Patent Application
20090074016
This invention relates to apparatus for Terahertz wave generation and more particularly to such apparatus generating Terahertz waves from water including vapor of water and solutions.
Terahertz (THz) waves, T-rays, or submilimeter/far-infrared waves, refer to electromagnetic radiation in the frequency interval from 0.1 to 10 THz. They occupy a large portion of the electromagnetic spectrum between the mid-infrared and microwave bands. A 1 Terahertz wave has a period of 1 picosecond, a wavelength of 300 μm. In other units, a 1 THz wave is equivalent to 33 cm−1, 4.1 meV and 47.6 K. In the past decades, especially since the advent of THz time-domain spectroscopy (THz-TDS) far-IR spectroscopy has found extensive applications in various fields including gas sensing, explosives detection and security screening, pharmaceuticals, biological and biomedical study, etc. Compared to relatively well-developed sensing and imaging in microwave, mid-infrared and optical bands, basic research, advanced technology developments and real-world applications in the THz band are still in their infancy. Recent advances in THz science and technology make it one of the more promising research areas in the 21st century for sensing and imaging, as well as in other interdisciplinary fields [3-8]. Numbers in brackets refer to the references appended hereto, the contents of which are incorporated herein by reference. It is expected that Terahertz wave research will enable innovative imaging and sensing capabilities for application in material characterization, microelectronics, medical diagnosis, environmental control and chemical and biological identification [1, 2]. Recent research suggests that intense THz radiation can be used to destroy cancerous tissue, not just to image it [31].
In the time-domain THz spectroscopy community, photoconductive dipole antennas and electro-optic crystals are commonly used for emitting and detecting pulsed THz waves. In general, most THz generation schemes provide quite low powers, severely restricting certain applications such as THz nonlinear optics. THz wave generation in ambient air has attracted considerable attention recently [9-13]. The first reported THz wave generation was achieved in the early 1990's by focusing an intense (peak power 1012 W) laser beam into air [9]. Through the mixing of an optical fundamental wave with its second harmonic (SH) wave, generation of intense THz wave pulses in air has been demonstrated. Optical power dependence measurements across the air breakdown threshold suggest that ionized air (plasma) plays an important role in generation of THz radiation. Recently, a THz field strength greater than 100 kV/cm has been reported by using a similar experimental arrangement with shorter optical pulses [13].
Developing a high-power THz emitter is thus very crucial for real-world applications, especially for standoff detection of threats (i.e., explosives or chemical/biological hazards) concealed in clothing or packages.
In one aspect, the apparatus for Terahertz wave generation according to the invention includes a femtosecond (fs) amplified laser generating a pulsed optical fundamental beam and a crystal for passing the optical fundamental beam to generate a second harmonic beam of the fundamental beam. A lens focuses the mixed fundamental and second harmonic beams and a gas cell containing water receives the focused beams to produce Terahertz waves. An off-axis concave parabolic mirror may be used in place of a lens. Water may be in its various physical states such as gaseous, vapor, liquid and including water clusters. In a preferred embodiment, the optical fundamental beam has a central wavelength of approximately 800 nm. A suitable laser is a Ti:sapphire regenerative amplifier generating approximately 1 mJ 100 fs pulses at a 1 kHz repetition rate. A suitable crystal is a type-I beta barium borate (BBO) crystal.
In a preferred embodiment, the water vapor has a partial pressure in the range of 6.9-23.6 torr. The gas cell may include a quartz window for receiving the focused beams and a high density polyethylene window through which the Terahertz waves emerge. A suitable BBO crystal thickness is 100 μm and the quartz window is a 100 μm thick quartz plate. A preferred embodiment of the invention further includes a gas distribution system connected to the gas cell to control pressure of the water vapor in the cell. A suitable laser power is in the 600-650 mW range.
The instrument disclosed and claimed herein uses water vapor as a nonlinear medium under pulsed optical excitation to generate intense broadband Terahertz wave emissions. The instrument contains a water vapor cell and a femtosecond amplified laser. The focused optical beam in the water vapor cell generates THz radiation in the forward direction. The ratio of the measured THz radiation electric field to the partial pressure of the water vapor is the strongest among all of the gases and organic vapors that were tested. Without being limited to any theory, the strong T-ray emission of water vapor is attributed by the inventors to the unique THz-frequency vibronic properties of protonated water nanoclusters that are believed to be a significant constituent of water vapor.
In yet another embodiment, the optical beam is focused onto a high-pressure stream of gas including water eliminating much of the loss due to water absorption.
