2014年6月12日星期四

美国最新T射线技术将光转化为声音（转）

美国最新T射线技术将光转化为声音（转）


来源：凤凰科技
2人参与 1评论
凤凰科技讯 北京时间5月26日消息科学日报报道，近日科学家们研发的一种能够检测光波的最新设备或能帮助打开电磁光谱的最后边界——太赫兹(Terahertz)光谱。这个名为T射线的光波太长以至于人眼无法看到，它可以帮助机场保安检测化学和其它武器，还可以让医生对身体组织进行成像，同时保证对健康区域的伤害尽量最少。此外，这或能给天文学家提供新的工具研究其它太阳系的行星。而这些只是少数可能的应用领域。    
光的演示图。用于倾听光波的设备将帮助打开电磁光谱的最后边界。  
由于目前只有检测光的专门工具具备检测太赫兹频率的能力，工程师尚未有效的利用它们。美国密西根大学研究人员演示了一种独特的太赫兹探测器和成像系统，可以有效的填补太赫兹空隙(Terahertz Gap)。  
“我们将T射线光转化为声音，” 美国密西根大学电子工程和计算机科学、机械工程以及（化学）大分子科学与工程的郭杰（Jay Guo）教授这样说道。“我们的探测器非常敏感紧凑，能够在室温下工作，我们利用了一种非传统的方法制造它。”这一探测器产生的声音非常高，人耳无法听到。
太赫兹空隙介于电磁光谱中微波和红外波段之间。这个光谱从最长最低能量的电磁波到最短高能量的伽马射线，后者在核弹爆炸时以及放射性原子衰变时会释放。在这两个极端之间存在微波频率，后者可以用于煮熟食物或者传输手机信号；使热视觉技术变为可能的红外频率；我们的世界所看到的光和颜色的可见波长；以及让医生可以窥看我们身体内部的X射线。  
太赫兹频段是“科研沃土”，郭和同事这样表示。然而，现在的探测器要么非常笨重且必须在低温下工作，要么无法实时操作。这限制了它们应用的有效性，例如武器和化学检测，或者医疗成像和诊断，郭这样解释道。  
郭和同事发明了一种特殊的换能器，能够实现光-声音的转换。换能器主要是将一种能量形式转换为另一种。在这个例子里便是将太赫兹光转换为超声波然后传输它们。  
这种换能器是由一种名为聚二甲硅氧烷（PDMS）的海绵状塑料制品，以及碳纳米管混合物制成的，以下是它的工作原理：当太赫兹光遇到换能器，纳米管会将它吸收，转化为热量，然后将这种热量传递给PDMS。加热的PDMS会膨胀，创造一种输出的压力波，这便是超声波。它大概是人耳能够听到的上限的1000倍。  
“检测超声波的方式有很多，”郭说道。“我们将一个非常困难的问题转化为一个已经被解决的问题。”尽管超声波探测器已经存在——包括那些用于医疗成像的——研究人员自己制造了一种非常敏感的显微镜可见塑料环，名为微环谐振器。这种结构测量的大小只有几毫米。  
研究人员将他们研发的系统与电脑相连，并演示了这一系统可以用于扫描和产生铝交图像。这种最新探测器的反应速度只有几百万分之一秒，郭表示它能够支持很多领域的实时太赫兹成像。  
这个系统与其它基于热的太赫兹检测系统有所不同，因为它是针对单个太赫兹光脉冲的能量，而非持续的T射线流做出反应。因此，它对温度范围以外的变化并不敏感。这项研究是由国家科学基金会（NSF）和美国空军科研办公室(US Air Force Office of Scientific Research)资助进行的。（编译/严炎刘星）

补充阅读：

太赫茲


太赫茲波是介於微波波段的終點與紅外線波段的起點之間.

太赫茲輻射被大氣層強烈的吸收，限制了通信距離。這個圖包含了太赫茲頻譜的低頻部分，從0.3到1 THz。Shown is the zenith atmospheric transmission of electromagnetic radiation from space to the summit of Mauna Kea, assuming a precipitable water vapor level of 0.001 mm (simulated). The downward spikes in the graph correspond to strong absorption lines due to various absorbances of different atmospheric molecules

這個圖補充上圖，顯示大氣層傳輸太赫茲頻譜的高頻部分，從1到3 THz。Shown is the zenith atmospheric attenuation of the electromagnetic spectrum from space to the summit of Mauna Kea, assuming a precipitable water vapor level of 0.001 mm (simulated). The decreasing transmission with increasing frequency, indicates greater absorption
太赫茲（Tera Hertz，THz）是波動頻率單位之一，又稱為太赫，或太拉赫茲。波動頻率的基本單位是赫茲，採千進位制，1太赫茲等於1012赫茲，也就是1000吉赫茲。
THz波（太赫茲波）包含了頻率為0.3到3 THz的電磁波。該術語適用於電磁輻射的毫米波波段的高頻邊緣之間的頻率，300 gigahertz（3×1011Hz），和低頻率的遠紅外光帶邊緣，3000 GHz (3×1012 Hz）。對應的波長的輻射在該頻帶範圍從1mm到0.1mm（或100μm）。
目前，國際上對太赫茲輻射已達成如下共識，即太赫茲是一種新的、有很多獨特優點的輻射源；太赫茲技術是一個非常重要的交叉前沿領域，給技術創新、國 民經濟發展和國家安全提供了一個非常誘人的機遇。它之所以能夠引起人們廣泛的關注、有如此之多的應用，首先是因為物質的太赫茲光譜（包括透射譜和反射譜） 包含着非常豐富的物理和化學信息，所以研究物質在該波段的光譜對於物質結構的探索具有重要意義；其次是因為太赫茲脈衝光源與傳統光源相比具有很多獨特的性 質。[1]

