Fourth Harmonic Generation Research Articles

Pushing the limits of deep-ultraviolet scanning near-field optical microscopy

Deep-ultraviolet (DUV) microscopy and microspectroscopy have received much attention in label-free live-cell imaging, selective molecular analysis, and optical characterizations of ultrawide bandgap materials. Far-field optics approaches usually suffer from the diffraction limit of light. Meanwhile, near-field optics technology is immature in the DUV spectral region. Herein, we develop a DUV scanning near-field optical microscope (SNOM) with an excitation wavelength of 210 nm. The fourth harmonic generation of a continuous-wave Ti:sapphire laser is the excitation source. Two negative feedback control systems stabilize the power and pointing, allowing scanning microscopy to be carried out over a long period of time even in the DUV spectral region. A reflective objective couples the excitation beam onto an optical fiber probe. The optical fiber probe is solarization-resistant and is not coated by metallic materials. The reflective-based objective optical design does not introduce chromatic aberrations in the detection signals. Our DUV-SNOM is used to measure the photoluminescence (PL) spectra of the quantum well structure of aluminum gallium nitride, which is an ultrawide bandgap material. The PL mapping images indicate that emissive localization centers can be individually visualized via our DUV-SNOM with a lateral resolution exceeding 150 nm.

Large aperture and non-critical phase-matched fourth harmonic generation of Nd:Glass lasers

A fourth harmonic generation (FHG) scheme in focusing beams is proposed and demonstrated for large aperture Nd:glass laser facilities. By placing the focusing lens before the FHG crystal, the problem of ultraviolet damage can be overcome, largely without affecting FHG conversion efficiency owing to the large angular acceptance of the non-critical phase matching technique. A numerical simulation of the FHG process indicates that angular acceptance can be appropriately increased by lowering the working temperature and jointing the two adjacent compensating angles, so that FHG in focusing beams with relatively small F numbers becomes feasible. With a 170 mm × 170 mm × 7 mm and 65% deuterated potassium dihydrogen phosphate crystal mounted in a high-precision, temperature-controlled system, high-efficiency FHG has been demonstrated in the focusing beam with a full beam convergence angle of 36 mrad. When driven with a 223 J, second harmonic radiation (2ω), 1 ns flat-top pulse with a beam area of 130 cm2, corresponding to 1.7 GW/cm2 2ω input intensity, 182 J of fourth harmonic radiation (4ω) were generated.

Cs3Na(H2C3N3O3)4·3H2O: A Mixed Alkali‐Metal Hydroisocyanurate Nonlinear Optical Material Containing π‐Conjugated Six‐Membered‐Ring Units

Practical ultraviolet nonlinear optical (NLO) crystals are indispensable in modern scientific equipment and solar‐blind communication systems. Herein, a new hydroisocyanurate NLO crystal, Cs3Na(H2C3N3O3)4·3H2O (1), was grown by a facile aqueous solution method. It features a two‐dimensional layered structure composed of coplanar large π‐conjugated 2∞[H2C3N3O3]– isocyanurate ribbons. More importantly, it exhibits concurrently short ultraviolet phase‐matching capability (λpm ≈ 227 nm) and considerable second‐harmonic‐generation response (deff ≈ 2/3 KDP) resulting from the π‐conjugated (H2C3N3O3)– six‐membered ring units. First principles calculations were employed to elaborate its delocalized π‐bonds and strong interaction between the Cs+ cations and (H2C3N3O3)– anionic groups. Consequently, 1 will be a promising ultraviolet NLO material for the fourth harmonic generation of Nd‐based lasers. Successful synthesis of the title compound also provides a new roadmap to discover new ultraviolet linear optical or NLO materials in metal hydroisocyanurates.

Fourth harmonic generation based on miniaturized tunable bow-tie resonators

A compact continuous-wave deep-ultraviolet laser system is developed with fourth harmonic generation technique. Two miniaturized bow-tie resonators with dimensions of 95 mm × 66 mm × 53 mm are applied to realize the frequency conversion from infrared to deep-ultraviolet. Based on a 1.17 W 1014.8 nm continuous-wave fundamental laser, a 480 mW 507.4 nm laser is generated. And then a 25 mW 253.7 nm deep-ultraviolet laser is generated with 420 mW input pump power. The experimental data are in agreement with simulation results. As a result, this system is working well and suitable for the low power continuous-wave laser.

