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We report on a laser-diode (LD)-pumped master-oscillator power amplifier (MOPA) mid-infrared laser system based on an LD side-pumped Er:YSGG seed laser that can operate in both free-running and Q-switched regimes. In the free-running mode of the seed laser, the maximum amplified single-pulse energy was 83.4 mJ. In Q-switched mode of the seed laser, a maximum single-pulse energy of 7.8 mJ was achieved at 100 Hz repetition rate with the pulse width of 90 ns, corresponding to the peak power of 86.7 kW and the single-pass amplification factor of 1.66. The results indicate that the LD side-pumped MOPA structure is an effective way to realize a nanosecond ∼3µm mid-infrared laser with high repetition rate and high pulse energy.

In this paper, a single-frequency quasi-continuous-wave partially pumped slab (Innoslab) amplifier at 1064 nm is reported. The 4.4-W single-frequency seed laser was amplified to 148.1 W with optical-optical efficiency of 30.4%. The output pulse duration was 141.4 μs at the repetition of 500 Hz. The beam quality factors of M2 were 1.41 and 1.37 in the horizontal and vertical direction respectively. The experimental results match well with the numerical simulation. To the best of our knowledge, this is the first report on single-frequency Nd:YAG Innoslab amplifier with such a high output power and efficiency.

We report on a quasi-continuous Er:YAG planar waveguide laser operated at 2.94 µm based on the major oscillator power amplification configuration. With the total pump peak power of 32.01 kW, a maximum output peak power of 1.14 kW was obtained at the seed injection peak power of 184.4 W operated at 400µs, 40 Hz. Furthermore, the numerical simulation results indicate that better performance of the laser could be obtained with the higher injected seed laser power. To the best of our knowledge, this is the first experimental demonstration of 2.94 µm planar waveguide laser with an Er doped host material.

We demonstrated a high-power laser diode (LD) side-pumped two-rod Er:YSGG mid-infrared laser by optimizing the location of the two laser heads in resonator based on the matrix formalism A maximum output power of 61.02 W was achieved at repetition rate of 100Hz and pulse width of 750 μ s , corresponding to optical–optical efficiency of 12.6%. The experimental results illustrated that the technology of double rods in series can effectively improve the output power of the LD side-pumped Er:YSGG laser.

Unintentional gallium incorporation was observed and investigated in the epitaxial growth of InGaN by metalorganic vapor phase epitaxy. InGaN was grown without intentional gallium precursor and the gallium incorporation rate was found not dependent on TEGa source but was significantly influenced by temperature and TMIn source flow. The source of the unintentional gallium incorporation is confirmed to be from the flow distributor of the reactor. The incorporation mechanism was analyzed to be the diffusion of resultant of transmetalation reaction between TMIn or its decomposed products (for example DMIn) and residual gallium. Due to the unintentional gallium incorporation, the growth rate and indium content of InGaN layer are determined by indium source, gallium source and the growth temperature.

We designed a high-slope-efficiency and high-power laser diode (LD) side-pump Er:YSGG laser based on the analysis for the effect of the crystal dimension and the LD distribution on the temperature and energy distributions in laser crystal. A maximum output power of 34.9 W for a 2.8 µm mid-infrared laser was achieved at 200A, 120 Hz, and 500 µs pulse width, corresponding to the slope efficiency of 13.7% and optical-optical efficiency of 12.7%. Moreover, the beam quality $M_x^2/M_y^2$Mx2/My2 factors of the laser were measured to be 5.15/5.19 and the far-field divergences ${\Theta _x}/{\Theta _y}$Θx/Θy were 9.52/9.75 mrad.

We demonstrate the quasi-continuous-wave long-pulsed operation of a compact all-solid-state modeless laser based on intracavity frequency-shifted feedback by an acousto-optic modulator. The laser active medium is an Nd:YAG crystal rod, side-pumped by high-brightness laser diodes of 808 nm. When the pump repetition and duration are 200 Hz and 250 μs, the maximum single pulse energy of 85 mJ is produced at an optical conversion efficiency of ~34%, and also a good beam quality of M2 X = 1.80 and M2 Y = 1.78 is obtained in the horizontal and vertical directions respectively. Moreover, a narrow continuous spectrum is achieved with the 3-dB linewidth of 75 pm (i.e. 20 GHz) and the center wavelength of 1064.16 nm. Via the sum frequency generation, such a long-pulse (several hundred μs), narrow-linewidth modeless laser source is preferred for solving the major problem of saturation of the mesospheric sodium atoms and can create a much brighter sodium guide star to meet the needs of adaptive imaging applications in astronomy.

