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Terahertz - Science topic

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Dear Colleagues,
It is claimed that terahertz waves have an interesting penetration capacity in dielectrics.
Is there a mathematical method to evaluate this penetrability of a dielectric medium by a terahertz wave?
Thanks and Regards
A BENOUDINA
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Maybe you can find the old but very good books by Arthur on Hippel :Dielectrics and waves; and also (same Author) Dielectric materials and applications
and of course in Wikipedia and (much less well known " Electropedia" (from IEEE) via the web
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It is quite challenging to build a telescope of terahertz beam as it is invisible. Can anyone give some suggestions so that the lab life can be easier?
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It is not possible to discuss the details of your optical scheme without drawings. You can contact me by e-mail <aatarasov@laseroptek.com> if you need.
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Hi there,
I wish you are doing great.
I would like to know the state-of-the-art terahertz technology for Imaging (0.5 - 6 THz). Do you know, which company is selling the best one in terms of fast scanning of layered materials, THz power, Frequency range, flexibility, portability, resolution, field-of-view, etc.
I appreciate your time and any feedback.
Best regards,
Shyamal
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Adnan Zahid thank you very much for mentioning the paper.
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I would like to use the simulations to assess the feasibility of non-destructive testing using the terahertz method for composite materials of various structures.
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Yes.
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I found on comsol database one online webinar/workshop where a terahertz metamaterial was modeled and there, input "(px*py/4/2/Z0_const)*E0^2" this equation is used. I need to know why is this used?
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I have read a few papers on this topic but the simulation procedure hasn't been mentioned anywhere. Only results have been shown.
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First of all you need specifically determine detection mechanism, then you be able to find description path. In my view, if the target virus shows distinct absorption and /or reflection versus THz radiation, you can develop a meta-surface according to target frequencies. Backing to your question regarding simulations, I have to say that several possible simulations are available to you. You can use COMSOL, CST, Lumerica and MATLAB to describe your device. Even you can use equivalent circuit model for speedier simulations. In this way you have to describe your materials and structures. For instance see:
Biabanifard, Sadegh, et al. "Tunable ultra-wideband terahertz absorber based on graphene disks and ribbons." Optics Communications 427 (2018): 418-425.
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I am recently working on terahertz generation using tilted pump front method in LN crystal. I basically followed the setup in Hirori 2011 apl paper but unfortunately I couldn't see any THz although some SHG can be seen. I used 800 nm, 70 fs laser. I think maybe I am wrong somewhere but I dont know. Can anyone share their practical tips please? For example, which part (angle of grating, location of lens etc. ) is most important? Can the scattering light from the crystal tell us any information? How to tell that we have the teraherz? Thank you very much!
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Chris Bohr Spectrally broad spectrum. You can look for some papers on the internet. I would recommend to collect all the pump residue from LNO and inject it into the spectrometer. Be careful not to damage the spectrometer.
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Usually the terahertz source used for terahertz communication is a continuous terahertz wave generated by differential frequency, then can terahertz pulses generated by femtosecond laser be used for terahertz communication?
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In principle yes however there is serious problem: how to achieve good sensitivity? To extract information, one needs to perform down-conversion or envelope demodulation / pulse position detection. For Thz continuous wave one can do down-conversion by some kind of mixer however of Thz pulses we mostly left with option of square (envelope) detector and it requires much stronger signal. Some limited distance can obtained by providing rather high power of transmission.
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I believe that Terahertz will be used in 6G system. What do you think?
Spectrum is the basis and scarce resource of mobile communication. The growing demand for traffic requires future mobile communication systems to expand available spectrum resources. Terahertz and Visible Light will be two kinds of attractive candidate spectrum. The development and utilization of terahertz spectrum in communication and other fields has been highly valued by countries and regions from Europe, the United States, Japan and other countries. It has also received strong support from the International Telecommunication Union(ITU). Visible light communication technology is a new communication mode developed with the support of high-speed switch by lighting source. It can effectively alleviate the current problem of radio frequency communication bandwidth tension, and provide a new choice for short-distance wireless communication.
Terahertz wave refers to the electromagnetic wave whose spectrum is between 0.1 and 10THz and whose wave length is 30 to 3000 microns. The spectrum is between microwave and far-infrared light, adjacent to millimeter wave in its low band, and adjacent to infrared light in its high band, located in the transition region between Macro electronics and micro-photonics. Terahertz, as a new frequency band between microwave and optical wave, has not been fully developed. Terahertz communication has the advantages of rich spectrum resources and high transmission rate. It is a very advantageous broad band wireless access (Tb/s level communication) technology in future mobile communications.
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In short THz communications will be part of 6G. Due to its extremely short range due to poor propagation performance. Such a technology is promising very low latency and very high data-rates, but not intended to provide all kind of services. only expected for some specific use-cases like Indoor connectivity, inter-datacenter communications, UAV/satellite communications, etc.
