BBO Crystal Research Articles

Nonlinear mirror mode-locked Pr:YAlO3 laser

Mode-locking of a praseodymium based solid-state laser by a nonlinear mirror is reported, for the first time to the best of our knowledge. Pr:YAlO3 crystal having 0.6 at.% of Pr3+ ions was used as an active medium. For mode-locking, a nonlinear BBO crystal together with a properly designed dichroic mirror were employed to form an output coupler with intensity dependent reflection. A continuous-wave mode-locked laser operation with a mean output power of 100 mW was achieved under 3.5 W InGaN laser diode pumping. The laser emitted 250 ps pulses with a repetition rate of 105 MHz at a center oscillation wavelength of 747 nm.

Proposal for electro-optically tunable Kerr-Lens in BBO crystal by cascaded second-order process

In this report, a linear electro-optically tunable non-phase matched second-order nonlinear process is transformed into a cascaded second-order nonlinear process in a bulk BBO crystal to generate the effect of electro-optically tunable Kerr-type nonlinearity. A phase mismatch condition is created by applying an electric field to the x–z plane of the crystal which introduces a nonlinear phase shift between the launched and reconverted fundamental waves from the generated second harmonic wave. The sign of this nonlinear phase shift is easily changed by changing the polarity of applied electric field. The nonuniform radial intensity distribution of incident Gaussian beam introduces a curvature to the fundamental wavefront which focuses or defocuses the incident beam while propagating through the medium. Hence, a positive or negative lens is established within the crystal. The focal length of that lens is also tuned by tuning the applied electric field. The maximum calculated effective nonlinear refractive index due to nonlinear phase shift is ±6.6508×10−14cm2/W.

Third-harmonic-generation of a diode laser for quantum control of beryllium ions.

We generate coherent ultraviolet radiation at 313 nm as the third harmonic of an external-cavity diode laser. We use this radiation for laser cooling of trapped beryllium atomic ions and sympathetic cooling of co-trapped beryllium-hydride molecular ions. An LBO crystal in an enhancement cavity generates the second harmonic, and a BBO crystal in a doubly resonant enhancement cavity mixes this second harmonic with the fundamental to produce the third harmonic. Each enhancement cavity is preceded by a tapered amplifier to increase the fundamental light. The 36-mW output power of this all-semiconductor-gain system will enable quantum control of the beryllium ions' motion.

Second-order cascading-assisted filamentation and controllable supercontinuum generation in birefringent crystals.

We experimentally investigate filamentation and supercontinuum generation in a birefringent medium (BBO crystal), in the self-focusing regime where intrinsic cubic nonlinearity is either enhanced or reduced by the second-order cascading due to phase-mismatched second harmonic generation. We demonstrate that the supercontinuum spectral extent is efficiently controlled by varying the phase mismatch parameter. In the range of negative phase mismatch, we achieve full control of the blue-shifted spectral broadening, which is very robust and independent on the input pulse energy. In the range of positive phase mismatch, both the blue-shifted and the red-shifted spectral broadenings are controlled simultaneously, however showing a certain dependence on the input pulse energy. The results are interpreted in terms of complex interplay between the self-phase-matched second harmonic generation, which is a process inherent to narrow ultrashort pulsed laser beams and concurrent self-steepening processes which arise from cubic and cascaded-quadratic nonlinearities.

Experimentally testing Hardy’s theorem on nonlocality with entangled mixed states**Project supported by the National Natural Science Foundation of China (Grant Nos. 61308008 and U1330201).

Hardy’s theorem on nonlocality has been verified by a series of experiments with two-qubit entangled pure states. However, in this paper we demonstrate the experimental test of the theorem by using the two-photon entangled mixed states. We first investigate the generic logic in Hardy’s proof of nonlocality, which can be applied for arbitrary two-qubit mixed polarization entangled states and can be reduced naturally to the well-known logic tested successfully by the previous pure state experiments. Then, the optimized violations of locality for various experimental parameters are delivered by the numerical method. Finally, the logic argued above for testing Hardy’s theorem on nonlocality is demonstrated experimentally by using the mixed entangled-photon pairs generated via pumping two type-I BBO crystals. Our experimental results shows that Hardy’s proof of nonlocality can also be verified with two-qubit polarization entangled mixed states, with a violation of about 3.4 standard deviations.

