Review pubs.acs.org/cm
Deep Ultraviolet Nonlinear Optical Materials T. Thao Tran,† Hongwei Yu,† James M. Rondinelli,‡ Kenneth R. Poeppelmeier,§ and P. Shiv Halasyamani*,† †
Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States Department of Materials Science and Engineering and §Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3108, United States
‡
ABSTRACT: Deep ultraviolet (absorption edge 6.2 eV) nonlinear optical (NLO) materials are of current interest owing to their technological applications and materials design challenges. Technologically, the materials are used in laser systems, atto-second pulse generation, semiconductor manufacturing, and photolithography. Designing and synthesizing a deep UV NLO material requires crystallographic noncentrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient (dij > 0.39 pm/V), moderate birefringence (Δn ∼ 0.07), chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. This review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence. Finally, future directions with respect to new deep UV NLO materials are discussed.
■
borate fluorides are purple, and the miscellaneous materials are given in black. For reference, the 157 nm F2 and 193 nm ArF excimer wavelengths (dashed blue lines), the fifth, sixth, and seventh Nd:YAG (1064 nm) harmonic wavelengths (dotted green lines), and the 6, 7, 8, and 9 eV energies (dashed red lines) are given. There are only five materials with an absorption edge below the F2 excimer line of 157 nm: BPO4, LiB3O5 (LBO), Sr2Be2B2O7 (SBBO), CsBe2BO3F2 (CBBF), and KBe2BO3F2 (KBBF). Of these, only KBBF and CBBF are useful in the deep UV, attributable to the poor birefringence of LBO and SBBO. As mentioned earlier, six conditions must be satisfied for a functional deep UV NLO material.3,7 1. Symmetry. Materials that may exhibit deep UV NLO behavior must crystallize in one of the 20, of the 21, noncentrosymmetric crystal classes (the cubic crystal class 432 is acentric, but the nonzero third-rank tensors are of equal and opposite magnitude, resulting in a zero SHG response).8 2. Wide UV transparency range, i.e. absorption edge 6.2 eV). Obviously, given the absorption edge, the materials need to be colorless. This would indicate that cations in these materials cannot have any d−d or f−f transitions. Thus, most transition metal and lanthanide cations cannot be incorporated. Cations with fully occupied d or half-occupied f electronic shells, such as Zn2+, Gd3+, and Y3+ can also be considered as their electronic shells effectively inhibit unfavorable electronic transitions. In addition, many main group metals have absorption edges well above 200 nm. Thus, the range of cations that can be used in any deep UV NLO material is limited. Even with this limitation, there are a variety of cations and anionic groups that can be used in the design and
INTRODUCTION Materials capable of generating coherent deep ultraviolet (UV) light, that is wavelengths below 200 nm (Eg > 6.2 eV), are of intense interest from both an academic and technological standpoint. Technologically, these materials are used in semiconductor manufacturing, photolithography, laser systems, atto-second pulse generation, and advanced instrument development.1−4 Coherent light in the deep UV is also needed for laser-based ultrahigh resolution photoemission spectrometry and photoelectron emission microscopy. Coherent radiation in the UV and deep UV is possible with excimer lasers, e.g. ArF excimer at 193 nm and F2 excimer at 157 nm. However, solidstate lasers in these wavelength ranges are often preferred, which is attributable to handling ease, narrow bandwidth, tunability, energy density, and peak power density. An excellent manner in which to generate coherent deep UV light is through solid-state lasers using cascaded frequency conversion with nonlinear optical (NLO) materials, i.e. sixth harmonic generation of 1064 nm (Nd:YAG) radiation: 177.3 nm, i.e. 1064 nm/6.5 For years, there has been a “200 nm wall”; that is, no material existed for which sixth harmonic generation was possible.6 As we will discuss, this wall has been surmounted by a few materials. From a material’s design and synthetic chemistry perspective, it is an ongoing challenge to discover a new material with the following attributes: crystallographically noncentrosymmetric (NCS), wide band gap (Eg > 6.2 eV), large second-harmonic generating (SHG) coefficients (dij > 0.39 pm/V (d36 KDP), moderate birefringence (Δn ∼ 0.07), chemically stable with a large laser damage threshold (LDT > 5 GW/cm2), and easy growth of large (centimeter size) highquality single crystals. The deep UV spectrum from ∼225 to 125 nm is shown in Figure 1. The materials discussed in this review, with welldefined deep UV absorption edges, are noted at the appropriate places on the spectrum. The borates are colored orange, the © 2016 American Chemical Society
Received: June 13, 2016 Revised: July 7, 2016 Published: July 12, 2016 5238
DOI: 10.1021/acs.chemmater.6b02366 Chem. Mater. 2016, 28, 5238−5258
Review
Chemistry of Materials
Figure 1. Deep UV spectrum from ∼225 nm to 125 nm depicting the absorption edges of the materials discussed in this review, along with excimer wavelengths (F2 and ArF), 5th, 6th, and 7th harmonics of Nd:YAG (1064 nm), and the 6, 7, 8, and 9 eV energies.