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Basically, a software that helps to fit the terahertz transmission and conductivity data with the Lorentz model or drude model or drude-Lorentz model or any other material-dependent model to get various related parameters. parameters
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Hey,
Just a little disclaimer I am an active researcher in THz but i do not tend to do spectroscopy so my information may not be completely up to date.
As far as i am aware this is no commerical software that does this, some THz antennas setups are sold with their own software (such as the BATOP setups)
In the vast majority of cases THz still tends to be lab based research, as such each individual lab tends to have their own fitting codes they have produced themselves. One option would be for you to do this, then you would have a code working exactly like you wished. Depending on the parameters you wish to extract if you shoot me a message/email I can give you some guidance on how to achieve this :)
Alternatively i know there is a github project focusing on this, although i haven't ever tested it myself i've attatched a link here: https://github.com/THzbiophotonics/Fit-TDS
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Are there any programmable metasurfaces available for purchasing? they should be designed for frequencies above 200 GHz, and can modulate the phase or amplitude of the input terahertz wave rapidly. 
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You may consider using some silicon wafer with high brightness light image projected on it e.g. https://core.ac.uk/download/pdf/41979793.pdf
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Millimetere wave (mmWave) channel model is widely known in the literature [1, eq. (7)], [2, eq. (1)]. [3, (55)].
What is the difference between mmWave vs. Terahertz Channel Models? In other words, is there any paper providing the channel model for Terahertz bands (including path loss, path gains, AoD/AoA, etc.)?
[1] A. Koc and T. Le-Ngoc, "Hybrid Millimeter-Wave Massive MIMO Systems with Low CSI Overhead and Few-Bit DACs/ADCs", Accepted in IEEE 92nd Vehicular Technology Conference: VTC2020-Fall , Victoria, BC, Canada, October 2020.
[2] L. Yan, C. Han and J. Yuan, "A Dynamic Array-of-Subarrays Architecture and Hybrid Precoding Algorithms for Terahertz Wireless Communications," in IEEE Journal on Selected Areas in Communications, vol. 38, no. 9, pp. 2041-2056, Sept. 2020
[3] A. Morsali, A. Haghighat and B. Champagne, "Generalized Framework for Hybrid Analog/Digital Signal Processing in Massive and Ultra-Massive-MIMO Systems," in IEEE Access, vol. 8, pp. 100262-100279, 2020
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Good question
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I am a communication engineering student and I am interested in doing research in Terahertz generation. So my question is what are the subjects or topics that I do need to have a sound knowledge before going into it?
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Hello:
I would suggest a decent background in:
1) Electromagnetism--Dipole models, plasma and material interactions, dispersion, Gaussian beam propagation, etc.
2) Wave/Fourier Optics--We take lots of Fourier transforms for spectroscopy
3) Nonlinear Optics--Some of the best sources are made by optical rectification
4) Lasers--most coherent sources require ultra-short pulsed laser
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I want to calculate terahertz spectra for my system but I am confuse about how to do it. The program I am using (crystal17) has two options, one is to perform frequency calculations at gamma point and calculate IR spectra which by default comes from 0 to 4000 cm-1 . Another option is to do phonon dispersion calculations (http://tutorials.crystalsolutions.eu/tutorial.html?td=dispersion&tf=dispersion-ht) which generate supercell and perform calculations on it. I am confused about if the treahartz spectra will be the regular IR spectra taken between 0 to 100 cm-1 or it should be calculated from the phonon dispersion calculations.
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Thank you Manoj Kumar Banjare sir for the publication. I am reading it now
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Plz tell all possible antenna type which can be used for Terahertz
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For THz can used nanowire dipole and monople antennas, dielectric resonators antennas as well.
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A preliminary study of device orientation and its effect has been investigated in the following paper.
Laplace model for device orientation has been validated through experimental measurements.
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Hello friends
Do you know an easy method (e.g. software, open-source code, etc.) for the extraction of materials properties (e.g. permittivity and loss) from TDS data?
I am new to this field and have no background in physics.
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Hi Masoud,
this open access article seems to give a good overview of the required algorithms for the extraction of the relative permittivity and loss constant:
Usually, most research institutions with TDS systems also have the scripts and are able to post process the raw data after the measurements.
If this is not your case, I found an open source code written in Python:
The code is explained in the following paper:
I hope it helps!
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Hi everyone,
I am working on a project about Terahertz antenna. I fabricated an antenna with CPW-feeding structure, with size of 200 um. The s11 parameters can be obtained by GSG proble+VNA+Thz Module.
However, i have no idea on how to measure the radiation of the antenna since it has to been tested on a wafer probe system, which is not ready to be moved into a Chamber.
It is said that the Total Radiated Power is the the potential parameter that can be measured.
Does anyone could please help on how to setup and calibrate the system for Total Radiated Power for 140-220GHz planar antennas?
Thank you very much in advacne!
Best,
Bin
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I am not sure whether this would be applicable to your specific setup but a general method is to measure the input impedance :
1. while antenna is radiating in free space ( at such high frequencies chamber may not be required). This would show power loss plus power radiated.