Tunable terahertz waves from 4-dimethylamino-N′-methyl-4′-stibazolium tosylate pumped with dual-wavelength injection-seeded optical parametric generation

We developed a widely tunable terahertz (THz)-wave source covering the sub-THz frequency by difference frequency generation using a 4-dimethylamino-N′-methyl-4′-stibazolium tosylate (DAST) crystal. Near-infrared waves generated by dual-wavelength injection-seeded β-BaB2O4 optical parametric generation (is-BBO-OPG) were used for pumping the DAST crystal, which had separated wavelengths in the spectrum with a difference frequency of sub-THz. Furthermore, the non-collinear phase-matching condition was designed to compensate the walk-off effect of the BBO crystal. Consequently, tunable THz-waves from 0.3 to 4 THz were generated by tuning the wavelength of one of the seeding beams. The generated sub-THz-waves were monochromatic (dν < 33 GHz) with a maximum energy of 80 pJ at 0.65 THz.

Quality of polarization entanglement in spontaneous parametric down conversion

We experimentally demonstrated a high degree of polarization entanglement known as entanglement visibility through spontaneous parametric down conversion process pumped by a femtosecond laser. The entangled-photon pair was obtained using two type-I BBO crystal. The down-converted photons from these crystals demonstrates a high visibility of 98.7% (θ2 = 0°) and 90% (θ2 = 22.5°). These results are in agreement with the theory which expects high visibility from such arrangement.

High-accuracy deep-UV Ramsey-comb spectroscopy in krypton

In this paper, we present a detailed account of the first precision Ramsey-comb spectroscopy in the deep UV. We excite krypton in an atomic beam using pairs of frequency-comb laser pulses that have been amplified to the millijoule level and upconverted through frequency doubling in BBO crystals. The resulting phase-coherent deep-UV pulses at 212.55 nm are used in the Ramsey-comb method to excite the two-photon 4p^6 rightarrow 4p^5 5p [1/2 ]_0 transition. For the {}^{84}hbox {Kr} isotope, we find a transition frequency of 2829833101679(103) kHz. The fractional accuracy of 3.7,times10^{-11} is 34 times better than previous measurements, and also the isotope shifts are measured with improved accuracy. This demonstration shows the potential of Ramsey-comb excitation for precision spectroscopy at short wavelengths.

Parametric amplification of broadband radiation of a cw superluminescent diode under picosecond pumping

It is proposed to use cw superluminescent diodes with a spectral width of about 300 and high spatial coherence as seed radiation sources in parametric amplifiers with picosecond pumping in order to form broadband picosecond pulses. A two-cascade parametric amplifier based on (BBO) crystals is pumped by pulses of the second harmonic of an Nd : YAG laser. For a superluminescent diode spectral width of (centre wavelength ), the spectral width of picosecond pulses at the parametric amplifier output is . At a total pump energy of for BBO crystals, the energy of the enhanced emission of the superluminescent diode is found to be .

Selecting the pre-detection characteristics for fiber coupling of parametric down-converted biphoton modes

Photon modes have an important role in characterizing the quantum sources of light. The two main pre-detection factors affecting the biphoton mode coupling in SPDC are the pump beam focusing parameter and the crystal thickness. We present the numerical and experimental results on the effect of pump focusing on conditional down-converted photon modes for a Type-I BBO crystal. We experimentally verify that biphoton coupling efficiency decreases asymptotically with pump beam focusing parameter. We attribute this behaviour to (a) the asymmetry in the spatial distribution of down-converted photons with the pump beam focusing parameter and (b) the ellipticity of biphoton modes introduced due to the focusing of the pump beam. We also show the ellipticity experimentally as well as quantify it with the focusing parameter. These results may be useful in selecting optimum conditions for generating efficient fiber coupled sources of heralded single photons and entangled photons for quantum information applications.

