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Terahertz technology has been developed rapidly in the past 30 years. Numerous applications in medicine, biology, agriculture, materials, security, communication and astronomy have been demonstrated. Terahertz sources can be divided into narrowband (monochromatic) source and broadband source according to their spectral characteristics. From a spectral perspective, coherent broadband and narrowband terahertz sources are mutually complementary, each having its own characteristics and scope of applications. Broadband terahertz sources can be used for quick access to the hybrid spectra of rotational and vibrational molecular fingerprints or imaging in a wider spectral range. Narrowband terahertz source with good spectral resolution and sensitivity, is suitable for pump-probe, fine structure resolution of molecular fingerprints and terahertz remote detection and imaging. Therefore, developing the tunable high peak power and narrowband terahertz sources is very important for the applications in the detection and identification of molecular fingerprints. The difference frequency generation is one of the most important techniques for obtaining widely tunable, high power and narrowband terahertz sources. In this review, the recent progress of tunable terahertz sources based on the difference frequency generation in the last five years is reviewed, including the two fields of optical laser-based difference frequency sources and quantum cascade laser-based difference frequency sources. For the former class, the experimental results from reports with different difference frequency sources and several typical nonlinear crystals are classified, and the corresponding experimental techniques and results are introduced. For terahertz wave generation, different optical difference frequency sources by a dual-wavelength laser, double laser, a laser and an optical parametric oscillator (OPO), the signal and idler waves of an OPO, and double OPOs are demonstrated in increasing their tunabilities. Significant progress has been made in the nonlinear crystals used to generate terahertz wave by the difference frequency process, for example, by improving the property of inorganic crystals with ion doping, taking advantage of waveguide and PPLN structures, and especially developing novel nonlinear organic crystals. For the quantum cascade laser-based difference frequency sources, the latest advances in the techniques of difference frequency generation and wavelength tunability are presented. GaAs-based terahertz quantum cascade lasers are powerful semiconductor THz sources but cryogenic cooling is still a necessity. Recently, difference frequency generation was combined with the mid-infrared quantum cascade laser technology, thus becoming a leading room temperature semiconductor source in the terahertz range. To improve the frequency tuning range in the difference frequency terahertz quantum cascade laser, wavelength tuning techniques of the inner cavity and the external cavity have been developed. The difference frequency generation quantum cascade terahertz laser source has been the only technique workable at room temperature for the quantum cascade laser so far, which opens the door for developing the compact and widely tunable room temperature terahertz sources.

Ultrashort pulse laser with a repetition rate of below 10 MHz is suitable for a variety of micromachining applications to avoid plasma shielding effects. Besides, the parabolic pulse possesses clean wings, short pulse duration, and large peak power because only the linear chirp is accumulated during the propagation. Based on these two points, a similariton oscillator with a repetition rate of below 10 MHz is a most perfect seed source of an amplification system for micromachining. In this paper, an amplifier similariton oscillator with dispersion map based on a piece of 10 m Yb-doped large-mode-area single-polarization photonic crystal fiber is demonstrated. The semiconductor saturable absorber mirror is employed in the linear cavity as an end mirror to initiate and maintain the mode-locking operation. An adjustable slit is adopted between the end mirror and grating pair in another arm, as a central wavelength adjuster and the spectral filter to ensure the laser operational wavelength in accordance with the working wavelength of semiconductor saturable absorber mirror and the stability of mode-locking operation. The stable single-pulse mode-locking operation can be achieved by adjusting the intracavity dispersion and the operating wavelength. With the net cavity dispersion of-0.89 ps2, a spectrum with steep and smooth edges is obtained, which means that the laser does not operate in the soliton regime but in the dispersion-mapped amplifier similariton regime. A highest output power of 820 mW is obtained with a pulse duration of 6.2 ps and spectral width of 3.84 nm under a pump power of 12.8 W. The repetition rate is 8.6 MHz, corresponding to a pulse energy of 95 nJ. It is the first time that the similariton with a repetition rate of lower than 10 MHz and a highest pulse energy of 95 nJ from a similariton laser has been achieved, to the best of our knowledge. Numerical simulation results confirm that the self-similar evolution is achieved in the gain fiber, and the parabolic-and gauss-shaped pulse can be emitted at the zero-order reflection of the grating and after the slit, respectively.