2. Cover the antenna structure with a metallic casing and measure the input resistance. (this would show the power loss)
3. Subtract 2. from 1.
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I am concerned about the energies of EM radiations. Like the visible has an energy in the range of 1eV, UV has an energy of 10 eV. So when we shine the EM on a semiconductor, how does it affect the charge carrier concentration of it.
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It depends on the thermal part of incident energy, which in turn rising the charge carriers from valence to conduction band
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  • I'm new in the field, working on characterization for a model of air , cotton , air in the terahertz band.
  • I need to calculate the Permittivity of the cotton in the range from 0.5- 1.5 THZ needed to calculate the total pathloss.
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Hello Esraa,
You might be interested by our work in silk foam THz characterization:
We measuredthe refractive index of silk foam. Because of it's high porosity, we found a refractive index of 1.06 very close to that of air. I guess cotton would show similar values very close to 1 (if its porous).
Otherwise, you can also check our review on THz imaging. Specifically, section 2.2 where we show how to calculate refractive index from transmission/reflection measurements. Here:
Also, check the terahertz literature, characterization of cotton may have already been demonstrated.
Best
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A highly structured semiconductor material with a conductivity type is available. What material can be modified to obtain a terahertz radiation source?
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When you refer to a THz laser source are you referring to something similar to a QCL? In which case i am not aware of anything which can be synthesised without expensive equipment. To my knowledge every THz-QCL requires photolithography and wet etching processes (an example paper on high paper THz-QCL: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6746300).
If however you are after Terahertz Time-Domain spectroscopy, there are plenty of inexpensive options for generation materials, such as ZnTe, GaAs, PCAs, InAs etc... This technique has the advantage of being broadband but requires a femetosecond laser source to drive the operation which in itself can be quite expensive.
However i hope that some inexpensive options for QCLs will become available soon, and if anyone has any further information on this please let me know!
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Why is only femto second laser used. How is it related to terahertz generation?
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THz radiation is made up of femtosecond pulses so you need a femtosecond laser (or femtosecond something) to generate it, as Paul Kinsler describes.
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Can you give me an idea regarding THz wave propagation in human body, any model or any experimental measurements to get some sort of ideas.
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The absorption of the water molecules to the electromagnetic waves is due to the polarization mechanism where there is orientation polarization at relatively low frequencies after which comes the absorption due to ionic polarization at the long infrared and after that comes the electronic polarization at extremely high frequency. There is also the attenuation due to the conductivity of the water because of the salt contained in it. May be the salty water is very similar in response to the human body.
Best wishes
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I am simulating a THz structure with discrete port in current mode ( not S-parameter or Voltage mode), now my problem is how to obtain S-parameter and efficiency with that?
here is not in 1D-results S-parameters and efficiency results?
regards
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You must have a load resistor connected in the circuit. Also you cannot use a current source since its available power is infinite. You must add a source resistance in parallel with the current source, equal to the load. Then the efficiency can readily be calculated, and is left to an exercise by the student.
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In 0.1 - 10 THz regime, how to reduce the path loss?, what are issues associated with path loss?
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As said by Zahed Hossain you should not select THz frequencies for wireless communication that have high water vapor absorption. In addition to wise selection of THz frequencies and high gain antennas you can overcome high pathloss penality by beamforming with ultra-massive MIMO antennas.
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is there any reference to display the development of methods?
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I just wants to measure the protein sample using THz Spectroscopy. Can anybody suggest me what concentration is appropriate??
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I am developing a CCTV Terahertz patent - but I have my own use cases - please read my suggestions - mainly in German - please use a translator.
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How can I access to your special suggestions , give me the link please .
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Hi
I want to enter the conductivity of superconductor in cst but according to this paper, it is complex and depend on the angular frequency of incident terahertz wave. how can I do it?
thanks for your help
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It depends on frequency. Note that at 13 GHz the imaginary part of
conductivity of YBCO is about 10^8 S/m while the real part is two orders of
magnitude smaller. At 1.3 THz the real and the imaginary part of conductivity
will be similar (the imaginary part vary with frequency as f^-(1), while the real part
can be consider as a constant value).
Also when you use Drude model for the real part of conductivity it is necessary
to know plasma frequency for YBCO.
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Hello,
Could anyone suggest a paper giving the far infra-red or terahertz complex refractive index of graphene?
Thank you!
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Thank You All,
Indeed infra-red properties of graphene monolayers depend strongly on the doping, and temperature as established by L.A. Falkovsky. With appropriate modelling of Fresnel coefficient as well as the permittivity, it possible to extract the complex refractive index.
With reference to the to the paper suggested by Dr Azeez, I think the real index would be approximately 2.0 in THz range at weak doping.
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Dear Respectful Researchers,
With 5G still under going (well and fast), 6G falls into the radars of quite a bunch of pioneers in the telecommunications and related areas. To be honest, It may be not too early to "worry about" it, since around every 10 years there comes a newer generation of wireless communication.
