Coupling effects of the sum-frequency process and difference-frequency process on upconversion terahertz-wave detection via a DAST crystal.

Frequency upconversion technology with good performance including high sensitivity, fast response, and room-temperature operation is a promising method for terahertz-wave detection. The sum-frequency conversion and difference-frequency conversion jointly affect the detection ability for upconversion detection using organic crystals as nonlinear media. The concurrence of both processes has been ignored in past studies, which results in discrepancies between theoretical simulations and experimental results. In this paper, four-wave interaction equations involving two nonlinear conversion processes are proposed, and the effect of the sum-frequency process is analyzed in upconversion terahertz-wave detection via a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. The ratio of the sum-frequency signal to the difference-frequency signal varies for different terahertz frequencies and crystal thicknesses. Experiments suggest that theoretical simulations are good at predicting physical processes. Under certain conditions, the detection efficiency can be improved by simultaneously utilizing the two signals. The total signal photon number is not sensitive to the crystal thickness. Furthermore, the theoretical exploration of terahertz single-photon detection provides a noteworthy reference for future experiments.

Continuous tuning of unidirectional emission wavelength by bending a notched-elliptical polymer microdisk.

An approach of continuously tunable unidirectional emission through bending a notched-elliptical polymer microdisk is proposed. The characteristics of the bending-dependent action are carefully analyzed, and the resonance wavelength for unidirectional emission can be tuned continuously through bending the device. Such a whispering-gallery-mode microresonator enables unidirectional emission with ultra-low divergence, of which the emission efficiency and Q factor are stabilized, demonstrating the whole structure is robust and relatively insensitive within a certain bending angle range. A maximum resonance wavelength shift of ∼100 nm and Q factor of 1500 can be achieved with the total size of the microdisk less than 10 µm. This kind of microresonator is promising for applications in multilevel integrated photonics circuits and may open the door to new functionalities of resonator devices, from sensing to optical amplification.

Short-wave infrared real-time high dynamic range imaging and display based on correlated double sampling.

A real-time high dynamic range (HDR) imaging and display method based on correlated double sampling is proposed for short-wave infrared (SWIR) cameras in order to effectively improve its range of brightness and contrast, as well as to obtain more image details. The method utilizes the correlated double sampling technique of the SWIR detector to extend the 14-bit raw image into a 16-bit HDR image and achieve 4 times the HDR imaging. Subsequently, a dynamic range compression process, including logarithmic mapping and histogram equalization, is performed for the 16-bit HDR image to be mapped to an 8-bit display. Finally, the experimental results show that the method can enrich the details of SWIR images under the premise of ensuring real-time imaging.

All-solid-state widely wavelength-tunable and high-efficiency Yb:YSr3(PO4)3 laser.

We demonstrate an all-solid-state widely wavelength-tunable Yb:YSr3(PO4)3 (Yb:YSP) laser with high efficiency. The free-running Yb:YSP laser oscillating at multiple wavelengths in the range of 1024-1054 nm is realized with different crystal lengths and output coupler transmittances. The maximum output power of 2.72 W is obtained under the absorption pump power of 7.30 W. The highest slope efficiency is 66.9%, using the crystal of 6.5-mm-length. Simultaneous dual-wavelength operation can be realized as well. Furthermore, the widely wavelength-tunable Yb:YSP laser with a range of more than 60 nm (from 1004 to 1066 nm) is achieved using a birefringent filter. The experimental results indicate that the Yb:YSP crystal can be a promising candidate for ultrafast lasers in the 1 µm region.

Multi-wavelength microresonator based on notched-elliptical polymer microdisks with unidirectional emission.

A three-dimensional notched-elliptical microdisk with a wavelength-size notch on the boundary is proposed as a multi-wavelength and unidirectional emission lasing source. The device contains multiple properly designed two-dimensional whispering gallery mode-based polymer notched microdisks with different dimensions for use as a multi-wavelength source. It can have a relatively high optical quality factor of 4000, unidirectional emission with low far-field divergence ∼4°, and the efficiency of emission is as high as 84.2%. The effect of the notch size on the far-field divergence is analyzed, and the multi-wavelength lasing performance is characterized, demonstrating that the resonator is robust and reliable. This work paves a unique but generic way for the design of compact multi-wavelength microlasers.

