonstrated experimentally with both USAF resolution target and biological sample that, the spatial resolution of 5.52 μm and the field of view of 147.08 mm2 can be reached with the DMD based PIE method. In a word, by using the DMD to replace the translation stage, we can effectively overcome the main shortcomings of common PIE related to the mechanical scanning, while keeping its advantages on both the high resolution and large field of view.

Active-polarization-controlled long-depth focus generated by orthogonal nanoslit array

In order to realize long-range directional excitation and coupling, active-polarization-controlled Bessel beams with an orthogonal nanoslit array are proposed. Excited with left or right circular polarization light, long-depth focus from Bessel beams can be generated with different propagation directions. Moreover, multiple long-depth foci are also designed according to dual-conical phase settings. Proved with numerical simulations, it is considered that the active-polarization-controlled system can be potentially used in future logic photonic and plasmonic systems for optical switching and multichannel coupling.

Cellular phase observations and measurements on red blood cells affected by lithium and lead ions with quantitative interferometric microscopy

As an important marker in disease diagnosis, red blood cell morphology measurement is necessary in biological and medical fields. However, traditional setups as microscopes and cytometers cannot provide enough quantitative information in morphology detections. In order to capture tiny variations of red blood cells affected by metal ions in external environment, quantitative interferometric microscopy is applied: combining with phase retrieval and cell recognition, cellular phases as well as additional quantitative cellular parameters can be acquired automatically and accurately. The research proves that quantitative interferometric microscopy can be potentially applied in cellular observations and measurements for both biological and medical applications.

Portable smartphone based quantitative phase microscope

To realize portable device with high contrast imaging capability, we designed a quantitative phase microscope using transport of intensity equation method based on a smartphone. The whole system employs an objective and an eyepiece as imaging system and a cost-effective LED as illumination source. A 3-D printed cradle is used to align these components. Images of different focal planes are captured by manual focusing, followed by calculation of sample phase via a self-developed Android application. To validate its accuracy, we first tested the device by measuring a random phase plate with known phases, and then red blood cell smear, Pap smear, broad bean epidermis sections and monocot root were also measured to show its performance. Owing to its advantages as accuracy, high-contrast, cost-effective and portability, the portable smartphone based quantitative phase microscope is a promising tool which can be future adopted in remote healthcare and medical diagnosis.