From the frequency domain, 5G wireless system designs are already moving to millimeter wave (mmWave) bands as high as 100 GHz. Will Terahertz (>300 GHz) be the next domain 6G will conquer in terms of the spectrum. However, what type of challenges would be using THz for mobile communications ? Since THz are more close to behave as "optical" than mmWave, how fast/soon could we expect it to be used for real-life communication, based on the R&D progress of current state-of-the-art designs ?
Moreover, what else significant technological evolutions or revolutions do we expect/predict to deeply impact the future 6G and form it ? AI, LiFi, Quantum Computing, etc. And, what kinda of fancy applications scenarios could you think of/expect/predict ? (E.g. holographic gaming, super deep immersive video, etc. )
Dear researchers, thanks for your discussion and inputs.
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1) continue with the left overs of 5G (Massive MIMO, mmWave, Terahertz, D2D, UAVs, NOMA, ...etc )
2) Adding Artificial Intelligence, specifically Machine Learning to the Loop
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How spp waves are generated in transmitting type of graphene based patch antenna at terahertz frequency?
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SPP is supported by the plasmonic metamaterials named spoof surface plasmon polaritons like graphene. Surface plasmon polaritons (SPP) are collective oscillations of free electrons trapped at metal-dielectric interfaces. When electrons in graphene are excited by an incoming electromagnetic wave, they start moving back and forth. This global oscillation of electrical charge results in a confined electromagnetic wave on top of the graphene layer. For transmitting, the SPP waves can be created by injecting electrons into the dielectric layer beneath the graphene sheet. In antennas, microstrip SPP transmission line is used as the feeding line to improve the beam scanning range.
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There are currently two mainstream theories when describing the generation mechanism of terahertz waves. One is an ionized photocurrent model, and the other is a four-wave mixing model. The four-wave mixing model is designed to the third-order nonlinear process in the filament. However, third-order nonlinear processes have encountered difficulties in describing the strength of terahertz waves. It has been reported in the literature that higher order nonlinear effects are more effective than third order nonlinear effects. We know that higher order nonlinearities require higher light intensity. My confusion is that the four-wave mixing model is suitable for laser energy with low laser energy, which corresponds to the pulsed laser energy when the plasma has not yet formed. Then why is the higher-order nonlinear effect for such low energy more important than the third-order nonlinear effect? My confusion stems from the b diagram in Figure 12 in the following article.
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For figure12 (b) In this article, it is stated that the each of the higher orders (5th and 7th) have had some arbitrary scaling applied to fit on the same figure as the 3rd order process. I believe this is the root of your confusion.
To my knowledge third-order processes are more efficient for THz generation than the higher order processes
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One sentence from reference say: “ Unlike nonlinear crystals, a lack of phonon modes in plasma prevents THz absorption, resulting in broadband THz radiation with no spectral gap”。 Can we understand that this plasma does not absorb terahertz waves? But, we also often see literature mentioning the absorption of terahertz waves by plasma. So what is the relationship between the nature of the plasma and the absorption of terahertz waves?
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It mostly depends on the density of the carriers. You could use Drude model to simulate the optical response of a film under different density. The carriers' effect would influence the film's optical state (transparent-loss increasing-loss decreasing-reflectant) with the increase of the density.
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The femtosecond laser can form a plasma in the air, and broadband terahertz radiation is formed in the plasma due to the oscillation of positive and negative charges. We know that there are two main frequencies involved. One is the oscillation frequency of the plasma and the other is the peak frequency of the terahertz spectrum. The usual study considers the peak frequency of terahertz to be the intrinsic oscillation frequency of the plasma. Is the oscillation frequency of the plasma and the peak frequency of the terahertz really equal? What are the main factors determining the peak frequency of the terahertz spectrum?
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Terahertz range can work with massive MIMO or NOMA??
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NOMA is useful when two (or more) users have similar channel directivity, so that they can be covered by the same beam, but different pathlosses. One of the key properties of massive MIMO is called "favorable propagation" and means that the chance of having two users with similar channel directivity reduces as you add more antennas. If you go up in frequency (and keep the aperture fixed), then the beams will be even more directive.
In other words, more antennas and higher frequency are two properties that reduce the NOMA gain until the point where it disappears. At terahertz frequencies, I doubt that NOMA will be useful.
This is not a bad thing, it is a good thing! It means that using massive MIMO is sufficient - no need for an additional NOMA layer.
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Terahertz range of frequencies is the future range of mobile communications what are its pros and cons??
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The "only" reason to go up in frequency is to have access to more bandwidth. More bandwidth also requires higher sampling rate and higher frequencies require transmitter and receivers that are capable of modulating signals of that kind.
Visible light is one example of terahertz signals. Just as visible light is blocked walls and other objects, terahertz applications will be limited in the same way.
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I was wondering if LT-GaAs or other III-V materials such as InGaAs/InAlAs that are grown in MBE for Terahertz application can be grown in MOCVD ?
What are the differences? Pros and cons of each method?
Has any one tried growing III-V materials in MOCVD for terahertz application?
