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August 29, 2017 English Posts , Light 77,774 Views

With the new Laser He-Ne (described in the Laser He-Ne post), you can easily test the physical properties of the diffraction grating . We propose, in particular, to measure the pitch of the grating through the measurement of the diffraction produced on the He-Ne laser beam.

The Diffraction Grating

When a collimated beam of light passes through an aperture, or if it encounters an obstacle, it spreads out and the resulting pattern contains bright and dark regions. This effect is called diffraction , and it is characteristic of all wave phenomena. It can be understood by considering the interference between different parts of the wavefront, which was altered in passing through the aperture. The angle of of diffraction is of order  λ / d with λ  the wavelength and d the dimension of the aperture. Thus, for visible light, apertures in the range 10-100 μm produce easily resolved diffraction patterns. The diffraction phenomena has been treated in the post  Light as a Wave : Slit Diffraction .

If instead of a single slit, two slits are illuminated by a plane wavefront, a series of interference fringes parallel to the slits will appear on a far screen, as shown in the image below.

This is the classical experiment of Thomas Young (1800). If the spacing between the slits is d and the width of the slits  b  is greater than the wavelength, the Fraunhofer diffraction equation gives the intensity of the diffracted light as:

Where the sinc function is defined as sinc( x ) = sin( x )/( x ) for  x  ≠ 0, and sinc(0) = 1. The sinc function includes the effects of diffraction due to the width of the slits.

The intensity of the principal maxima can be calculated and it decreases as the diffraction order is increased, as shown in the image below.

Experimental Setup and Gratings Measurements

The experimental setup is very simple and consists in pointing the beam laser emitted from the He-Ne source on the diffraction grating. The beam undergoes diffraction and produces on the screen behind the grating the diffraction pattern with the first and second order maxima. Measuring the distance between the grating and the screen and measuring the position of the maxima is immediate to obtain the angles  θ m  and from these we can calculate the grating pitch, using the equation previously described and knowing that λ  is 632.8 nm. In the image below you can see the laser, the diffraction grating and the screen on which you can see the luminous spots corresponding to the diffraction maxima.

From the measurements made with the Paton – Hawksley grating on the first and second order diffraction maxima we obtained the following data:

First Order – θ1  = 0.402 rad – d = λ / sin( θ1 )   = 1.62 μm which corresponds to a pitch of  617 l/mm Second Order – θ2  = 0.873 rad – d = 2λ / sin( θ2 )   = 1.65 μm which corresponds to a pitch of  605 l/mm

With the holographic grating for the first order we obtained the following data:

First Order – θ1  = 0.675 rad – d = λ / sin( θ1 )   = 0.99 μm which corresponds to a pitch of 1012 l/mm

As you can see, the results obtained fit quite well with nominal grating data .

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Detection of beta and alfa radiation with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In previous posts, we described the device in general terms and its functionality as a gamma spectrometer. In this post, we describe its use as a beta and alpha radiation detector. To detect beta and alpha particles, the device uses a PIN-type semiconductor sensor positioned on the back of the device.

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A Laser Fusion Breakthrough Gets a Bigger Burst of Energy

By Kenneth Chang

In July, scientists at the National Ignition Facility at Lawrence Livermore National Laboratory in California generated a burst of energy by bombarding a pellet of hydrogen with 192 lasers, reproducing for a brief moment the process of fusion that powers the sun. It was a repeat of an experiment last December , but this time the scientists generated even more energy with nearly a factor of two in gain compared with the energy of the incoming lasers.

“We again repeated ignition,” Richard Town, the associate program director of the laser fusion program at Livermore, said in an interview. He gave a talk about the July experiment on Monday at a conference in Denver.

The Livermore results raise hopes that fusion can one day be used to generate bountiful amounts of electricity without producing greenhouse gases or long-lived radioactive waste.

The Key Number: 3.88 megajoules.

The experiment in December generated a whirlwind of accolades when it produced about three megajoules of energy — equivalent to about 1.5 pounds of TNT, or about 1.5 times the energy of the incoming lasers. It was the first time that a fusion reaction in a laboratory setting produced more energy than it took to start the reaction.

