In recent years whispering gallery mode (WGM) resonators have attracted interest due to their various potential passive (filters, resonators, sensors) and active (lasers, four-wave mixing) applications. By choosing an appropriate material with very low absorption, and fabricating a very smooth surface, WGM resonators can reach ultra-high quality (Q) factors. Additionally, the surface of the WGM resonator can be functionalized with nanoparticles or nanomaterial layers, which can enhance optical properties. Recently, we have been interested in the functionalization of the WGM resonator surface for active applications. WGM resonators are suitable for nonlinear optical interactions due to their ultra-high Q factors, significantly lowering necessary pumping power. WGM resonators can be used to generate optical frequency combs (OFCs), which have many applications in optical clocks, spectroscopy, and communications. After coating WGM resonator with quantum dots, besides the OFC generation, we have observed the third harmonic generation. Functionalization with erbium ions leads to the observation of lasing.
A computational imaging technique using a lens and Lucy-Richardson-Rosen algorithm (LRRA) has been developed for 3D imaging. A deep 3D point spread function (PSF) was recorded in the first step. A single camera shot of an object was recorded next. Using the 3D PSF and the LRRA, the complete 3D information of the object was reconstructed. In this configuration, direct imaging and indirect imaging concepts co-exist: when the imaging condition is satisfied, an image of the object is directly obtained and in other cases it is indirectly obtained. The proposed single lens incoherent digital holography system will be attractive for numerous imaging applications.
Current activities on our research of soft X-ray lasers and higher harmonic generations using intense, subpicosecond lasers are reviewed. Especially for soft X-ray lasers we experimentally demonstrate that by longitudinally pumping 2-mm-long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-X-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submillirad order, and only requires pump laser energy of 150 mJ. Simulations show that the pedestal in the main pump pulse can generate electron density and gain profiles with large spatial gradients, which result in the selective amplification of low-order transverse modes. The present result is the demonstration of an efficient and alternative method of improving the spatial coherence of X-ray lasers with amplified spontaneous emission medium, with possibilities of becoming an excellent tool to explore various application experiments. In higher harmonic generation, topics related to observation of blue shift due to collisionless absorption process is described.
The values of tensor components of nonlinear susceptibilities responsible for self-action effects have been measured in quadratic nonlinear-optical crystals at the Nd:YAG laser wavelength. The angular dependences of the nonlinear susceptibilities have been determined on the base of measured relations between tensor components. The variation of the high-order nonlinear susceptibilities at various spectral ranges has been discussed. The measurements have been conducted on the basis of the analysis of polarization dependences of focused radiation transmittance in the Z-scan scheme.
We demonstrate the first observation of significant resonance enhancement of a single high-order harmonic in the extreme ultraviolet region. Such intense harmonics are generated during the interaction of a femtosecond Ti:sapphire laser pulse with low-ionized indium ablation. A strong 13th harmonic (61.2 nm) with conversion efficiency of 8×10-5 and output intensity almost two orders of magnitude higher than neighbouring harmonics is demonstrated. Such an approach paves the way for efficient single-harmonic enhancement in the extreme ultraviolet range using different ablation sources.
We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited boron plasma produced by a prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. The harmonic generation in these conditions assumed to occur due to the interaction of femtosecond pulses with ions. The conversion efficiencies in the plateau region were varied between 10-7 to 8×10-5 depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the free-electrons-induced self-defocusing of main pulse.
We demonstrate two methods of high-order harmonic generation, which has the potential of generating high-order harmonics with high intensities. The first method is solid surface harmonics. Using the second harmonic output of the 10 TW, 60 fs Ti:sapphire laser system at the INRS, we have observed multiple soft x-ray harmonics of the 397 nm pump laser. The highest order (23rd harmonic at 17.3 nm) observed in our experiments are limited by the 17 nm absorption edge of the thick 1.6 μm Al foil, which is used to eliminate the high intensity pump laser. The second method is harmonics from an ablation plume generated using a relatively low intensity prepulse. We demonstrate the generation of up to the 63rd harmonics (λ=12.6 nm) of a Ti:sapphire laser pulse (150 fs, 10 mJ), using pre-pulse (210 ps, 24 mJ) produced boron plasma as the nonlinear medium. The influence of various parameters on the harmonic conversion efficiency was analyzed. Typical conversion efficiencies were evaluated to be between 10-4 (for third harmonic) and 10-7 (within the plateau range).
Backscattered 2 ω and 3 ω/2 harmonics were investigated during the interaction of femtosecond radiation ( λ=795 nm,
t=150 fs, 10-Hz pulse repetition rate) with various targets. The harmonics were generated inside the drilled hole without
changing the position of target from shot to shot. No 3 ω/2 harmonic was generated in the case of single shots
irradiated the fresh surface of the target. Various characteristics of harmonic radiation were analyzed and their
dependences on pump radiation parameters were discussed.
Simulations are performed to clarify the mechanisms that generate high spatial coherence x-ray lasers with the longitudinally pumped nickellike molybdenum scheme. Various factors that affect the x-ray laser output is also investigated, and we clarify the experimental conditions that maximize its performance.
The measurements of nonlinear refractive indices and third-order susceptibilities responsible for self-action effects in nonlinear-optical crystals (KDP, KTP, BBO, LiNbO3) in various spectral ranges (1064 and 532 nm) were carried out by the Z-scan method. It was obtained that investigated media (excepting KTP crystal λ=1064 nm) had self-focusing properties. The significant values of nonlinear losses were recorded at λ=532 nm. The analysis and comparison of experimentally and theoretically obtained values of nonlinear susceptibilities are given. The angular and polarization dependencies of Kerr nonlinearities were analyzed.