A high-power tunable Littrow grating external-cavity tapered semiconductor laser is designed in this paper. By using the grating external cavity to lock the wavelength and narrow the linewidth, we gained a series of single-frequency laser with the central wavelength at 927 nm, tunable range >20nm and linewidth < 980 fm with the continuous pumping. When the wavelength is 926.8 nm, the threshold current is 1.25A and the oblique efficiency is 0.682 W/A. When the injection current increases to 4A, the maximum output power is up to 1.906W, with the electro-optic efficiency of 21.7%, the linewidth of 700fm, and the beam quality of 1.948 and 3.788 in the fast and slow axis respectively.

A 208 W all-solid-state modulated-longitudinal-mode quasi-continuous-wave sodium beacon laser was developed, and the photon return flux was up to 56800 photons/s/cm2 during the pulse length without obvious saturation effect.

In this paper, a two-stage partially pumped slab (Innoslab) microsecond amplifier at 1064 nm is reported. The 4.4-W single-frequency seed laser is amplified to 303.6 W, with an overall optical–optical efficiency of 25.7%. The overlapping efficiency of the first- and second- amplifier stage is 67% and 55.6%, respectively. The pulse width is 145.0 µs, at a repetition rate of 500 Hz, and the beam quality factor of M 2 is 1.84 and 1.71 in the horizontal and vertical directions, respectively. With higher overlap between the pump volume and the seed laser mode, the output power and optical–optical efficiency can be further improved.

At the surfaces of crystals, linear susceptibility tensors would differ from their counterparts in the interior of the bulk crystal. However, this phenomenon has not been shown in a visible way yet. In previous researches, numerous types of nonlinear Cherenkov radiation based on different materials have been studied, while linear Cherenkov radiation is barely reported. We experimentally prove the generation of linear Cherenkov radiation on the potassium dihydrogen phosphate (KDP) crystal surface and theoretically analyze its phase-matching scheme. In our study, o-polarized light and e-polarized light can mutually convert through the linear Cherenkov process. According to this result, we figure out new nonzero elements at off-diagonal positions in the linear susceptibility tensor matrix at crystal surfaces, compared with the normal form of a bulk KDP.

Surface morphology and optical properties of InGaN-based multiple quantum wells grown on c-plane freestanding GaN substrates by metalorganic vapor phase epitaxy was investigated. The surface shows a dependence on the substrate crystallographic orientation, presenting wavy and mirror-like feature with the substrate off-cut towards the [ 11 2 ¯ 0 ] direction and the [ 1 1 ¯ 00 ] direction, respectively. To get sharp hetero-interfaces, InGaN multiple-quantum-wells-based blue laser diode wafers were grown on substrates with off-cut towards the [ 1 1 ¯ 00 ] direction. The photoluminescence peak wavelength of the blue laser wafers shows a linearly blue shift as the substrate off-cut angle increases. A minimum spectra line width of 90 meV@450 nm can be obtained with a substrate off-cut angle of about 0.5°. These surface and optical properties were ascribed to the step growth kinetics of the wurtzite type materials.

We report on a long-pulse, quasicontinuous wave partially end-pumped slab (INNOSLAB) amplifier at 1319 nm. The low-pulse-energy seed laser was amplified to 8.24 mJ at repetition of 500 Hz by five-pass amplification. The beam quality factors are 1.70 and 1.28 in the horizontal and vertical directions, respectively. The optical–optical efficiency was 1.16% under the pump power of 187.5 W. The experimental results match well with the numerical simulation. The optical–optical efficiency can be improved to about 10% with higher absorption efficiency of the pump source and higher overlapping efficiency between the pump light and 1319-nm seed laser. And further improvements can be realized by optimizing the input pulse energy density and the pass of amplification.

We first report a 1319 nm long-pulsed duration laser with relaxation oscillation suppressed by placing lithium triborate crystals into the laser cavity. Two more etalons were added into the laser cavity to further increase the difference in cavity loss between oscillating and non-oscillating modes. With very little reduction in average output laser power, the smooth temporal profile laser pulse was obtained with a pulse duration of 150 μs. The average output power was 5.45 W, and the beam qualities Mx2 and My2 were 1.12 and 1.20.

