This paper presents a comparison between ablation results for various modes of laser operation: single pulsing, GHz bursting, and regular bursting. We start with the comparison of ablation thresholds of single ultrafast pulses with pulse widths ranging from 300 fsec to 3 psec. Those are contrasted with regular fsec pulse bursting at 20 ns pulse-to-pulse spacing, as well as GHz bursting with pulse spacing of 1.4 nsec. A variety of GHz milling results are presented and contrasted with single pulsing, as well as with 20nsec bursting, as the number of fsec pulses in a burst is adjusted from 3 pulse per burst, to 9, and 17, and for various scanning speeds. Material removal results are presented for milled transparent hard materials, specifically, fused silica, borosilicate glass, sapphire, and diamond. Aluminum and silicon are also included in this milling test for comparison. To maintain proper correlations, similar experimental conditions are used throughout, such as focused spot diameters, fluence, frequency of operation, and scan speed. The lasers used for this study are internally developed at IMRA for material processing. The data include results for 1045nm, 523nm, frequencies up to 1MHz, and power up to 5W.
We address recent fiber-based femtosecond laser technology. Specifically, fiber-chirped pulse amplifier is discussed for
the enabling the concept of real-world applications. We review recent selected material applications demonstrating advantages of ultrafast dynamics of highly repetitive pulse train in nanoparticle generation in pulsed-laser deposition and reliable Si wafer singulation.
A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.
A scanning near-field microscope provides nano-scale imaging capability of field induced THz wave emission spectra from semiconductor surfaces and interfaces. Combined with a scanning probe tip and femtosecond optical pulse excitation, THz wave emission with sub-100 nm spatial resolution has been demonstrated. The scanning probe tip modulates semiconductor surface field with nano-scale accuracy through the imaging charge dipole, the tunneling current, or the contact current. The modulated THz wave from the highly localized area under the scanning tip is detected in time-domain. This aperture-less imaging method leads the way to study nano-scale to atomic level emission spectroscopy at THz frequency range.
Pulsed THz imaging is a promising non-destructive technology based on its high transmission through selected dielectric materials and its capability to provide time-of-flight and spectral information. The traditional method of the pulsed THz imaging is a point-to-point reflective scanning system. The image is acquired by analyzing the peak amplitude information of the THz pulse in the time-domain at each pixel. It requires the THz beam or sample scanned. In this paper, we present our approach of large scale, focal plane THz wave imaging. In our 2-D focal plane THz wave imaging, the THz beam is expanded to be 60 mm in diameter. The THz beam illuminates the target in a reflective manner, in which a polyethylene lens projects the image onto a 40 mm by 40 mm by 2 mm ZnTe sensor crystal. The probe beam is expanded to be 40 mm in diameter and overlap with the THz beam on the sensor. The modulated probe beam profile carrying the image information is captured by a CCD camera. This technique enables us to view the objects which are optically opaque but transparent in THz frequency and shows feasibility in remote sensing, security inspection, and military defense applications.
This paper describes a study conducted into the limit on spectral resolution due to the dynamic range of a T-ray spectrometer. The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time-domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical pulsed terahertz sources in terms of their dynamic range and hence achievable spectral resolution.
Gas sensing and identification in far infrared or THz band is useful because many polar molecules have unique spectral fingerprints in this range, which are from the rotational transitions of the molecules. We have investigated the potential of THz time-domain spectroscopy (THz TDS) as a quantitative analysis technique for gas sensing. Ammonia vapor has been chosen as a sample gas. The absorption cross section at 0.572 THz of ammonia in the pressure range of 0.2-20 Torr was extracted to be (5.7±0.3)×10-20 cm2/molecule. In addition, a pressure calibration curve based on pure ammonia measurements was obtained. Using this calibration curve, we made quantitative analysis on the mixture of ammonia and air at 100 Torr. The result shows that THz TDS is an appropriate technique for quantitative analysis of polar gas and gas mixture. We measured the THz spectra of ammonia at different partial pressures in ~590 Torr nitrogen (78% nitrogen in atmosphere), and obtained a pressure calibration curve. THz spectra of ammonia at different partial pressures in 760 Torr atmosphere were measured. Based on the principle of differential optical absorption spectroscopy (DOAS) and the pressure calibration curve got in ~590 Torr nitrogen, we obtained the ammonia partial pressures. The result is compared with the value measured by vacuum gauge and the maximum error is 30%. This indicates that THz TDS based on principle of DOAS is an applicable quantitative technique for sensing ammonia or other polar gases in atmosphere.
Water, at both the liquid and gas phase, maintains a high absorption coefficient in the terahertz (THz) frequency range. As a result, a major limitation of THz time-domain spectroscopy (THz-TDS) for real-world applications is water attenuation. The humidity in the atmosphere affects THz waves (T-ray) for long distance measurement and tracing materials, such as explosive materials. We measure air at various humidity and we report how humidity affects THz-TDS measurement. We also report the changes to spectrum amplitudes by measuring water vapor absorption in a vacuum chamber.
Terahertz (THz) time-domain spectroscopy (TDS) is a powerful measurement tool for characterizing materials with potential fingerprint capability. Due to its pulsed nature, the spectral resolution of THz-TDS is limited by its temporal scanning measurement and its dynamic range. A novel THz-TDS system with a large signal-to-noise ratio (SNR) improves the spectral resolution. Techniques that will enhance the performance of THz-TDS are demonstrated.
The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz - 70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.
The spectroscopic properties and 3-micrometer lasing of Er3+ doped in yttrium vanadate (YVO4) were investigated in this paper. It is found that the Er3+ concentration has a negative effect on the emission of the transition 4I13/2 yields 4I15/2 (1.55 micrometer), and a positive effect on that of the 4I11/2 yields 4I13/2 transition (2.68 micrometer). With direct upper-state pumping and a plane-concave cavity a self terminating laser was achieved at the wavelength of 2.724 micrometer in the 30 at.% Er3+ doped sample. No laser activities were realized in the crystals with lower Er3+ concentrations. The laser activities of Er3+:YVO4 and Er:YAG were compared and the differences between both were discussed on the basis of the features of the upper and lower state and the population dynamics of the two levels. It is proposed that the lack of effective cross relaxations between ions in the 4I13/2 state in the Er3+:YVO4 are responsible for the termination of its 3 micrometer laser. The possible ways to improve the laser behavior were also suggested based on the discussions.