The July experiment was essentially identical to the December one. “We expected a similar yield,” Dr. Town said. “On the order of three megajoules.”

The actual output was 3.88 megajoules.

The better-than-predicted result indicates that with a few tweaks, laser fusion can become markedly more efficient. But minuscule variations could yield fusion duds as well.

A fusion experiment in June, just a month earlier, was also predicted to produce about three megajoules, but it generated only between 1.6 and 1.7 megajoules, Dr. Town said.

A more recent attempt this month, as part of efforts to maintain nuclear weapons without underground nuclear tests, yielded slightly more than two megajoules, breaking even with the laser energy.

“It was a little bit surprising that we did not achieve ignition on all of them,” Dr. Town said.

Why It Matters: Much more to learn.

Analyzing the results, the Livermore scientists now think they better understand what is going on.

For one, the 192 lasers are not perfect. “There are some variations every time you shoot the laser,” Dr. Town said.

Instead of laser energy arriving perfectly balanced to compress the hydrogen fuel capsule, a slight imbalance nudges the capsule off in one direction. Some of the energy is lost, and the inward implosion does not heat the hydrogen as much.

There are also slight variations in the fuel capsules that affect the fusion reactions. Computer simulations now indicate there can be a wide range in the output energy.

“It could fall as low as 1.4 megajoules,” Dr. Town said. “And if the stars align and everything works perfectly, it could get up to seven megajoules.”

What Happens Next: Upgrading and optimizing the experiment.

Siegfried Glenzer, a scientist at the SLAC National Accelerator Laboratory in Menlo Park, Calif., who led the initial fusion experiments at the Livermore facility years ago, said of the July advance, “The fact that the gain has gone up on the last shot is encouraging news and shows that the current implosions are not yet fully optimized.”

A new series of experiments is about to begin at the National Ignition Facility as it aims to generate higher fusion yields more consistently. The energy of the facility’s lasers is being upgraded to 2.2 megajoules from 2.05. The latest advances occurred after the last upgrade from 1.9 megajoules. Additional energy is expected to lead to further improvements.

“If you can couple effectively more energy to the hot spot, you should get more yield,” Dr. Town said. “You can do that by having a bigger hammer.”

Kenneth Chang has been at The Times since 2000, writing about physics, geology, chemistry, and the planets. Before becoming a science writer, he was a graduate student whose research involved the control of chaos. More about Kenneth Chang

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• Published: 05 February 2024

Nuclear fusion

A boost for laser fusion

• Vladimir Tikhonchuk   ORCID: orcid.org/0000-0001-7532-5879 1 , 2  

Nature Physics volume  20 ,  pages 682–683 ( 2024 ) Cite this article

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An Author Correction to this article was published on 12 February 2024

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Inertial confinement represents one of two viable approaches for producing energy from the fusion of hydrogen isotopes. Scientists have now achieved a record yield of fusion energy when directly irradiating targets with only 28 kilojoules of laser energy.

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Photograph by Eugene Kowaluk, Laboratory for Laser Energetics, University of Rochester

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A Correction to this paper has been published: https://doi.org/10.1038/s41567-024-02433-z

Abu-Shawareb et al. Phys. Rev. Lett. 129 , 075001 (2022).

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Williams, C. A. et al. Nat. Phys. https://doi.org/10.1038/s41567-023-02363-2 (2024).

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Gopalaswamy, V. et al. Nat. Phys. https://doi.org/10.1038/s41567-023-02361-4 (2024).

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Basov, N. G., Krokhin, O. N. & Sklizkov, G. V. In Laser Interaction and Related Plasma Phenomena (eds Schwarz, H. J. & Hora, H.) 389–408 (Springer, 1972).

Gopalaswamy, V. et al. Nature 565 , 581–586 (2019).

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The wave pattern for electrons passing through a double slit, one-at-a-time. If you measure “which ... [+] slit” the electron goes through, you destroy the quantum interference pattern shown here. However, the wave-like behavior remains so long as the electrons have a de Broglie wavelength that's smaller than the size of the slit they're passing through.

