Challenges and Opportunities

1. Experiment Setup Fig.4, Layout of experimental set up. The experimental set up for THz generation is based on Michelson interferometer geometry.A liquid helium cooled silicon bolometer (Infrared Labs, LN-6/C) was used to detect the THz radiation. A 0.96 mm thick <110> oriented CdTe crystal was used as the THz emitter. Laser pulses from the ultrafast Ti-Sapphire oscillator used in the experiment (Coherent Chameleon-XR, 90 MHz repetition rate; <140 fs pulse duration) were tuned from 710 nm to 970 nm. A pellicle film split the laser beam into the two arms of the Michelson interferometer. The recombined beams were focused onto the CdTe crystal. The emitted THz beams interfere with each other when the moving arm of the Michelson interferometer was scanned, and the autocorrelation of the THz waveform was presented. CdTe as Emitter for Compact THz Imaging Systems Xu Xie, Jingzhou Xuand X.-C. Zhang Center for THz Research, Rensselaer Polytechnic Institute, Troy, NY 12180 Contact: zhangxc@rpi.edu Results and Discussion Challenges and Opportunities Currently most pulsed THz systems are based on Ti-Sapphire laser systems with ZnTe crystals as THz emitters and detectors. The bulky size and high cost of Ti-sapphire lasers limit wide use of compact THz systems. Recent advances of ultrafast fiber laser systems have yielded smaller laser sources. Since optical rectification as well as EO sampling method are mostly determined by phase matching conditions between THz radiation and laser pulses, these two processes are laser wavelength sensitive. This can be described by coherent length as where THzis the frequency of THz radiation, nopt is the refractive index of emitter/sensor crystal in THz frequency region and nTHz is the group index of the laser pulse. The longer the coherent length, the stronger the THz radiation will be generated and more sensitive detection is possible. Calculations show ZnTe is not suitable for use with fiber laser wavelengths at 1550 nm and 1050 nm. Destination Terahertz radiation is gentle but piercing. THz imaging technology has been used in industrial non-destructive evaluation and is promising in mail and luggage examination, biological sensing and imaging. A low cost and compact THz system is crucial for its future use. Fig.5, (a) Emitted THz power from a CdTe crystal and a ZnTe crystal as a function of laser wavelength. (b) Emitted THz power and power transmission of CdTe crystal as a function of laser wavelength. Fig.1, CW THz images of three kinds of drugs. The small polyethylene bags contain from left to right: MDMA, aspirin, and methamphetamine. The bags were placed inside the envelope during imaging. (Credit: Kodo Kawase, Optics Express 11 2549 ) ZnTe crystal was compared with CdTe at different laser wavelength as a THz emitter. Experimental results show consistency with the calculations. As a result, in the wavelength range from 710 to 970 nm, the longest laser wavelength can give the most powerful THz radiation from CdTe which is stronger than ZnTe at 970 nm . Fig.1 CW THz image of a space shuttle thermal insulating form. Conclusion We measured THz radiation from CdTe crystal at different wavelengths via the process of optical rectification. It is demonstrated that in the range from 710 nm to 970 nm, phase mismatching affects the emitted THz power. As a result the longer laser wavelength can give more powerful THz radiation in this wavelength range. This has made it possible to use Yb doped fiber lasers with wavelength of about 1050 nm as laser source to build compact THz imaging systems. Acknowledgement This work was supported in part by CenSSIS, the Center for Subsurface Sensing and Imaging Systems, under the Engineering Research Centers Program of the National Science Foundation (Award Number EEC-9986821). Fig.3, Calculated coherent lengths at 2 THz of different materials as a function of laser wavelengths. (Credit: Masaya Nagai, Applied Physics Letters 85 3974 ) Calculations show that CdTe has a maximum coherent length when the laser wavelength is 1050 nm that is close to a Yb doped fiber laser wavelength. This implies possible use of CdTe crystals with fiber laser systems for THz generation and detection. Research of CdTe properties in THz generation and detection at different laser wavelengths has become important. Here we report a study of CdTe in wavelengths ranging from 710 nm to 970 nm. Our result shows that at 970 nm CdTe can generate stronger THz power compared with ZnTe crystal. Fig.2, A CW THz imaging system with a GUNN diode as THz source and a Schottky diode as THz detector. In order to obtain abundant spectrum information of the image target, an image system with broadband THz generation and detection capabilities must be realized. Our goal is to make a low cost, compact pulsed broadband THz imaging system by using optical rectification for THz generation and EO sampling for THz detection.