A femtosecond laser single-stage nonlinear amplification system composed of Yb-doped large-mode-area single-polarization photonic crystal fibers is demonstrated. Effects of net cavity dispersion and pump power on oscillator output parameters and the evolution dynamics of the amplified pulse after compression are discussed for different seed pulse parameters. Under the experimental conditions in this paper, the longer and less chirped pulses are obtained with a larger negative net intracavity dispersion in the oscillator. When a nearly-transform-limited pulse is chosen as seed pulse nder the condition of oscillator pump power of 4.53 W, the shortest nearly-pedestal-free amplified pulse is achieved under the amplifier pump power of 60 W after the dispersion is compensated by a grating pair, in which the pulse duration is 45.7 fs with an average power of 28 W at a refletition frequency of 42 MHz. When the oscillator pump power is increased to 5.08 W and most nearly-transform-limited pulses under the pump condition are selected as the seed pulses, the maximum average power of 34.5 W with a duration of 53.5 fs is obtained at an amplifier pump power of 70 W.

Mass segmentation plays an important role in computer-aided diagnosis (CAD) system. The segmentation result seriously affects classifying mass as benign and malignant. By combining the simplified pulse coupled neural network (SPCNN) and the improved vector active contour without edge (vector-CV), a novel method of mass segmentation in mammogram is proposed in this paper. First, the parameters and termination conditions of SPCNN are obtained through mathematical analysis and the initial contour is segmented by SPCNN. Then, the vector CV model is accordingly modified to overcome the shortcomings of traditional CV model. Finally, combined with the initial contour, the improved vector-CV is used to segment the mass contour. The experiments implemented on the public digital database for screening mammography (DDSM) and the clinical images which are provided by the Center of Breast Disease of Peking University People’s Hospital indicate that the proposed method is better than the existing methods, especially when dealing with the dense breasts of Oriental female.

Mass detection in mammograms usually has high false positive (FP) rate. Content based mass retrieval can effectively reduce the FP rate by comparing the image which is to be determined with mass images which have already been diagnosed. In this paper, a method combining discriminating anchor graph hashing (DAGH) and linear neighborhood propagation (LNP) is proposed for mammogram mass retrieval. Original AGH image representation does not consider pathological relevance in defining image similarity. To solve this problem, DAGH is put forward as a new image representation, which introduces the pathological class into image similarity. Furthermore, LNP is employed as a relevance feedback technique. Finally, interactive retrieval for mammogram masses is implemented based on the learning strategy between the underlying features and high-level semantic for images. Mammograms provided by the Breast Center of Peking University People's Hospital (BCPKUPH) are used to test the proposed method. Experimental results show that the DAGH image representation introducing pathological class is superior to original AGH in analyzing the similarity of mass images. Compared with existing methods, the proposed method shows obvious improvement in mass retrieval performance.

Using the method of adaptive kernel learning based relevance vector machine (ARVM) and combining the morphological filtering and the clustering criterion recommended by Kallergi, a new algorithm for microcalcification (MC) clusters processing in mammograms is investigated. Firstly, the detection of MC is formulated as a supervised-learning problem. Then the ARVM is used as a classifier to determine whether an MC object is present at each location in the mammogram and a morphological processing is used to remove the isolated spurious pixels. Finally, the identified MC clusters are obtained by Kallergi criterion. To improve the computational speed, a fast processing method based on ARVM is developed, in which the whole image is decomposed first into sub-image blocks for parallel operation. Experimental results indicate that the ARVM method outperforms the RVM method and, in particular, the fast processing method could greatly reduce the testing time.

The properties of light guiding mechanism and modes relating to the core refractive index in a hybrid photonic crystal fiber (PCF) are numerically investigated by the full-vectorial finite element method. The light guiding mechanism of core mode is the double-bandgap-guiding when the core refractive index is smaller than the effective index of the air-hole cladding in the hybrid-PCF. If the core refractive index is between the refractive indexes of the two kinds of hybrid-claddings, there is a co-operative action of the index-guiding and the bandgap-guiding on the guide mode in the core. Only the total internal reflection guiding occurs in the fiber when the core refractive index is greater than the effective index of the high-index rod cladding. Comparison of these guiding properties in the hybrid-PCF are also carried out. The results provide guidance for the design of hybrid photonic crystal fiber for special purposes.