When we divide up matter into the smallest possible chunks that it's made of — into the stuff that can be divided or split no further — those indivisible things we arrive at are known as quanta. But it's a complicated story each time we ask the question: how does each individual quantum behave? Do they behave like particles? Or do they behave like waves?

The most puzzling fact about quantum mechanics is that the answer you get depends on how you look at the individual quanta that are part of the experiment. If you make certain classes of measurements and observations, they behave like particles; if you make other choices, they behave like waves. Whether and how you observe your own experiment really does change the outcome, and the double-slit experiment is the perfect way to show how.

This diagram, dating back to Thomas Young's work in the early 1800s, is one of the oldest pictures ... [+] that demonstrate both constructive and destructive interference as arising from wave sources originating at two points: A and B. This is a physically identical setup to a double slit experiment, even though it applies just as well to water waves propagated through a tank.

More than 200 years ago, the first double-slit experiment was performed by Thomas Young, who was investigating whether light behaved as a wave or a particle. Newton had famously claimed that it must be a particle, or corpuscle, and was able to explain a number of phenomena with this idea. Reflection, transmission, refraction, and any ray-based optical phenomena were perfectly consistent with Newton's view of how light should behave.

But other phenomena seemed to need waves to explain them: interference and diffraction in particular. When you passed light through a double slit, it behaved just the same way that water waves do, producing that familiar interference pattern. The light-and-dark spots that appeared on the screen behind the slit corresponded to constructive-and-destructive interference, indicating that — at least under the right circumstances — light behaves as a wave does.

If you have two slits very close to one another, it stands to reason that any individual quantum of energy will go through either one slit or the other. Like many others, you might think that the reason light produces this interference pattern is because you have lots of different quanta of light — photons — all going through the various slits together, and interfering with one another.

So you take a different set of quantum objects, like electrons, and fire them at the double slit. Sure, you get an interference pattern, but now you come up with a brilliant tweak: you fire the electrons one-at-a-time through the slits. With each new electron, you record a new data point for where it landed. After thousands upon thousands of electrons, you finally look at the pattern that emerges. And what do you see? Interference.

Electrons exhibit wave properties as well as particle properties, and can be used to construct ... [+] images or probe particle sizes just as well as light can. Here, you can see the results of an experiment where electrons are fired one-at-a-time through a double-slit. Once enough electrons are fired, the interference pattern can clearly be seen.

Somehow, each electron must be interfering with itself, acting fundamentally like a wave.

For many decades, physicists have puzzled and argued over what this means must really be going on. Is the electron going through both slits at once, interfering with itself somehow? This seems counterintuitive and physically impossible, but we have a way to tell whether this is true or not: we can measure it.

So we set up the same experiment, but this time, we have a little light we shine across each of the two slits. When the electron goes through, the light is slightly perturbed, so we can "flag" which one of the two slits it passed through. With each electron that goes through, we get a signal coming from one of the two slits. At last, each electron has been counted, and we know which slit every one went through. And now, at the end, when we look at our screen, this is what we see.

If you measure which slit an electron goes through when performing a one-at-a-time double slit ... [+] experiment, you don't get an interference pattern on the screen behind it. Instead, the electrons behave not as waves, but as classical particles.

That interference pattern? It's gone. Instead, it's replaced by just two piles of electrons: the paths you'd expect each electron to take if there were no interference at all.

What's going on here? It's as though the electrons "know" whether you're watching them or not. The very act of observing this setup — of asking "which slit did each electron pass through?" — changes the outcome of the experiment.

If you measure which slit the quantum passes through, it behaves as though it passes through one and only one slit: it acts like a classical particle. If you don't measure which slit the quantum passes through, it behaves as a wave, acting like it passed through both slits simultaneously and producing an interference pattern.

What's actually going on here? To find out, we have to perform more experiments.

By setting up a movable mask, you can choose to either block one or both slits for the double slit ... [+] experiment, seeing what the outcomes are and how they change with the motion of the mask.

One experiment you can set up is to put a movable mask in front of both slits, while still firing electrons through them one-at-a-time. Practically, this has now been accomplished  in the following fashion:

• a movable mask with a hole in it starts off by blocking both slits,
• it moves to the side so that the first slit is then unmasked,
• it continues moving so that the second slit is also unmasked (along with the first),
• the mask continues its motion until the first slit is once again covered (but the second is still unmasked),
• and finally both slits are covered again.

