Jianhong Ge

Breaking the Diffraction Barrier Using Fluorescence Emission Difference Microscopy

We propose a novel physical mechanism for breaking the diffraction barrier in the far field. Termed fluorescence emission difference microscopy (FED), our approach is based on the intensity difference between two differently acquired images. When fluorescence saturation is applied, the resolving ability of FED can be further enhanced. A detailed theoretical analysis and a series of simulation tests are performed. The validity of FED in practical use is demonstrated by experiments on fluorescent nanoparticles and biological cells in which a spatial resolution of <λ/4 is achieved. Featuring the potential to realize a high imaging speed, this approach may be widely applied in nanoscale investigations.

Enhancing the performance of fluorescence emission difference microscopy using beam modulation

Fluorescence emission difference microscopy (FED) has been proved to be an effective method to break the diffraction barrier which had restricted the spatial resolution of optical microscopy in the far-field for a long time. In this paper, we theoretically demonstrate that the attainable resolution of FED can be further enhanced by using beam modulation. By applying beam shape modulating, polarization state modulating, phase encoding and beam blocking to the illumination beams in the process of FED imaging, we optimize the size of the corresponding PSFs and thus improve the resolving ability of FED. The simulation tests demonstrate that in the optimal case after beam modulation, the resolution of FED can be enhanced by a factor of 26% compared with a conventional one.

Beam quality improvement by gain guiding effect in end-pumped Nd:YVO 4 laser amplifiers

In end-pumped Nd:YVO₄ amplifiers, beam quality improvement is obtained both in theory and in experiments. A theoretical model of gain-guided laser amplifier is developed by comprehensively considering thermal effect, gain guiding and gain saturation effect. Several key parameters of the amplifier are discussed such as the input beam quality, the beam filling factor between input beam and pump beam, the ratio between input power and pump power, and the length of laser crystal. The theoretical results are confirmed by the experiments.

Broad-area laser diode with 0.02nm bandwidth and diffraction limited output due to double external cavity feedback

External cavity feedback for a broad-area laser diode (BAL) with antireflection coating on the front facet is investigated experimentally. The feedback is created by a high-reflection mirror and a blazed grating, which reflect two parts of the laser emission back to the BAL, thereby forming an external resonator. When a Fabry–Perot etalon was inserted into the resonator an output beam with a bandwidth of 0.02nm and power of 150mW was obtained. The beam diffraction limited factor amounted to M2=1.16.

Beam Profile Evolution and Beam Quality Changes Inside a Diode-End-Pumped Laser Oscillator

Experimental observations of beam profile evolution and beam quality changes in an asymmetrical diode-end-pumped laser oscillator are presented. The beam intensity distribution and beam size show great differences between the two intra-cavity counter-propagating beams. Measurements also show that the beam quality changes when the beam passes through the gain medium. These phenomena are attributed to the thermally induced aberrations and soft aperture induced diffractions. The results present a novel clue of designing a high power laser oscillator with fundamental mode output.

Optical super-resolution by subtraction of time-gated images

Applying image subtraction strategy together with time-gated detection, we can reveal finer details in a fixed cell sample using moderate light intensities. The subtractive imaging is obtained from the subtraction of a suitably weighted short lifetime intensity-image from a long lifetime one, while the weight coefficient can be selected using the analysis of an optical transfer function. Compared with other subtractive methods, ours is regarded as an optimized variation of gated stimulated emission depletion microscopy and can be implemented in a simpler manner, providing lateral super-resolution, and it is theoretically possible to further expand its application scope.

Influences of spherical aberration on resonator’s stable zones and fundamental mode output power scaling of solid state laser oscillators

A parameter x is introduced to characterize the strength of thermal lens spherical aberration, whose influences on resonators stable zones are analyzed theoretically. Some new and helpful results are obtained. For symmetrical plane-plane cavity, spherical aberration has just influence on the back edge of stable zone. For asymmetrical plane-plane cavity, spherical aberration has influence on the back edges of the two stable zones and the front edge of the second stable zone. Effects of transverse mode collapsing to TEM00 mode and stable zones separation of different orders transverse modes are pointed out, which is the foundation of TEM00 mode output power scaling for solid state laser oscillator. Influences of parameters such as resonators long arm length, short arm length, and pump beam radius on the extent to which of stable zones separation of different order transverse modes are discussed. An experimental setup of a high power diodes dual-end pumped Nd:YVO4 TEM00 mode laser oscillator is built up and investigated experimentally. 51.2 W TEM00 mode output power in CW operation is achieved with an optical-to-optical efficiency of about 50% and beam quality factor M2 being 1.2.

Axial nano-displacement measurement with high resolution and wide range based on asymmetrical illumination.

We propose a novel axial nano-displacement measuring approach. Based on asymmetrical illumination, the axial drifts of the sample plane can be measured by detecting the position of the centroid of the focal spot. Both CCD and QD are used as the detector in the system and two data processing models are designed. With a relatively simple and applicable configuration, the proposed system can realize a wide measuring range of >4λand a high axial resolution of 2nm. Moreover, the presented approach is immune to the influence caused by the energy fluctuation of the laser source. Possessing these advantages, this measuring method has big potential to be applied in modern engineering and scientific researches.

Sub-diffraction imaging with total internal reflection fluoresence (TIRF) microscopy by stochastic photobleaching

Total internal reflection fluorescence (TIRF) microscopy is widely used in fluorescent imaging. Evanescent wave fields generated by the internal reflection are used to illuminate the sample, and only fluorophores within a thickness of 100 nm thick from the surface are activated, improving the signal-to-noise ratio (SNR) of the image. Sub-diffraction imaging with TIRF microscopy by stochastic photobleaching is studied. Each fluorophore can be localized from the recorded streaming followed by image subtraction. This method can yield contrast-enhanced images with a higher SNR and improve the lateral resolution to approximately 120 nm.

Beam quality improvement of broad-area laser diode with external cavity feedback

A broad area laser diode (BAL) external cavity is experimentally investigated and analyzed using ray transfer matrices. In the experiment, a grating that is placed at the image plane of the output facet of the BAL is used as an external cavity mirror for the slow axis and as a wavelength selective component for the fast axis at the same time. By tilting the grating with respect to the slow and the fast axis direction, respectively, the number of transverse modes oscillating in the cavity can be limited and the spectral line width of the laser diode can be reduced. With this setup, a laser beam with an improved beam quality, an output power of 230 mW and a spectral line width of 0.6 nm (300 GHz) is obtained when operating the BAL at 2.7 times the threshold drive current. We have also investigated a different cavity configuration to achieve alignment insensitivity and stable operation in which the grating was replaced by a flat HR mirror. With a similar beam quality, at the same operation current of the BAL a laser beam with a power of 320 mW and a spectral line width of 1.5 nm (750 GHz) is obtained with this setup.