Dr. Shambhu Ghimire is a principal investigator at Stanford PULSE Institute in SLAC National Accelerator Laboratory. Ghimire leads the Attosecond X-ray Photonics Group. He is a recipient of the Young Investigator award from the Office of Science, U.S. Department of Energy. His research interests are on strong-field and attosecond electron dynamics. He received his PhD in physics from Kansas State University in 2007 and went to University of Michigan for a post-doc work before joining SLAC in 2009.
Current Role at Stanford
Principal Investigator in a DOE-funded research area: High-order Harmonic Generation (HHG)
Member, Stanford PULSE Institute
Honors & Awards
Early Career Award, U.S. Department of Energy (2014-2019)
Education & Certifications
Ph.D, Kansas State University, Physics (2007)
Post-doctoral, University of Michigan, Ultrafast Science (2009)
My research interests are in ultrafast condensed matter, atomic, molecular and optical physics. In our experiments, we use high-intensity ultrafast laser systems in table-top settings, as well as X-ray free-electron lasers such as the Linac Coherent Light Source (LCLS). Recently, we are studying non-linear optical response of quantum materials such as two-dimensional crystals, topological insulators and Weyl semi-metals. We are particularly interested in the regime where the strength of the light-field approaches the interatomic binding strengths in these materials. In this regime, the typical approximations of conventional nonlinear optics breakdown spectacularly, and we enter into the regime of strong-field physics and attosecond science. In this regime, materials respond to the intense laser-field by producing its high-order harmonics. We first identified this regime by observing high-order harmonics from zinc oxide crystals subjected to intense mid-infrared laser fields in 2010. Since then there has been a lot of exciting work in the field, which includes the use of microscopic generation mechanism to probe valence charge distributions, electronic band structure, Berry curvature, and topological phase transitions.
- Strong-field physics in three-dimensional topological insulators PHYSICAL REVIEW A 2021; 103 (2)
- Attosecond synchronization of extreme ultraviolet high harmonics from crystals JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 2020; 53 (14)
Attosecond science based on high harmonic generation from gases and solids.
2020; 11 (1): 2748
Recent progress in high power ultrafast short-wave and mid-wave infrared lasers has enabled gas-phase high harmonic generation (HHG) in the water window and beyond, as well as the demonstration of HHG in condensed matter. In this Perspective, we discuss the recent advancements and future trends in generating and characterizing soft X-ray pulses from gas-phase HHG and extreme ultraviolet (XUV) pulses from solid-state HHG. Then, we discuss their current and potential usage in time-resolved study of electron and nuclear dynamics in atomic, molecular and condensed matters.
View details for DOI 10.1038/s41467-020-16480-6
View details for PubMedID 32488005
- Strong-field physics in three-dimensional topological insulators Strong-field physics in three-dimensional topological insulators 2020
- High-harmonic generation from an epsilon-near-zero material NATURE PHYSICS 2019; 15 (10): 1022-+
- Interferometry of dipole phase in high harmonics from solids NATURE PHOTONICS 2019; 13 (2): 96-+
Crystal orientation-dependent polarization state of high-order harmonics
2019; 44 (3): 530–33
We analyze the crystal orientation-dependent polarization state of extreme ultraviolet high-order harmonics from bulk magnesium oxide crystals subjected to intense linearly polarized laser fields. We find that only along high-symmetry directions do high-order harmonics follow the polarization direction of the laser field. In general, there are strong deviations that depend on harmonic order, strength of the laser field, and crystal orientation. We use a real-space electron trajectory picture to understand the origin of polarization deviations. These results have implications in all-optical probing of electronic band structure in momentum space and valence charge distributions in real space, and in producing attosecond pulses with time-dependent polarization in compact setups.
View details for DOI 10.1364/OL.44.000530
View details for Web of Science ID 000457292400017
View details for PubMedID 30702671
- High-harmonic generation from solids NATURE PHYSICS 2019; 15 (1): 10–16
High-Harmonic Generation from Topological Insulators
View details for Web of Science ID 000482226301135
- Probing periodic potential of crystals via strong-field re-scattering JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 2018; 51 (11)
- Locking the waveform with a quartz crystal NATURE PHOTONICS 2018; 12 (5): 256–57
Polarimetry of High Harmonics in Bulk Crystals
View details for Web of Science ID 000526031000241
Emission Phase of Extreme Ultraviolet High Harmonics from Bulk Crystals
View details for Web of Science ID 000526031000355
- Orientation dependence of temporal and spectral properties of high-order harmonics in solids PHYSICAL REVIEW A 2017; 96 (6)
High-harmonic generation in amorphous solids
2017; 8: 724
High-harmonic generation in isolated atoms and molecules has been widely utilized in extreme ultraviolet photonics and attosecond pulse metrology. Recently, high-harmonic generation has been observed in solids, which could lead to important applications such as all-optical methods to image valance charge density and reconstruct electronic band structures, as well as compact extreme ultraviolet light sources. So far these studies are confined to crystalline solids; therefore, decoupling the respective roles of long-range periodicity and high density has been challenging. Here we report the observation of high-harmonic generation from amorphous fused silica. We decouple the role of long-range periodicity by comparing harmonics generated from fused silica and crystalline quartz, which contain the same atomic constituents but differ in long-range periodicity. Our results advance current understanding of the strong-field processes leading to high-harmonic generation in solids with implications for the development of robust and compact extreme ultraviolet light sources.Although higher harmonic generation from solids has become of interest in many fields, its observation is typically limited to crystalline solids. Here, the authors demonstrate that higher harmonics can be generated from amorphous solids.
