Li2CdGeS4, A Diamond-Like Semiconductor with Strong Second

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Li2CdGeS4, A Diamond-Like Semiconductor with Strong SecondOrder Optical Nonlinearity in the Infrared and Exceptional Laser Damage Threshold Jacilynn A. Brant,† Daniel J. Clark,‡ Yong Soo Kim,‡,⊥ Joon I. Jang,‡ Jian-Han Zhang,† and Jennifer A. Aitken*,† †

Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, United States ⊥ Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, South Korea ‡

S Supporting Information *

will be difficult to find other materials with larger nonlinearities in the IR region, and this statement has since been viewed as prophetic.3b,4 Although the diamond structure is among the most commonly encountered, the full potential of diamond-like materials surprisingly remains to be realized. The logical successors of these diamond-like materials with proven technological utility are quaternary DLSs, and taking these materials to the next level is an attractive means for discovering forthcoming NLO materials. A driving force behind much materials discovery is the production of coherent laser beams in IR and THz regimes,5 which can be accessed by commercially available means but with limited efficiencies due to a lack of suitable materials. Many factors are involved in selecting outstanding NLO materials: (1) phase-matchability, (2) strong second-order nonlinear susceptibility, (3) wide optical transparency, (4) thermal stability, (5) availability of large single crystals, (6) environmental stability, etc.3b Typically, narrow-bandgap materials have been considered to enhance the second-order NLO coefficients,6 but highly efficient generation of long wavelengths (λ) requires sufficiently wide bandgaps. In fact, most IR NLO crystals have relatively narrow bandgaps, which tend to impart low laser damage thresholds (LDTs) and twophoton absorption (TPA) limitations.7 Metal oxide NLO materials are widely developed but are generally unsuitable for broadband IR applications due to absorption. Organic materials and polymers can exhibit high optical nonlinearity but lack sufficient transparency and have poor thermal stability and low damage thresholds.8 Due to these shortcomings, improved NLO materials are sought after. Numerous approaches to discovering new NLO materials are based on assembling asymmetric building blocks to target noncentrosymmetric structures. Building blocks can be based on (BO3)3− units,9 second-order Jahn−Teller (SOJT) distorted d0 transition metal cations (e.g., Nb5+and V5+),10 polar displacement of d10 cations (e.g., Pb2+, Cd2+),9b,11 and stereochemically active lone pairs, such as those in SeO32− and IO3−.12 Although these approaches are valuable, accessing

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he discovery of new nonlinear optical (NLO) materials for coherent generation of electromagnetic waves in the infrared (IR) and terahertz (THz) region is essential for the advancement of important technologies that rely on tunable lasers, including minimally invasive surgery,1 the sensing of hazardous and high-risk materials for homeland security, industrial process controls, etc.2 Inherently noncentrosymmetric diamond structures can be exploited in the pursuit of improved properties, such as strong second-order nonlinearity in the IR, since the structures are predictable and accessible with a range of compositions. Using a basic design strategy, a higher level of versatility is introduced to diamond-like semiconductors (DLSs) in order to manipulate properties, as the hierarchy is extended from ternary chalcopyrites to a quaternary NLO material with the wurtz-stannite structure, Li2CdGeS4 (Figure 1). DLSs are of interest since they dominate the list of current benchmark NLO materials in the IR region, such as chalcopyrite-type AgGaS2, AgGaSe2, and ZnGeP2, as well as sphalerite-type GaAs and GaP.3 Levine stated in 1973 that the chalcopyrite structure is so favorable for NLO properties that it

Figure 1. (Left) I and III cations combine with a chalcogenide to assemble the chalcopyrite structure. (Right) Quaternary DLSs with the wurtz-stannite structure are targeted by reconstituting III cations with II and IV tetrahedral cations. © 2014 American Chemical Society

Received: March 23, 2014 Revised: April 24, 2014 Published: April 29, 2014 3045

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and ∼70× α-quartz, respectively, when exposed to a Nd:YAG laser (1064 nm).20 Herein, using the Kurtz powder method,21 the wavelength and particle size dependences of SHG in Li2CdGeS4 are reported over a broad fundamental range. The impact of TPA on SHG in Li2CdGeS4 was investigated under typical picosecond excitation (1064 nm). These data indicate that Li2CdGeS4 greatly surpasses the benchmark AgGaSe2 from the perspective of the IR-generation efficiency in a conventional DFG scheme. The phase-purity of Li2CdGeS4, as well as the AgGaSe2 references, was confirmed using data that were collected on the high-resolution powder diffraction beamline, 11-BM, at the Advanced Photon Source, Argonne National Laboratory. In order to assess the quality of the reference, the SHG response of AgGaSe2(MC), a microcrystalline powder sample obtained using conventional solid-state synthesis, was compared with that of an optical-quality single crystal of AgGaSe2 obtained from Gooch & Housego that was ground into powder, AgGaSe 2 (OQ). The SHG efficiency of AgGaSe2(MC) is lower than that of AgGaSe2(OQ) (Supporting Information Figure S5), likely due to a higher defect concentration in AgGaSe2(MC). Likewise, the defect concentration of Li2CdGeS4 is expected to be higher than if it were grown as an optical-quality single crystal. Thus, the χ(2) value of Li2CdGeS4 was estimated using AgGaSe2(MC). The validity of using a quality homemade reference in the estimation of the χ(2) value is not unprecedented.22 A χ(2) value for Li2CdGeS4 using AgGaSe2(OQ) is also given in Supporting Information Figure S5, which corresponds to the lower bound of χ(2). The particle size dependence of the SHG was measured with incident λ of 1.1−3.3 μm, on particles with diameters up to 106 μm (Figure 3). A pronounced dip in the broadband SHG near