We reported a high-power narrowband blue diode laser which is suitable for subsequent nonlinear frequency conversion into the deep ultraviolet (DUV) spectral range. The laser is based on an external cavity diode laser (ECDL) system using a commercially available GaN-based high-power blue laser diode emitting at 448 nm. Longitudinal mode selection is realized by using a surface diffraction grating in Littrow configuration. The diffraction efficiency of the grating was optimized by controlling the polarization state of the laser beam incident on the grating. A maximum optical output power of 3.1 W in continuous-wave operation with a spectral width of 60 pm and a side-mode suppression ratio (SMSR) larger than 10 dB at 448.4 nm is achieved. Based on the experimental spectra and output powers, the theoretical efficiency and output power of the subsequent nonlinear frequency conversion were calculated according to the Boyd– Kleinman theory. The single-pass conversion efficiency and output power is expected to be 1.9×10-4 and 0.57 mW, respectively, at the 3.1 W output power of the ECDL. The high-power narrowband blue diode laser is very promising as pump source in the subsequent nonlinear frequency conversion.

For many applications, laser diodes with very narrow and stable emission wavelength are needed. The realization of mode selection by an external cavity system with a grating is widely used. The influence of longitudinal mode selection by external grating on the filamentation and catastrophic optical mirror damage (COMD) of 970 nm broad area single emitters is studied in this paper. The emitters worked at three configurations: free running, with 10% mirror feedback (mirror lasers), and 10% grating feedback (grating lasers). The grating lasers showed very short lifetime caused by COMD, while the free-running lasers and mirror lasers show no power degradation. The COMD was confirmed by optical microscope showing cracking and melting of the optical antireflective (AR) coatings. By observing the near-field pattern of the three lasers, the COMD of the grating lasers was attributed to the pronounced filamentation induced by the grating feedback. What’s more, the filamentations vary when the locked wavelength change which indicates that the carrier dynamics thus the refractive index profile is very sensitive to the locked lasing wavelength.

We developed a tunable-line-width 101 W average-power all-solid-state 589nm double spectral line sodium beacon laser. The laser was based on the technical route of 1064nm and 1319nm Nd:YAG laser extra cavity sum frequency generation. The laser contained two spectral lines: 589.1591 nm and 589.1571 nm. The former line was matched to the sodium D2a absorption line with the average power of 81W, while the other line was matched to the sodium D2b absorption line with the average power of 20W. The beam quality of the two spectral line lasers was both less than 1.3. The two lasers were polarized-combined to transmit coaxially. The initial line width of the laser was about 0.3GHz, which was in the comb-like discrete structure of about three longitudinal modes. We used a white noise generator to modulate the 1064nm single frequency seed laser in frequency domain. The line width’s tunability was accomplished by tuning the driving power of the white noise generator. The final line width tuning range of the 589nm laser was ~0.3GHz to ~1.1GHz.

In this article, we demonstrate that the angles at which second-harmonic (SH) waves are generated at ferroelectric domain walls satisfy the Snell law for nonlinear media. Nonlinear reflection and refraction are observed experimentally and the relation is found to be in good agreement with theoretical predictions. The ratio of the intensities of refracted and reflected waves has been measured. Under an anomalous-dispersion-like condition, the forbidden nonlinear reflection and refraction is analyzed and found to have a behavior similar to that of the total internal reflection in linear optics. In the periodic domain structure, the coherent superposition of SH waves has been observed, on the basis of which we have proposed a comprehensive theory to explain nonlinear effects in multilayered structures.

True blue nitride laser diodes (LDs) are one of the key challenges for epitaxy of nitrides due to the variety of its potential applications. The growth of high temperature p-type layers may cause thermal degradation of the InGaN-based multiple quantum wells (MQWs) active region because of the annealing effect, since thick p-AlGaN layers were introduced as upper optical cladding layer in the LDs. The degradation was found in blue LDs grown on both Si and sapphire substrate. In the degraded LD wafer samples, “Dark” non-radiative MQWs regions were observed by microscopic photoluminescence. Formation of metallic indium precipitates and voids in these regions were confirmed by transmission electron microscope. The thermal degradation is attributed to the decomposition of indium-rich InGaN materials in the MQWs. The indium-rich InGaN materials were supposed to be accumulated at dislocation related V-shaped pits according to the surface morphology by atomic force microscope. To obtain high quality InGaN-based MQWs, one of the four methods can be introduced to eliminate the degradation. A lower thermal budget can suppress the decomposition of indium-rich InGaN clusters by a lower p-cladding layer growth temperature. The use of low threading dislocation density substrates results in low density indium-rich InGaN clusters. The introducing of H 2 carrier gas during the quantum barriers growth or a 2-step growth scheme with a higher quantum barrier growth temperature etches off the indium-rich InGaN clusters. The suppression of the thermal degradation in the MQWs makes it possible for lasing of blue laser diode directly grown on Si.

