Laser Crystal Research Articles

Free-space laser emission from Nd:YAG elliptical microdisks.

The utilization of deformed microcavities, such as elliptical microdisks, has been widely acknowledged as an effective solution for achieving free-space emission in microcavity lasers. However, the deformations introduced in the microcavity structure tend to decrease the quality factor (Q factor), resulting in weakened output intensity. To address this issue, one potential approach is to employ highly efficient laser gain media that can compensate for the negative impact of the structure on the output intensity. In this study, we employed the exceptional laser crystal material Nd:YAG as the laser gain medium and successfully fabricated an elliptical microdisk laser with a major semiaxis of 15 µm and an eccentricity ratio of 0.15. By utilizing an 808 nm laser for pumping, we were able to achieve free-space laser emission with a slope efficiency of 1.7% and a remarkable maximum output power of 58 µW. This work contributes toward the advancement of the application of deformation microcavity lasers.

64‐3: Laser Crystallization of Amorphous Silicon via Spot Beam Annealing Method

In spot beam laser annealing, fiber laser and the polygon scanner‐based optical system play a key role by precisely controlling the laser spot position and linear velocities. Key parameters of the laser crystallization are laser power, spot overlap and line overlap. AFM, Raman, and SEM analyzes are used to evaluate polysilicon crystallization state, optimize process parameters, and understand spot beam annealing mechanism. This paper presents the spot beam annealing system configuration and experimental results for low‐temperature poly‐silicon fabrication.

Laterally coupled photonic crystal surface emitting laser arrays

We propose and investigate a novel coherent laser array design based on laterally coupled photonic crystal surface-emitting lasers (PCSELs). As a new type of semiconductor laser technology, PCSELs have field confinement in a planar cavity and laser beam emission in the surface normal direction. By engineering lateral couplings between PCSELs with heterostructure photonic crystal designs, we can achieve coherent operations from an array of PCSELs. In this paper, we demonstrate coherent operation from a passively coupled PCSEL array design. We fabricated PCSEL array devices on a GaAs-based quantum well heterostructure at a target wavelength of 1040 nm. Experimental results show that the 2-by-2 PCSEL arrays have spectral linewidth of 0.14–0.22 nm. Beam combining performance was characterized by self-interference experiments. Similar coherency between the PCSEL array and single PCSEL device was observed. Our compact PCSEL array designs by passive lateral coupling have potential applications in fields of on-chip photonic computing, quantum, and information processing.

Impact of temperature on optical properties of Tm3+-doped disordered oxide crystals for cryogenic lasers emitting near 1.9 μm: a comparative study

Impact of temperature on optical properties of Tm3+-doped disordered oxide crystals for cryogenic lasers emitting near 1.9 μm: a comparative study

10-mJ 300-fs 1-kHz cryogenically cooled Yb:CaF2 regenerative amplifier

In this paper, we report on a cw-pumped cryogenically cooled Yb:CaF2 laser crystal regenerative amplifier that can produce >10.3 mJ, <285 fs laser pulses at 1034-nm and repetition rate of 1-kHz. We achieve record high optical pump to optical amplified compressed output efficiency at >14%. Using a symmetric cavity design to compensate thermal lensing effect, we were able to achieve excellent beam quality with measured M2 at 1.19 and 1.16 at horizontal and vertical directions, respectively.

Fractional thermal load in cryogenically cooled Yb:YLF and Yb:YAG lasers

We present a method for the direct measurement of the fractional thermal load (FTL) in cryogenically cooled laser crystals. The experimental methodology involves characterizing the liquid nitrogen evaporation rate in a dewar containing the laser crystals, allowing for the accurate determination of FTL. The FTL is measured to be 1.7 × quantum defect (QD) for Yb:YLF and 1.5 × QD for Yb:YAG under continuous wave lasing conditions. The measured FTL values are then used to calculate the temperature distribution inside the crystals as a function of pump power, and the simulation results are found to be in very good agreement with the in-situ temperature measurements using contactless optical luminescence thermometry. The method and findings presented in this work hold great potential to benefit laser engineers and scientists working with cryogenic lasers to address and overcome temperature-dependent handicaps.