and controlling the assembly of these building blocks can be challenging, compositions can be limited, and competing centrosymmetric structures are often difficult to avoid. Here, we employ tetrahedral building blocks that are predisposed to assemble diamond-like structures by aligning along a single crystallographic direction, rendering the structures noncentrosymmetric, Figure 1. The readily accessible diamond structure, which is often termed the default structure for 4-connected nets,13 is accompanied by minimal centrosymmetric contenders when design principles are in play. On the basis of electrostatic valency, vast combinations of elements that are capable of exhibiting tetrahedral coordination can be employed to target diamond-structured materials of nine quaternary formulae.14 The I2−II−IV−VI4 formula15 is attractive since both chalcogenides and alkali metals (e.g., Li) can be incorporated while many divalent and tetravalent ions can be selected for property tuning. Much progress has been made in the past decade through the discovery of Li-containing chalcogenides that exhibit wide bandgaps16 befitting improved laser damage thresholds and reduced probability of TPA when pumping with intense ultrafast near-IR lasers. For example, LiInS2, with a bandgap of 3.56 eV, is a mature NLO material for difference frequency generation (DFG) of powerful radiation in the deep mid-IR due to the wide transparency (0.4−12 μm), relatively high nonlinear susceptibility (χ(2) ∼ 14 pm/V), and a high laser damage threshold (200 kW/cm2) that is an improvement over AgGaS2 (25 kW/cm2).16b Further improvements in LDT in combination with improved second harmonic generation (SHG) and transparency are warranted for practical applications. Indeed, exploration of composition space among I2−II−IV−VI4 DLSs has led to the discovery of Li2CdGeS4, which outperforms LiInS2 because it has a higher damage threshold > 4 GW/cm2 with no apparent TPA effect, is phasematchable in the IR region, and exhibits stronger nonlinearity, χ(2) ∼ 51 pm/V. The compound melts at ∼890 °C (Supporting Information Figure S6) and is stable in air and water. According to optical measurements, Li2CdGeS4 has a direct bandgap of 3.15 eV (Supporting Information Figure S7). Our electronic structure calculations support the presence of a direct gap (Supporting Information Figure S8), which conflicts with the calculations by Li et al.17 Li2CdGeS4 is transparent from ∼0.5 to 22 μm (Figure 2), which is wider than typical clarity

Figure 3. (Left) Particle size dependence of the SHG for Li2CdGeS4. (Right) The broadband SHG response.

the conversion λ of 1.0 μm is common for all measurements due to fundamental absorption by the capillary sample tubes but does not affect the estimation of the SHG coefficient. In the near-IR (λ < 1.7 μm), Li2CdGeS4 shows a significantly stronger SHG response than AgGaSe2 (MC and OQ). This arises essentially from the narrow bandgap of the reference (1.7 eV), causing substantial bandgap absorption of SHG radiation as well as TPA of the fundamental beam. In sharp contrast, Li2CdGeS4 is free of this serious problem due to its wider bandgap energy, 3.15 eV. In addition to the outperformance, Li2CdGeS4 is type-I phase-matchable for incident λ > 1.5 μm, which likely extends up to the far-IR region as evidenced by the rather constant SHG response for incident λ > 3.0 μm.6 The absolute χ(2) value for Li2CdGeS4was estimated in this static limit, in which both the sample and the reference are phase-

Figure 2. Diffuse reflectance UV−vis−NIR (left) and FT-IR (right) spectra of Li2CdGeS4.

windows of commercial NLO crystals, such as AgGaSe2 (0.76− 17 μm), AgGaS2 (0.48−11.4 μm), ZnGeP2 (0.74−12 μm),3b and LiInS2 (0.4−12 μm),16b as well as the new NLO materials Na2Ge2Se5 (0.521−18.2 μm)18 and K2P2Se6 (0.596−19.8 μm).19 We previously reported that wurtz-stannite-type Li2CdSnS4 and Li2CdGeS4 have SHG responses that are ∼100× α-quartz 3046