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The decomposition temperatures for different precursors used in MOCVD vary but are all so high that you cannot grow at extremely low temperature, so no LT-GaAs.
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Hey, I am new to COMSOL and I am trying to simulate terahertz antenna. For the same, I want to excite the port via femto-pulse. Can anyone suggest me how do i provide local femto-excitation e.g., near anode or cathode. I am working on wave optics module and looking the radiation pattern in far-field.
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hey nimisha...you need to have two modules for that... RF module or wave optics module and semiconductor module... I have designed PCA using COMSOL...if you need any help then drop me a mail at Abhishek.Gupta@eli-alps.hu
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I have used a QMC ltd. manufactured liquid Helium cooled bolometer, model no. QGeB/2 for Terahertz detection from two color laser produced plasma source. The Terahertz radiation we get is of 1-2 picosecond duration at a repetition of 10 Hz and generates a 6-8 volt signal at 40 dB setting of the bolometer. I am referring the following attached document, taken from the thesis of J van Tilborg (http://alexandria.tue.nl/extra2/200611221.pdf) for calibrating the results for our case. To use the equation (A.6), the values of S (Bolometer voltage responsivity, V/W), η (efficiency of the radiation absorption by the bolometer) and τ (response time of bolometer, given by C/(G-I02 dR/dT), where C is the heat capacity of Germanium and G is the thermal conductance of metallic layer) are required. Correct knowledge of these parameters will only allow me to implement the technique. Voltage responsivity is given in the manual as 16.5 kV/W. η, I suppose is the same as β they have used to calculate the incident power Pdet in chapter 6, page no. 27. The temperature in lab was maintained at 23 °C and humidity was maintained in a range of 60-70% throughout the experiment. I have mailed the QMC people for parameters. Has anyone used this bolometer for similar purpose?
How might I calibrate the energy of picosecond long Terahertz pulses while using a continuous wave Germanium detector based bolometer?
If some other calibration technique is possible, kindly guide me!
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Hi, I am working with Terahertz signals, generally reference and sample, both collected in time domain. When I FFT, I will get a range of frequencies up to 12THz. However, most of the time, above 3.5THz, the FFT signal will be all noise, but sometimes noise can come in at a lower frequency. And given my understanding that the equipment operating range is from 0.1THz - 3.5Thz, and this can vary according to the subject of study, can I ask if there is a way to determine the actual range of Terahertz frequency which can give useful information about signal by making use of FFT information?
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Basically, there is 'working' range of THz TDS apparatus given by its construction and can be reached for very low absorbing samples. It is defined by the signal-to-noise ratio > 1 in the spectrum of the collected time-domain signal.
In praxis, due to absorption in the tested object, the useful 'working' range
of the apparatus is reduced, typically on the high-frequency side of the THz spectrum.
I recommend you to read this paper
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I am trying to find the refractive index (n) and extinction coefficient(k) of the complex refractive index of my sample- a fish. The data are collected in time domain via time domain Terahertz reflection spectroscopy. The data are then FFT and n and k are obtained through further calculation. However, the problem is I get positive and negative extinction coefficient over a range of frequencies for my sample (k). What does this mean? Is there any way to rectify this problem?
Any help is deeply appreciated. Thank you!
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A negative extinction coefficient essentially means that you have amplification or gain rather than absorption. If gain is unlikely with your sample then you must have some kind of error either in the measurement or calculation.
I hope this helps.
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I am looking for the surface conductivity of graphene in THz range. if chemical potential is 1eV. If you suggest any paper to me then that will be okay.
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There is a short and rude answer to this question. Zero.
The conductivity is the product of mobility and the concentration of charge carrier at the Fermi level. Mobility is known 15000 cm2⋅V^−1⋅s^−1 (from Wikipedia). There are no charge carriers at the Fermi level in a separate sheet of graphene. It has to be doped to shift the Fermi level. Everything depends on doping.
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As  i am working on terahertz antennas ,i need to use graphene as a conducting material so that i want to know the parameters of graphene conductivity,permittivity,permeability etc.
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Conductivity of typical high quality epitaxial graphene is 
about 6x10^6 S/m (APL 96, 082101, 2010) but it would
depend on technology of its deposition. It is nonmagetic material 
so permeability can be assumed as unity. Permittivity up to THz
frequencies is not measurable because its imaginary part related
to conductivity is much larger than the real part.
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I mean in near future.
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hope...........is still on the ground
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Because i do not how to fabricate the protein or biological sample without water to detect under terahertz time domain spectroscopy (TDS) setup like the thin films fabrication processing. 
Thanks a lot!!!!!
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You can use amber.
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Hello, 
I would like to calculate the refractive index from Terahertz measurement. I'm aware of the established equation to do so from the literature, whish is: 1 + [phi*c/(2*pi*f*d)] where phi is the phase between the sample and the reference signals.
