Radiometric calibration of a multiphoton microscope capable of measuring absolute photon flux of single photon sources.

Precise photon flux measurement of single photon sources (SPSs) is essential to the successful application of SPSs. In this work, a novel method, to our knowledge, was proposed for direct measurement of the absolute photon flux of single photon sources with a femtosecond laser multiphoton microscope. A secondary 2-mm-diameter aperture was installed under the microscope objective to define the numerical aperture (NA) of the microscope. The defined NA was precisely measured to be 0.447. An LED-based miniaturized integrating sphere light source (LED-ISLS) was used as a standard radiance source to calibrate the photon flux responsivity of the multiphoton microscope, with the defined NA. The combined standard uncertainty of the measured photon flux responsivity was 1.97%. Absolute photon flux from a quantum-dot based emitter was measured by the multiphoton microscope. The uncertainty of the photon flux was evaluated to be 2.1%. This work offers a new, to our knowledge, radiometric method for fast calibration of photon flux responsivity of microscopes, and absolute photon flux calibration of single photon sources.

Transmission and reflection cloaking by using a zero-refractive-index photonic crystal in the microwave region.

We propose a rectangular column two-dimensional square lattice photonic crystal to realize zero refractive index. Through analysis of the energy band structure of the photonic crystal structure, the lattice constant and side length of the rectangular columns can be optimized, and the Dirac cone dispersion appears at the center of the Brillouin zone. The Dirac cone is formed by the interaction of a monopolar eigenstate and a dipolar eigenstate to form a triple accidental degenerate state. The effective medium theory is used to invert the effective electromagnetic parameters of the photonic crystal with a double zero refractive index. The zero-phase change and the focusing characteristic of the concave lens of this kind of zero-refractive-index material are verified. Importantly, we have achieved transmission and reflection cloaking with this zero-index medium. Through the analysis of the amplitude and phase distribution characteristics of the electromagnetic field, it is proved that the designed cloaking devices have obvious cloaking effect.

Design of two invisibility cloaks using transmissive and reflective metamaterial-based multilayer frame microstructures.

Ultrathin metamaterials provide new possibilities for the realization of cloaking devices because of their ability to control electromagnetic waves. However, applications of metamaterials in cloaking devices have been limited primarily to reflection-type carpet cloaks. Hence, a transmissive free-space cloak was developed using a multilayer frame structure, wherein highly transparent metamaterials were used to guide incident waves into propagating around an object. The cloaking effect was quantitatively verified using near-field and far-field distributions. Metamaterials allow for the cloaking shells of transmissive cloaks to be developed without spatially varying extreme parameters. Moreover, a transmissive invisible cloak with metamaterial-based mirrors was designed. The design principle of this cloak with a frame structure consists of four metamaterial-based mirrors and two metal mirrors. After covered with the designed metamaterials-based mirrors cloak, the outgoing electromagnetic wave is restored greatly as if the wave passes directly through the obstacle without distortion. This cloak used the metamaterials mirrors to adjust the reflected angle, so that the outgoing electromagnetic wave does not change direction, thereby achieving the cloaking effect.

Manipulation of the arbitrary scattering angle based on all-dielectric transmissive Pancharatnam Berry phase coding metasurfaces in the visible range.

In order to improve the transmitted efficiency of the metasurface in the visible range, an all-dielectric Pancharatnam-Berry phase unit structure was proposed. Using these Pancharatnam-Berry phase element particles with different rotation angles, all-dielectric encoding metasurfaces can be constructed. The encoding metasurface connects the physical coding particles with digital coding in digital signal processing. The manipulation of the continuous transmission angle requires the continuous change of the encoding metasurface period. Since the size of encoding particles on the coded metasurfaces cannot be designed to be infinitesimally small, it is impossible to obtain the continuously changing period of the coded metasurfaces. To manipulate effectively and freely the angle of scattering in the visible range, Fourier convolution principle in digital signal processing was introduced on all-dielectric encoding metasurfaces with Pancharatnam-Berry phase meta-atoms. The addition and subtraction operations on two initial encoding sequences can be implemented to obtain a new encoding sequence. The manipulation of the arbitrary scattering pattern after Fourier convolution operations on different encoding sequences can be realized, especially for larger abnormal deflection angles. The checkerboard encoding metasurface was also designed to further prove the applicability of the Fourier convolution principle. Moreover, by using the proposed all-dielectric highly efficient Pancharatnam-Berry phase encoding meta-atoms, these coded particles with different rotation angles can be precisely arranged to build the generators of the orbital angular momentum beam with different topological charges.

Miniaturized integrating sphere light sources based on LEDs for radiance responsivity calibration of optical imaging microscopes.