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square-law dependence for Li2CdGeS4 persists within our experimental range. This shows that the damage threshold of Li2CdGeS4 for picosecond pulses is higher than 4 GW/cm2. The DFG of AgGaSe2, for example, using 1064 nm at 4 GW/cm2, is almost 10× more inefficient compared with the ideal case, due to significant TPA. Note that this 10× inefficiency is only for a ∼100 μm thickness. Since TPA effects scale with thickness, the inefficiency can be 1000× for a commercial AgGaSe2 crystal having a 1 cm thickness. Remarkably, this serious problem is absent in Li2CdGeS4 since it is entirely TPA inactive with an outstanding damage threshold. The lack of TPA effects and the non-phase-matching behavior at this low wavelength imply that Li2CdGeS4 does not undergo any fundamental depletion by TPA or phasematched SHG conversion into 532 nm, ensuring ideal, 100%, input for DFG mixing with another beam source. Therefore, Li2CdGeS4 could potentially be utilized for high-efficiency/ high-power DFG for IR and THz generation. It is noteworthy that several new NLO materials such as NaAsSe2,26 K2P2Se6,19 and Na2Ge2Se518 have this TPA issue essentially due to their narrow bandgaps; this underscores the truly unique nature of Li2CdGeS4 for highly efficient wave-mixing applications.27 Additionally, the compositional flexibility and structural simplicity of the I2−II−IV−VI4 family provides an avenue to develop an intimate understanding of the effects of composition on properties. Knowledge-based optimization of the material parameters by compositional tuning for further enhanced NLO properties is in process.

matchable with minimal multiphoton absorption, yielding ∼51.1 ± 4.4 pm/V, similar to the χ(2) value of AgGaSe2. Interestingly, SHG conversion at 750 nm (incident λ = 1.5 μm) marks the low end of the phase-matching range of Li2CdGeS4, which is shorter than the initiation of phasematchability of both AgGaSe2 and AgGaS2, at conversion λ of 1.55 μm and 900 nm, respectively. It is also remarkable that SHG in Li2CdGeS4 is efficient at the conversion λ of 1.55 μm, which corresponds to the current telecommunication wavelength. This indicates that Li2CdGeS4 could potentially be an active component in the information science and technology based on wavelength-division multiplexing networks.23 Since Li2CdGeS4 is transparent up to 22 μm, the upper bound of the DFG λ likely extends to the THz regime. To confirm the potential for high-power DFG, the SHG response at the fundamental Nd:YAG line (1064 nm), which is a typical wavelength for picosecond DFG mixing, was measured as a function of input intensity (Figure 4). Note that the input λ

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. SHG power dependence of Li2CdGeS4 and AgGaSe2 plotted on a log−log scale, superimposed by square-law fits. AgGaSe2 undergoes significant TPA as indicated by a TPA fit.

Supporting Information includes the following: synthesis; synchrotron powder diffraction, Rietveld refinement; diffuse reflectance UV−vis−NIR spectra; SHG; LDT; differential thermal analysis; and electronic structure. This material is available free of charge via the Internet at http://pubs.acs.org.

is resonant with the TPA band of AgGaSe2 but is below that of Li2CdGeS4 due to its wide bandgap. The input intensity, I, was varied in the range of 0.04−4 GW/cm2, and the spectrally integrated SHG counts from Li2CdGeS4, AgGaSe2(MC), and AgGaSe2(OQ) were plotted. The solid lines represent the maximum SHG case in which fundamental depletion is absent, i.e., ISHG = aI2, where ISHG and I are the SHG and fundamental intensities with a being a proportionality constant that incorporates |χ(2)|2. The a values for Li2CdGeS4 and AgGaSe2 were carefully determined by fitting the low-intensity regime where TPA is minimal. Considering the similar SHG coefficients for the sample and the reference, the salient difference in a, i.e., difference in the two solid lines, results from bandgap absorption of SHG in AgGaSe2, which is absent in Li2CdGeS4. The measured SHG counts from both AgGaSe2 samples deviate from the solid line for I > ∼0.2 GW/cm2, indicating that the fundamental beam is significantly depleted by TPA. We estimated the TPA coefficient, β, of AgGaSe2 by fitting the observed SHG power dependence using a modified fundamental intensity by TPA, ITPA:

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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(eq 1)

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant DMR-1201729). Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. We thank Gooch & Housego for supplying the AgGaSe2 single crystal. We especially thank Gary C. Catella, VP of Technology, Gooch & Housego (Ohio). Y.S.K. acknowledges the support of the Basic Science Research Program (Grant No. 2012-1010369) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

where d = 90−106 μm is the particle size for our reflection geometry.24 The value of β (38 cm/GW) is consistent with a theoretical value based on a two-band model within a factor of 2 but likely overestimated due to optical damage induced by significant TPA at higher excitation levels.25 In contrast, the

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ISHG = aITPA 2 ,

ITPA = I /(1 + Iβ d)

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