High-power 266 nm laser generation with a NaSr3Be3B3O9F4 crystal.

We demonstrate a 266nm ultraviolet (UV) picosecond laser by fourth-harmonic generation of a Nd:YAG laser with a 5.4mm thick NaSr3Be3B3O9F4 (NSBBF) crystal. A maximum output power exceeding 1W at 266nm was obtained (the highest output power being 1.6W), corresponding to a conversion efficiency of 10.3%. The stability measurements on the NSBBF crystal with a fluctuation of 3.34% at 200mW within 1h indicate that it is a promising UV nonlinear optical material for practical applications. In addition, for the first time, to the best of our knowledge, we measured the effective nonlinear coefficient of NSBBF crystal at 266nm and compared it with that of β-BaB2O4 crystal.

Synthesis and features of nonlinear optical switches based on dithienylethene unit

Two nonlinear optical (NLO) chromophores C1 and C2 based on dithienylethene were designed and synthesized as the ultraviolet NLO switches. The open/close behavior of C1 and C2 were investigated through the UV–vis spectra. Through quantum chemical calculations of the dipole moments, first and second hyperpolarizabilities, etc. we analyzed both opening and closure states of C1 and C2. The NLO switch ability of the chromophores were studied through monitoring of the SHG at the opening/closure states, which were performed under illumination of fourth harmonic generation of the nanosecond Nd:YAG laser (λ = 266 nm). The obtained results indicated that the maximal SHG changes were observed at energy density equal to about 90 J/m2, and the samples C1, 2′ closure states possess significantly higher value of the second order susceptibility compared to the opening states. After switching off of the external UV light, the induced second order susceptibilities remain up to 150 h without a decrease, which indicated that the obtained chromophores have the potential application as the NLO switches applied in photonics.

An all-dielectric metasurface as a broadband optical frequency mixer

A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.

High-power ultraviolet 278-nm laser from fourth-harmonic generation of an Nd:YAG amplifier in CsB3O5 crystal**Project supported by the National Natural Science Foundation Program of China (Grant Nos. 11504389 and 61505226), the National Key Research Development Program of China (Grant Nos. 2016YFB040203 and 61535013), and the National Development Project for Major Scientific Research Facility of China (Grant No. ZDYZ2012-2).

We report on the experimental investigation and theoretical analysis of a nanosecond pulse high power ultraviolet (UV) 278 nm laser by fourth-harmonic generation (FHG) of a 1112-nm Nd:YAG amplifier in LiB3O5 (LBO) and CsB3O5 (CBO) crystals. The UV laser delivers a maximum average power of 10.3 W at 278 nm with peak power of 36.8 kW under input pump power of 41 W at 556 nm. This is, to the best of our knowledge, the highest output power at the specific UV wavelength of 278 nm. We also performed the theoretical investigation on the FHG with a model in the Gaussian approximation of both spatial and temporal profiles, especially accounting for the two-photon absorption effect in CBO crystal for the first time. The average output power, pulse width, and beam spatial distribution of the UV laser were simulated. The theoretical calculations are in close agreement with the experimental results.

Highly efficient frequency doubling and quadrupling of a short-pulsed thulium fiber laser

We report the second harmonic generation and fourth harmonic generation of the output from a short-pulsed (~ 80 ps) thulium-doped fiber laser, generating 976 and 488 nm wavelengths with high efficiency. With a narrow-linewidth (0.5 nm) pump at a power of 3.2 W, a second harmonic power of 2.4 W was generated at 976 nm with a conversion efficiency reaching 75%. For FHG, 690 mW of power at 488 nm was obtained from frequency doubling of 976 nm with a conversion efficiency of 30%.

5.6 kW peak power, nanosecond pulses at 274 nm from a frequency quadrupled Yb-doped fiber MOPA.

