Quantum level working of IR sensor

[eptheta]

Quantum level working of IR sensor...

There are datasheets of IR sensors like the TSOP1738 that conveniently tell me that when I make an IR LED flash at 38Khz, a current is produced. However, i would like to know what happens on an atomic level.
Is it just a matter of electron promotion because of the infrared light ? Then how does the sensor only allow IR to cause excitation of the electrons ?
Is it the material they use for these sensors that reacts only to the IR light ?
How exactly does infrared light cause electrons to flow in this sensor ?

I would be very grateful of anyone could either answer my question or point me to links where this is nicely explained. I need to know what is behind these sensors as I am researching IR communication and wish to know more about the sensing mechanism.

Thank you.

 

[Lok]

Normal CCD's (hope the term is not outdated) are IR sensitive so they see it, we don't. My cell phone camera can see IR as I point it to IR cameras that also emit. I don't see the IR leds but my cell shows a bright green-ish light. Photo cameras don't see IR as they have an IR filter to produce quality pictures without weird lights you don't normally see.

In order to get only IR sensitivity you just put a filter that blocks the rest.

 

[eptheta]

Thank you Lok for taking the time to answer my question...
Alright, assuming you need just a filter, does that imply that the IR sensor is just a photo-diode of some sort ?
It is physical barriers that account for it being able to sense IR only and not the inherent property of the material?

On that note, can anyone explain, again on a quantum level, how photon-detectors work then?
I have seen equations, but do not understand why electrons are caused to flow when EM waves of a constant power are applied on to it.
Is this property based on the work function of the material being used?
Are semi-conductors preferred for photo-detection and why ?

Thank you,
I look forward to a response.

EDIT: after a little poking around, i noticed that the 'Photo-electric effect' seems to be valid for EM radiation of "very short wavelength, such as visible or ultraviolet light."--Wikipedia:Photoelectric effect
How then does IR fit into the picture ? It seems that this is not a simple case of the photo-electric effect.
Thanks

 

[Lok]

http://en.wikipedia.org/wiki/Charge-coupled_device

"Most common types of CCDs are sensitive to near-infrared light, which allows infrared photography, night-vision devices, and zero lux (or near zero lux) video-recording/photography. For normal silicon-based detectors, the sensitivity is limited to 1.1 μm. One other consequence of their sensitivity to infrared is that infrared from remote controls often appears on CCD-based digital cameras or camcorders if they do not have infrared blockers."

Not to go too much into the details about the many types of detectors. The idea is that the photoelectric effect is material and band-gap dependent. For shortwave light atomic absorbtion and PE effect are used ( short IR and above) and for longer wavelengths a simple radio antenna of specific size might do (microwaves or lower). The bigger the wavelength the bigger the detector (or detector cell) for electronic sensing.

 

[Lok]

Electrons ca be stripped from an atom by "hitting" with an photon of it's escape energy or more. In some molecular or metalic structures the "escape energy" (not a real escape) is not neccesarily the strippind away of the electron but rather just moving it to a higher energy state in the structure without kicking it out of it, that can result in a small electric current (or potential).

 

[eptheta]

Hello,
CCDs seem to be rather old technology... Can anyone here make out what type of tech is used in devices like the http://www.datasheetcatalog.org/datasheets/208/301092_DS.pdf"

Not the 38khz part, just the sensing part. (the frequency dependence is externally created by some other circuit elements and not by the sensor material itself right ?)

Thank you.

 

[eptheta]

Would anyone else like to contribute? I have a lot more questions to ask !
Does anyone here have an opinion on the matter ?

 

[ClamShell]

Would anyone else like to contribute? I have a lot more questions to ask !
Does anyone here have an opinion on the matter ?

I am brave...my fear of being wrong, is gone, I just also
have gaps here, so be kind.

1) the 38 kHz requirement IS performed by electronic circuits
that measure the IR sensor voltage. This is a way to "filter"
out all IR signals that are NOT flashed at a 38 kHz rate...the
rate that the remote sends IR pulses.