How does the pattern change?

The results of the 'masked' double-slit experiment. Note that when the first slit (P1), the second ... [+] slit (P2), or both slits (P12) are open, the pattern you see is very different depending on whether one or two slits are available.

Exactly like you might expect:

• you see a one-slit (non-interfering) pattern if only one slit is open,
• the two-slit (interference) pattern if both slits are open,
• and a hybrid of the two in the in-between times.

It's as though if both paths are there as available options simultaneously, without restriction, you get interference and wave-like behavior. But if you only have one path available, or if either path is restricted somehow, you won't get interference and will get particle-like behavior.

So we go back to having both slits in the "open" position, and shining light across both of them as you pass electrons one-at-a-time through the double slits.

A tabletop laser experiment is a modern outgrowth of the technology that enabled proving the absurd: ... [+] that light didn't behave like a particle.

If your light is both energetic (high energy per photon) and intense (a large number of total photons), you won't get an interference pattern at all. 100% of your electrons will be measured at the slits themselves, and you'll get the results you'd expect for classical particles alone.

But if you lower the energy-per-photon, you'll discover that when you drop below a certain energy threshold, you don't interact with every electron. Some electrons will pass through the slits without registering which slit they went through, and you'll start to get the interference pattern back as you lower your energy.

Same thing with intensity: as you lower it, the "two pile" pattern will slowly disappear, replaced with the interference pattern, while if you dial up the intensity, all traces of interference disappear.

And then, you get the brilliant idea to use photons to measure which slit each electron goes through, but to destroy that information before looking at the screen.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No ... [+] alterations of one particle at its destination affect the outcome of the other. You can combine principles like the quantum eraser with the double-slit experiment and see what happens if you keep or destroy, or look at or don't look at, the information you create by measuring what occurs at the slits themselves.

This last idea is known as a quantum eraser experiment , and it produces the fascinating result that if you destroy the information sufficiently, even after measuring which slit the particles went through, you'll see an interference pattern on the screen.

Somehow, nature knows whether we have the information that "marks" which slit a quantum particle passed through. If the particle is marked in some fashion, you will not get an interference pattern when you look at the screen; if the particle is not marked (or was measured and then unmarked by destroying its information), you will get an interference pattern.

We've even tried doing the experiment with quantum particles that have had their quantum state "squeezed" to be narrower than normal, and they not only exhibit this same quantum weirdness , but the interference pattern that comes out is also squeezed relative to the standard double slit pattern .

The results of unsqueezed (L, labeled CSS) versus squeezed (R, labeled squeezed CSS) quantum states. ... [+] note the differences in the density-of-states plots, and that this translates into a physically squeezed double slit interference pattern.

It is extremely tempting, in light of all of this information, to ask what thousands upon thousands of scientists and physics students have asked upon learning it: what does it all mean about the nature of reality?

Does it mean that nature is inherently non-deterministic?

Does it mean that what we keep or destroy today can affect the outcomes of events that should already be determined in the past?

That the observer plays a fundamental role in determining what is real?

A variety of quantum interpretations and their differing assignments of a variety of properties. ... [+] Despite their differences, there are no experiments known that can tell these various interpretations apart from one another, although certain interpretations, like those with local, real, deterministic hidden variables, can be ruled out.

The answer, disconcertingly, is that we cannot conclude whether nature is deterministic or not, local or non-local, or whether the wavefunction is real. What the double slit experiment reveals is as complete a description of reality as you're ever going to get. To know the results of any experiment we can perform is as far as physics can take us. The rest is just an interpretation.

If your interpretation of quantum physics can successfully explain what the experiments reveal to us, it is valid; all the ones that cannot are invalid. Everything else is aesthetics, and while people are free to argue over their favorite interpretation, none can lay any more claim to being "real" than any other. But the heart of quantum physics can be found in these experimental results. We impose our preferences on the Universe at our own peril. The only path to understanding is to listen to what the Universe tells us about itself.

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