So my question is how to calculate phi? is it just the difference between the peaks of the material pulse and the reference pulse? in that case, it doesn't depend on the frequency which doesn't make sense to me,, 
Would someone help me with this,,
thanks all
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Hi,
The peak difference you are talking will only give you the Rough estimate to begin your calculation (Time delay between peaks will give you N and Height difference will give you K). The references given above by Márta Unferdorben will show you the precise method to calculate them. I hope this helps. 
Happy New Year :-)
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Please show me some ways of improving the resolution of the terahertz imaging system 
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Your question was how can one improve the resolution. The limit if of course diffraction. However, there are some practical ways to improve resolution. First, I'd recommend using focusing mirrors instead of lenses whenever possible. Off-axis paraboloids are conic sections and show no spherical aberrations when focusing an incident plane wave. Use two of them for a confocal imaging system. Even better, if you can measure amplitude and phase, then you don't even need a mirror, but the knowledge of phase and amplitude in the diffraction pattern will allow you to reconstruct the object, much like it is done in holography. The problem here may be signal to noise for time domain systems. I also suggest you set up your system with a visible light source. If it looks good for visible light, it will look good for THz.   
The fundamental problem with many THz imaging systems is that we use fairly small optics to get the job done, often with unsatisfactory results. Remember, THz wavelengths are 1000 times longer than visible wavelenghts, but we don't have 1000 times larger optics of course. Still, go big, like 5 inch mirrors, or even larger. This can become costly if you make such a mirror at visible light quality, but you don't have to. You can save a lot of money by relaxing the tolerances to THz wavelength quality. 
Lastly, coherence can be a problem in the form of stray light, multi-path, etc. Make sure you take that into account when setting up your imaging system. 
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For example many reports are available with thickness dependent terahertz generation. Organic single crystals exhibits higher THz efficiency at particular thickness. Could you please explain the reason for thickness dependent THz efficiency? 
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If you consider the physical meaning of the phase matching term within the generation process,
ITHz ~ L2Sinc2(dK L/2)
for a particular THz and optical frequency we can say that delta k will be constant but non-zero, now by increasing the length of the crystal (L) the generation efficiency grows. It continues to grow until 
L = 2pi/dK
at this point the any newly generated terahertz radiation is pi out phase with any terahertz radiation generated L before it and so destructive interference means they cancel out and the efficiency decreases. 
From the same equation 
ITHz ~ L2Sinc2(dK L/2)
It is also possible to see now that by allowing dK to vary with THz frequency it is possible to have a frequency dependent efficiency. For instance it can occur that if the crystal is a certain thickness, the condition 
L = 2pi/dK
is met for low frequencies but not met for frequencies in a higher range. In its simplest form each frequency component has its own optimal crystal thickness (given by  L = 2pi/dK) and so for a given thickness each frequency component will have a slightly different efficiency.
I hope this helps.  
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Hello I would like to simulate that but I don't know to use which of the physics and order of simulation? I had simulated a lot of antenna with comsol but for THz region really I'm beginner in it. Does anyone simulate the terahertz antenna in comsol? Regards,
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Hell
I've worked in the field of THz antenna simulation. I work with software COMSOL.  but I not could get an acceptable answer,With this software. Because this software solution is  FEM not FDTD. It is better to use you of the CSTsoftware.
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Hi, Folk!
I would like to measure sample in low-temperature (several 10 Kelvins) using THz TDS. Could anybody suggest a commercially available thermostat for that? A smaller one is better, as there is not large space (30 cm) between two antennas.
Thanks a lot!
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Thanks for your kind information, Mahmut!
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We have the refractive index of organic crystal OH1, measured by THz-TDS.
But it seems there is no formula which can describe the refractive index of OH1. I have read a paper 'Terahertz source at 9.4 THz based on a dual-wavelength infrared laser andquasi-phase matching in organic crystals OH1'. It says a formula can fit the measurement well by Lorentz 8-oscillator mode. 
So I am looking for the formula fitted by  Lorentz 8-oscillator mode. Is there anyone who know the formula or the other formula which can also fit the measurement of refractive index of OH1 in the terahertz range. Thank you so much
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Just the Lorentz formula, 
ε = cte + summation_i [ Ai/(  ω20,i - ω2 - j Γiω) ]
where ε is the complex dielectric constant (can be converted to absorption and refractive index). The fitting values are cte, Ai, ω20,i Γi. Where A is the strength of the i th resonance at frequency ω0 with a damping of Γi (determines the broadness of the resance).
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Dear all,
         Where can I find the complex refractive index of Ge and Te in Terahertz (THZ) region like (0.1 to 2.5 THZ)?
          Thanks.
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Database, or academic articles. Just go searching with key words 'Terahertz database', or 'Terahertz semiconductor', etc.
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I want to design a various focus liquid lens for terahertz driven by electrode, can you give me some advice or recommend some papers? Thank you 
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graphene
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Hello,
Could you tell me how can I calculate frequency dependent dielectric matrix in terahertz regime for molecule.
I have created a super-cell with vacuum layers.
Any ideas?
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Dear Dmitriy,
The attached review describes the equations for calculating a frequency dependent dielectric matrix in terahertz regime.