2) Now to the sensor...A "photon" is "absorbed" into the
"electron cloud" of an "atom" and/or "molecule" of the
IR sensor...or is not. If "absorbed", an "electron cloud"
"expands" to a higher "energy" and either "kicks" an
"electron" to a wire(s) mounted in the sensor, and/or emits
other "photons", to get the "atom" and/or "molecule" to "shrink"
back to its "preferred" (or "rest") state of "energy". An IR detector
would prefer "kicking" whereas, a "photon" source (eg, LASER)
would prefer the "re-radiating" effect. My quoted concepts are
surely incomplete, so take it with a bit of salt.

 

[eptheta]

Honestly, any opinion is a good opinion ... Thanks
From what you said, I assume that an IR sensor is any regular EM sensor with the appropriate IR filters put into place...
Am i correct to assume that ?

If so, then my quest for knowledge ends here...

 

[ClamShell]

IR sensors that I am aware of, are semiconductors, ie, diodes (PN),
transistors (PNP or NPN), or PIN diodes...(check wiki).

I assume that the silicon impurities(doping) has a lot to do with
the wavelength sensed as well as light filters. "regular" seems
like "one rule fits all" logic?...the devil is in the details.

 

[Dr Lots-o'watts]

The spec sheet does say it's a PIN diode. Are you asking what specific material they use?

 

[eptheta]

I assume that the silicon impurities(doping) has a lot to do with
the wavelength sensed as well as light filters. "regular" seems
like "one rule fits all" logic?...the devil is in the details.

Is there some sort of equation relating %doping with whatever material in silicon to wavelength sensed ?
Or forget the equation, can you explain physically why doping (which i know increases/decreases conductor/semi-conductor behavior) could result in different wavelengths being sensed ?
The "I assume" part sort of scares me...But i did some of my own research and it does seem to make sense...

Are you asking what specific material they use?

I looked around and everywhere there are lists of materials that can be used for IR detection... It would be very(very) helpful if you could find out specifically what material the http://www.datasheetcatalog.org/datasheets/208/301092_DS.pdf" sensor uses..

Any inputs will be valued greatly...
Thanks to Lok, ClamShell and Dr Lots-o'watts for their help so far.

 

[eptheta]

I am rather convinced that doping is responsible for peak wavelength sensing.
That is a specific ratio of doping results in EM waves of a specific wavelength causing maximum photon emission.
Am i correct in that belief ?

Also, if anyone could provide a reason for why this happens ? In terms of physical concepts like electron promotion and work function and other concepts, and not only by mathematical equations...

Thank you !

 

[Dr Lots-o'watts]

I am rather convinced that doping is responsible for peak wavelength sensing.
That is a specific ratio of doping results in EM waves of a specific wavelength causing maximum photon emission.
Am i correct in that belief ?

Also, if anyone could provide a reason for why this happens ? In terms of physical concepts like electron promotion and work function and other concepts, and not only by mathematical equations...

Thank you !

In PIN diodes, the wavelengths that are absorbed are the ones which correspond to the energy differences between pairs of energy levels. The structure of the energy levels is determined when the materials are processed. This may include doping.

 

[eptheta]

Ah, Great, that was simple enough..
So there is no definite wavelength the material can eject electrons for, it is just that when the wavelength energy matches the energy level difference, it emits more electrons than when it doesn't match ?(or something like that)

Would the graph between wavelength of light to current produced be something like this ?
[PLAIN]http://img194.imageshack.us/img194/8545/91107135.png

The reason i needed all this information is because I'm doing a project on infrared communication and i wanted to know the physics behind it and not just the engineering jargon. Is there any other information i should know about ?

Thank you

 

[Dr Lots-o'watts]

There is always a width to the allowable wavelengths, because :

1. It isn't necessarily useful to have a narrow width.
2. It isn't necessarily possible to manufacture an arbitrarily narrow width (defects are inherent).
3. Materials have a temperature T>0, so its atoms have movement, making the energy gap non-constant.
4. Ultimately, Heisenberg's principle prevents a single wavelength (perfectly defined energy) from happening. A true single wavelength belongs to an infinitely long monochromatic wave, an idealistic mathematical representation, which does not exist.