Crystal growth and spectroscopic properties of Nd3+ -doped SrB4O7 single crystal

Single crystal of Nd3+ -doped SrB4O7 (SBO) has been successfully grown by the Czochralski method. The spectroscopic properties were investigated and spectral parameters were calculated based on the Judd–Ofelt theory. The results suggest that the Nd:SBO crystal may have great potential as laser crystal, and through self-frequency doubling, it is expected to achieve the frequency conversion of the characteristic output of Nd3+ in the near infrared to obtain a visible laser.

ZnSe:(Cr,Fe) laser crystal matrices: Challenges related to doping

We investigate co-doped ZnSe: (Cr, Fe) laser crystals with an approach that includes both optical spectroscopy and theoretical modelling. We found that concentrations of chromium and iron up to 1018 cm−3 are optimal for obtaining a homogeneous solid solution in designing laser crystals. Higher dopant concentrations lead to the formation of clusters with spinel structure in the ZnSe matrix. The optimal effectiveness of ZnSe laser media with active Cr2+ and Fe2+ elements is achieved at temperatures of around 100 K. Heating induces charge transfers like Fe2+→Fe3+ and Cr2+→Cr3+, while cooling results in phonon freezing. A theoretical model has been developed to explain the observed temperature evolution of the absorption spectrum. The types and values of distortions of the Cr- and Fe-based coordination complexes are determined. We have found that the Jahn-Teller distortions are crucial for modelling the temperature-dependent changes of the absorption spectra.

Low energy femtosecond pulsed laser ablation of Nd:YAG laser crystal in high-pressure oxygen and nitrogen background gas

A low-energy femtosecond (fs) pulsed laser was utilized to ablate the Nd:YAG laser crystal target in high-pressure oxygen (O2) and nitrogen (N2) background gases. Only minute amounts of neodymium (Nd), yttrium (Y), and aluminum (Al) were detected in the deposited material due to the thermalization resulting from low laser pulse energy and significant scattering caused by the high background gas pressure. We used a collision-based model and peak-fitting method of the angular distribution of the plume to explain the result. The collision-based model was used to explain and determine the probability distribution of the elements arriving from the target to the substrate. The magnitude of the probability distribution obtained with O2 background gas is higher as compared to N2 background gas owing to the former’s higher collision frequency. The peak-fitting method of the angular distribution of Nd, Y, and Al provided better approximations of the ratio of atoms arriving after ablation as compared to the collision-based model. Further improvements can be carried out to better predict the deposited material ratio.

Structural, optical properties, and laser spectroscopy of erbium-doped low-melting borate glasses

Glasses that contain Erbium oxide as a dopant exhibit garnered considerable interest for its use in photonic applications, primarily because of the intra-4f sublevel transitions of Erbium ions, which enable emission in the technologically significant 1.5 μm communications window. In this work, a series of Erbium oxide doped sodium lithium borate glasses were synthesized via melt quenching with compositions xEr2O3-(55-x)B2O3–25Li2O–10Na2O–5CaO–3SrO–2Al2O3 (x = 0, 1, 2, 3, and 5 mol%). An investigation was conducted to examine the consequences of Er3+ on the crystal structure and laser spectroscopy. Furthermore, the optical properties of the samples that were created. XRD analysis showed degradation of the glass network with increasing Er2O3 content due to disruption of boroxol rings. FTIR spectra indicated variations in BO3/BO4 ratios and Er–O vibrations with doping. Absorption spectra revealed characteristic Er3+ f-f transitions and a redshift in the UV edge. The bright field imaging proved the effect of Er3+ on the homogeneity of the surfaces. Meanwhile, 3D laser Raman mapping chemically proves the good distribution of Er2O3 on the glass surface. With an excitation under 980 nm, visible emissions at 550 and 660 nm, along with NIR emission at 1550 nm arising from Er3+ 4f levels, were observed. The results demonstrate that Er3+ doping effectively modifies the glass structure and optical properties. The borate glass host provides an excellent platform for customized erbium-doped photonic devices operating in the telecom C-band.

Wide-wavelength-tunable operation of Tm:GYAP disordered crystal laser with birefringence filtering

A disordered crystal of Tm:GYAP with a doping concentration of 5 at% was designed and grown. Test results show that the fluorescence spectral half-height width of this crystal is 203 nm, which implies that this crystal is capable of realizing a wide wavelength-tunable laser operation in the 2 µm band. In this study, the Tm:GYAP crystal based laser with a center wavelength of 1936.2 nm and a spectral width of 7.56 nm had been realized by using a compact straight cavity structure. Under a pumping of 8.55 W, a maximum output power of 0.841 W was obtained, and the corresponding optical-to-optical conversion efficiency and the slope efficiency were 9.8% and 12.7%, respectively. In addition, the wavelength-tunable laser operation of Tm:GYAP crystal was realized for the first time by inserting a birefringent filter. The tuning range of the output laser exceeded 120 nm (from 1911.8 nm to 2039.5 nm) even in the compact straight cavity.