View details for DOI 10.1038/s41467-017-00989-4
View details for Web of Science ID 000411989800008
View details for PubMedID 28959029
View details for PubMedCentralID PMC5620047
Laser waveform control of extreme ultraviolet high harmonics from solids
2017; 42 (9): 1816-1819
Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.
View details for DOI 10.1364/OL.42.001816
View details for Web of Science ID 000400487700044
View details for PubMedID 28454168
- Anisotropic high-harmonic generation in bulk crystals NATURE PHYSICS 2017; 13 (4): 345-349
- High-harmonic generation from an atomically thin semiconductor NATURE PHYSICS 2017; 13 (3): 262-?
Phase-coherence of high-order harmonics from bulk crystals using homodyne detection
View details for Web of Science ID 000427296200295
High-order harmonics from bulk and 2D crystals
View details for Web of Science ID 000432564600541
Waveform control of high-harmonic generation in solids
View details for Web of Science ID 000427296200292
- Nonsequential two-photon absorption from the K shell in solid zirconium PHYSICAL REVIEW A 2016; 94 (4)
Solid-state harmonics beyond the atomic limit
2016; 534 (7608): 520-?
Strong-field laser excitation of solids can produce extremely nonlinear electronic and optical behaviour. As recently demonstrated, this includes the generation of high harmonics extending into the vacuum-ultraviolet and extreme-ultraviolet regions of the electromagnetic spectrum. High harmonic generation is shown to occur fundamentally differently in solids and in dilute atomic gases. How the microscopic mechanisms in the solid and the gas differ remains a topic of intense debate. Here we report a direct comparison of high harmonic generation in the solid and gas phases of argon and krypton. Owing to the weak van der Waals interaction, rare (noble)-gas solids are a near-ideal medium in which to study the role of high density and periodicity in the generation process. We find that the high harmonic generation spectra from the rare-gas solids exhibit multiple plateaus extending well beyond the atomic limit of the corresponding gas-phase harmonics measured under similar conditions. The appearance of multiple plateaus indicates strong interband couplings involving multiple single-particle bands. We also compare the dependence of the solid and gas harmonic yield on laser ellipticity and find that they are similar, suggesting the importance of electron-hole recollision in these solids. This implies that gas-phase methods such as polarization gating for attosecond pulse generation and orbital tomography could be realized in solids.
View details for DOI 10.1038/nature17660
View details for Web of Science ID 000378270300047
View details for PubMedID 27281195
High harmonics from solids probe Angstrom scale structure
View details for Web of Science ID 000391286401034
- Anomalous nonlinear X-ray Compton scattering NATURE PHYSICS 2015; 11 (11): 964-970
- High-harmonic generation from Bloch electrons in solids PHYSICAL REVIEW A 2015; 91 (4)
- Strong-field and attosecond physics in solids JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 2014; 47 (20)
Measurement of coherence Lengths of Below Threshold Harmonics in Solid Argon
View details for Web of Science ID 000355262505282
- Generation and propagation of high-order harmonics in crystals PHYSICAL REVIEW A 2012; 85 (4)
- Scaling of High-Order Harmonic Generation in the Long Wavelength Limit of a Strong Laser Field IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 2012; 18 (1): 419-433
High-order harmonic generation in solid argon
Conference on Lasers and Electro-Optics (CLEO)
View details for Web of Science ID 000310362403283
Redshift in the Optical Absorption of ZnO Single Crystals in the Presence of an Intense Midinfrared Laser Field
PHYSICAL REVIEW LETTERS
2011; 107 (16)
We report time-resolved electroabsorption of a weak probe in a 500 μm thick zinc-oxide crystal in the presence of a strong midinfrared pump in the tunneling limit. We observe a substantial redshift in the absorption edge that scales with the cube root of intensity up to 1 TW/cm(2) (0.38 eV cm(2/3) TW(-1/3)) after which it increases more slowly to 0.4 eV at a maximum applied intensity of 5 TW/cm(2). The maximum shift corresponds to more than 10% of the band gap. The change in scaling occurs in a regime of nonperturbative high-order harmonic generation where electrons undergo periodic Bragg scattering from the Brillouin zone boundaries. It also coincides with the limit where the electric field becomes comparable to the ratio of the band gap to the lattice spacing.
View details for DOI 10.1103/PhysRevLett.107.167407
View details for Web of Science ID 000296371800010
View details for PubMedID 22107430
- Single-cycle terahertz pulses with > 0.2 V/angstrom field amplitudes via coherent transition radiation APPLIED PHYSICS LETTERS 2011; 99 (14)
Nonlinear Atomic Response to Intense Ultrashort X Rays
PHYSICAL REVIEW LETTERS
2011; 106 (8)
The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.
View details for DOI 10.1103/PhysRevLett.106.083002
View details for Web of Science ID 000287714900007
View details for PubMedID 21405568
- Observation of high-order harmonic generation in a bulk crystal NATURE PHYSICS 2011; 7 (2): 138-141
Strong-field Induced Optical Absorption in ZnO Crystal
Conference on Lasers and Electro-Optics (CLEO)
View details for Web of Science ID 000295612403321
Femtosecond electronic response of atoms to ultra-intense X-rays
2010; 466 (7302): 56-U66
An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.
View details for DOI 10.1038/nature09177
View details for Web of Science ID 000279343800034
View details for PubMedID 20596013
High-Harmonic Generation in Strongly Driven Bulk Periodic Solid
Conference on Lasers and Electro-Optics (CLEO)/Quantum Electronics and Laser Science Conference (QELS)
View details for Web of Science ID 000290513600391
- Measurement of attosecond XUV pulses generated with polarization gating by two-dimensional photoelectron spectroscopy SPIE-INT SOC OPTICAL ENGINEERING. 2007