2D-DOD and 2D-DOA Estimation for a Mixture of Circular and Strictly Noncircular Sources Based on L-Shaped MIMO Radar.

In this paper, a joint diagonalization based two dimensional (2D) direction of departure (DOD) and 2D direction of arrival (DOA) estimation method for a mixture of circular and strictly noncircular (NC) sources is proposed based on an L-shaped bistatic multiple input multiple output (MIMO) radar. By making full use of the L-shaped MIMO array structure to obtain an extended virtual array at the receive array, we first combine the received data vector and its conjugated counterpart to construct a new data vector, and then an estimating signal parameter via rotational invariance techniques (ESPRIT)-like method is adopted to estimate the DODs and DOAs by joint diagonalization of the NC-based direction matrices, which can automatically pair the four dimensional (4D) angle parameters and solve the angle ambiguity problem with common one-dimensional (1D) DODs and DOAs. In addition, the asymptotic performance of the proposed algorithm is analyzed and the closed-form stochastic Cramer-Rao bound (CRB) expression is derived. As demonstrated by simulation results, the proposed algorithm has outperformed the existing one, with a result close to the theoretical benchmark.

Real-Time Dynamic 3D Shape Reconstruction with SWIR InGaAs Camera.

In this paper, a real-time, dynamic three-dimensional (3D) shape reconstruction scheme based on the Fourier-transform profilometry (FTP) method is achieved with a short-wave infrared (SWIR) indium gallium arsenide (InGaAs) camera for monitoring applications in low illumination environments. A SWIR 3D shape reconstruction system is built for generating and acquiring the SWIR two-dimensional (2D) fringe pattern of the target. The depth information of the target is reconstructed by employing an improved FTP method, which has the advantages of high reconstruction accuracy and speed. The maximum error in depth for static 3D shape reconstruction is 1.15 mm for a plastic model with a maximum depth of 36 mm. Meanwhile, a real-time 3D shape reconstruction with a frame rate of 25 Hz can be realized by this system, which has great application prospects in real-time dynamic 3D shape reconstruction, such as low illumination monitoring. In addition, for real-time dynamic 3D shape reconstruction, without considering the edge areas, the maximum error in depth among all frames is 1.42 mm for a hemisphere with a depth of 35 mm, and the maximum error of the average of all frames in depth is 0.52 mm.

Miniature resonator sensor based on a hybrid plasmonic nanoring.

A miniature resonator sensor based on a hybrid plasmonic nanoring with a gold layer coated uniformly on the outer boundary is described and investigated. By using the Lumerical finite-difference-time-domain (FDTD) method, the optimized sizes of the plasmonic layer thickness and the central hole are given and insight into the dependence of spectral displacements, Q factors, sensitivity and detection limits on the ambient refractive index is presented. Simulation results reveal that the miniature resonator sensor featuring high sensitivity of 339.8 nm/RIU can be realized. The highest Q factor can reach ∼60,000 with this nanoring and the minimum detection limit is as low as 1.5 × 10-4 RIU. The effects on the resonance shifts and Q factors due to geometric shapes of the inner boundary of the nanoring are discussed as well. This miniature resonator sensor has good potential for highly sensitive ultracompact sensing applications.

Electron-initiated low noise 1064 nm InGaAsP/InAlAs avalanche photodetectors.

We report an electron-initiated 1064 nm InGaAsP avalanche photodetectors (APDs) with an InAlAs multiplier. By utilizing a tailored digital alloy superlattice grading structure, a charge layer and a p type InAlAs multiplier, an unity gain quantum efficiency of 48%, a low room temperature dark current of 470 pA at 90% breakdown voltage, and a low multiplication noise with an effective k ratio of ∼0.2 are achieved. The measured maximum gain factor is 5 at room temperature, which is currently limited by the non-optimized electric field profiles, and can be readily enhanced by modifying the doping and thickness parameters for the multiplier and the charge layer.