簡介

THz波（太赫茲波）或稱為THz射線（太赫茲射線）是從上個世紀80年代中後期，才被正式命名的，在此以前科學家們將統稱為遠紅外射線。太赫茲波 是指頻率在0.1THz到10THz範圍的電磁波，波長大概在0.03到3mm範圍，介於微波與紅外之間。實際上，早在一百年前，就有科學工作者涉及過這 一波段。在1896年和1897年，Rubens和Nichols就涉及到這一波段，紅外光譜到達9um（0.009mm）和20um（0.02mm）， 之後又有到達50um的記載。之後的近百年時間，遠紅外技術取得了許多成果，並且已經產業化。但是涉及太赫茲波段的研究結果和數據非常少，主要是受到有效 太赫茲產生源和靈敏探測器的限制，因此這一波段也被稱為THz間隙。隨着80年代一系列新技術、新材料的發展，特別是超快技術的發展，使得獲得寬帶穩定的 脈衝THz源成為一種准常規技術，THz技術得以迅速發展，並在實際範圍內掀起一股THz研究熱潮。

產生源

自然產生源

太赫茲輻射的發射從任何大於約10開爾文（kelvin）的溫度下的黑體輻射的一部分。

人工產生源

在2012年，幾種太赫茲輻射的產生源有:

研究

  • 醫學成像
  • 安全檢查
  • 科學使用和成像
  • 通信
  • 製造

無線數據通信記錄

在2012年5月，從日本東京工業大學[6]的研究人員的一個團隊發表在en:Electronics Letters使用T-射線的無線數據傳輸已創下新的紀錄，並建議在未來它們被用來作為數據傳輸的頻寬。該團隊的概念驗證裝置使用的諧振隧穿二極管（en:resonant tunneling diode, RTD），其中的電壓下降的電流增加，造成二極管「共振」，併產生在太赫茲波段的波。使用該RTD，研究人員發送出542 GHz的信號，得到的數據傳輸速率是每秒3千兆位（Gigabits）。該演示比目前的Wi-Fi標準的快20倍的速度，和比此前的11月份的數據傳輸設 置的記錄快一倍[7]。太赫茲Wi-Fi可能僅能在大約10米（33英尺）範圍內工作，但「理論上」數據傳輸速度可以高達100 Gbit/s。[8]

Terahertz radiation

From Wikipedia, the free encyclopedia
"Terahertz" and "THz" redirect here. For The unit of frequency, see Hertz. For the transistor design, see Intel TeraHertz.
"T-ray" redirects here. For other uses, see T-ray (disambiguation).
Tremendously high frequency
Frequency range 300 GHz to 3 THz
Wavelength range 1 mm to 100 μm
ITU Radio Band Numbers
1 2 3 4 5 6 7 8 9 10 11 12
ITU Radio Band Symbols
ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF THF
NATO Radio bands
A B C D E F G H I J K L M
IEEE Radar bands
HF VHF UHF L S C X Ku K Ka V W mm
Television and radio bands
I II III IV V VI
v d e

Terahertz waves lie at the far end of the infrared band, just before the start of the microwave band.
In physics, terahertz radiation, also called submillimeter radiation, terahertz waves, tremendously high frequency,[1] T-rays, T-waves, T-light, T-lux, or THz, consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz, 1 THz = 1012 Hz). Wavelengths of radiation in the Terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.
Terahertz radiation occupies a middle ground between microwaves and infrared light waves, and technology for generating and manipulating it is in its infancy, and is the subject of research. This lack of technology is called the terahertz gap. It represents the region in the electromagnetic spectrum that the frequency of electromagnetic radiation becomes too high to be measured by directly counting cycles using electronic counters, and must be measured by the proxy properties of wavelength and energy. Similarly, in this frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques.

Introduction

Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, and it shares some properties with each of these. Like infrared and microwave radiation, terahertz radiation travels in a line of sight and is non-ionizing. Like microwave radiation, terahertz radiation can penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. The penetration depth is typically less than that of microwave radiation. Terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal.[2]
The earth's atmosphere is a strong absorber of terahertz radiation in specific water vapor absorption bands, so the range of terahertz radiation is limited enough to affect its usefulness in long-distance communications. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems, especially indoor systems. In addition, producing and detecting coherent terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.3–1.0 THz range (the lower part of the spectrum), including gyrotrons, backward wave oscillators, and resonant-tunneling diodes.