A 208 W all-solid-state modulated-longitudinal-mode quasi-continuous-wave sodium guide star (SGS) laser was developed by sum-frequency of a 1064 nm laser and a 1319 nm laser. The laser contained two spectral lines separated by 1.72 GHz for re-pumping the sodium atoms. To suppress absorption saturation effect of the sodium atoms induced by the high light intensity, we used a white noise generator to modulate the 1064 nm single frequency seed laser in the frequency domain. The line width of the modulated-longitudinal-mode 589 nm laser was maximally broadened to 0.74 GHz compared to the initial line width of ~0.30 GHz. A bright SGS with photon return flux of 56800 photons/s/cm2 during the pulse length was obtained.

Collinear phase-matching of sum-frequency generation (SFG) has been studied thoroughly previously, while non-collinear schemes are sometimes more flexible in application. However, this phase-matching type is more difficult to meet and control. We employ a convenient method to obtain harmonic generation in bulk potassium dihydrogen phosphate (KDP), using an incident wave vector and a reflected wave vector to create a triangle phase-matching relationship. With a simple, flexible set-up, we can observe 351 nm SFG, and the conversion efficiency is up to ∼3.6% per reflection. Furthermore, we believe this approach has potential application value and improvement space.

In this Letter, a new method is presented to calculate the interactive length between the fundamental wave and second harmonic generation (SHG) for the configuration of total internal reflection on the inner surface of a nonlinear crystal. Three independent experiments are designed to measure the bandwidths of this second harmonic wave. The theoretical expression of the intensity of SHG is obtained through a nonlinear coupled wave equation. The interactive length of this phase-matched SHG can be calculated mathematically by utilizing the measured bandwidths and the intensity equation. There is no existing method to obtain the interactive length either from theoretical calculations or by experimental measurement. This method can be applied to estimate the extremely short interactive volume in nonlinear processes.

Cherenkov type sum-frequency and frequency doubling process, which has relatively high converscion efficiency with relaxed phase matching requirements, is a good way for high-order harmonic generation. Cascaded high order Cherenkov harmonic generation on the interface with second order nonlinear coefficient χ(2) from −1 to +1 (i.e., domain wall) has been research in the past. The high-order harmonic generation is produced via enhanced second-order nonlinear susceptibility at domain wall, where Cherenkov second harmonic cascaded with successive multistep sum-frequency generation with simultaneously longitudinal phase-matching. On the nonlinear crystal surface, i.e., the interface with χ(2) from 1 to 0, the nonlinear wave-mixing effect is much more complex, because of the presence of total internal reflection. Besides the high-order nonlinear Cherenkov radiation, there are many other types of cascaded processes, which interplays with each other. This research sorted out the complex high-order nonlinear processes on crystal surface by analysis of their phase matching angles, wavelengths, spot morphology and so on. Through the polarization analysis of phase mismatched high-order nonlinear process on crystal surface, we can determine the cascaded process with χ(2) rather than high-order susceptibility. Cherenkov sum-frequency of the two laser beams with different wavelengths proved that the nonlinear polarization wave propagating is confined on the surface of the nonlinear crystal as on the domain wall, which is an important basis to analyze high-order Cherenkov process. We demonstrate that on the crystal surface high-order Cherenkov type harmonics phase matches several ways, including cascade mode with lower-order Cherenkov radiation participation and without. In addition, a four-wave mixing process at the frequency of third harmonic generation is found, which only appears at the position meeting the complete phase matching, and is relatively weak to the cascaded Cherenkov harmonics, since the third-order nonlinear coefficient is less affected by surface symmetry breaking.

We report on a third-order nonlinear phenomenon four-wave mixing that occurs on the nonlinear interface. The phase-matching is associated with the scattered light and the reflected light of the fundamental waves.

We investigate the generation of ultraviolet (UV) second-harmonic radiation at the boundary of a UV transparent crystal, which is derived from the automatic partial phase matching of the incident wave and the total internal reflection. By adhering to another UV non-transparent crystal with a larger second-order nonlinear coefficient χ(2), a nonlinear interface with large disparity in χ(2) is formed and the enhancement of UV second-harmonic radiation is observed experimentally. The intensity of enhanced second harmonic wave generated at the nonlinear interface is up to 11.6 times that at the crystal boundary. As a tunable phase-matching method, it may suggest potential applications in the UV, and even vacuum-UV region.