A modified Z-scan experimental setup based on a Yb-doped photonic crystal fiber amplifier was used to achieve polarization-resolved measurement of the nonlinear properties of bulk intrinsic GaP. The experiment results reveal that three-photon absorption dominated the nonlinear absorption processe at room temperature. The anisotropy and crystal orientation dependence of the nonlinear absorption and nonlinear refractive index was experimentally studied and the nonlinearities showed saturation at high pump intensities.

The mode-controlled nonlinear process in a highly nonlinear photonic crystal fiber is studied experimentally. The central wavelength of frequency-upconverted dispersive wave and its transverse intensity profile are controlled through a selective coupling of the pump field to the higher order modes of the fiber and a polarization-sensitive phase matching process. However, due to their large effective index difference, the two orthogonal modes cant be excited simultaneously. The index difference is lowered by injecting wax into the air holes which effectively modifies the core structure. This modification degenerates the two orthogonal high-order modes. As a result, they are excited simultaneously and form a hollow beam output.

The eigenvalue equation associated with seven-core photonic crystal fiber was derived from coupled-mode equation. Mode properties were analyzed by its eigenvalues and eigenvector. Using the frequency domain finite difference method, the relationship between fiber structure and coupling coefficient was fully discussed through changing the wavelength, hole pitch and core distance. Mode shaping and higher-order supermode cut off was realized. The results were helpful to the fiber structure design and in-phase supermode selection.

Nitrogen doping of fluorinated diamond-like carbon (FN-DLC) films were deposited on p-type (100) silicon wafers at different deposition condition. The contact angle of water on the FN-DLC films surfaces was measured by a sessile-drop method. The surface morphology of films was characterized by atomic force microscopy (AFM). The Raman spectrum, infrared spectrum and X-ray photoelectron spectrum (XPS) demonstrate that the hydrophobic nature of FN-DLC films lies on chemistry structure, polarization intensity and roughness of films surface. As the ratio of sp3/sp2 decreases, the quantity of CF2 and roughness of film surface increases, thus contact angle increases. Otherwise, contact angle decreases. The contact angle is correlated with the deposition condition, which low temperature, low radio frequency power and high flux ratio r(r=CF4/［CF4+CH4］) brings the contact angle to ascend.

Copper nitride (Cu 3 N) thin films were deposited on glass substrates by reactive radio frequency magnetron sputtering under different radio frequency power ( P ) and nitrogen partial pressure r ( r =N 2 /［N 2 +Ar］). The thickness, crystalline structure and surface morphology of films were characterized by profilometer, X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The optical transmission spectrum was obtained by an ultraviolet-visible (UV-VIS) spectrophotometer and the optical band gap ( E g ) was calculated. The results suggest that the films' deposition rate increases with P and r . The surface of the films reveals a compact structure, and the grain size of Cu 3 N is about 30nm. Meanwhile, with increasing r , the grain size and optical band gap of Cu 3 N increase, of which E g ranges from 1.47 to 1.82eV, and the films' growth prefers the (111) direction at low r and the (100) direction at high r .

Based on a tight-binding disordered model describing a single electron band, we establish a model of one-dimensional binary disordered systems with off-diagonal correlations, and derive the alternating current (ac) conductance formula. By calculating the ac conductivity, the function of disorder and off-diagonal correlations in electronic transport are analyzed, and the dependence of the ac conductivity on the field frequency and the temperature is studied. The results indicate that the ac conductivity of one-dimensional binary disordered system decreases with the increasing of the degree of lattices energy disorder. And the ac conductivity of the system is also dependent on the probabilities p, which represents the degree of compositional disorder of the system to some extent. The ac conductivity of system first decreases with the increasing of p, then increases with farther increasing of p. We also find that off-diagonal correlations lead to delocalization and enhance the ac conductivity of the system，namely, the ac conductivity of one-dimensional binary disordered system with off-diagonal correlations is larger than that of uncorrelated system in the Anderson limit. Moreover, the ac conductivity of one-dimensional binary disordered system decreases with the increasing of temperature, and increases drastically with the increasing of the frequency of the electric field.