The novel emitter disclosed herein provides much stronger THz emission than ambient air and commonly used electro-optic crystals. It is expected that the novel THz emitter disclosed herein will provide commercial solutions to many contemporary problems. It could become a new-generation THz source for THz time-domain spectrometers in the future.
The THz emitter disclosed herein is also very inexpensive compared with electro-optic crystals and is easy to replace once it is damaged or used for a long time. The emitter provides extremely intense THz electric fields and can provide a sub-picosecond far-infrared pulse. The THZ emission is highly directional [32] and the THz wave is ultra-broadband, up to 7 THz.
a is a schematic illustration of the technique disclosed herein for the generation of THz waves using water vapor as the emitter.
b is a schematic illustration showing the emitter cell connected to a gas handling and distribution system, including a valve and pump, that allows pure vapors to be input into the gas cell and to control pressure of the vapor in the gas cell.
a is graph of amplitude versus time showing plots of recorded temporal waveforms of THz field signal generated from water vapor at 23.6 torr and from ambient air (752 torr), by changing a time delay between the THz field and an optical probe beam.
b is a graph of amplitude versus frequency showing the spectrum corresponding to the temporal waveforms of
c is a graph of amplitude versus time for pure water.
With reference first to
Terahertz wave generation will now be described in more detail with reference to
In this embodiment the gas cell 28 is a glass cell with a diameter of 30 mm and length 50 mm. A front window 38 through which the beam 20 passes is quartz. A rear window 40 from which the THz wave emanates is preferably high density polyethylene. As shown in
In the embodiment shown in
Referring again to
The detection of the Terahertz wave is standard electro-optic sampling methodology utilizing a 2.5-mm thick ZnTe crystal.
The emitted THz field amplitude is proportional to the pulse energy of the ω beam and the square root of the pulse energy of the 2ω beam, once the total optical pulse energy is above the plasma formation threshold. The optimal efficiency of the THz field is achieved when all the waves (ω, 2ω, and THz waves) share the same polarization. We tested several pressures at 6.9, 9.1, 11.9 and 23 torr, respectively. Within this pressure range (6.9 torr to 23.6 torr), the emitted THz field increases with the water vapor pressure. It should be noted that water vapor also has strong absorption for THz waves. If the distance between the optical focal spot and the exit window in the vapor cell increases, less THz radiation is expected. Current measurements with water vapor was performed at room temperature. Higher temperature leads to higher water vapor pressure. Upon heating, higher pressures should generate more intense THz radiation.
The four-wave-mixing (FWM) in the water vapor plasma is the major mechanism of THz wave generation. We also prove that the optimal efficiency of the THz field is achieved when all the waves (ω, 2ω, and THz waves) are at the same polarization corresponding to χ(3)xxxx in the FWM process, while the contribution from χ(3)xxyy is very small.
In the FWM THz rectification process, the frequencies of the three input beams add to nearly zero (THz frequency). When a pulsed laser is used, the nonlinear response is driven by the envelope of the input fields. This envelope composes the rectified field which is the source of the THz wave. Mathematically, this third-order nonlinear process is related with χ(3)(Ω: 2ω+Ω, −ω, −ω), where Ω is the frequency of emitted THz wave. Predicted by the four-wave-mixing theory, the THz field at Ω has the form as,
ETHz(t)∝χ(3)E2ω(t)Eω*(t)Eω*(t)cos(φ) (1)
Where E2ω(t)=½E400 exp(i2ωt)+c.c., Eω(t)=½E800 exp(iωt)+c.c. and the phase factor φ=k2ωΔl is the relative phase difference between the ω and 2ω beams, with k2ω being the wave vector of the 2ω beam and Δl being the path difference between the ω and 2ω beams along the beam propagation direction. Equation (1) is based on the plane-wave approximation. When describing the THz field as a function of optical beam power, Equation (1) can also be written as
ETHz∝χ(3)√{square root over (I2ω)}Iω cos(φ) (2)
If the nonlinear media is spatially isotropic, there are three independent components in the third order susceptibility tensor: χ(3)xxxx, χ(3)xyxy and χ(3)xxyy, with the four subscripts corresponding to polarizations of Ω, 2ω, ω, ω beams, respectively.
As an example of the generation of intense THz wave radiation, we select ambient air as the nonlinear medium reference which has been extensively studied recently in our group. Our experimental results confirm that the threshold of THz wave generation is related to the air ionization threshold. We observed a turning point around 150 μJ (combined pulse energy of ω and 2ω beams). This is explained by the four-wave-mixing process in the plasma when the laser pulse energy exceeds the air ionization threshold. By considering the combination of different wavelength laser beams, we estimated the power density of 1.5×1014 W/cm2 assuming the focal spot with 30 μm in diameter. Meanwhile, using the standard value of air χ(3) with the 1.5×1014 W/cm2 laser power intensity [15], the calculated THz field is about four orders smaller than our measured field. This behavior reveals the laser induced plasma with a greatly enhanced χ(3) is the nonlinear media in which the THz wave is generated.