Buried Depressed-Cladding Waveguides Inscribed in Nd3+ and Yb3+ Doped CLNGG Laser Crystals by Picosecond-Laser Beam Writing.

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca3Li0.275Nb1.775Ga2.95O12 (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE3+-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

High-power 2.3 µm Tm:YLF laser with intracavity upconversion pumping by a Nd:ASL laser at 1051 nm.

A Tm:LiYF4 laser operating on the 3H4 → 3H5 transition is embedded in a high-power diode-pumped Nd:ASL laser for intracavity upconversion pumping at 1.05 µm. This leads to a record-high output power at 2.3 µm for any bulk thulium laser pumped by an upconversion process. The continuous-wave Tm:LiYF4 laser delivers 1.81 W at 2.3 µm for 32 W of laser-diode pump power, making this kind of pumping competitive with direct diode pumping. The intracavity pumping process allows for counteracting the low absorption inherent to upconversion pumping and to dispatch the thermal loads on two separate laser crystals. The proposed laser architecture also features a relatively weak heating of the Tm:LiYF4 crystal and an increased tolerance to Tm3+ absorption. This laser design opens a new paradigm that holds great promise for high-power 2.3-µm solid-state lasers based on thulium ions.

Laser‐Crystallization of TiO2 Nanotubes for Photocatalysis: Influence of Laser Power and Laser Scanning Speed

AbstractTitanium dioxide (TiO2) nanotubes have raised significant attention in the field of photocatalysis. However, achieving locally‐confined, tunable, and efficient crystallization remains challenging. Laser crystallization that enables surface‐confined, fast, and localized annealing has emerged as an alternative to calcination of TiO2 nanotubes. Nonetheless, questions regarding crystallization parameters and mechanism remain open. In this work, amorphous TiO2 nanotubes are synthesized by a two‐step anodization process and laser exposed (515 nm, 1 MHz, 250 fs, 256–569 mW, 1–200 mm s−1) to study the impact of laser parameters and the mechanism of laser crystallization. A thorough analysis via Raman spectroscopy, X‐ray diffraction, scanning electron microscopy, UV–vis spectroscopy, transmission electron microscopy, and degradation of methylene blue is performed and the influence of laser processing parameters on the crystallization are studied and discussed. As laser power increases, a gradual transition from amorphous to anatase and rutile phase takes place, consistent with a thermal crystallization mechanism. Contrary, with increasing laser scanning speeds, thus lower pulse numbers, anatase is converted to rutile and melting occurs. The degradation of methylene blue with anatase samples crystallized by laser and calcination exhibits equal kinetic degradation constants of 0.013 s−1cm−2. This study enhances the understanding of laser crystallization of TiO2 nanotubes.

Upconversion-pumped femtosecond thulium laser at 2309 nm mode-locked by a GaSb-based SESAM

We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.

3.73 W all-solid-state dual-wavelength Nd:Lu2O3 crystal laser

The laser output characteristics of Nd:Lu2O3 crystals were investigated in detail to obtain a dual-wavelength all-solid-state laser. Using 806 nm LD end-face pumped Nd:Lu2O3 crystals with lengths of 6 mm, a 1076 &amp; 1080 nm laser outputs with a maximum output power of 3.73 W were obtained, with a slope efficiency of 30.4%, an optical-to-optical conversion efficiency of 28.5%, and a power stability of 0.41% for 4 h of continuous measurement. Furthermore, by suppressing the higher-order modes, a high beam quality laser output with beam quality factors of 2.092 and 1.589 in the x and y directions, respectively, and a maximum output power of 1.27 W were obtained. In addition, it was experimentally verified that both wavelengths of the output laser were elliptically polarized.