LED-based integrating sphere light sources (LED-ISLSs) in the size of typical microscope slides were developed to calibrate the radiance responsivity of optical imaging microscopes. Each LED-ISLS consists of a miniaturized integrating sphere with a diameter of 4 mm, an LED chip integrated on a printed circuit board, and a thin circular aperture with a diameter of 1 mm as the exit port. The non-uniformity of the radiant exitance of the LED-ISLSs was evaluated to be 0.8%. The normal radiance of the LED-ISLSs in the range of (5∼69) W m-2 sr-1 was measured with a standard uncertainty of 1.3% using two precision apertures and a standard silicon photodetector whose spectral responsivity is traceable to an absolute cryogenic radiometer. The LED-ISLSs were applied to calibrate the radiance responsivity of a home-built optical imaging microscope with a standard uncertainty of 2.6∼2.9%. The LED-ISLSs offer a practical way to calibrate the radiance responsivity of various optical imaging microscopes for results comparison and information exchange.

Measurement of the distance between two parallel precision apertures using time-of-flight method for cryogenic radiometry.

Time-of-flight method was adopted to measure the distance between two parallel precision apertures utilized in a vacuum chamber for cryogenic radiometry. The diameters of the apertures are 9 mm and 8 mm, respectively. A 1550-nm femtosecond pulse laser, a 70-GHz photodetector, and a 30-GHz oscilloscope were used to measure the round-trip flight time difference between the flat front surfaces of the two precision apertures. The distance between the apertures was analyzed to be 0.36423 m with a relative standard uncertainty of 0.004%. The non-contact distance measurement method is useful for applications such as low background infrared radiance measurement system based on an absolute cryogenic radiometer.

Enhancement of bandwidth and angle response of metasurface cloaking through adding antireflective moth-eye-like microstructure.

Ultrathin metasurface provides a completely new path to realize cloaking devices on account of their fascinating ability to control electromagnetic wave. However, the conventional cloaking devices are limited by their narrow bandwidth. To overcome this challenge, we present the realization of ultrabroadband and wide angle metasurface cloaking through high refractive index dielectric layer and antireflective "moth-eye-like" microstructure in this work. Two options are proposed and demonstrated numerically in terahertz region. By using local phase compensation, the proposed carpet cloaks can suppress significantly the unexpected scattering and reconstruct wavefront. The cloaking effects of the proposed design are verified from 0.65THz to 0.9THz with a wide range of angles. Moreover, the proposed metasurface cloaking is probable to extend to the optical and microwave domains and can be applied in stealth, illusion optic, radar and antenna systems.

Temporal response of laser power standards with natural convective cooling.

Laser power detectors with natural convective cooling are convenient to use and hence widely applicable in a power range below 150 W. However, the temporal response characteristics of the laser power detectors need to be studied in detail for accurate measurement. The temporal response based on the absolute laser power standards with natural convective cooling is studied through theoretical analysis, numerical simulations, and experimental verifications. Our results show that the response deviates from a single exponential function and that an ultimate response balance is difficult to achieve because the temperature rise of the heat sink leads to continuous increase of the response. To determine the measurement values, an equal time reading method is proposed and validated by the laser power calibrations.

Hidden image recovery using a biased photorefractive crystal in the Fourier plane of an optical imaging system.

Self-diffraction can be induced using a biased photorefractive crystal in the Fourier plane of an imaging system where the light beam intensity is naturally high due to the concentration effect of an optical lens. The spatial frequency spectrum of the output image is proportional to the optical power density distribution in the Fourier plane. A photorefractive crystal with small size can be used and hence an reduced amount of biased voltage is needed to obtain significant diffraction effect in the image plane. When the input image is an overlay of a signal and a noise pattern, theoretic model reveals that the induced diffraction in the Fourier plane may be preferably applied on the noise pattern. In order to illustrate the effect experimentally, a signal from a weakly illuminated object is coupled with an overwhelming noise pattern and then the hidden signal is successfully recovered using a SBN61 crystal with an applied voltage of 800 V in the Fourier plane. Such technology can be employed in encrypted spatial communication systems for security purposes.

Improvement of measurement uncertainties in laser power detector calibration by convolving with the detector's impulse response function.

For the calibration of thermal type laser power detectors with slow response time, instability of the input laser significantly contributes to the measurement repeatability. A convolution method is adopted to reduce the impact of source instability. The equivalent incident power is calculated by convolving the real-time power input and the detector impulse-response function (IRF). The value is applied in place of the traditional input power value for the calibration. The IRF is measured using the (1-70) W laser power primary standard at National Institute of Metrology of China. The measurement repeatability of the transfer detector's responsivity is improved from 1.1% using the traditional method to 0.19% using this method. The systematic errors, primarily due to source drift are also reduced. The proposed method can be applied in the calibration of general thermal type laser power detectors.