A 2 W deep-ultraviolet (DUV) source at 274 nm with 5.6 kW peak power is demonstrated by frequency quadrupling a diode-seeded, polarization-maintaining (PM), Yb-doped fiber master oscillator power amplifier (MOPA) system delivering 1.8 ns pulses at a repetition rate of 200 kHz. The second harmonic generation (SHG) and the fourth harmonic generation (FHG) are achieved by using Lithium Triborate (LBO) crystal and β-BaB2O4 (BBO) crystal in sequence, with an IR-to-green and green-to-UV conversion efficiency of up to 65% and 26%, respectively. This is the first kW peak power pulsed UV system reported at 274 nm which has great potential for machining insulators, 2D materials, isotopic separation of Calcium-48, and fluorescence analysis of biological molecules.

High-Power, Solid-State, Deep Ultraviolet Laser Generation

At present, deep ultraviolet (DUV) lasers at the wavelength of fourth harmonics of 1 μm (266 nm/258 nm) and at the wavelength of 193 nm are widely utilized in science and industry. We review the generation of these DUV lasers by nonlinear frequency conversion processes using solid-state/fiber lasers as the fundamental frequency. A DUV laser at 258 nm by fourth harmonics generation (FHG) could achieve an average power of 10 W with a beam quality of M2 < 1.5. Moreover, 1 W of average power at 193 nm was obtained by sum-frequency generation (SFG). A new concept of 193-nm DUV laser generation by use of the diamond Raman laser is also introduced. A proof-of-principle experiment of the diamond Raman laser is reported with the conversion efficiency of 23% from the pump to the second Stokes wavelength, which implies the potential to generate a higher power 193 nm DUV laser in the future.

Bismuth ferrite dielectric nanoparticles excited at telecom wavelengths as multicolor sources by second, third, and fourth harmonic generation.

We demonstrate the simultaneous generation of second, third, and fourth harmonics from a single dielectric bismuth ferrite nanoparticle excited using a telecom fiber laser at 1560 nm. We first characterize the signals associated with different nonlinear orders in terms of spectrum, excitation intensity dependence, and relative signal strengths. Successively, on the basis of the polarization-resolved emission curves of the three harmonics, we discuss the interplay of susceptibility tensor components at different orders and show how polarization can be used as an optical handle to control the relative frequency conversion properties.

Periodic structures on germanium induced by high repetition rate femtosecond laser

Laser-induced periodic surface structures (LIPSS) are studied on germanium surface in air by the femtosecond pulsed laser with repetition frequency of 76 MHz and wavelength λ of 800 nm. Three types of LIPSS were found and they are low-spatial-frequency LIPSS (LSFL), high-spatial-frequency LIPSS (HSFL), and LSFL superimposed with HSFL. The period ΛLSFL of LSFL shrinks quickly from approximately 650 nm to 400 nm (∼λ/2) when lowering the scanning speed. Comparatively, the period ΛHSFL of HSFL keeps almost constant between 90 and 100 nm (∼λ/8) when the scanning speed and the laser pulse energy vary. LSFL and HSFL coexist when the laser pulse energy is around 3.3 nJ/pulse and the scanning speed ranges between 3 and 8 mm/s. The surface plasmon polariton waves make a contribution to the formation of LIPSS and the fourth harmonic generation (FHG) might be involved in the formation of HSFL.

Room temperature noncritical phase matching fourth harmonic generation properties of ADP, DADP, and DKDP crystals

Noncritical phase-matching (NCPM) fourth harmonic generation (FHG) of a 1053 nm laser was realized at room temperature on an ammonium dihydrogen phosphate (ADP), 35% deuterated DADP and 62% deuterated potassium dihydrogen phosphate (DKDP) crystals grown by point-seed rapid growth method. The NCPM temperature is 33.7°C for ADP, 32.1°C for DADP, and 31°C for DKDP crystal. The temperature, angle tuning properties, and performance in NCPM FHG of ADP DADP and DKDP crystals are calculated and compared in detail. The 2ω-to-4ω conversion efficiency has been demonstrated up to 67% with ADP crystal, 63.45% with DADP crystal, and 63.78% with DKDP crystal. Large-aperture high-efficiency fourth harmonic generation would be realized by improving the temperature control and crystal quality with all these crystals and all these results could be a good reference in crystal chosen of NCPM FHG.

High-efficiency UV generation at 266 nm in a new nonlinear optical crystal NaSr3Be3B3O9F4.