We report on a new character of single domain wall (DW) of electrically-poled ferroelectric crystal which can modulate parametric processes via Cherenkov-type phase matching. Experimental result shows that the effective nonlinear polarization is confined in DW, and its phase velocity can be modulated when incident light is off the domain wall's direction. These effects lead to novel Cherenkov second harmonic generation (CSHG), and other modulated parametric process, such as Cherenkov sum frequency generation (CSFG).

In this article, on the basis of our previous research on bandwidth of non-collinear phase-matching second harmonic generation (SHG) and nonlinear Cherenkov radiation (NCR) generated on the boundary of bulk nonlinear crystal, we demonstrated theoretically the coupled wave equation of SHG generated by a Gaussian beam on the boundary of 5%/mol M g O : L i N b O 3. Using the Fourier-transform technique, we solved this coupled wave equation. The results can explain theoretically the generation mechanism of two different experimental phenomena which corresponded to non-collinear phase-matching SHG under anomalous-dispersion-like dispersion condition and nonlinear NCR under normal dispersion condition, respectively. The theoretical results show a good agreement with the experimental data. This work can provide theoretical fundamental for more complicated research on the boundary of nonlinear media.In this article, on the basis of our previous research on bandwidth of non-collinear phase-matching second harmonic generation (SHG) and nonlinear Cherenkov radiation (NCR) generated on the boundary of bulk nonlinear crystal, we demonstrated theoretically the coupled wave equation of SHG generated by a Gaussian beam on the boundary of 5%/mol M g O : L i N b O 3. Using the Fourier-transform technique, we solved this coupled wave equation. The results can explain theoretically the generation mechanism of two different experimental phenomena which corresponded to non-collinear phase-matching SHG under anomalous-dispersion-like dispersion condition and nonlinear NCR under normal dispersion condition, respectively. The theoretical results show a good agreement with the experimental data. This work can provide theoretical fundamental for more complicated research on the boundary of nonlinear media.

Non-collinear continuous frequency doubling with relatively high conversion efficiency is demonstrated in periodically-poled lithium niobate. It is shown that about 80% of the total second harmonic energy can be collected to one output channel by tuning incident angle. The fractional phase-matching condition is introduced to interpret the phenomenon. These results make it possible for direct power applications of continuous frequency doubling over the broad spectral range from the visible to the infrared without temperature or angle tuning.

We present investigations into a narrow-linewidth, quasi-continuous-wave pulsed all-solid-state amplified spontaneous emission (ASE) source by use of a novel multiple-pass zigzag slab amplifier. The SE fluorescence emitted from a Nd:YAG slab active medium acts as the seed and is amplified back and forth 8 times through the same slab. Thanks to the angular multiplexing nature of the zigzag slab, high-intensity 1064-nm ASE output can be produced without unwanted self-lasing in this configuration. Experimentally, the output energy, optical conversion efficiency, pulse dynamics, spectral property, and beam quality of the ASE source are studied when the Nd:YAG slab end-pumped by two high-brightness laser diode arrays. The maximum single pulse energy of 347 mJ is generated with an optical efficiency of ~5.9% and a beam quality of 3.5/17 in the thickness/width direction of the slab. As expected, smooth pulses without relaxing spikes and continuous spectra are achieved. Moreover, the spectral width of the ASE source narrows versus the pump energy, getting a 3-dB linewidth of as narrow as 20 pm (i.e. 5.3 GHz). Via the sum frequency generation, high-intensity, smooth-pulse, and narrow-linewidth ASE sources are preferred for solving the major problem of saturation of the mesospheric sodium atoms and can create a much brighter sodium guide star to meet the needs of adaptive imaging applications in astronomy.

An 81 W average-power all-solid-state sodium beacon laser at 589 nm with a repetition rate of 250 Hz is introduced, which is based on a novel sum frequency generation idea between two high-energy, different line widths, different beam quality infrared lasers (a 1064 nm laser and a 1319 nm laser). The 1064 nm laser, which features an external modulated CW single frequency seed source and two stages of amplifiers, can provide average-power of 150 W, beam quality M2 of ~1.8 with ultra-narrow line width (< 100 kHz). The 1319 nm laser can deliver average-power of 100 W, beam quality M2 of ~3.0 with a narrow line width of ~0.3 GHz. By sum frequency mixing in a LBO slab crystal (3 mm x 12 mm x 50 mm), pulse energy of 325 mJ is achieved at 589 nm with a conversion efficiency of 32.5 %. Tuning the center wavelength of 1064 nm laser by a PZT PID controller, the target beam’s central wavelength is accurately locked to 589.15910 nm with a line width of ~0.3 GHz, which is dominated mainly by the 1319 nm laser. The beam quality is measured to be M2 < 1.3. The pulse duration is measured to be 150 μs in full-width. To the best of our knowledge, this represents the highest average-power for all-solid-state sodium beacon laser ever reported.