Based on the negative eigenvalue theory we discuss the non-diagonal disorder and dimensional effects for the electronic structure in low-dimensional disordered systems. The result shows that: It is the non-diagonal disorder and the dimensio nal effect that strongly affects the electronic structure in the low-dimensional disordered system. The non-diagonal disordered mainly shows the changes of the structure and the boundary effects of the particles in the system. From one- dim ensional system to narrow gap one, the number of the electronic bandgap decrease s, the degree of local states increases, and the structure of the energy gap wil l be more complicated.

Birefringent microstructure fibers are shown to allow efficient gereration of anti-Stokes line emission as a result of four-wave-mixing in higher mode by using unamplified femtosecond Ti:sapphire laser pulses.Intense blue-shifted lines with different central wavelength were generated in the high-delta (i.e.high-air filling in the cladding) microstructure fiber with axis sizes 50 and 46 μm by femtosecond laser pulses with 35-fs in duration,820nm in central wavelength and 4nJ in energy per pulse.The experimental result shows that phase-matched four-wa ve mixing in higher-order modes of microstructure fibers allows unprecedentedly high efficiencies of anti-Stokes frequency conversion to be achieved for subnano joule femtosecond laser pulses.The dependence factor of the four waves mixing in birefringent microstructure fiber is compared and analyzed.Good agreement betwe en the theory and experiment is achieved.

For the first time to our knowledge, it is put forward that GNSE can be resolved by adaptive split-step Fourier method (ASSFM) basing on split-step Fourier method (SSFM) in this paper. It is found that ASSFM must be used to resolve GNSE f or ensuring precision when the frequency shift corresponding to soliton peak val ue is remarkable and microstructure fibers' parameters change along with frequen cy remarkably. It is found that the influence of dispersion on laser pulses evol ution and supercontinuum generation in microstructure fiber is remarkably. Mech anism and scope of expanded spectrum are different in fiber different dispersion regions. The precision of numerical simulation by ASSFM is higher than SSFM in the process of laser pulses propagating in microstructure fibers by reason that the variation of wavelength corresponding to pulse peak power can be considered adequately.

An experimental study of supercontinuum generation in a photonic crystal fiber with zero dispersion wavelength 780?nm by femtosecond laser and its dependence factor is presented. Supercontinuum light with a spectrum more than one octave broad (500-1100?nm) was generated in a photonic crystalfiber by femtosecond pulses from a selfstarting, selfmode locked Ti:sapphire oscillator running at center wavelength of 810-840?nm. Different supercontinuums were generated by the 35?fs pulses with different power, central wavelength, prechirp, and the continuous noise in the femtosecond pulses. Some reasonable analysis result is obtained from the comparisons of the experimental results, and the mechanism of the supercontinuum generation is analyzed.

It Is reported that the supercontinuum spectrum can be generated in holey microstructure fibers by femtosecond laser pulses with a 800nm center wavelength. Broadband continua extending from 440 to 890nm are generated in holey microstructure fibers. Based on the theory of scalar approximation, the effective refractive index of the cladding and effective area for the fundamental space-filling mode and chromatic dispersion of fibers are calculated. It is found that the holey microstructure fibers have specifically the ability of controlling dispersion and waveguide property.The mechanism of supercontinuum generation in holey microstructure fibers is explicated preliminarily. The theoretic analysis is in good accodance with the experimental result.It is considered that supercontinuum broadband spectrum can be generated in holey microstructure fibers even if their cladding constitutes of random gas-line.

A random distributed microstructure fiber in which strong asymmetry was introduced around a puresilica core was reported. Femtosecond laser pulses with duration of 35?fs from a Ti:sapphire laser oscillator were coupled into this fiber, and then periodic color rings were observed from the side of the fiber when the supercontinuum light propagated along the fiber. We attributed this phenomenon to the polarization beating of the supercontinuum light in this highly birefringent microstructure fiber. Measurement showed that the fiber had a beat length of approximately 1?mm at a central wavelength of 600?nm, which is in good agreement with the result of calculation. A new method is proposed to measure the beat length directly in a highly birefringent microstructure fiber.