Table 1 lists the gases studied, their chemical symbols, 1st photo-ionization energy, vapor pressures measured and their saturated pressure at 298 K, relative THz field to the ambient air, and the Field Figure of Merit. Ambient air emits about twice the THz field as water vapor, but its pressure is 764 torr, while in comparison, the vapor pressure of water is only 23 torr. Here, we introduce a Field Figure of Merit (FOM) as the THz Field/Partial Pressure. The Field Figure of Merit from the water vapor is 18.5 times stronger than the air, and it is the strongest one among all measured THz field from gases and vapors, as shown listed in Table 1. The number of molecules in a fixed volume is linearly proportional to the partial pressure. When the Figure of Merit is defined as the THz Power/Partial Pressure, then the Power FOM from the water vapor is a factor of 380 stronger than that from ambient air!
Strong Terahertz Emission from Water Clusters
We expected that the four-wave-mixing rectification in the laser-induced water vapor plasma is the main mechanism of the THz wave generation in the air plasma through the use of individual control of the ω and 2ω beams. However the presence of water clusters could be the major reason for the strongest radiation when a 400 nm optical beam is introduced with the excitation of 800 nm beam (second harmonic wave of the fundamental wave), as discussed below:
Recent scientific interest in water clusters has been motivated by their possible roles in atmospheric and environmental phenomena [16,17], biology [18], and astrophysics [19], as well as by their relevance to the structure and properties of liquid water and ice [20]. Experiment and theory agree that not only can such clusters be produced, but also they exist optimally in certain “magic numbers” and configurations of water molecules [21-28]. Prominent among the magic-number water clusters are ones possessing an approximately pentagonal dodecahedral structure. Ideally, these clusters have a closed, icosahedral symmetry formed by 20 hydrogen-bonded water molecules, with their oxygen atoms at the vertices of 12 concatenated pentagons and with 10 free exterior hydrogen atoms.
Ab initio density-functional molecular-orbital levels for the archetype (H2O)21H+ cluster of
Anomalous emission and absorption of submillimeter (THz) radiation from the atmosphere were first identified by Gebbie [29] as possibly associated with aerosols of water clusters undergoing solar optical pumping. He argued that at sea-level densities such aerosols are separated by 104 times their cluster radii and, under this condition of isolation, can be pumped by photons into vibrational modes of lowest frequency analogous to a Bose-Einstein condensation, thus acquiring giant electric dipoles. Their interaction with radiation is thereby greatly enhanced. For example, atmospheric aerosol absorption at 50 cm−1 is comparable with that of a water molecule rotation line at 47 cm−1, which has a transition dipole of 1.1 Debyes in an air sample containing 1017 cm−3 water molecules. Even if the aerosol density of water clusters is only approximately 104 cm−3 [16, 30], then an effective aerosol transition moment of 106 Debyes can be inferred. In other words, Gebbie [29] attributed this greatly enhanced submillimeter (THz) absorption and emission from comparatively low-density aerosols to solar optical pumping, cooperative stimulated emission, and maser action of the constituent water clusters.
The electronic structure (
As originally suggested by Carlon [16] and Gebbie [29], it is reasonable to expect that ambient air of modest humidity should contain aerosols comprised of water clusters such as (H2O)21H+. Thus, the above water cluster theory may explain the origin of intense T-ray emission from water vapor when combined near-ultraviolet (400 nm) and near-infrared (800 nm) laser beams are applied in the above-described invention.
Finally, it is worth pointing out that if we switch the order of the THz field and the second harmonic field in the third order susceptibility in the four-wave-mixing optical process, it should be possible to measure the THz wave by using water vapor as a nonlinear sensor. The reciprocal nature of such nonlinear optical process with the resonance nature in water vapor (molecules and their clusters) should provide unprecedented instrumental performance and innovation when we use water vapor as the THz wave emitting source and THz wave detecting medium.
To estimate the THz emission power of water clusters in apparatus disclosed herein, we begin with the standard formula for electromagnetic radiation power emission from an oscillating electric dipole (in cgs units):
P=p
2ω4/3c3
where p is the dipole moment, ω is the (angular) frequency of the dipole vibration—in a protonated water cluster (
Another embodiment of the invention is shown in
It is recognized that modifications and variations of the invention described herein will occur to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims.
