Optics Communications 282 (2009) 1452–1454

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Optics Communications

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Efficient harmonic generation of second, third, and fourth orders fromFourier-transform-limited CO2 laser beam at 10.6 lm in GaSe crystals

Yi Jiang, Yujie J. Ding *

Department of Electrical and Computer Engineering, Lehigh University, 19 Memorial Drive West, Bethlehem, PA 18015, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 August 2008Received in revised form 19 December 2008Accepted 19 December 2008

PACS:42.65.Ky

Keywords:Frequency conversionHarmonic generation including high-orderharmonic generation

0030-4018/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.optcom.2008.12.049

* Corresponding author. Tel.: +1 610 758 4582; faxE-mail address: yud2@lehigh.edu (Y.J. Ding).

Second, 3rd, and 4th order harmonics from a Fourier-transform-limited nanosecond CO2 laser beam at10.6 lm were efficiently generated within one or two GaSe crystals. The peak output powers were deter-mined to be 443, 23, and 2 W, respectively, corresponding to conversion efficiencies of 3.0%, 0.13% and0.0093%, which were significantly improved, compared with all the previous results. The linewidths ofthe harmonic beams were estimated to be 80, 104, and 116 MHz, respectively.

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1. Introduction

Despite of the significant development of quantum-cascadelasers, a CO2 laser remains to be one of the most powerful lasersproducing the discrete output wavelengths in the range of 9.2–11.6 lm. Through harmonic generation in nonlinear-opticalcrystals, one can extend these wavelengths to as short as 2.3 lm.There are a few nonlinear crystals that can be used to achieve sucha goal [1–6]. Among them, GaSe has a wide transparency range(0.65–18 lm), large birefringence, and a large nonlinear coefficient(d22 � 54 pm/V) [7]. In the past, frequency doubling of CO2 lasers[8,9], optical parametric generation pumped at 3 lm [10], and dif-ference-frequency generation [11], were achieved by using GaSecrystals.

In this article, we report our new results on the phase-matchedsecond-harmonic generation (SHG), third-harmonic generation(THG), and fourth-harmonic generation (FHG) from a CO2 laserbeam at 10.6 lm, corresponding to the output wavelengths of5.3, 3.53, and 2.65 lm, respectively, from GaSe crystals at roomtemperature. The normalized conversion efficiency for SHG hasbeen significantly increased, compared with all the previous re-sults. In the past [12–14], both THG and FHG were investigatedby using CO2 laser radiation as the fundamental beam. To the bestof our knowledge, our result represents the first report on THG and

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FHG by cascading two second-order nonlinear processes in GaSecrystals. Since the output powers from quantum-cascade lasersare still quite low, frequency conversion may be still is the onlyeffective approach for generating high powers in the mid-IR region.

2. Experimental details

We have first measured the absorption coefficient of the GaSecrystals in the range of 1.6–16 lm, see Fig. 1. One can see fromFig. 1 that the 10-mm-long GaSe crystal has absorption coefficients<0.5 dB/cm within the wavelength range of 2–12 lm. Our experi-mental setup for cascading two second-order nonlinear processesis illustrated by Fig. 2. The GaSe crystals used in our experimentswere neither doped nor polished and coated. They were grownby Siberian Physico-Technical Institute at Tomsk State University,Russia. The two GaSe crystals, labeled as the first and second, withlengths of 15 and 10 mm measured along optic axis, respectively,were used for SHG (xþx) 2x) and SHG (2xþ 2x) 4x) orsum-frequency generation, SFG (xþ 2x) 3x), where x standsfor the output frequency of the CO2 laser. They were respectivelymounted on two rotational stages for us to access different inci-dent angles. A short-pulse repetition-frequency-excited waveguideCO2 laser at 10.6 lm was used as a fundamental beam for all thefrequency-mixing experiments. Temporal profiles of the laser out-put pulses are measured to be nearly Gaussian with the pulsewidth of 12 ns. The repetition rate of the laser was set to 60 kHz.Lasing occurred only within a single longitudinal mode. Since the

Fig. 3. Internal phase-matching angles measured for SHG and SFG (dots). Solid anddashed curves correspond to phase-matching angles calculated using the Sellmeierequations.

Fig. 1. Spectrum of absorption coefficient of GaSe crystal determined frommeasurements of transmittances (inset). Inset: transmission spectrum measuredon 10-mm-long GaSe crystal. Dashed horizontal line represents transmittance ifonly Fresnel-reflection losses are taken into consideration.