Investigations of nonlinear optical parameters of AU, Ag, Pt and Cu colloidal solutions by Z-scan method are presented. Nonlinear refractive indices of these solutions on the wavelength of Nd:YAG laser radiation and its second harmonic have been measured. Two-photon absorption and nonlinear susceptibilities of these solutions have been studied.
Investigations on harmonic generation in laser-matter interaction of Nd:glass laser radiation with solid surfaces have been performed. Detailed measurements were made on the intensity of second, third and fourth harmonics, their polarization properties and spectral distribution, as well as dependencies of these characteristics on intensity and polarization of the incident laser radiation. Intensity dependence of second, third and fourth harmonics for p- polarized laser pump shoed a power law scaling of 1.5, 1.8 and 3.8 respectively. Maximal conversion efficiencies for second, third and fourth harmonic generation were observed in the range of 10-8 to 10-12. For p- polarized laser radiation, the generation efficiency was more than ten and hundred times higher in comparison to that for the s-polarized radiation for second and third harmonics respectively. Among other features, rotation of second harmonic polarization as a function of laser intensity showed a quite different behavior for p- and s-polarized laser radiation, and the second harmonic observed in the specular reflection direction was red-shifted for laser intensity exceeding 5 by 1014 W cm-2.
KEYWORDS: Difference frequency generation, Harmonic generation, Frequency conversion, Nd:YAG lasers, Molecules, Picosecond phenomena, Ultraviolet radiation, Systems modeling, Chemical species, Electrons
Investigations of nonlinear-optical frequency conversion of Nd:YAG laser radiation in naphthalene vapors are presented. Third harmonic generation caused by difference frequency generation in six-photon process was offered. Optimum temperature for naphthalene vapor was found to be 170 degree(s)C, at which the synchronous conversion of pump radiation to the third harmonic radiation was carried out. Third harmonic generation conversion efficiency varied within the 10-10 depending on pump intensity and cell temperature. Third-and fifth-order nonlinear-optical susceptibilities of naphthalene vapors were calculated.
Frequency conversion of high-intensity radiation of Nd:glass and YAG lasers in different media have been investigated. Various factors leading to limitation of conversion efficiency are examined. On the base of these experiments a sources of coherent radiation in broad UV and VUV spectral ranges both at fixed and tunable frequencies based on harmonics generation have been studied.
Harmonics generation of laser radiation ((lambda) equals 1.06 (mu) , (tau) equals 3 ps) in low-temperature atmospheric plasma and target plasma is investigated. Maximum efficiency was achieved for third harmonics generation ((eta) equals 10-3). Results of odd harmonics generation (up to eleventh, (lambda) equals 96 nm) are reported.
Harmonics generation of laser radiation ((lambda) equals 1.06 (mu) , (tau) equals 3 ps) in low- temperature atmospheric plasma and target plasma is investigated. Results of odd harmonics generation (up to eleventh, (lambda) equals 96 nm) are reported.
St.able pulses with a duration of 2ps derived from a special designed pulsed Nd:glass oscillator with negative feedback were amplified by the injection seeding of Q-swit.cned laser. Amplification fact.or of ~ 10• was achieved in this scheme. Chirped pulses amplification technique is used in further amplification in many staged laser system. In the works of recent. years to advance in the region of high power light. fields the Nd:glass lasers were suicessfully used and tfie highest J<OWer densities al. nearly 1020 W/cm level have been obt. ained1 • . Two problems would be solved for the beginning when such high PQwer picosecond laser system is creating.The first is the designing of J<icosecond oscillator with performed parameters. Compared witfi cw-raser system, pulsed mode-locked solia state laser offer the advantages of the increased out.put energy and short.er pulses. To overcome the shortcomings of a passive mode-locking concept the idea of feedback controllea generation was _proP.osed by many authors. N~at.ive feedback operation results in armost. 100% inode-Iocking and highly contrast. pulses at the out.put. due to controlled prolongation of the nonlinear stage of generation process. One of the main feature is that the pulse intensity in the cavity mar be maintained by negative feecffiack on the level when considerab e pulse shortening would occur. In this way, pulse duration which is close to the theoretical limit set. by the fluorescence linewidth of the laser material may be obtained. Generation of reproducible subpicosecond li~ht pulses in pulsed Nd: glass oscillator was perfectly realized in ' 4 • Second problem is concerned with the efficient preamplificat.ion of relatively weak pulse from the oscillator out.put.. One of the widely used method in preient is the ul t.rashort. pulses amplification in regenerative amplifier . However, one have to satisfy in this case to sufficiently severe reqqirement.s for the cavities length detuning Ct.o within 0.001- 0.01YJ bet.ween the oscillator and regenerative amplifier.
A compact nuclear radiation dosimetric complex consisting of a photodensitometer and three types of detectors based on the quartz glasses and permitting us to measure the transmittance of glasses having been irradiated by large doses up to 108 Gy and to find the doses value by calibrating dependence between the transmittance of quartz glasses and the doses value is described.
An express method for determination of spatial characteristics of laser radiation allowing us to reduce some operations in photometric process, and as a consequence to decrease systematic mistake and to accomplish the measurement faster, is described.
A device for duration measurements of the picosecond and subpicosecond optical pulses in single shot is described. The elaborated scheme allows us to measure autocorrelation trace in a dynamic range no less than 103. Operation is demonstrated by investigation of the temporal properties of the optical pulse on the output of the mode-locked Nd:glass laser.
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