IGA-rule 17 for performance estimation of wavelength-extended InGaAs photodetectors: validity and limitations.

A rule of thumb, denoted as IGA-rule 17, has been developed to describe the temperature and cutoff wavelength-dependent dark currents of wavelength-extended InGaAs photodetectors in a 2-3 µm band. The validity and limitations of the rule are discussed. This rule is intended as an index for device developers to evaluate their technologies in processing, a simple tool for device users to estimate reachable performance at various conditions in their design, and an effective bridge between the two.

Impact of etching on the surface leakage generation in mesa-type InGaAs/InAlAs avalanche photodetectors.

Effects of mesa etching and geometry on InGaAs/InAlAs avalanche photodiodes (APDs) were investigated by using both wet and inductively coupled plasma (ICP) etching with different mesa shapes as well as etchants. It was found that the mesa geometry had no evident impact on APDs' characteristics regardless of the etching techniques applied. Besides, ICP-etched APDs showed faster punch-through, suppressed premature surface breakdown and lower dark current behaviors compared to the wet-etched devices. Substantially suppressed surface leakage was also observed for ICP-etched devices, showing 1 and 3 orders of magnitude better at room temperature and 77 K respectively, and over 1 order of magnitude higher surface resistivity up to 4×107 Ω cm, in comparison to the wet-etched APDs. Introduction of extra hydrogen and Ar plasma in ICP etching led to detrimental effects to APDs' performance by enhancing the tunneling or recombination at surfaces. Those experimental results were clearly interpreted based on the surface state theories.

Passively Q-switched Nd:YAG ceramic laser with V(3+):YAG saturable absorber at 1357 nm.

1357 nm single-wavelength continuous-wave and passively V(3+):YAGQ-switched Nd:YAG ceramic crystal lasers have been experimentally demonstrated for the first time to our knowledge. The output power of the continuous-wave laser was as high as 2.4 W under the incident pump power of 14.8 W. The corresponding optical to optical efficiency was about 16.2% and the slope efficiency was about 18.6%. The Q-switched laser remained single-wavelength output with a maximum average output power of 628 mW at the incident pump power of 12.4 W. The corresponding pulse width was 21 ns and the repetition frequency was 15 kHz.

Optical Projection Tomography Using a Commercial Microfluidic System.

Optical projection tomography (OPT) is the direct optical equivalent of X-ray computed tomography (CT). To obtain a larger depth of field, traditional OPT usually decreases the numerical aperture (NA) of the objective lens to decrease the resolution of the image. So, there is a trade-off between sample size and resolution. Commercial microfluidic systems can observe a sample in flow mode. In this paper, an OPT instrument is constructed to observe samples. The OPT instrument is combined with commercial microfluidic systems to obtain a three-dimensional and time (3D + T)/four-dimensional (4D) video of the sample. "Focal plane scanning" is also used to increase the images' depth of field. A series of two-dimensional (2D) images in different focal planes was observed and compared with images simulated using our program. Our work dynamically monitors 3D OPT images. Commercial microfluidic systems simulate blood flow, which has potential application in blood monitoring and intelligent drug delivery platforms. We design an OPT adaptor to perform OPT on a commercial wide-field inverted microscope (Olympusix81). Images in different focal planes are observed and analyzed. Using a commercial microfluidic system, a video is also acquired to record motion pictures of samples at different flow rates. To our knowledge, this is the first time an OPT setup has been combined with a microfluidic system.

Scale effects of low-dimensional relaxor ferroelectric single crystals and their application in novel pyroelectric infrared detectors.

Scaling effects of low-dimensional relaxor ferroelectric single crystals have induced large delocalization of domain switching, leading to a dramatic increase in pyro-electric performances by 2-5.5 times, and promoting the detectivity of fabricated pyroelectric detectors to an international leading level of 2.21 × 10(9) cmHz(1/2) /W at 10 Hz, 4 times higher than that of commercial LiTaO3 -based detectors.