Sources

Natural

Terahertz radiation is emitted as part of the black-body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10–20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. The Atacama Large Millimeter Array, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

Artificial

As of 2012, viable sources of terahertz radiation are:
  • In 2012, a source was announced that used a resonant tunneling diode (RTD) to produce waves in the terahertz band at 542 GHz,
The first images generated using terahertz radiation date from the 1960s; however, in 1995, images generated using terahertz time-domain spectroscopy generated a great deal of interest, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays", even showed up in a contemporary novel by Tom Clancy.
In 2002 the European Space Agency (ESA) Star Tiger team,[9] based at the Rutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand.[10] By 2004, ThruVision Ltd, a spin-out from the Council for the Central Laboratory of the Research Councils (CCLRC) Rutherford Appleton Laboratory, had demonstrated the world’s first compact THz camera for security screening applications. The prototype system successfully imaged guns and explosives concealed under clothing.[11]
There have also been solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers.
In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Welp of Argonne's Materials Science Division.[12] This new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: When an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two millivolts per junction – can induce frequencies in the terahertz range, according to Welp.
In 2008, engineers at Harvard University demonstrated that room temperature emission of several hundred nanowatts of coherent terahertz radiation could be achieved with a semiconductor source. THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Until then, sources had required cryogenic cooling, greatly limiting their use in everyday applications.[13]
In 2009, it was shown that T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging of the adhesive tape and subsequent discharge.[14]
In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation.[15]
In 2013, researchers at Georgia Institute of Technology's Broadband Wireless Networking Laboratory and the Polytechnic University of Catalonia developed a method to create a graphene antenna: an antenna that would be shaped into graphene strips from 10 to 100 nanometers wide and one micrometer long. Such an antenna would broadcast in the terahertz frequency range.[16][17]

Research

  • Medical imaging:
    • Unlike X-rays, terahertz radiation has a relatively low photon energy for damaging tissues and DNA. Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g., fatty tissue) and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with an imaging system that is safe, non-invasive, and painless.
    • Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate than conventional X-ray imaging in dentistry.
  • Security:
    • Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. Passive detection of terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.[18][19] In January 2013, the NYPD announced plans to experiment with the newfound technology to detect concealed weapons,[20] prompting Miami blogger and privacy activist Jonathan Corbett to file a lawsuit against the department in Manhattan federal court that same month, challenging such use: "For thousands of years, humans have used clothing to protect their modesty and have quite reasonably held the expectation of privacy for anything inside of their clothing, since no human is able to see through them." He seeks a court order to prohibit using the technology without reasonable suspicion or probable cause.[21]
  • Scientific use and imaging:
    • Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
    • Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples that are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long-term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and spectrally broad, so such images can contain far more information than a conventional image formed with a single-frequency source.
    • Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11 tesla), the electron spin Larmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequency EPR experiments, such as the National High Magnetic Field Laboratory (NHMFL) in Florida.
    • Submillimetre astronomy.
    • Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.[22]
  • Communication:
    • Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.[citation needed]
  • Manufacturing:
  • Power generation:
    • NASA has done recent work with using terahertz radiation in the "5-30THz range" to vibrate a nickel lattice loaded with hydrogen in order to induce low energy nuclear reactions (LENR) but has found that generating the radiation using existing technologies to be very inefficient.[23]

Wireless data transmission record

In May 2012, a team of researchers from the Tokyo Institute of Technology[24] published in Electronics Letters that it had set a new record for wireless data transmission by using T-rays and proposed they be used as bandwidth for data transmission in the future.[25] The team's proof of concept device used a resonant tunneling diode (RTD) in which the voltage decreased as the current increased, causing the diode to "resonate" and produce waves in the terahertz band. With this RTD, the researchers sent a signal at 542 GHz, resulting in a data transfer rate of 3 Gigabits per second.[25] The demonstration was twenty times faster than the current Wi-Fi standard[25] and doubled the record for data transmission set the previous November.[26] The study suggested that Wi-Fi using the system would be limited to approximately 10 metres (33 ft), but could allow data transmission at up to 100 Gbit/s.[25][clarification needed]

Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1 and 1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.[citation needed]

Safety

The terahertz region is between the radio frequency region and the optical region generally associated with lasers. Both the IEEE RF safety standard[27] and the ANSI Laser safety standard[28] have limits into the terahertz region, but both safety limits are based on extrapolation. It is expected that effects on tissues are thermal in nature and, therefore, predictable by conventional thermal models[citation needed]. Research is underway to collect data to populate this region of the spectrum and validate safety limits.[citation needed]
A study published in 2010 and conducted by Boian S. Alexandrov and colleagues at the Center for Nonlinear Studies at Los Alamos National Laboratory in New Mexico[29][30] created mathematical models predicting how terahertz radiation would interact with double-stranded DNA, showing that, even though involved forces seem to be tiny, nonlinear resonances (although much less likely to form than less-powerful common resonances) could allow terahertz waves to "unzip double-stranded DNA, creating bubbles in the double strand that could significantly interfere with processes such as gene expression and DNA replication".[31] Experimental verification of this simulation was not done. A recent analysis of this work concludes that the DNA bubbles do not occur under reasonable physical assumptions or if the effects of temperature are taken into account.[32]
 
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