Terahertz generation and detection using nonlinear frequency conversion

Jerry C. Chen, M. Jalal Khan, Zong-Long Liau and Sumanth Kaushik M.I.T. Lincoln Lab, 244 Wood St, Lexington, MA 02420

Abstract—Ti-sapphire is nonlinearly converted to 1 mW average terahertz power at 9.9% photon conversion efficiency. Terahertz is mixed with 1550 nm pump in quasi-phase-matched GaAs, for ultra-sensitive (78 fW/Hz1/2), room-temperature detection with nanosecond timing resolution.F

Terahertz sensors have attracted much interest recently, for imaging and spectroscopy of drugs, explosives, galaxies, semiconductors, and tissues [1]. All these sensors would benefit from better sources and detectors. As optical technology is further developed, many researchers (eg, [2-5]) have used χ(2) crystals and optical pumps to frequency convert optical to terahertz for bright sources and to convert terahertz to optical for sensitive detectors. With an amplified ti-sapphire pump and lithium niobate crystal, our terahertz source emits 1 mW average and 1 MW peak, from 0.1 to 1.6 THz at a 1 kHz repetition rate. For the terahertz detector we improve our previous sensitivities by over 50 times. This noise equivalent power of 78 fW/Hz1/2 and power conversion efficiency of 1.2 × 10-3 are the best reported, to our knowledge. Pulses can be resolved with 1 ns resolution.

The ti-sapphire amplifier creates terahertz in a nonlinear frequency conversion process called optical rectification. Because the 115 fs pulse contains many terahertz of spectrum, the “red” and the “blue” frequency components of a single optical pulse mix in a χ(2) crystal, producing broadband terahertz radiation. A diffraction grating and focusing lens disperses the “red” and “blue” components into different angles, so the generated terahertz exits at a large angular deviation from the optical pump’s direction. This tilting greatly enhances velocity matching, which leads to improved conversion efficiency and the world’s record for highest average power (3 mW) from optical frequency conversion [4]. Here, we use a more modest pump, Coherent’s Libra HE-USP, which gives 3.8 mJ at 1 kHz rate. The χ(2) crystal is 0.6% MgO-doped stoichiometric LiNbO3 prism, cut at 63 and 73 degree angles. These angles permit the terahertz and pump to exit easily. The grating to lens distance is 24 cm and the lens to prism distance is 12 cm; this doubles the size of the ti-sapphire beam.

This setup depicted in Figure 1 produced over a 1 mW average terahertz power, which corresponds to a 2.6× 10–4 power conversion efficiency and a 9.9% photon conversion. Spiricon’s Pyrocam III camera shows good terahertz beam quality, which is expected as the ti-sapphire has a M2 spec of 1.3. To measure the terahertz pulse width and spectrum, we used the electro-optic effect, where the presence of terahertz in a 0.1 mm thick ZnTe crystal rotates the polarization of the ti-sapphire probe. The pulse width is 0.75 ps. Since the pulse repetition rate is 1 kHz, the peak terahertz power is 1.3 MW. Figure 2 plots the terahertz spectrum, where there is appreciable power from 0.1 to 1.6 THz. Further details on the generation setup and the EO measurement can be found in [4].

The upconversion experimental layout is shown in Figure 3. To test our receiver, we use a Virginia Diodes’ amplifier multiplier chain as a terahertz source. At 820 GHz it emits 120 μW into a near Gaussian mode. The optical pump is similar to that described previously [5]. The seed is now an Agilent tunable laser diode operated at 1550 nm. After the final EDFA, the optical power is 1 to 2 W average or 3 to 6 kW peak. The terahertz and pump are focused on a GaAs crystal to produce an optical idler. An optical spectrum analyzer measures the idler wavelength to be 1556.6 nm, as expected from energy conservation.