Efficient generation of the second and third harmonics of high-power femtosecond Yb:KGW-laser radiation in nonlinear-optical BBO crystals

The frequency conversion of radiation into the second and third harmonics of a high-power femtosecond laser based on Yb:KGd(WO4)2 (Yb:KGW) crystals in BBO (β-BaB2O4) crystals has been experimentally and quantitatively investigated. Under the optimum compression conditions of femtosecond pulses of width 300 fs, the conversion efficiency into the second harmonic reached 50%, and that into the third harmonic reached 20%. The introduction of additional phase modulation reduced the generation efficiency of the second harmonic to an extent that is different for positive and negative signs of the phase modulation. The results of numerical calculations quantitatively agree with the experimental results.

The nanosecond optical parametric amplifier of a weak signal based on BBO crystals

Parameters of the optical parametric amplifier (OPA), based on two BBO crystals were studied. The OPA was made with the schematic of the extraordinary wave walk off compensation. Efficient amplification of the weak signal (λ = 1053 nm) in the field of the strong pumping wave (λ = 532 nm) was obtained. The measured value of the amplification was equal to ~106. The noise level of the parametric amplifier was less than 10−3 from the signal level.

Nucleation and Crystal Growth in Laser-Patterned Lines in Glasses

Laser-induced crystallization is a new method for the design and control of the crystallization of glasses and opens a new door in the study of nucleation and crystal growth in glasses. Nonlinear optical Sm-doped -BaB2O4 (-BBO) crystal lines were patterned by continuous wave Yb:YVO4 fiber laser (wavelength 1080 nm) in 8Sm2O3-42BaO-50B2O3 glass as an example, and nucleation and crystal growth behaviors in the laser-patterned bending and crossing lines were examined. It was confirmed that the growth of c-axis oriented -BBO crystals follows along the laser scanning direction even if laser scanning direction changes. The model of self-organized homo-epitaxial crystal growth was demonstrated for the orientation of -BBO crystals at the crossing point of two lines, in which the first crystal line at the crossing point acts as nucleation site for the second crystal line. This study proposes a new crystal growth technology.

Simple Autocorrelation Measurement by Using a GaP Photoconductive Detector

We developed a simple and real-time readout autocorrelator for several tens and sub-10fs pulses, based on the two photon absorption phenomena of a commercial GaP photodetector including a transimpedance amplifier. With a suitable gain adjustment, we demonstrated that the interferometric autocorrelation for sub-nJ pulses delivered as a high output voltage as to resolve all fringes in an autocorrelation trace with features of low noise and a low offset voltage. By fitting the measured quadratic power dependence of output voltages, we obtained the quantum efficiency of TPA for the GaP detector comparable with those of a GaAsP diode and an SHG with a thin BBO crystal. The autocorrelator of a TPA based GaP photodetector is highly suitable for sensitively measuring a few cycle pulses with a broad spectral distribution from 600 nm to 1100 nm.

Tunable broadband intense IR pulse generation at non-degenerate wavelengths using group delay compensation in a dual-crystal OPA scheme.

A robust group delay compensated dual-crystal optical parametric amplification (DOPA) scheme is proposed that will be used to prove the positive effect of group delay compensation on a DOPA as predicted by the simulations in the previously published literature. Through simple adjustments, it is also capable of providing 20 fs pulses (theoretically compressible to 12 fs, corresponding to sub-four-cycle for 1300 nm components), broadband IR pulses at non-degenerate wavelengths using short pulse (broadband) pump laser. In our table-top DOPA system, group delay compensation has been realized using a simple optical crystal. Our design provides output power in order of 100 mW. We managed to achieve minimum 20 nm improvement on the bandwidth, compared to single-crystal OPA (SOPA) structure whilst keeping total conversion efficiency above 30%. Adjusting our configuration by optimizing the phase-matching angles of the two BBO crystals, we also have realized a practical scheme that benefitting from group delay compensation can obtain 75 nm bandwidth improvement while keeping the conversion efficiency constant. This achievement will open the doors to the realm of multiple crystals OPA systems and provide a solution to the imposed limitation on the effective lengths of applicable non-linear crystals and hence limited power gain of such broadband OPA systems.