PHYSICAL REVIEW E 75, 036614 2007
Terahertz plasmonic composites
Syrus C. Nemat-Nasser,1,* Alireza V. Amirkhizi,1 Willie J. Padilla,2,† Dimitri N. Basov,2 Sia Nemat-Nasser,1,‡
Derek Bruzewicz,3 and George Whitesides3
1
Center of Excellence for Advanced Materials, Department of Mechanical and Aerospace Engineering, University of California,
San Diego, Mail code: 0416, 9500 Gilman Drive, La Jolla, California 92093-0416, USA 2
Department of Physics, University of California, San Diego, Mail code: 0416, 9500 Gilman Drive, La Jolla,
California 92093-0416, USA 3
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA
Received 27 October 2006; revised manuscript received 20 December 2006; published 27 March 2007
The dielectric response of a polymer matrix composite can be substantially modified and tuned within a broad frequency band by integrating within the material an artificial plasmon medium composed of periodically distributed, very thin, electrically conducting wires. In the microwave regime, such plasmon/polymer
composites have been studied analytically, computationally, and experimentally. This work reports the design, fabrication, and characterization of similar composites for operation at terahertz frequencies. Such composites require significant reduction in the thickness and spacing of the wires. We used numerical modeling to design
artificial effective plasmonic media with turn-on frequencies in the terahertz range. Prototype samples were produced by lithographically embedding very thin gold strips into a PDMS polydimethylsiloxane matrix.
These samples were characterized with a Fourier-transform infrared interferometer using the frequencydependent transmission and Kramers-Kronig relations to determine the electromagnetic properties. We report the characterization results for a sample, demonstrating excellent agreement between theory, computer design,
and experiment. To our knowledge this is the first demonstration of the possibility of creating composites with tuned dielectric response at terahertz frequencies.
Hoping this will be helpful,
Rafik
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I have found piezoelectric resonance peaks for my sample so in further I have to take terahertz characterization.
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Its available in TIFR Mumbai
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In THz frequencies we use quasi-static capacitance for calculation of output power,
what explanation has the calculation of quasi-static capacitance in high frequencies (terahertz freq.)???
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Assalam......
In quasistatic measurements, capacitance is measured directly by integrating charging current. However, CV measurements can also be made by superimposing a small sinusoidally oscillating (AC) signal on the voltage sweep and measuring the corresponding impedance directly as a function of bias voltage. (This requires the use of a high precision impedance meter.) In this case, capacitance measured under conditions of accumulation and depletion can be expected to be the same as observed in quasistatic measurements, i.e., conditions still remain near equilibrium. However, if the frequency of the AC signal is sufficiently high, capacitance measured under a condition of inversion is not the same as in the quasistatic case. The explanation for this is quite simple and is a direct consequence of non-equilibrium behavior of the inversion layer. Physically, any inversion layer must be formed from minority carriers generated in the depletion region and swept to the surface by the electric field. (Of course, minority carriers may be also generated in the bulk and diffuse into the depletion region.) Equilibrium conditions imply that there is sufficient time (by definition) for the inversion layer carrier concentration to respond to any changes in applied field. However, if the material quality of the silicon is good, carrier generation-recombination processes occur very slowly, with
a time constant on the order of milliseconds. Therefore, for an applied AC voltage in the megahertz range, the response of the inversion layer is simply too slow to “follow” the signal and similar to ionized dopant impurity atoms, the inversion layer appears fixed with respect to the AC component of the bias. (Of course, the inversion layer does respond to the primary voltage sweep.) Therefore, for high frequency conditions, the capacitance per unit area measured in inversion is the series combination of oxide capacitance per unit area and capacitance per  unit area of the depletion region. Furthermore, since, the depletion width reaches a
maximum value, the combined capacitance per unit area saturates at Cmin.
for more..
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As we all know, the laser pulse duration is in fs. But the terahertz is happend in ps. Does the terahertz wave come within the time scale of fs when the laser is on? 
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The physical mechanism is that filament induced birefringence in gases provides a phase delay between the two orthogonal components of the THz field, leading to an elliptically polarized THz emission. By sending a sequence of two femtosecond IR laser pulses at 800 nm separated by less than 3 ns, forming two overlapping filaments in air (so called bifilamentation), they can generate 1 order of magnitude more intense THz emission than the transient-Cherenkov THz emission from the plasma filament. The origin is attributed to the emission from a bimodal transmission line created by a pair of neighboring plasma filaments. (http://www.col.org.cn/abstract.aspx?id=COL201311011401-06)
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In THz antennas, an AuGe alloy and a layer of Ti/Au (Titanium/Gold) are employed as the electrode material, what is the AuGe and what does it do in antenna performance?
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Dear Kamiar,
Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. The power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model it can be concluded that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.
More detailed information is contained in:
Regards, Leonid A. Skvortsov
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I read an article about a terahertz antenna with a Interdigitated structure (figure attached), and I have a question about the structure,
what the 2nd Au layer does on the top of the whole structure?(main question)
and why SiO2 layer has coated the interdigitated structure?
the link of the main article attached.