Topological photonic crystal nanowire array laser with bulk states

A topological photonic crystal InGaAsP/InP core-shell nanowire array laser with bulk states operating in the 1550 nm band is proposed and simulated. By optimizing the structure parameters, high Q factor of 1.2 × 105 and side-mode suppression ratio of 13.2 dB are obtained, which are 28.6 and 4.6 times that of a uniform nanowire array, respectively. The threshold and maximum output are 17% lower and 613% higher than that of the uniform nanowire array laser, respectively, due to the narrower nanowire slits and stronger optical confinement. In addition, a low beam divergence angle of 2° is obtained due to the topological protection. This work may pave the way for the development of high-output, low-threshold, low-beam-divergence nanolasers.

Highly efficient continuous-wave solid-state Raman crystal lasers at 555 and 559 nm

High-power efficient continuous-wave Nd:YVO4/KGW Raman lasers at 555 and 559 nm are achieved by using a double-sided dichroic coating output coupler to improve the resonance quality factor. The Np-cut potassium gadolinium tungstate (KGW) is used to generate the Stokes waves at 1159 and 1177 nm by placing the polarization of the1064 nm fundamental wave parallel to the Ng and Nm axes, respectively. The lithium triborate (LBO) crystal with the cut angle in the XY plane for the type-I phase matching is used to perform the intracavity sum frequency generation for yielding the green light at 555 nm and the lime light at 559 nm at the optimal phase matching temperature. Experimental results were systematically accomplished to comprehend the optimal cavity length for the conversion efficiency. Under the optimal cavity length, the output powers can reach 6.6 and 6.3 W at a pump power of 22 W for the wavelengths of 555 and 559 nm, respectively. The conversion efficiencies can be up to 30% and 28.6% for 555 and 559 nm, respectively.

Local ligand distortion broadens the near-infrared emission of Kramers neodymium ions by partially replacing La with Gd in NaLa(WO4)2 crystals

A series of x at% Gd3+: NaLa0.975-xNd0.025Gdx(WO4)2 mixed crystals were grown by means of the Czochralski method. The mixed crystals are formed by replacing La3+ ions in a part of NaLa(WO4)2 host with Gd3+ ions, and the first principle proves that doping Gd3+ improves the disorder and activity of NaLa(WO4)2 crystals. For NaLa0.915Nd0.025Gd0.06(WO4)2 mixed crystals, the physical and thermal properties were characterized by using structural refinement, infrared spectrum and thermal expansion coefficient. The results show that the grown crystal belongs to the space group Ι41/a with high phase purity, and has excellent thermal stability and anisotropy. Compared with NaLa0.975Nd0.025(WO4)2 single crystal, with the introduction of Gd3+, the local distortion electric field is generated, the original degenerate state of Nd3+ is changed, and the intensity of Nd3+ emission peak at 1062 nm and 1332 nm is increased by 2.34 and 1.89 times, respectively. The spectral bands of absorption and emission spectra are broadened by about 4 nm. The broadening of spectral band is discussed in detail by using the theory of uniform and non-uniform broadening. For uniform band broadening, the density functional theory is used to calculate the phonon state density and vibration mode of the mixed crystal, and the Raman spectrum proves that the spectral band broadening is caused by the introduction of Gd3+, which generates a new vibrational level and increases the electron-phonon coupling, thus widening the band. For the non-uniform broadening of the spectral band, combined with the variation of the dipole moment caused by the electric field distortion, the spectral band is proved. Finally, the J-O theoretical parameters, absorption and emission cross-sections of mixed crystals are calculated. The experimental results show that NaLa0.915Nd0.025Gd0.06(WO4)2 mixed crystals are excellent laser crystal materials, which can improve the utilization of laser pump light and improve the efficiency of laser.

Optimal waveguide structure for low-threshold InGaN/GaN-based photonic-crystal surface-emitting lasers

We analyzed the optimal waveguide structure of two types of InGaN-based photonic crystal surface-emitting lasers (PCSELs) to suppress the coupling with leaky modes via mode simulations. To minimize the threshold material gain (gth), we calculated the confinement factor and quality factor of PCSELs with varying waveguide layer thicknesses in the separate confinement heterostructure (SCH) layer. The optical mode intensity profile revealed the coupling between the fundamental mode of SCH and parasitic leaky modes in the cladding layer or substrate as the primary root cause of the low-quality factor and high threshold gain of PCSELs. The asymmetric nature of the SCH structure yielded the optimal waveguide structure to be dependent on the position of the air holes. With a proper waveguide thickness and air hole depth, the optimized threshold modal gain of PCSELs with the n-side air holes can be less than 30 cm−1.