fundamental beam power; tω and T2ω = transmission coefficients of the fundamental and the harmonic waves, respectively; R(θ) = incident multiple-reflection correction; p(θ) = projection factor; β(θ) = beam size correction; L = crystal thickness; λ = wavelength of the fundamental beam. If
synthesis of new deep UV NLO materials, including alkali and alkaline-earth metals, as well as borate, nitrate, carbonate, and phosphate groups. 3. Large SHG coefficient, dij, compared to d36 KDP (0.39 pm/V). The SHG coefficients of NLO materials are directly related to the SHG conversion efficiency. Materials with large SHG coefficients are capable of high conversion efficiencies. In the deep UV region, a SHG coefficient dij, that is comparable to or larger than d36 KDP (0.39 pm/V), is required to obtain high conversion efficiencies. Individual NLO coefficients, dij’s, are often determined by Maker Fringe measurements.9,10 These measurements require cut parallel plate single crystals that have been indexed and polished. The faces of these crystals must be on the order of 5 mm. The number of independent nonzero dij’s are dependent on the crystal class, and also take into account Kleinman symmetry.11,12 In most instances the d36 NLO coefficient of KDP is used as a reference, i.e. d36(KDP) = 0.39 pm/V. Maker Fringe measurements can be described as follows. The generated second harmonic power, P2ω, may be expressed by9,10
f (n , θ ) =
1 tω4T2ω R(θ ) p2 (θ ) β(θ ) − n22ω)2
then P2ω can be simplified to P2ω(θ ) =
512π 2d 2pω2 cw 2
f (n , θ ) sin 2 Ψ
The f(n,θ) function depicts the envelope of the Maker fringe, and sin2 Ψ determines the minimum oscillating position. By fitting the calculated and measured Maker fringes, a constant, C = 512π2d2pω2/(cw2), can be obtained. The magnitude of the second-order coefficient of a NLO crystal can be determined relative to d36 (KDP), and the equation can be expressed as dsample =
512π 2 d 2Pω2 tω4T2ωR(θ )p2 (θ )β(θ ) sin 2 Ψ P2ω(θ ) = 2 2 cw (nω − n22ω)2
Csample CKDP
× d36(KDP)
In this manner, the dij’s of a properly cut, polished, and indexed deep UV crystal can be determined and compared to d36(KDP). 4. Moderate birefringence, Δn ∼ 0.07−0.10, to achieve phase-matching in the UV and deep UV. In order to effectively generate coherent light by the SHG processes, a phase-matching condition is required; that is, the refractive index of the harmonic radiation equals the refractive
where Ψ=
(nω2
2πL (nω cos θω − n2ω cos θ2ω) λ
With the above, c = speed of light in air; w = radius of the laser beam; nω and n2ω = refractive indices at the fundamental and harmonic wavelengths, respectively; d = NLO coefficient; Pω = 5239
DOI: 10.1021/acs.chemmater.6b02366 Chem. Mater. 2016, 28, 5238−5258
5240
eV)109 eV)110 eV)110 eV)110 eV)110 eV)111
6.2 eV)111
(>6.2 (>6.2 (>6.2 (>6.2 (>6.2 (>6.2
(6.67 eV)97 (6.53 eV)98 (6.53 eV)100 (6.53 eV)99 (6.2 eV)184
GdBe2B5O11111
nm nm nm nm nm nm
nm nm nm nm nm