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The top layer of metal is to shield alternate electrode spacings.  If you look at figure (b) the authors have noted the E field direction in each inter-electrode spacing - the field alternates between a "positive" and "negative" direction.  As explained in the earlier papers on this technique (links below) if both positive and negative directions were allowed to radiate terahertz radiation then the generated pulses would cancel each other in the far field.  The overcoating of gold prevents the excitation by the laser pulse thereby only generating terahertz pulses of the same polarity, producing constructive interference in the far field.  Fig. 2a in the APL link below shows the effect of not having the top layer of gold to prevent the destructive interference.
The SiO2 layer is used to insulate the second layer from the first.
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How can I find out?
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Dear Johneph,
in terahertz area of frequencies index of refraction of substance n (ω) and its coefficient of absorption α (ω) calculate by means of measured amplitudes and phases of the radiation which passed through a sample by means of expressions [1]:
                     n(w) = 1 +[c/wd] Im[lnEtr(w)/E0(w)]   ,                                     
where E0 (ω) and Etr (ω) – intensity of electric field of radiation before passing of radiation through a sample respectively; d – sample thickness; ω – radiation frequency; c – velocity of light.
Reflection coefficient R (ω) and index of refraction n (ω) are connected with each other by an approximate ratio (near normal falling):
R(w) ~ [(n(w)-1)/n(w)+1]2
[1]. Kemp M. Proc. of SPIE, 6402, 64020D (2006).
See also:
“Determination of complex refractive index of thin metal films from terahertz time-domain spectroscopy”, Началоформы
Da-Xiang Zhou, E. P. J. Parrott, Douglas J. Paul and J. Axel  ZeitlerJ. Appl. Phys. 104, 053110 (2008).
2.      “Measurement of Terahertz Refractive Index of Metal with Terahertz Time-Domain Spectroscopy”, Hiroaki Yasuda and Iwao Hosako  Jpn. J. Appl. Phys. 47, 1632 (2008). 
3.      Yun-Shik Lee. “Principles of Terahertz Science and Technology”, 2009.
https://books.google.ru/books?id=4ZqkuUURxFcC&pg=PA161&lpg=PA161&dq=refractive+index+of+the+metal+in+the+terahertz+frequency+range&source=bl&ots=tbnvcAPsAw&sig=_ezofjaejcch- RIc4PJ2WLFqZyA&hl=ru&sa=X&ei=luCfVNuJDITXyQOIj4LoCw&ved=0CB0Q6AEwADgK#v=onepage&q=refractive%20index%20of%20the%20metal%20in%20the%20terahertz%20frequency%20range&f=false
4. THz Spectral Database - the NIST WebBook http://webbook.nist.gov/chemistry/thz-ir/
I hope that this information will be useful to you.
 Best regards, Leonid
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What is the value of epsilon(infinity) of Aluminium in terahertz regime?
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I recommend you to use as a reference M.A. Ordal et al., Optical Properties of metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti and W in the infrared and far-infrared, Applied Optics 22, 1099, (1983). There you can find all the necessary values to fit your dielectric function (of several metals) to experimental data.
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I am studying Terahertz Time Domain Spectroscopy for Ph.D work.
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What about the natural sources  in THz range ?
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What is the most effective and best method for THz generation?
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See Thz Photonics book by X.-C.Zhang.  Also some of his review articles discuss these issues.
It also depends on what you are looking at?  Highest power/energy of THz radiation?  Wide bandwidth?? Low cost??
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I am setting up a CW THz system and have put together both transmission as well as reflection mode setups. Although the maximum photocurrent I am getting for the transmission mode is around 30 nA at 0.1 THz, the maximum I can get for the reflection mode is around 15 nA at 0.1 THz.
Do I need to work more to align the system or this is as much as I can get in a reflection mode.
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Hi Anselm.  
Thanks for your clarification. We have been actually using two more parabolic mirrors to direct the THz beam to the sample and guide the reflected beam to the parabolic mirror infront of the photodetector. So I guess, we are not being able to properly guide and collect the waves from the sample surface. 
Yes, 50 % loss is there, we can get more or less the same bandwidth but the SNR is obviously low. 
Guess, I need to put in more effort. Thanks once again.
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And what are the components you suggest to work on to boost the SNR in table top systems (generation, pump stability, electric noise immunity, electro-optical sampling vs antenna etc.)?
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Just FYI, you can obtain much higher DR with a CW system based on harmonic multiplication of stabilized microwave sources, up to about 1.4 THz. The ABmillimetre VNA produces about 10 microwatts at 1 THz. Not much help if you've already invested in a time-domain system, I understand.
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I have to perform THz time domain spectroscopy measurements in transmission geometry on biological sample (cell cultures). Can anyone suggest a material for the sample holder that does not absorb too much of the THz signal?
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Also see:
and:
Optical Materials Express, Vol. 3, Issue 2, pp. 260-269 (2013)