We study the configuration of efficient nonlinear Cherenkov radiation generated at the inner surface of a one-dimensional nonlinear photonic crystal, which utilizes the combination of both quasi-phase matching and total internal reflection by the crystal surface. Multi-directional nonlinear Cherenkov radiation assisted by different orders of reciprocal vectors is demonstrated experimentally. At specific angles, by associating with quasi-phase matching, the incident fundamental wave and total internal reflection wave format completely the phase-matching scheme, leading to great enhancement of harmonic wave intensity.

We observe conical sum-frequency generation in a bulk anomalous-like dispersion medium, which is attributed to complete phase-matching of one fundamental wave and the scattering wave of the other fundamental wave. In addition, efficient sum-frequency output is achieved making use of total internal reflection with conversion efficiency of 7.9% by only one reflection. The experiment proposes a new phase-matching mode under an anomalous-like dispersion condition, which suggests potential applications in efficient frequency conversion.

We investigate several bandwidths of non-collinear phase-matching second harmonic generation, which is generated by sum-frequency of the incident and reflected wave on the inner surface of a z-cut 5%/mol MgO : LiNbO₃ crystal. The bandwidths of angle, temperature and wavelength in this configuration are measured to be about 0.51°, 4.1°C and 6 nm, respectively. The large acceptance of non-collinear phase-matching second harmonic generated on the surface shows attractive potential in the application of wavelength conversion.

We demonstrate a method to generate enhanced nonlinear Cherenkov radiation (NCR) in the bulk medium by utilizing the total reflection on the physical boundaries inside the crystal. This is the experimental demonstration of enhanced NCR by a sharp χ(2) modulation from 0 to 1, and a new way for generating enhanced NCR in addition to using the waveguide structures and the ferroelectric domain walls, which also possesses a better beam quality for applications.

Time-reversal symmetry is important to optics. In linear optics, a time-reversed process to laser emission enables total absorption of coherent light fields into an optical cavity of loss by time-reversing the original gain medium. In nonlinear optics, time symmetry exists for some well-known processes such as parametric up/down conversion, sum/difference frequency generation, however, combining exact time-reversal symmetry with nonlinear wave mixings is yet explored till now. Here, we demonstrate time reversed wave mixings for second harmonic generation (SHG) and optical parametric amplification (OPA). This enables us to observe the annihilation of coherent beams under time-reversal symmetry by varying the relative phase of the incident fields. Our study offers new avenues for flexible control in nonlinear optics and potential applications in efficient wavelength conversion, all-optical computing., Comment: 18 pages, 4 figures

We discuss the nonlinear response due to the spatial modulation of the second-order susceptibility at the interface between two nonlinear media, and experimentally demonstrate that the nonlinear Cherenkov radiation is enhanced by the interface of two nonlinear crystals with a large disparity in χsup(2)/sup. In our experiment, the intensity of the nonlinear Cherenkov radiation generated at the nonlinear interface was approximately 4 to 10 times that at the crystal boundary. This result suggests potential applications to efficient frequency conversion.

We reveal a variety of nonlinear Cerenkov radiation (NCR) patterns that occur in a single photonic crystal modulated by domain walls, which manifest themselves as normal, degenerated, and anomalous-dispersion-like NCR type sum-frequency generation. The phase-matching geometries of the evolution of Cerenkov radiation varying with the dispersion relationship among the interaction waves are exploited, respectively.

By using ultrathin nonlinear media, the phase velocity of nonlinear polarization waves can be continuously adjusted. We realized nonlinear Cherenkov radiation (NCR) in an anomalous dispersion medium experimentally, which breaks the minimum speed limit of NCR. This modified nonlinear Cherenkov radiation has two new special states with respect to its prior, i.e., the degenerate state and extreme state. Also, periodically arranged ultrathin nonlinear media can enhance NCR and realize the nonlinear Smith-Purcell effect.

By employing angle modulation, phase-matched efficient Cherenkov second harmonics can be generated by different domain walls, which is a novel mechanism totally distinguished from Quasi-phase-matching.