Terahertz spectroscopy and its infinite possibilities
#Tech #Terahertz #Spectroscopy #THz-TDS
by clement
For those who are familiar with the fields of chemistry or physics, spectroscopy is a powerful tool that could provide spectral and temporal fingerprints of materials or elements. For most people nonetheless, this technology does not affect our daily lives and we couldn’t care less. Terahertz, on the other hand, is a word that is even more bizarre for most of us. Since on paper, terahertz (10¹²Hz) merely means the frequency 3 orders higher than the commonly heard Gigahertz (GHz,10⁹Hz).
When we talk about the latest technological advancements, what comes in our minds is often consumer products such as the new iPhone or Tesla’s new automobile. Breakthroughs in fields of science seem distant from our daily lives, yet they are the building blocks for future tech products, which is why I would like to introduce you to this fascinating field of science and technology.
How can terahertz benefit our lives?
What could transmission frequency as high as terahertz possibly mean to our lives? Imagine one day, you as a future netizen could have wireless internet access up to 100 Gbits/s, which means that downloading hundreds of movies and TV shows can be done within an instant, taking medical images no longer puts you at risk of cancer, companies can examine the quality of their food products or IC chips on their assembly line with unprecedented precision. These are the possible potentials of terahertz technology. Consequently, the next question is, what lies behind terahertz technology?
Terahertz, the chimera of microwave and photonic technology
Terahertz radiation waves with wavelengths between 10m to 1mm sit in a region among the electromagnetic spectrum between radio waves and far-infrared.
Like radio waves, terahertz waves can penetrate through non-conducting materials such as paper, wood, clothing, etc. Therefore, terahertz is suitable to create tomographic images or used as a safer alternative for X-ray security scanners in airports. Like far infrared, terahertz waves are non-ionizing, which means that atoms in contact with terahertz will not lose their electrons and become ions. Therefore, terahertz is theoretically non-carcinogenic, unlike X-ray or Gamma rays. It also travels in a line-of-sight manner, which means the wave would propagate as a straight line from the source. Hence, terahertz waves could diffract, reflect, refract just like how “conventional” visible light and infrared would.
The terahertz gap
Even though terahertz has many interesting and extensive properties, some practical concerns have hindered its widespread. Terahertz possesses wavelength short enough to create complications for MMIC (Monolithic Microwave Integrated Circuits) design. Meanwhile, current laser-based terahertz systems could not provide highly efficiencies emission. If terahertz waves could successfully propagate through the atmosphere, it would also suffer from high attenuation from air particles such as water vapor and oxygen molecules.
Due to this absorbing effect, low radiation power would result in a short effective range of terahertz signals. For communication applications, this will pose a big threat for long-distance information transfer. As a result, terahertz may be more suitable for short-range indoor wireless access.
To build terahertz systems in optoelectronic laboratories, we often utilize ultrashort pulse lasers(which are quite expensive) such as titanium-sapphire lasers or other types of mode-locked lasers.
Pump-probe spectroscopy, the way to find the perfect terahertz transmitter
A widely used method to generate terahertz radiation is by using a specially designed antenna chip called the photoconductive antenna (PCA). To keep things simple, we can say that PCA is a device that converts laser pulses to terahertz waves. The PCA device consists of a semiconductor chip with an antenna at the sides and a gap at the center.
PCA devices work like this:
・We shine a pulse laser with sufficient energy levels at the central gap to create photo-induced electrons within the semiconductor material based on the photoelectric effect.
・By applying a bias voltage at the antenna electrodes, we can drive the generated electrons to produce a current pulse.
・This current pulse will generate terahertz radiation from the antenna.
Okay cool, this means that if we have a well-designed antenna and a laser system, then hooray! We get terahertz radiation! Unfortunately, things aren’t that simple. To generate EM waves up to terahertz (10¹²Hz), we will require a signal pulse that is about the picosecond range. Using tens or hundreds of femtosecond pulse width lasers to drive the device, we will need a corresponding electron response time no longer than the picosecond range to generate terahertz radiations. This electron density response time has a scientific name called the carrier-lifetime, which is sensitive to the smallest amounts of impurities or defects in the material.
To find the semiconductor material that provides adequate carrier-lifetime for terahertz generation, we often use a tool called the pump-probe spectroscopy.
Spectroscopy is the experimental technique to study the spectra of any kind of matter interacting with electromagnetic radiations. Here we will focus on the reactions of materials when they are in contact with laser beams.
Pump-probe spectroscopy, in particular, is the technique of utilizing two pulse laser beams, one powerful pump beam to excite and generate electrons within the semiconductor material, and a weak probe beam to detect the reflectivity or transmittance difference of the material caused by the pump beam. This material property changing effect is caused by photo-generated electrons blocking the passage of the probe laser beam.
By recording the transmittance or reflectivity difference of the material with the different delay time between the pump beam and the probe beam, the duration of occurrence and dynamics of photo-generated electrons could be visualized and recorded.
Using the method above, the ultrafast carrier-lifetime of semiconductors can be obtained with good reliability and could determine whether the material is a good candidate for terahertz antenna devices or not.
THz-TDS, the state-of-art technique to uncover secrets beneath materials
Now, here comes the tricky (or interesting) part. In the previous paragraph, we used spectroscopy to find the perfect material for Terahertz generation, and the generated terahertz waves will open the door towards the powerful and versatile terahertz time-domain spectroscopy (THz-TDS).
To maintain the simplicity of our discussion, we can describe THz-TDS as traditional pump-probe spectroscopy with the detection (probing) medium swapped for terahertz waves instead of a weak probe laser beam. The bulky pulse laser system will be used to drive our terahertz emitter and detector, which interestingly are both PCA devices.
By detecting the amplitude or phase difference of the terahertz pulse wave after it passes through the material under test, we can uncover the secrets hidden in the matter.
There is much useful information the matter could leak. For example, the vibration frequency of a certain molecule may fall in the terahertz region. As a result, the radiation energy of a certain frequency will be absorbed by the matter and create an observable attenuation at the detection side, creating the so-called “fingerprint” of the matter similarly to spectroscopies at different frequency ranges.
THz-TDS is the ultimate tool to realize the wonders I mentioned in the beginning. Materials being transparent at terahertz spectral range such as drugs, narcotics, explosives or even the part of the human body could produce unique spectral fingerprints for identification. With certain modifications, THz-TDS could be used for imaging purposes such as security checking machines or bio-medical applications. Some results even show the capability to detect skin cancer and monitor scar growth.
Outlook
At the age of 5G communications, the industry and academia have been gradually shifting their attention to millimeter waves or even submillimeter waves (which is the spectrum of terahertz waves). Once high-performance emitters and detectors could be manufactured, compact terahertz systems or even IC chips may become a reality and terahertz technologies can benefit our lives as consumer products. Terahertz based systems may provide solutions to improve current IoT and LiDAR systems such as autonomous vehicle vision and smart city infrastructures.
Will terahertz replace the current method of communication and imaging? The answer is most likely no. Instead, I believe it will co-exist with great technologies that are available in the meantime, yet the properties of terahertz will make its application one of a kind without a doubt.
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Author/clement
Half physics, half EE senior student at NTHU. Cherish every moment in life, work hard for a better tomorrow.