That shape you drawn is an idealistic possibility. In the "technical sheet" from the following link, you will find 3 photodiodes of different designs, each optimized for a certain range. They each have a "peak sensitivity", to which corresponds the maximum current. In the "user manual", you can find the wavelength where that peak sensitivity occurs.

http://www.gentec-eo.com/en/products/photo-detectors.52.htm

 

[eptheta]

That's exactly what i was looking for ! Thanks !
There are a few things I don't quite understand:

1.

Precise Calibration
Wavelength selection in 1 nm steps

How exactly do they calibrate it ? I thought a sensor once made to sense a particular wavelength, always senses that wavelength... How can one change this range in such small steps(1 nm)?
Some sort of a physical mechanism or electrical(dealing with changing energy levels of atoms somehow??)

2.

Choice of Attenuators
OD-1 OD-2
10% Transmission 1% Transmission

What are attenuators in this context ? (I'm guessing they are some sort of filters, but that's just a guess) How exactly do they work (if they are filters or wavelength sensor adjusting devices)

Please do reply fast. Thank you.

 

[Dr Lots-o'watts]

1. How exactly do they calibrate it ? I thought a sensor once made to sense a particular wavelength, always senses that wavelength... How can one change this range in such small steps(1 nm)? Some sort of a physical mechanism or electrical(dealing with changing energy levels of atoms somehow??)

As I said, there is an inherent range. And for these instruments in particular, it is a plus, because it allows the user to measure a variety of different lasers with the same detector. The width of the range is put to use.

2. What are attenuators in this context ? (I'm guessing they are some sort of filters, but that's just a guess) How exactly do they work (if they are filters or wavelength sensor adjusting devices)

Think sunglasses. They absorb some of the light, so that only 10% (or 1%) reach the sensing element. This prevents saturation (max current), above which there is obviously no measurement.

 

[eptheta]

Hi,
This 'inherent range' you talk about... if i understand the tech sheet correctly, it says the same sensor can be used to sense wavelengths at steps of 1 nm (within its inherent range)...

By this, assuming light intensity is same for all wavelengths i use, do they mean:
a) Judging by the voltage generated, one can estimate the wavelength (therefore, one must refer to standard values which have been found experimentally)

OR

b) by calibrating something, one can pick a wavelength and get maximum voltage in all cases within its range...

if it's a), then i understand..
if it's b), then what is this something that is calibrated ?
and if its something else, then i'd be grateful if you could tell me...

Thank you.

 

[Dr Lots-o'watts]

To use the unit, you enter what wavelength you want to measure (799 nm, or 800 nm, or 801 nm, or 802 nm etc.). Then you point the laser towards it, and it tells you how many watts in the beam.

...?

 

[eptheta]

Hello,
I have 2 questions:
Suppose i choose the wavelength of violet light and shine a monochromatic beam of red light, will the indicator show 0 watts ?
or will the power displayed correspond to set of ranges where red light should ideally fall ?
Say if set for violet, and i put violet light, 4W(or whatever the max should be) should be shown, if i put red light 1.2W should be shown... Something like that ?

Also, these sensors are still based on the same old photoelectric effect with work functions right ? Nothing more complex ?

Thank you.

 

[eptheta]

Anyone willing to answer my 2 questions please ?

 

[Dr Lots-o'watts]

Suppose i choose the wavelength of violet light and shine a monochromatic beam of red light, will the indicator show 0 watts ?

No, it will show a value that is not correct. A certain percentage off the correct value.

These sensors are based on these effects (PN or PIN) http://en.wikipedia.org/wiki/Photodiode

In photodiodes (photovoltaic effect for instance), the electrons jump levels due to visible or IR photons, but they stay inside the material.

In the photoelectric effect, the wavelength is shorter (UV), such that they have sufficient energy to make the electrons actually jumps out the material. This term is sometimes used incorrectly to refer to the photovoltaic effect.

 

[eptheta]

Hi,
So now that you've established that the phenomena is just the photoelectric effect, and that a material is chosen such that the difference between the energy levels matches the EM energy of IR for maximum power...

How do calculate the 'correct' value for a photo-detector ?
Is it just $$\Delta$$E(difference in energy levels)=hc/$$\lambda$$ ?
and then solve for $$\lambda$$?

Also, I still do not understand how doping actually changes the difference between energy levels of atoms in a lattice...
I know of doping as increasing/reduce the number of free electrons by substituting their place in the crystal with a dopant atom...
How does sufficient doping enhance the photo-detecting ability of a material ?