266 nm laser output in NaSr3Be3B3O9F4 crystal by the fourth harmonic generation process with a picosecond mode-locked Nd-based YAG laser has been done for the first time. When the input pumping energy was 870 μJ at 532 nm, a 280 μJ 266 nm UV laser was obtained and the corresponding conversion efficiency was 35.9%. Further investigations identified that NaSr3Be3B3O9F4 has a large acceptance angle width of 0.47 (mrad • cm), a small walk-off angle of 35.43 mrad and a large deff as 0.62 pm/V for the fourth harmonic generation. These results indicate that NSBBF is applicable for high-power 266 nm laser generation.

High-power and high-conversion efficiency deep ultraviolet (DUV) laser at 258 nm generation in the CsLiB6O10 (CLBO) crystal with a beam quality of M2

We report a 10kHz, 10.5W deep ultraviolet (DUV) laser at 258nm with a pulse duration of 3ns by fourth-harmonic generation (FHG) in a CsLiB6O10 crystal with high-conversion efficiency. The fundamental laser is at 1030nm with 35W output power consisting of a Yb:YAG ceramics thin rod amplifier. The M2 of the DUV laser was confirmed to be <1.5. To the best of our knowledge, this is the best beam quality for DUV laser power higher than 10W generated by FHG of a solid-state laser, as well as the highest conversion efficiency of 31% from infrared to DUV so far.

High-average-power femtosecond laser at 258 nm.

We present an ultrafast fiber laser system delivering 4.6W average power at 258nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M2 value of 1.3×1.6. The pulse duration is 150fs, which, potentially, is compressible down to 40fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.

Structure and Optical Properties of K0.67Rb1.33Al2B2O7 Crystal

A UV nonlinear optical (NLO) crystal K0.67Rb1.33Al2B2O7 (KRABO) has been obtained by the top-seeded solution growth method for the first time. Its structure is formed of coplanar [BO3]3− triangle groups and [Al2O7]8− groups, which are built from two [AlO4]5− tetrahedra sharing one vertex. Rb and K atoms have the same coordinates and locate between each layer in a disorderly fashion. The crystal has a larger NLO effect compared with its analog K2Al2B2O7 (KABO), as evidenced by powder second harmonic generation (SHG) test. The shortest SHG phase-matching wavelength is down to 231 nm according to the first-principle calculation, which indicates that KRABO is possible for the fourth harmonic generation of Nd:YAG at 266 nm.

Growth of High Quality Al-Doped CsLiB6O10 Crystals Using Cs2O-Li2O-MoO3 Fluxes

High quality and large size Al-doped CsLiB6O10 (CLBO) single crystals have been successfully grown by top-seeded solution growth (TSSG) technique using Cs2O–Li2O–MoO3 fluxes. The advantages of this newly developed flux system were investigated by viscosity measurements and growth experiments. Al-doped CLBO presents a very high transmittance in the visible region and the weak absorption values at 1064 nm along a and c axes are only 140 and 50 ppm/cm, respectively. The measured LIDT of Al-doped CLBO at λ = 1064 nm and τ = 5.0 ns is 5.10 GW/cm2. Moreover, Al-doped CLBO exhibits an apparent enhancement of the hygroscopic nature in contrast with the undoped crystal as determined by the humidity experiments. Finally, a high fourth harmonic generation (FHG) conversion efficiency of 63% utilizing Al-doped CLBO has been achieved by a picosecond mode-locked Nd:YAG laser, the results also reveal that Al doping has no obvious impact on the FHG conversion efficiency.

Bandwidth analysis of non-collinear fourth and fifth harmonic generation in nonlinear uniaxial crystals

The optimal angle bandwidth and wavelength bandwidth of fourth-harmonic generation (FHG) and fifth-harmonic generation (FIFHG) of the 1064[Formula: see text]nm laser are analyzed based on the numerical calculation results of non-collinear type-I and type-II phase matching processes for general nonlinear uniaxial crystals with 1[Formula: see text]cm length. The non-collinear phase matching angles and effective nonlinear coefficients of FHG and FIFHG are calculated. The optimal angle bandwidth and wavelength bandwidth are obtained. The results are beneficial to broadband and efficient non-collinear phase matching FHG and FIFHG experiments and studies.