To improve conversion efficiency and receiver sensitivity, we switched from bulk GaAs [5] to quasi-phase matched (QPM) GaAs. Diffusion bonding [6] fused two 4 mm thick bulk GaAs crystals of orthogonal orientation together at 700 degrees Celsius. Using this crystal, we demonstrated a near 5 dB improvement in the conversion efficiency; the theoretical limit is 6 dB. We also developed an anti-reflection (AR) coating at 1550 nm using aluminum oxide and tantalum pentoxide [6], which allow more optical pump light into and idler light out of the GaAs, without absorbing much THz. The result is an additional 2.5 dB of improvement.

The idler was coupled into fiber then routed via an attenuator into a Princeton Lightwave Geiger mode avalanche photo-diode (GM-APD). The GM-APD records the total number of 1 ns gates with photons over a 1 s time interval. The APD’s dark counts limit its sensitivity or noise equivalent power (NEP) to 9.3× 10−17 W/Hz1/2. By attenuating the idler to the APD’s noise floor, we measure the THz NEP to be 78 fW/Hz1/2. This corresponds to a This work is sponsored by the OSD / DDR&E Quick Reaction Fund under NSWCDD oversight and the United States Air Force under Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.

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minimum detectable pulse energy of 1.1 × 10-20 J in the 1 ns gate; of course, we must integrate over 5 × 104 pulses in 1 s. Dividing the THz NEP by the optical NEP gives the system power conversion efficiency of 1.2 × 10-3. The system photon conversion efficiency of 5× 10-6 follows from the quantum defect.

We can delay the GM-APD’s 1-ns gate with respect to the upconverted pulse and thereby measure the idler’s temporal characteristics (Figure 4). We can see that the upconverted idler signal has a temporal width of about 6 ns. The THz source is continuous wave so the idler’s temporal shape mirrors that of the optical pump. However, if we had used a pulsed THz source, our THz detector would provide a 1 ns time-resolved measurement of the THz pulse. The figure also shows there is no upconverted idler signal when the THz source is turned off.

We have used optical frequency upconversion at 1550 nm to create a fast, ultra-sensitive room temperature terahertz receiver. Nonlinear conversion was enhanced by quasi-phase matching, increased pump powers, and AR coatings, resulting in record breaking power conversion efficiency of 1.2 × 10-3 and an unprecedented 78 fW/Hz1/2

noise equivalent power at room-temperature. The source’s 1 mW average power (and 1.3 MW peak power) is one of the largest from optical frequency conversion. The enhanced velocity matching gives 10% photon efficiency. In the future, we hope to combine this terahertz source and detector for standoff sensor applications.

Acknowledgement—Authors thank Peter O’Brien of M.I.T. Lincoln Laboratory for AR coating GaAs crystals and for help with imaging the diffusion bond interfaces. We thank Harold Hwang and Matthias Hoffmann of M.I.T. for getting us started on terahertz generation and EO measurements.

Figure 1. Experimental layout of THz source. Figure 2. Terahertz amplitude versus frequency.

Figure 3. Experimental layout of THz detector

Figure 4. Idler’s pulse shape versus time measured by GM-APD.

References [1] M. Tonouchi, Nature Photon., vol. 1, pp. 97-105, Feb. 2007. [2] Y. J. Ding and W. Shi, Solid State Electron., vol. 50, pp. 1128-1136, June 2006. [3] H. Minamide and H. Ito, Proc. SPIE, vol. 7179, pp. 71790C1-8, Jan. 2009. [4] K.-L. Yeh, J. Hebling, M. C. Hoffmann, and K. A. Nelson, Opt. Commun., vol. 281, pp. 3567, Jul. 2008. [5] M. J. Khan, J. C. Chen, and S. Kaushik, Opt. Lett., vol. 33, pp. 2725-7, Dec. 2008. [6] M. J. Khan, J. C. Chen, Z.-L. Liau, and S. Kaushik, 15th Coherent Laser Radar Conf., Toulouse, France, Jun. 22-6, 2009.

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