Does the excess of electrons just mean that the work function reduces ? Or is it something else ?

Thanks

 

[Dr Lots-o'watts]

Hi, So now that you've established that the phenomena is just the photoelectric effect, and that a material is chosen such that the difference between the energy levels matches the EM energy of IR for maximum power...

I haven't established anything. Your specs sheets says you have a PIN diode, mine says I have a photo detector (silicon, germanium etc,). For the purpose of this discussion, they are the same thing. And you're not listening, I just said the photoelectric effect is unrelated to this thread.

Yes, that is the basic relation between energy levels and absorbed photons.

When the objective is to change the energy levels, we talk about alloying. This changes the crystal structure i.e. the distances between the atoms, which is what defines the energy levels.

When the objective is to increase conductivity (of a semi-conductor), we talk about doping.

The work function is related to the PHOTOELECTRIC effect, and just like a Japanese watermelon, it is completely UNRELATED to photo diodes and IR radiation, which basically work on the PHOTOVOLTAC effect. http://en.wikipedia.org/wiki/Photovoltaic_effect

 

[AndersonMD]

Quantum may be the wrong way to think about the system. This in a solid state device do not act like individual isolated atoms. Rather you should think about it probably more in a solid state band picture. An EM wave has an electric portion that resonates at some frequency and shakes the electrons in the solid (since they have an electric charge). If the NRG of the photon is absorbed it knocks an electron into a higher band (giving it NRG), this could be an atomic excitation (then you might get something like a Frenkel exciton), but more likely the electron takes off hopping around the other atoms leaving an empty spot (positive charge or hole) where it used to be. Generally you will have a bias on the system (a field) which will pull the electons to some location to be collected and create a current which you are seeing.

 

[eptheta]

Very, very sorry. I could have sworn you wrote it the other way around. Right, photovoltaic, not photoelectric.. .. .. ..Got it..
"and just like a Japanese watermelon" Its similes like this that should be featured and translated into several thousand languages...

"This in a solid state device do not act like individual isolated atoms"
-Now that most of my concepts have been crushed,(I know very little about crystal structures and their atomic interactions) I wanted to know what all variables determine the energy levels.
You mentioned distance between atoms, is that the only factor?
-Also, should i even be going into the atomic level if they have sufficiently different 'crystal' behavior?

-Alloying is still the same concept as doping right, just a different name ? Introducing other elements to change crystal structures and properties? (Please excuse the terminology wrt crystals, to me crystal structure is almost the same as crystal lattice.. well not that bad, but still...)
I checked alloying on Wikipedia and it just says alloying "enhances its(the metal's) properties"...

"Quantum may be the wrong way to think about the system"
-You are right... Is there any way to move this topic to the appropriate forum section (if it will get more responses)?

Thank you.
[PS: 'NRG' is just energy right ? Either that or I will feel very silly in the near future]

 

[AndersonMD]

It could be moved into the solid state section... probably unnecessary though.

 

[Dr Lots-o'watts]

I wanted to know what all variables determine the energy levels.
You mentioned distance between atoms, is that the only factor? -Also, should i even be going into the atomic level if they have sufficiently different 'crystal' behavior?

The type of atoms, their relative proportions, and the way they've been put together determine the crystal structure, which in turn fixes the energy levels. And just to reassure you, the theory from crystal structure to energy levels needs quantum mechanics.

Alloying is still the same concept as doping right.

Doping is alloying that is specifically aimed at increasing the carrier concentration. The term is used with semiconductor, not so much with metals.

The crystal lattice is the framework. The crystal structure is a more general term that includes what is located at each point of the lattice.

 

[eptheta]

The type of atoms, their relative proportions, and the way they've been put together determine the crystal structure, which in turn fixes the energy levels. And just to reassure you, the theory from crystal structure to energy levels needs quantum mechanics.

And here I begin my hopeful quest for knowledge... Can you point me to a resource that establishes (with equations etc) a relationship between all the mentioned variables and the resulting energy level ?

Thank you.

 

[AndersonMD]

Standard solid state text's that I have used are by Kittel and Ashcroft/Mermin. Should both be in your library.