Y. Jiang, Y.J. Ding / Optics Communications 282 (2009) 1452–1454 1453

emitted pulses are Fourier-transform-limited, the spectral line-width of the pulses is about 52 MHz. The highest output peakpower was measured to be 11.9 kW. In our experiments, the high-est peak intensity focused onto the GaSe crystals was �3 MW/cm2.Under such an intensity there was no damage of the crystals, whichis consistent with one of the previous results [15]. The outputpower of the laser was continuously varied by using a k/2 wave-plate and a polarizer. All the harmonic beams at 5.3, 3.53, and2.65 lm propagated in the same direction as the fundamentalbeam. A short-pass filter was placed behind the 15-mm GaSe crys-tal to cut off the 10.6-lm radiation for FHG and a quartz filterplaced behind the 10-mm GaSe crystal was used to block most ofthe radiation at the wavelengths longer that 4 lm. A spectrometerwas used to measure the spectra of the fundamental and harmonic

Fig. 2. Experimental setup for SHG, THG, and FHG in GaSe crystals from a CO2 laserbeam at 10.6 lm: WP stands for a k/2 plate at 10.6 lm; M1 and M2 designatemirrors; L1 and L2 correspond to two ZnSe convex lenses; F1 and F2 are short-passfilter and quartz plate, respectively; and SP stands for spectrometer.

wavelengths. A power meter and a photovoltaic detector with thetime constant of 1 ns were used to measure the output powers andtemporal profiles of the output harmonic beams.

3. Results and discussions

Two SHG processes (xþx) 2x;2xþ 2x) 4x), achievedsequentially in the two crystals, and SFG (2xþx) 3x) in thesecond crystal were phase-matched for the oo-e and oe-e configu-rations, respectively. The phase-matching angles for SHG in thefirst crystal and SFG and SHG in the second crystal were measuredto be 14.4�, 13.8�, and 9.9�, respectively. These values are in verygood agreements with those calculated using the Sellmeier equa-tions in Ref. [16], i.e. 14.9�, 14.3�, and 10.7�, see Fig. 3. We havemeasured the spectra of the output harmonic beams using a spec-trometer. From these spectra, we have deduced the output wave-lengths of second-, third-, and fourth-harmonic beams to be 5.3,3.53, and 2.65 lm, respectively. They are the same as those directlycalculated from the fundamental wavelength. However, the line-widths of these harmonic beams are much narrower than thosemeasured by using the spectrometer. On the other hand, the pulsewidths of the output beams at 5.3, 3.53, and 2.65 lm were mea-

Fig. 4. Average output powers vs. input power for fundamental beam at 10.6 lmfor SHG, THG, and FHG, respectively. Solid, dashed, and dotted curves correspond tofitting to data using quadratic, cubic, and quartic dependences, respectively.

1454 Y. Jiang, Y.J. Ding / Optics Communications 282 (2009) 1452–1454

sured to be 7.8, 6.0, and 5.4 ns by using the photovoltaic detector,respectively. Since the harmonic beams were also expected to bemore or less Fourier-transform-limited, the linewidths were calcu-lated to be 80, 104, and 116 MHz, respectively. These are the nar-rowest linewidths ever achieved for the harmonic generation froma CO2 laser. One can see from Fig. 4 that the dependences of theoutput powers on the input power of the fundamental beam forSHG, THG, and FHG are quadratic, cubic, and quartic, respectively,which are all consistent with our theories for the harmonic gener-ation. For the average power of 6.50 W at 10.6 lm, the average out-put power at 5.3 lm from the first crystal was measured to be194 mW, corresponding to the peak power of 443 W and a conver-sion efficiency of 3.0%. These values are 6 times higher than theprevious results obtained on a GaSe crystal [9] and a factor of about3 higher than our previous results achieved using a ZnGeP2 crystal[17]. The normalized conversion efficiency was determined to be311% MW�1 cm�1, which is a factor of about 20 higher than thehighest conversion efficiency ever achieved from the GaSe crystalsfor SHG from 10.6 lm (15% MW�1 cm�1) [18]. On the other hand,the average output powers at 3.53 and 2.65 lm were measuredto be 8.30 mW and 600 lW (the corresponding peak powers of23.0 and 2.00 W), corresponding to the conversion efficiencies of0.13% and 0.0093%, respectively. Based on the expressions givenin Ref. [7], we have calculated the conversion efficiencies for theharmonic generation of 5.3, 3.53, and 2.65 lm from 10.6 lm tobe 5.1%, 0.79%, and 0.034%, respectively. These values are withinthe same orders of magnitude as our measured ones. To the bestof our knowledge, there has been no report on the measurementsof the conversion efficiencies for THG and FHG from the fundamen-tal wavelength of 10.6 lm in GaSe crystals in the past.

We have also observed THG from the first crystal. Since SFG inthe first crystal has a short coherence length of 240 lm, the highestoutput power at 3.53 lm was measured to be 17 lW (correspond-ing to the peak power of 47 mW). This is a factor of about 490 low-er than that generated by using the two GaSe crystals (8.30 mW).

4. Conclusion

Fourier-transform-limited pulses at 5.3, 3.53, and 2.65 lm weregenerated by frequency doubling or cascading two second-order

nonlinear processes in one or two GaSe crystals from a CO2 laserbeam at 10.6 lm. The highest average output powers for the threeoutput harmonic beams were measured to be 194 mW, 8.30 mW,and 600 lW, respectively, corresponding to the peak output pow-ers of 443, 23, and 2 W, respectively.

These high-power harmonic pulses will have potential applica-tions in countermeasure and remote sensing.

Acknowledgement

This work has been supported by US Air Force Office of Scien-tific Research.

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