Microsphere-based cantilevers for polarization-resolved and femtosecond SNOM

We present a cantilever-based near-field probe with integrated Mie scattering dielectric SiO2 microsphere (MSDM) for near-field optical imaging as well as femtosecond spectroscopy applications. In contrast to the state-of-the-art transmissive near-field probes, the MSDM reveals a transmission of almost unity known from far-field microscopy configuration. For proper handling, the microsphere is integrated at the apex of a conventional pyramidal aperture tip carried by an atomic force microscopy cantilever. It proved to be mechanically robust during the scanning process even if operating it in the contact mode. The spherical symmetry provides on the one hand a well-defined mechanical contact point with the sample irrespective of its inclination angle to the sample surface. On the other hand, the symmetry of the device preserves the polarization of light proving to be useful for the investigation of the polarization dependent behavior of plasmonic nanostructures. The high transmission combined with low dispersion renders spectroscopic investigations on the femtosecond timescale with a moderate lateral resolution. Second order autocorrelation experiments on a BBO crystal reveals a time resolution well below \(100\, {\hbox{fs at }} 191\, {\hbox{nm}}\) spatial resolution.

Optical parametric amplification by incoherent pump in the trapping regime

Optical parametric amplification of signal and idler waves in the field of the incoherent pump is investigated. The improvement of the coherence of the amplified waves in the trapping regime is demonstrated. Numerical simulations for the experimental situation in the BBO crystal are provided. It is elucidated that the wide range of wavelengths of the interacting waves can be used.

Continuous-wave, single-frequency 229 nm laser source for laser cooling of cadmium atoms.

Continuous-wave output at 229nm for the application of laser cooling of Cd atoms was generated by the fourth harmonic using two successive second-harmonic generation stages. Employing a single-frequency optically pumped semiconductor laser as a fundamental source, 0.56W of output at 229nm was observed with a 10-mm long, Brewster-cut BBO crystal in an external cavity with 1.62W of 458nm input. Conversion efficiency from 458nm to 229nm was more than 34%. By applying a tapered amplifier (TA) as a fundamental source, we demonstrated magneto-optical trapping of all stable Cd isotopes including isotopes Cd111 and Cd113, which are applicable to optical lattice clocks.

Generating quasi-single-cycle THz pulse from frequency-chirped electron bunch train and a tapered undulator

We propose a proof-of-principle experiment to test a new scheme to produce a single-cycle radiation pulse in free-electron lasers (FELs). Here, a few ${\it\alpha}$ -BBO crystals will be first used to produce an equally spaced laser pulse train. Then, the laser pulse train illuminates the cathode to produce a frequency-chirped electron bunch train in a photocathode rf gun. Finally, the frequency-chirped electron bunch train passes through a tapered undulator to produce a quasi-single-cycle THz pulse. This experiment should allow comparison and confirmation of predictive models and scaling laws, and the preliminary experimental results will also be discussed.

Frequency doubled femtosecond Ti:sapphire laser with an assisted enhancement cavity**Project supported by the National Basic Research Program of China (Grant Nos. 2013CB922401 and 2012CB821304) and the National Natural Science Foundation of China (Grant No. 61378040).

We report an enhancement cavity for femtosecond Ti:sapphire laser at the repetition rate of 170 MHz. An enhancement factor of 24 is obtained when the injecting pulses have an average power of 1 W and a pulse duration of 80 fs. By placing a BBO crystal at the focus of the cavity, we obtain a 392-mW intracavity doubled-frequency laser, corresponding to a conversion efficiency of 43%. The output power has a long-term stability with a root mean square (RMS) of 0.036%.